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(SUBMITTED TO THE ASTRONOMICAL JOURNAL - REVISED: AUGUST 30, 2011)
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A PETAL OF THE SUNFLOWER:
PHOTOMETRY OF THE STELLAR TIDAL STREAM IN THE HALO OF MESSIER 63 (NGC 5055)*
TAYLOR S. CHONIS1, DAVID MARTÍNEZ-DELGADO2, R. JAY GABANY3,
STEVEN R. MAJEWSKI4, GARY J. HILL5, RAY GRALAK6,AND IGNACIO TRUJILLO7, 8,9
(Submitted to The Astronomical Journal - Revised: August 30, 2011)
ABSTRACT
We present deep surface photometry of a very faint, giant arc-loop feature in the halo of the nearby spiral
galaxy NGC 5055 (M63) that is consistent with being a part of a stellar stream resulting from the disruption
of a dwarf satellite galaxy. This faint feature was first detected in early photographic studies by van der
Kruit (1979); more recently by Martínez-Delgado et al. (2010) and as presented in this work, the loop has
been realized to be the result of a recent minor merger through evidence obtained by wide-field, deep images
taken with a telescope of only 0.16 m aperture. The stellar stream is clearly confirmed in additional deep
images taken with the 0.5 m telescope of the BlackBird Remote Observatory and the 0.8 m telescope of the
McDonald Observatory. This low surface brightness (µR≈26 mag arcsec−2), arc-like structure around the
disk of the galaxy extends 14.0′(∼29 kpc projected) from its center, with a projected width of 1.6′(∼3.3
kpc). The stream’s morphology is consistent with that of the visible part of a giant, “great-circle" type stellar
stream originating from the recent accretion of a ∼108M⊙dwarf satellite in the last few Gyr. The progenitor
satellite’s current position and final fate are not conclusive from our data. The color of the stream’s stars is
consistent with dwarfs in the Local Group and is similar to the outer faint regions of M63’s disk and stellar
halo. Through our photometric study, we detect other low surface brightness “plumes"; some of these may
be extended spiral features related to the galaxy’s complex spiral structure and others may be tidal debris
associated with the disruption of the galaxy’s outer stellar disk as a result of the accretion event. We are able
to differentiate between features related to the tidal stream and faint, blue, extended features in the outskirts of
the galaxy’s disk previously detected by the GALEX satellite. With its highly warped HI gaseous disk (∼20◦),
M63 represents one of several examples of an isolated spiral galaxy with a warped disk showing recently
discovered strong evidence of an ongoing minor merger.
Subject headings: galaxies: dwarf — galaxies: evolution — galaxies: halos — galaxies: individual (NGC
5505) — galaxies: interactions — galaxies: photometry
1. INTRODUCTION
In the context of a cold dark matter (ΛCDM) universe, dark
matter halo mergers (and the subsequent merging of their
baryonic components) drive the evolution of galaxies (White
& Frenk 1991). While state of the art cosmological simula-
tions now show this (e.g., Springel et al. 2005), it has been
advanced for nearly 40 years that galaxy mergers create a
hierarchical framework describing galactic evolution, based
in part on the characterization of stellar populations in the
Milky-Way (MW) stellar halo (Searle & Zinn 1978) and on
views of extra-galactic interactions (Toomre & Toomre 1972).
While major galactic mergers are rare in ΛCDM at the present
*This paper includes data taken at The McDonald Observatory of The
University of Texas at Austin.
1Dept. of Astronomy, University of Texas at Austin, 1 University Station,
C1400, Austin, TX 78712, USA: tschonis@astro.as.utexas.edu
2Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidel-
berg, Germany
3BlackBird Observatory, Mayhill, New Mexico, USA
4Dept. of Astronomy, University of Virginia, 530 McCormick Rd., Char-
lottesville, VA 22904, USA
5McDonald Observatory, University of Texas at Austin, 1 University Sta-
tion, C1402, Austin, TX 78712, USA
6Sirius Imaging Observatory, Mayhill, New Mexico, USA
7Instituto de Astrofísica de Canarias, C/ Vía Láctea, s/n, E38205 - La La-
guna (Tenerife), Spain
8Departamento de Astrofísica, Universidad de La Laguna, La Laguna
(Tenerife), Spain
9Ramón y Cajal Fellow
epoch (Robaina et al. 2009), minor mergers (i.e., those oc-
curring between a massive galaxy and a much less massive
satellite) are expected to occur continuously and play an in-
fluential role in the ongoing evolution of present day galax-
ies. Minor mergers where the more massive galaxy is a spi-
ral generally leaves pre-existing stellar disks intact (Robertson
et al. 2006), though often dynamically altered (Velázquez &
White 1999). Even if no obvious interacting satellite can be
found, signatures of such an interaction may be present for
several Gyr, such as optical or HI disk warps (Sancisi 1976)
or heated disks (Tóth & Ostriker 1992; Hernquist & Quinn
1993; Velázquez & White 1999). In a ΛCDM cosmology, the
heating and subsequent survival of disks through many minor
mergers over cosmic time remains a debated topic of great in-
terest (e.g., Purcell et al. 2009; Moster et al. 2010). Recent
simulations (e.g., Bullock & Johnston 2005; Johnston et al.
2008; Cooper et al. 2010) have shown that another observable
signature of such minor mergers should be found in the form
of low surface brightness tidal features (e.g., stellar streams)
resulting from the disruption of the satellite while still in orbit.
These features extend into the stellar halo of the parent spiral
and could be viewed as evidence of the evolution of a spiral
galaxy stellar halo in-action, an event postulated as long ago
as Searle & Zinn (1978).
With the advent of wide-field digital sky surveys, such sub-
structure has been found in the stellar halo of the MW in
the form of faint over-densities and streams that are resolved
2 T. S. CHONIS ET AL.
into individual stars and reveal the evolutionary history of our
galaxy. Arguably, the most spectacular of such structures is
the well-studied Sagittarius dwarf galaxy and its tidal stream
(e.g., Majewski et al. 2003), which has been found to be quite
complex and rich in substructure (Belokurov et al. 2006). This
system has been extensively modeled (e.g., Law & Majew-
ski 2010, Peñarrubia et al. 2010a). Observational evidence
shows that its tidal tails appear to consist of long streams
of debris wrapping around the MW in a near polar, rosette-
like orbit (Martínez-Delgado et al. 2004). A wealth of other
streams have also been detected surrounding our galaxy, such
as the low galactic latitude Monoceros tidal stream (Yanny et
al. 2003), the Orphan stream (Belokurov et al. 2007), and the
Anticenter stream (Grillmair 2006). Similar structures have
also been observed around our nearest massive neighbor, the
Andromeda Galaxy (M31; Ibata et al. 2001a, McConnachie
et al. 2009), including giant features extending >200 kpc to-
wards M33 (Ibata et al. 2007; McConnachie et al. 2010). Such
features are evidence for the inside-out formation of galaxies
and the role of minor mergers in the evolution of spiral galaxy
stellar halos in the Local Group.
If such substructure can be observed around other nearby,
relatively isolated galaxies outside the Local Group, this con-
clusion could be generalized to late-type massive spirals in
the present epoch. This prospect has been explored through
the semi-analytical models of Johnston et al. (2001) and more
recently by the ΛCDM-based models of Bullock & Johnston
(2005), Johnston et al. (2008), and Cooper et al. (2010). In
fact, these authors predict that in a statistically significant
sample of isolated late-type Local Volume spirals, nearly ev-
ery galaxy should display such a fossil record of its recent
evolution if imaged to sufficient depth (e.g., µV≈30 mag
arcsec−2). Indeed, several examples of extra-galactic tidal
streams resulting from dwarf accretion currently exist as a
result of recent research. Malin & Hadley (1997) have per-
formed deep imaging on many galaxies and found loop-like
features around M83 and M104. A similar loop structure was
discovered by Shang et al. (1998) around NGC 5907, the pro-
totypical isolated galaxy displaying a significant optical and
HI warp. Deep imaging of NGC 5907 by Martínez-Delgado et
al. (2008) later revealed a stunning complex of streams in ad-
dition to the brightest one discovered by Shang et al. (1998).
Martínez-Delgado et al. (2009) discovered a stream resem-
bling models of the MW’s Monoceros stellar stream (Yanny et
al. 2003) around NGC 4013, which displays one of the largest
known HI warps despite its relative isolation. Wehner & Gal-
lagher (2005) discovered a set of streamers indicating a com-
plex merger history around the face-on starburst galaxy NGC
3310 and later performed a detailed study to find that the ma-
terial did not originate from NGC 3310 itself (Wehner et al.
2006). Recently, Mouhcine et al. (2010) discovered streams
and a thick stellar envelope around NGC 891, a MW ana-
logue, and performed the first ground-based resolved study of
stars in a tidal stream system outside of the Local Group. De-
spite these examples, there are still relatively few known tidal
stream systems outside of the Local Group.
Observing extra-galactic stellar streams due to dwarf accre-
tion analogous to those found in the Local Group can show
us that our own galaxy is not unusual, reveal different phases
of such interactions, and can open up discussions of different
mass and orbit combinations and accretion histories. More
fundamentally, it will provide statistical tests of the predic-
tions of galaxy formation and evolutionary models based on
the ΛCDM paradigm. The stunning perspective of extra-
galactic tidal streams obtained with modest instruments like
those used by Martínez-Delgado et al. (2008, 2009) for NGC
5907 and NGC 4013, respectively, encourages a more sys-
tematic look for these ghostly structures in the nearby uni-
verse. With that purpose, a pilot survey was conducted of stel-
lar tidal streams in a select sample of nearby, MW-like spiral
systems using modest aperture telescopes (0.1 - 0.5 m) operat-
ing under very dark skies (Martínez-Delgado et al. 2010). For
the first time, this pilot survey has allowed a comparison to
ΛCDM based simulations that model the evolution of stellar
halos through satellite disruption (see the cited works in the
preceding paragraph) with observations. There exists amaz-
ing morphological agreement between these simulations and
the range of tidal features observed around nearby “normal"
disk galaxies.
Here, we present surface photometry of similar low surface
brightness features around NGC 5055 (hereafter, M63), in-
cluding a large loop-like structure that is consistent with being
part of a stellar tidal stream displaying a “great-circle" mor-
phology (Johnston et al. 2008; Martínez-Delgado et al. 2010).
M63 is a large, relatively isolated SA(rs)bc galaxy (de Vau-
couleurs et al. 1991) situated at a distance of 7.2 Mpc (Pierce
1994), and belongs to the M51 galaxy group (Fouque et al.
1992). Its optical disk displays a fragmented and patchy pat-
tern of spiral arms that extend outwards, resembling a celestial
flower (hence its popular name: “The Sunflower Galaxy").
This characteristic places it in the class of flocculent spirals
(Elmegreen & Elmegreen 1987). Similar to discoveries made
for NGC 5907 and NGC 4013, a recent HI study of M63
by Battaglia et al. (2006) revealed a very pronounced warp
(∼20◦) in its gaseous disk that extends out ∼40 kpc from its
center. Additionally, M63 has recently been found through
a GALEX study by Thilker et al. (2007) to host a Type 1 ex-
tended UV (XUV) disk that is characterized by significant star
formation well beyond the anticipated star formation thresh-
old (i.e., the Hαedge; see Martin & Kennicutt 2001) at a
galactocentric distance of up to ∼20 kpc. According to the
HYPERLEDA database, M63 has a total Bband absolute
magnitude of -21.2 AB mag and a radius of 11.7′measured at
the µB= 25 AB mag arcsec−2isophote. For the reader’s refer-
ence, we note that the stellar mass and total dark matter halo
mass of M63 within 40 kpc are 8×1010 M⊙and 2×1011 M⊙,
respectively, as determined by Battaglia et al. (2006) by fit-
ting the observed rotation curve with a model that includes a
“maximum disk” for the stellar component and an isothermal
halo.
The faint arc-like structure in the outskirts of M63 was first
detected and speculated upon in a photographic study by van
der Kruit (1979). The current work will show that this struc-
ture is consistent with being part of the ongoing evolution
of M63’s stellar halo. Along with the strong HI warp, this
feature is evidence for a recent interaction with a low-mass
companion and becomes yet another example of the possible
link between disk warps and recent mergers. In the follow-
ing section, we describe the observations, data reduction, and
analysis in detail. In §3, we discuss the myriad of low sur-
face brightness features detected and measured in our deep
images. Finally, through various subsections in §4, we outline
and discuss our general conclusions. Throughout this paper,
all reported UBVRI magnitudes are on the Johnson (Vega)
system and all ugriz magnitudes are on the AB system, unless
otherwise specified.
THE TIDAL STREAM OF MESSIER 63 3
FIG. 1.— The M63 stellar tidal stream detection and confirmation images. The orientation and scale are indicated by the 6′(12.6 kpc) long arrows in the
lower-right corners of each image. Shown are the final 31.7 hour image obtained with the G-NMS 0.16 m (a) and the final 11.0 hour image obtained with the
BBRO 0.5 m (b) resulting from the combination of the images listed in Table 1 after histogram equalization. The images have been cropped to 35.2′×25.7′(a)
and 26.9′×17.3′(b).
TABLE 1
LIST OF DATA AND EXPOSURE TIMES
Telescope Filter Tot. Exp. Time
(minutes)
G-NMS 0.16 m CL 790
Red 390
Green 390
Blue 330
1900∗
BBRO 0.5 m CL 405
Red 90
Green 54
Blue 108
657∗
MDO 0.8 m B390†
R320†
∗Total exposure time for images in Figure 1.
†Total exposure time for images in Figure 2.
2. OBSERVATIONS AND DATA REDUCTION
To obtain a clearer view of the barely detectable features
first described by van der Kruit (1979), we obtained deep,
wide-field images of M63 with a small 0.16 m telescope and
first presented these data in Martínez-Delgado et al. (2010).
For completeness, these observations will be described be-
low. We confirm the detection and study these features in
more detail through new follow-up observations with two ad-
ditional telescope-instrument combinations. Those less con-
cerned with the technical observational details should skip di-
rectly to §3 for our findings.
2.1. Gralak-New Mexico Skies 0.16 m Telescope
As first presented in Martínez-Delgado et al. (2010), low
surface brightness features were detected in deep optical im-
ages of M63 taken with R. Gralak’s 0.16 m Astro-Physics
Starfire 160 f/5.9 Apochromatic refractor during dark-sky ob-
serving runs between April 2007 and February 2008 at New
Mexico Skies Observatory situated in the Sacramento Moun-
tains (New Mexico, USA - this telescope shall be referred to
hereafter as G-NMS 0.16 m). The 0.16 m telescope is out-
fitted with a Santa Barbara Instruments Group (SBIG) STL-
11000M11 CCD camera. This setup yields a large 131.2′×
87.5′field of view (FOV) at an image scale of 1.96′′ pixel−1.
The data set consists of deep exposures (with individual ex-
posure times of 20 or 30 minutes) taken through four non-
standard photometric filters: a wide, non-infrared “Clear-
Luminance" filter (CL; 3500 < λ (Å) <8500) as well as Red,
Green, and Blue filters from the SBIG-Custom Scientific filter
set. Observations in these filters were intended for construct-
ing a true-color image of M63. The details of the data acqui-
sition and reduction (including standard methods for bias and
dark subtraction as well as flat-fielding and image coaddition)
are identical to those described in similar work by our group
(Martínez-Delgado et al. 2008, 2009). As summarized in Ta-
ble 1, the total combined integration time of the data taken in
all filters is 31.67 hours (1900 minutes).
The final combined image contains a very large dynamic
range since the tidal stream is much fainter than M63’s disk.
As described in Martínez-Delgado et al. (2008, 2009), the fi-
nal image has been histogram equalized by the application of
an iterative, non-linear transfer function to optimize contrast
and detail in the faintest parts of the image. The resulting
image can be seen in Panel aof Figure 1. A complex, faint
outer structure can be seen in unprecedented detail surround-
ing M63, including the large loop feature that was alluded to
originally by van der Kruit (1979).
2.2. BlackBird Remote Observatory 0.5m Telescope
We confirmed the above detection by re-examining a set
of archived CCD images obtained during dark-sky observ-
ing runs in March and April of 2005 with the 0.5m f/8.3
Ritchey-Chrétien telescope of the BlackBird Remote Obser-
vatory (BBRO), also located in the Sacramento Mountains.
The data were acquired using an SBIG STL-11000M CCD
camera, which yields a 29.8′×19.9′FOV at an image scale of
0.45′′ pixel−1. As with the G-NMS data, the BBRO image set
consists of multiple deep exposures (with individual exposure
times of around 15 minutes) taken through the SBIG-Custom
Scientific filter set. Data acquisition and standard data reduc-
tion procedures are also as described in the works cited above
in §2.1. Table 1 summarizes the exposures included in 10.95
11 See http://www.sbig.com/sbwhtmls/large_format_cameras.htm#STL-
11000M for the camera’s specifications
4 T. S. CHONIS ET AL.
FIG. 2.— MDO 0.8 m B(left) and R(right) final combined images. The orientation and scale are indicated by the 6′(12.6 kpc) long arrows. The Band R
images result from the combination of 6.5 and 5.3 hours of exposures, respectively. Each has been background subtracted as described in the text and histogram
equalized. The field after trimming is 37.8′×34.9′. The dashed lines show the location of cuts through the image to demonstrate the effectiveness of the sky
background subtraction, and are referred to in Figure 3.
hours (657 minutes) of total integration. The resulting his-
togram equalized image is shown in Panel bof Figure 1.
The BBRO 0.5m telescope has been used by our group in
other studies of low surface brightness features around nearby
spiral galaxies as a part of the pilot survey discussed in §1
(Martínez-Delgado et al. 2010; see also Martínez-Delgado et
al. 2008, 2009, Trujillo et al. 2009, and Sollima et al. 2010).
From past experience (which will be confirmed later in this
work), we estimate that we are able to detect faint features in
the BBRO image (and the G-NMS final image) to a surface
brightness of µR.27 mag arcsec−2.
2.3. McDonald Observatory 0.8 m Telescope
Since the above observations yield only limited photometric
information, we have obtained follow-up observations using
the McDonald Observatory (MDO) 0.8 m telescope. These
observations will allow us to characterize portions of the faint,
outer structure and potentially differentiate the tidal stream
from other extended stellar halo and disk components.
The 0.8 m telescope utilizes a wide-field instrument, the
Prime Focus Corrector (PFC), at f/3.0 (Claver 1992). This
instrument contains a front-side illuminated Loral-Fairchild
2048×2048 CCD with 15µpixels that covers a 46.2′×46.2′
FOV at an image scale of 1.355′′ pixel−1. The detector has
a read noise of 5.87 e−and a gain of 1.60 e−ADU−1. The
instrument covers a wide spectral range in the optical and
near-IR from 3000 Å to 1 µm, which is divided by a stan-
dard Bessel UBVRI filter set. Deep images were acquired in
Band Rduring a dark-sky observing run in April 2009 and are
also summarized in Table 1. All on-sky images were taken at
an airmass X<1.6 (∼70% of all images were acquired with
X<1.2) and were dithered to reduce systematics. The ob-
serving conditions were generally very good; however, two
nights had relatively variable transparency conditions.
2.3.1. Standard Data Reduction
Standard data reduction procedures for overscan correction,
bias subtraction, and dome flat-fielding were carried out us-
ing the CCDRED package in IRAF.12 No dark subtraction
was applied as the dark signal is negligible in a 900 second
exposure, the longest used in this study. Illumination cor-
rection is performed by taking at least 3 blank-sky frames
off-target (since M63 and its faint outer structure occupy a
large fraction of the CCD’s imaging area) per filter, inter-
spersed evenly throughout each night at identical exposure
times to the science frames. These sky images were median-
combined with a σ-clipping algorithm to remove stars. This
simple method removes all but the faintest parts of the outer
stellar point spread functions (PSF) that happen to overlap be-
tween individual frames and evade rejection. The PSF resid-
uals were found to be ∼25% of the large-scale sky varia-
tion, randomly distributed throughout the combined frame,
and relatively small in extent (typically occupying <1% of
the total image area, or less than a few ×104pixels). These
resulting images were subsequently fit using a fifth order,
two-dimensional Legendre polynomial. The fit for each fil-
ter’s combined blank-sky frame serves as the illumination cor-
rection and was applied to each flat-fielded science image.
Each calibrated science image was then examined for variable
transparency through aperture photometry of several bright
stars in the field. Those frames with instrumental magnitudes
lying >2σfrom the mean were discarded. The remaining im-
ages were then average-combined. The final Band Rimages
are combinations of 26×900 second images (6.5 hours) and
32×600 second images (5.3 hours), respectively.
2.3.2. Background Sky Modeling and Subtraction
As will be seen, the components of the faint outer regions
of M63 we are most interested in are ∼10 ADU above the
background. Thus, careful background subtraction of the fi-
nal images must be obtained for useful surface photometry.
Subtraction of a constant value for the sky background is com-
12 IRAF is distributed by the National Optical Astronomy Observatories,
which is operated by the Association of Universities for Research in Astron-
omy, Inc., under cooperative agreement with the National Science Founda-
tion.
THE TIDAL STREAM OF MESSIER 63 5
TABLE 2
RANDOM NOISE SOURCES: MDO 0.8M-RIMAGE∗
Per Single Pixel 75×75 Pixel Bin
Source of Uncertainty (ADU) (%)#(ADU) (%)#
Image Read Noise†0.65 0.03 0.01 0.00
Image Photon Noise†6.70 0.29 0.09 0.00
Flat-Field Photon Noise‡4.42 0.19 0.06 0.00
Surface Brightness Fluctuation∗∗ 2.21 0.09 0.03 0.00
TotalRandom Noise 8.33 0.36 0.11 0.01
NOTE. — See §2.3.4 for details on the sources of uncertainty listed in this table.
∗Average source ADU = 9.88; average sky ADU = 2287.52.
#Percentage of the average total flux per pixel.
†Calculated for 32 science images.
‡Calculated for 25 flat-field frames with average 25000 ADU.
∗∗ Calculated from Tonry & Schneider (1988), Equation 12.
plicated by the fact that the night sky is not uniform over such
a large FOV (Wild 1997). Additionally, Figure 2 shows that
M63 and its faint outer regions occupy a large fraction of the
imaged area. With this in mind, a simple mask and global
modeling technique (using a 2-D analytical function, for ex-
ample) is likely not the best choice for background determina-
tion. Zheng et al. (1999) found that such a method systemati-
cally under or over-fits certain regions of their images used for
deep surface photometry. Instead, they fit the sky piece-wise
in a row-by-row and column-by-column fashion using low-
order polynomials. We adopt this method of sky subtraction
and briefly describe it here as it applies to our observations.
We begin by masking all sources in the Band Rfinal images
so that the wings of stellar PSFs, small galaxies, and all faint
regions of M63 were covered to .15% of the sky variation.
This level was chosen because there is a trade-off between
the surface brightness one can mask the PSF wings to and
leaving enough pixels after masking to properly sample the
fit (Zheng et al. 1999). For both the Band Rimages, ∼51%
of pixels were masked leaving ∼49% available for the fitting
process. We then twice model the background piece-wise us-
ing third order polynomials: once fitting each row and again
fitting each column, excluding masked pixels and those lying
±2σfrom the mean value of all non-masked pixels in the row
or column. The models fit from rows and from columns are
then averaged to produce a final background model that is bet-
ter sampled, and was finally smoothed using a large Gaussian
kernel (σ=80 pixels; 4σcutoff) to eliminate any small artifacts
from the modeling process. The result is then subtracted from
the final Band Rimages as derived for each individually. The
accuracy of the modeling and subtraction will be evaluated
below in §2.3.4. The resulting sky models show that the final
Band Rimages have backgrounds that are already quite free
of variation. The fit maximum-to-minimum variation from
average is 0.74% for Band 0.73% for R.
2.3.3. Photometric Calibration
M63 lies in a Sloan Digital Sky Survey (SDSS; York et al.
2000) field. Thanks to the PFC’s wide FOV, there are a large
number of isolated, dim stars (i.e., not saturated in SDSS or
our images) available by which we can obtain a photomet-
ric calibration for Band Rusing “tertiary" gri SDSS stan-
dards following the method and transformations of Chonis &
Gaskell (2008). Their transformations (Equations 1 and 3 in
that work) are valid only over the ranges 0.2<g−r<1.4
and 0.08 <r−i<0.5. To determine that these color ranges
are appropriate for our observations, we plot B−Rvs. g−r
and B−Rvs. r−icolor-color diagrams using the large sam-
ple of matched Stetson (2000) BVRI standard photometry and
SDSS DR4 (Adelman-McCarthy et al. 2006) ugriz photome-
try (>1200 stars) from Jordi et al. (2006). Using these di-
agrams, we verify the linearity between B−Rand the SDSS
colors over the limited color ranges. Given that the range of
B−Rgalaxy colors (0.4<B−R<1.8) from the large, mor-
phologically diverse sample of Jansen et al. (2000) comfort-
ably fits within these linear regions, we expect the transforma-
tions of Chonis & Gaskell (2008) to be adequate for the range
of B−Rcolors we might expect to observe in the M63 field.
Instrumental magnitudes of 113 stars in each Band Rimage
were obtained; SDSS photometry for these stars was obtained
from the DR7 SkyServer13 (Abazajian et al. 2009). Follow-
ing the selection criteria of Chonis & Gaskell (2008), 63 of
these stars were transformed from SDSS magnitudes to Band
Rand compared to the instrumental magnitudes. The residu-
als were fit with a constant zero-point offset as well as a small
color term in the instrumental B−Rcolor. The standard error
in the mean of the residuals after this correction is given by
σphot,x(63)−1/2, where σphot,xis the residuals’ standard devia-
tion in photometric band x(where xis either Bor R). This
results in an uncertainty of 0.010 mag and 0.007 mag for B
and Rrespectively. This uncertainty’s contribution to the total
error budget will be discussed below.
Note also that all reported magnitudes for M63 have been
additionally corrected for Galactic extinction, but not for
galaxy inclination. From Schlegel et al. (1998), AB= 0.076
and AR= 0.047; according to the NASA/IPAC Infrared Sci-
ence Archive,14 AVvaries over the MDO 0.8 m FOV by no
more than ±0.01 mag in this region of the sky.
2.3.4. Estimation of Measurement Uncertainties
We discuss the various sources of uncertainty in our surface
brightness measurements and describe the methods by which
we estimate them. The sources of uncertainty can be divided
into two main categories:
1. Random Noise - Sources of random noise, such as read
noise from the CCD read-out process and Poisson noise due to
13 DR7 SkyServer: http://cas.sdss.org/dr7/en/
14 NASA/IPAC Infrared Science Archive:
http://irsa.ipac.caltech.edu/applications/DUST
6 T. S. CHONIS ET AL.
FIG. 3.— Profiles showing the effectiveness of the sky background subtraction process. The location of the cuts can be seen in Figure 2. Each profile is
median-combined over a 30 pixel-wide strip. Cut 1 (top panels) is through a region with relatively few sources (which are masked) to show typical residuals of
the background subtraction. The background of the B(R) image has a typical value before subtraction of 364 (2283) ADU. Cut 2 (bottom panels) is through
the stellar stream and the outer stellar halo of the galaxy with no sources masked to show the relative signal strength of the photometric features of interest as
compared to the subtracted background. The arrows indicate the location of signal due to the stellar stream.
photon counting for both science frames and flat-field frames,
are calculated using standard methods. An additional source
of random noise is surface brightness fluctuations from the
measured photometric feature itself. Because the faint sub-
structures of interest are composed of individual stars, each
pixel contains noise from counting statistics (because there
are an average, finite number of stars contained in the solid
angle on the sky subtended by an individual pixel). The noise
contributed by this surface brightness fluctuation is given by
the square-root of Equation 10 in Tonry & Schneider (1988).
In their equation, we use d= 7.2 Mpc; for B(R), t= 900 (600)
seconds, the magnitude corresponding to 1 ADU/s m1= 20.91
(21.51) mag, and ¯
M= 1 (0, to be conservative). Finally, ¯
gis
the average signal (in ADU) of the area of the galaxy being
measured. Sampling more pixels in a given measurement bin
reduces random noise (e.g., as n−1/2, where nis the number of
pixels in the bin). Thus, we can reduce these noise sources to
near negligible levels, as will be the case for all signal levels
considered in this work, by sampling many pixels covering a
photometric feature. To illustrate this and to give a sense of
the level of random noise in our images, we sample a 75×75
pixel box centered in a region of the stellar stream in the R
band image containing on average 9.88 ADU pixel−1from
the stream and 2287.52 ADU pixel−1from the sky. Based
on the photometric calibration discussed in §2.3.3, this results
in a surface brightness for the stellar stream of µR= 26.0 mag
arcsec−2. The noise in this bin is compared to that in a single
pixel and is tabulated in Table 2. The noise levels are reported
both in ADU and as a percentage of the average total flux per
pixel.
2. Non-Random Errors - The main source of non-random
error in photometry of large, low surface brightness objects
results from the inevitably imperfect sky subtraction process.
Figure 3 shows profiles through the Band Rimages whose
locations are indicated by the dashed lines in Figure 2. Cut
1 shows that the mean of the background after subtraction is
very close to 0 and that there are only slight indications of
systematic errors introduced by the sky subtraction process,
which are small in amplitude and extent. Cut 2 shows the
signal from the stellar stream and faint disk light of M63 af-
ter background subtraction. For a more quantitative look at
the background after sky subtraction, we calculate the me-
dian of pixels contained in various sized bins placed in a
grid across the masked, background subtracted final images
in which we consider only bins with >85% unmasked pix-
els. On all scales, the distributions of bin medians are nearly
Gaussian in shape. As noted in Zheng et al. (1999), these dis-
tributions are slightly skewed in a positive sense given that
the faint, extreme outer parts of bright PSFs are not neces-
sarily masked since masking them entirely would leave very
few pixels with which to fit the background. This slightly
THE TIDAL STREAM OF MESSIER 63 7
FIG. 4.— The standard deviation σ(top) of the distribution of the median
of pixels contained in various sized bins placed in the non-masked portions
of the B(diamonds) and R(stars) images after sky subtraction as a function
of the total number of pixels ncontained in each bin. The thin solid lines
are power law fits for each set of data points. The same is also shown for
the quantity σall /σout (bottom), which is the ratio of the standard deviation
of the distribution of the median of pixels contained in bins placed over the
entire combined blank-sky illumination correction image after background
modeling and subtraction to that placed outside the dummy mask.
increases the distributions’ standard deviations. In the pres-
ence of the positive skew, we note that the modes of the dis-
tributions are 0, indicating that the backgrounds were properly
subtracted. The top panel of Figure 4 shows the trend of the
distributions’ standard deviations σwith the number of pixels
ncontained in the bin for both Band Rimages. Note that σ
measures both random noise in the background and the uncer-
tainty due to sky subtraction. By inspection of the top panel
of Figure 4, one can see that σis much larger than would be
indicated by random noise alone (see Table 2). This excess
uncertainty in the measured values of σis due to the system-
atic error in the background modeling and subtraction. As
such, the data are fit well with power laws in nhaving expo-
nents of -0.20 and -0.18 for Band R, respectively, rather than
-0.50 as would be expected from only random noise.
The pixels sampled above can only be those from the sky
that were unmasked and directly fit. Thus it is unknown to
what degree the sky fitting method properly interpolates the
sky values under the mask where measurements will later be
taken. To quantify this, we combine all frames taken off-target
for illumination correction for each filter and median-combine
them with a σ-clipping algorithm, removing the signature of
field stars as described in §2.3.1. The mask used for mod-
eling the background of the science frames was then applied
as a dummy mask. We use the remaining pixels to model
the blank-sky background, using third order polynomials as
in §2.3.2. The final fit was subtracted from the combined
blank-sky frames and the distribution of bin median values
was determined outside the dummy mask, as described in the
preceding paragraph. We denote this distribution’s standard
deviation as σout. We also sample bins over the entire im-
age with the dummy mask removed after subtracting the sky
model since there is no diffuse source under it. The standard
deviation of this distribution is also calculated and is denoted
σall. We find for both Band Rthat in general, σall/σout >1,
which is indicative of a less than ideal interpolation in the
masked region where the galaxy is located and the sky is not
directly fit. This quantity is not affected by the PSF residu-
als in the σ-clipped median combine since their effect cancels
when taking the ratio due to the random distribution through-
out the image. Finding that σall/σout >1 is not surprising
given that M63 occupies a large fraction of the total image
area. In the bottom panel of Figure 4, we show the trend of
σall/σout with nfor both Band Rimages. Both data sets are
well fit together with a single power law in nwith an exponent
of 0.10. This analysis with that described in the preceding
paragraph shows that the measurement uncertainty after back-
ground subtraction has only a weak dependence on the size of
the feature being measured. It also shows that the background
subtraction accuracy is limited by the faint, outer portions of
bright PSFs and the inability to directly model the sky under
the large galaxy mask.
The remaining source of uncertainty is in the photometric
zero-point determined in §2.3.3. Since this uncertainty is in a
systematic photometric offset, it is simply added into the total
error budget after its conversion to magnitudes.
Sample Error Estimate - Using the above empirical esti-
mates of the measurement uncertainty after subtracting the
modeled background, we give a sample error estimate for the
75×75 pixel box centered on the tidal stream in which we
estimated the contribution of uncertainty due to random noise
for the Rimage. We first estimate σ(top panel of Figure 4)
and multiply this value by the corresponding scaling factor
(bottom panel) to take into account the imperfect interpola-
tion under the mask where the stream is located. For a 75×75
pixel box, this corresponds to an uncertainty of 1.21 ADU.
We then add the small contribution from random noise (here,
0.11 ADU from Table 2). After conversion to magnitudes and
addition of the uncertainty in the photometric zero-point, this
yields a surface brightness of µR= 26.0+0.2
−0.1mag arcsec−2.
2.3.5. Surface Brightness Maps
We create a new mask covering the bright disk of M63 and
all intervening field stars while leaving the faint outer features
exposed. The remaining unmasked pixels are converted from
ADUs to magnitudes, producing Band Rsurface brightness
maps of the faint structures based on the images shown in
Figure 2. These maps are shown in Figure 5. The brightest
isophote outside of the mask is µR≈23 mag arcsec−2. As
can be seen, we can reliably measure features to µR≈27 mag
arcsec−2.
From these maps, we have created a B−Rcolor index map.
Since colors are inherently more uncertain than an individ-
ual surface brightness measurement, we bin the B−Rarray
in 7×7 pixel boxes, taking the median of the pixels in the
bin while ignoring those masked. Only bins with >60% un-
masked pixels are considered. The resulting color index map
can be seen overlaid on the G-NMS image in Figure 6. We
8 T. S. CHONIS ET AL.
FIG. 5.— B(left) and R(right) surface brightness contour maps, derived based on the description given in §2.3.5. Note that the contour scales for each are not
identical.
FIG. 6.— B−Rcolor index map, derived based on the description given in
§2.3.5 and overlaid on the G-NMS image. Typical errors in B−Rare ±0.2
mag for a feature with µR≈26 mag arcsec−2(refer to Figure 5 for the surface
brightness of various features).
find that B−Rcolors in M63’s faint outer structures range
from ∼0.8 to ∼1.7, with noticeable systematic color gradi-
ents present. The implication of both the surface brightness
maps and the B−Rcolor map are discussed in the following
section.
3. LOW SURFACE BRIGHTNESS FEATURES
Based on our deep imaging, we have identified 8 dis-
tinct photometric features of interest, the most prominent of
which is the giant loop structure first discovered by van der
Kruit (1979) to the north-east of the galaxy’s disk resembling
a flower petal (which is fitting, considering M63’s popular
name). We identify and label these features in Figure 7. For
all features, we have calculated Band Rsurface brightness
and B−Rcolor indices (averaged over the entire spatial extent
of each feature, as illustrated in Figure 8). The measurements
are presented in Table 3.
3.1. The Tidal Stream of M63
In Figure 7, feature ais the faint loop structure that appar-
ently emerges from the East side of M63’s disk and sweeps al-
most 180◦around the system to the North-East with its center
reaching 14.0′(∼29 kpc projected) from the galaxy’s center
and entering again at the North-West side. When first discov-
ered by van der Kruit (1979), there was little doubt that this
feature was real. However, it was unclear if it originated in
M63 itself, or if it was instead “high latitude reflection nebu-
losity in our Galaxy" (i.e., Galactic cirrus), analogous to that
contaminating the field of M81 and M82 (Sollima et al. 2010).
Our deep images show that this feature is part of M63 based
on similar morphology to the great-circle type tidal streams,
arising from the recent merger and disruption of a lower mass
satellite galaxy in a nearly circular orbit (Johnston et al. 2008;
Martínez-Delgado et al. 2010; see §4 below for further discus-
sion). This morphology is also reported for the stellar stream
associated with the Sagittarius dwarf (e.g., Ibata et al. 2001b;
Martínez-Delgado et al. 2004; Law & Majewski 2010) and
around other near-by systems, such as NGC 5907 (Shang et
al. 1998; Martínez-Delgado et al. 2008) and M83 (Malin &
Hadley 1997).
To quantify the morphological characteristics of feature a,
we have measured its projected orientation relative to the spi-
ral disk of M63 by fitting ellipses to the MDO images. The
ellipticity of M63’s disk, measured at the µB= 25.0 mag
arcsec−2isophote, is found to be ǫM63 = 0.80. The flatten-
ing is thus EM63 = 0.40, yielding an inclination of iM63 =
arccos(1−EM63)≈53.3◦. To check our ellipse fitting proce-
dure, we verify that this value is in good agreement with that
reported by the HYPERLEDA database (iM63 ≈56.0◦). The
stream’s light distribution was traced and fit with an ellipse
having a semi-major axis aof 8.95′(18.7 kpc) and a minor-
to-major axis ratio q= 0.57. We find that the position of the
major-axis of the stream is tilted with respect to the major-axis
of M63’s disk, with a measured difference in position angle
∆φ= 14.8◦. Note, however, that with no dynamical informa-
THE TIDAL STREAM OF MESSIER 63 9
TABLE 3
MEASURED PROPERTIES OF LOW SURFACE BRIGHTNESS FEATURES IN MESSIER 63
µBµRB-R
Feature (mag arcsec−2) (mag arcsec−2) (mag) Association/Comment
aLoop 27.6±0.2 26.1±0.1 1.5±0.2 Stellar Stream
Dim Break 28.3+0.7
−0.426.8±0.3 —
b................. 27.0±0.1 25.6±0.08 1.5±0.1 Tidal Debris
c................. 25.80+0.05
−0.04 24.48±0.03 1.32+0.06
−0.05 Tidal Debris
d................. 28.8+1.0
−0.527.4+0.7
−0.4— Stellar Stream
e................. 27.9±0.3 26.7±0.2 1.2±0.4 Tidal Debris/Spiral Association
f................. 26.46+0.10
−0.09 25.20±0.07 1.3±0.1 Tidal Debris/Spiral Association
gSmall Clumps 26.9±0.1 26.0±0.1 1.0±0.1 UV-Extended Disk
UGCA 342 24.83±0.03 23.96±0.03 0.87±0.04
hSmall Clumps 26.33±0.09 25.39±0.09 0.9±0.1 UV-Extended Disk
M63 Outer Disk/Stellar Halo 24.83±0.02 23.52±0.01 1.31±0.02 µR= 23.5 Isophote
FIG. 7.— Photometric features measured by the MDO 0.8 m are labeled a-hon the G-NMS image. For reference, a color image of M63’s disk constructed
from the BBRO data has been superimposed, indicating the more familiar extent of this galaxy. The blue arrows indicate the location where the width of the
main loop was measured. The red arrows in the insets indicate the location of an extremely dim feature (d). The middle inset is the MDO Rband image while
the lower is confirmation from the G-NMS image. For reference, we show in the top inset a less dramatic stretch of the G-NMS image to more clearly show the
inner regions of the stellar halo. Note that that each photometric feature is explicitly outlined in Figure 8.
tion at hand for the stream, we cannot use the ellipse fitting
to yield its orientation on the sky plane due to the degeneracy
with intrinsic orbital ellipticity.
We have measured the width of the stream, defined as the
Full Width at Half Maximum (FWHM) of a Gaussian fit as
in Martínez-Delgado et al. (2008). Visual inspection of the
stream shows that the width has little variability along its path.
However, it is difficult to verify this at the locations where the
stream appears to enter the galaxy’s outer regions as the faint
light and other features at high galactocentric radius contami-
nate our view of the stream. The stream width was measured
from an extracted 200 pixel wide strip in the MDO Rband
image that is perpendicular to the stream at the location in-
dicated in Figure 7. The strip was collapsed with a median-
combine resulting in a single profile through the stream. Since
the background is slightly brighter on the side of the stream
that is closer to the galaxy, simple measures of FWHM would
be biased toward larger values. To avoid this, we sample the
10 T. S. CHONIS ET AL.
FIG. 8.— MDO Rband image shown with foreground stars, background
galaxies, and the disk of M63 masked and leaving the faint outer disk and
stellar halo substructure exposed for measurement. Each photometric feature
labeled a-hin Figure 7 has been enclosed in red, showing the area of pixels
used for the average surface brightness measurements in Table 3. The mea-
sured areas of UGCA 342 and the dim break in the tidal stream are shaded
red while the measured area around the µR= 25 mag arcsec−2isophote is
shaded blue.
background faint light from the galaxy in 100 pixel long zones
on either side of the stream. Since the sampled strip is rela-
tively small in length compared to the size of the galaxy’s
diffuse outer light, we approximate the galaxy contribution to
the stream’s light profile as linear and subtract it; the resulting
profile is fit well with a Gaussian function. We estimate the
FWHM of the stream to be ∼95′′, which translates to 3.3 kpc
at the 7.2 Mpc distance to M63. To verify that the Rband mea-
surement is consistent with stream FWHM values reported in
previous work from images constructed with SBIG-Custom
Scientific filters (e.g., Martínez-Delgado et al. 2008, 2009),
we have also measured the stream width using the G-NMS
and BBRO images. To within 0.1 kpc, both measurements
agree with that found above.
The average surface brightness of the loop, as measured
along its length within the FWHM measured in the preced-
ing paragraph (see Figure 8), is µR= 26.1±0.1 mag arcsec−2
with an average color index of B−R= 1.5±0.2. The stream
appears to be fairly constant in surface brightness and color
along its length, except for an interesting small dim break in
the stream’s light distribution to the north, northeast of the
galaxy (see Figure 8) lasting for ∼2.7′(∼5.7 kpc in projec-
tion). The surface brightness in this dim break is very uncer-
tain, but it is probably close to 1 mag arcsec−2dimmer than
the rest of the stream. Possible causes of this surface bright-
ness discontinuity in the otherwise coherent arc of the stellar
stream are discussed below in §4.4.
As was found in Martínez-Delgado et al. (2009) for the tidal
stream of NGC 4013, the B−Rcolor index of this stream is
quite consistent with the red colors of S0 galaxies from the
sample of Barway et al. (2005). This color is also consis-
tent with the redder dwarf galaxies in the Local Group (Mateo
1998) and possibly suggests that the stream is dominated by
an old (&10 Gyr) stellar population, which is also predicted in
general by the models of Cooper et al. (2010). Within 1σ, the
measured stream color is similar to the faint outer regions of
the disk and stellar halo of M63 (as measured at the µR= 23.5
mag arcsec−2isophote; see Table 3). However, the B−Rcolor
FIG. 9.— A comparison of an optical image (shown here in grayscale from
the G-NMS 0.16 m telescope) with the distribution of HI gas around M63.
The HI map from Battaglia et al. (2006) is at 67′′ resolution, has a column
density detection limit of 0.10 M⊙pc−2, and is shown in blue overlaid on
the optical image. The red dashed curve represents the location of the stream
ellipse fit described in §3.1 and relevant photometric features are labeled.
index map (Figure 6) shows a slight systematic color gradient
towards redder colors at larger disk radii, most notably near
the outer eastern edges. This color gradient was also reported
in the photographic study of van der Kruit (1979).
Figure 9 shows a comparison of one of our optical images
with an HI map at 67′′ resolution from Battaglia et al. (2006).
The tidal stream extends well beyond the significant HI emis-
sion of the disk and apparently has no gaseous component at
the detection limit of this HI survey (0.10 M⊙pc−2). Visi-
ble in this map is the significant HI warp of ∼20◦as well as
extended spiral structure. Some of the other very faint struc-
tures visible in our deep images are clearly recognizable in
this map. For example, the conspicuous narrow trail of gas
visible in the northeast edge of the HI disk corresponds to the
optical feature labeled hin Figure 7. Additionally, the long
line of gas on the opposite side of the HI disk from hcorre-
sponds to optical feature gwith the large clump just inside of
it corresponding to the over-density carrying the designation
UGCA 342. As will be discussed in the following section, g,
h, and UGCA 342 are likely not directly associated with the
tidal stream. Finally, it is interesting to point out the enormous
“holes” in the HI distribution at the locations where the tidal
stream intersects the disk plane (see the ellipse fit tracing the
stellar stream’s light distribution in Figure 9). Further discus-
sion on the possibility of these voids being due to the passage
of the disrupted dwarf through the HI disk is given in §4.5.
3.2. Other Features
In addition to the large stellar loop discussed above, our
deep wide-field images also reveal a plethora of faint diffuse
light features that are identified in Figure 7 and are potentially
associated with the ongoing destruction of an accreted dwarf
galaxy. Recent simulations of 1:10 satellite-to-host mass ra-
tio minor mergers by Purcell et al. (2010) have shown that
such accretion events can dynamically eject disk stars into a
diffuse, azimuthally non-uniform stellar halo component with
heights of &30 kpc above the disk plane. As such, additional
features of interest that are potentially related to the accretion
event that resulted in the stellar loop are selected based on
their color similarity to feature a(and therefore also the inner
stellar halo and outer disk of M63) as well as their location
and spatially asymmetric nature.
THE TIDAL STREAM OF MESSIER 63 11
FIG. 1 0.— An archival GALEX UV image (1350 <λ(Å) <2800) showing
M63’s XUV disk, parts of which correspond to detected optical features g
and h. The blue contour is from Thilker et al. (2007) and indicates the µFUV
= 27.25 AB mag arcsec−2isophote. The red dashed curve represents the
location of the stream ellipse fit described in §3.1 and relevant photometric
features are labeled.
Feature bis an enormous protrusion appearing to begin
where feature aenters the galaxy’s outer isophotes and wrap-
ping around the southeastern edge, ending in a surface bright-
ness discontinuity to the south, southwest of the galaxy where
feature ebegins. Feature bis slightly brighter than a, but dis-
plays a very similar color of B−R= 1.5±0.1 and corresponds
to the redder edge of M63’s light distribution, which was al-
luded to by van der Kruit (1979) and can be seen in Figure
6.Feature cis a curious asymmetry protruding slightly to the
north at a smaller galactocentric radius than features aor b.
It is in close proximity to another location of slightly redder
colors on the B−Rcolor index map in Figure 6 and is sig-
nificantly brighter than feature a(probably due to the contri-
bution of M63’s faint outer light at the closer location of cto
the galaxy). Its measured color is consistent with M63’s outer
disk isophotes and the stream within the measurement errors.
Feature eis an enormous, dim plume that has similar µB
and µRas the dim break in feature aand is located at a similar
galactocentric radius. Its B−Rcolor is also consistent with
the tidal debris and the outer disk isophotes. However, there
is no HI gas associated with this feature down to the detection
limit of Figure 9. As such, this feature could be a part of the
stellar tidal stream, but we refrain from making any definitive
conclusions about it given that the photometric uncertainties
are relatively large.
To the west of the galaxy disk, it is possible to see a “wing”-
like feature labeled f. Like the features above, fhas a color
similar to the outer disk and stellar halo as measured at the
µR= 23.5 mag arcsec−2isophote. Although this giant plume
could be a component of the tidal debris, it may instead be
associated with an extension of a spiral arm into the outer
disk (note that the HI spiral structure closely aligned with
this feature extends up to a galactocentric radius of ∼40 kpc;
Battaglia et al. 2006).
Feature dis an extremely dim extension of light towards the
south of the galaxy and is visible only after very extreme non-
linear stretches of our images. Since it is below our limit for
accurate measurements, only a very uncertain surface bright-
ness can be reported, probably around 1 mag arcsec−2dimmer
than feature a. Since it appears in both the MDO Rband and
G-NMS images as a loop-like structure, it is unlikely to be a
sky subtraction residual. The models of Johnston et al. (2008)
show that the remnants of accreted dwarfs dim as a function of
time since the initial disruption as the satellite’s stellar distri-
bution becomes more diffuse. This implies that feature dmay
be a much more ancient merger fossil. A similar dim loop-like
structure can also be seen on the northern side of the galaxy
extending towards the V= 9.8 mag star SAO 44528. This
bright star hinders our ability to study this possible northern
component of the dim loop.
Our data also detect the optical counterparts of long,
clumpy filaments (features gand h) clearly visible in GALEX
ultra-violet (UV: 1350 < λ (Å) <2800) images of M63 (Gil
de Paz et al. 2007a), reproduced here from archival data in
Figure 10. As with the HI map, there are no hints of the
stream (feature a) in these UV data, even though it displays
a surface brightness similar to that of gand hin our optical
images. Given that they are seen in both the HI map and
GALEX imaging, this may suggest that gand hare com-
posed of a younger stellar population than the stream and are
“patchy” components of the XUV disk classified by Thilker
et al. (2007). This is upheld by the blue color indices of
these features (B−R≈0.9). In Figure 10, we show the
µFUV = 27.25 AB mag arcsec−2isophote from Thilker et al.
(2007). Patchy UV emission outside of this isophote helps
to define the Type 1 XUV disks, a classification to which the
M63 system belongs. As mentioned, these filaments can also
be associated with parts of the outer spiral structure visible
in the HI map in Figure 9, unlike the loop feature and other
suspected tidal debris. Embedded near feature gis the large,
bright clump of XUV emission, which looks to be a part of
the XUV disk according to the GALEX data. However, this
clump carries a UGCA (342) and PGC (46093) designation
and is classified as a Magellanic Irregular galaxy by de Vau-
couleurs et al. (1991) and tidal dwarf galaxy by Thilker et al.
(2007). Bremnes et al. (1999) deduced from deep photome-
try that it was part of the M63 system rather than a separate
galaxy. Given its blue B−Rcolor of 0.87±0.04 and its appar-
ent association with XUV disk features in the GALEX image,
we tend to agree with this claim. Thus, we do not consider
UGCA 342 to be a viable candidate for the progenitor dwarf
galaxy of the tidal stream, although kinematical data from op-
tical spectroscopy would be useful to at least confirm if it is
part of M63.
4. CONCLUSIONS AND DISCUSSION
4.1. Origin of Faint Light Features
We present surface photometry of a stellar tidal stream and
a deep panoramic view of a complex of substructure in the
outer regions of the nearby spiral galaxy M63. Our data, col-
lected independently from three different small telescopes, re-
veals an enormous arc-like structure around the galaxy’s disk
extending ∼29 kpc projected from its center, tilted with re-
spect to its strong HI gaseous warp and the stellar disk. This
strong indication of a recent merger event provides yet another
example (in addition to NGC 5907 and NGC 4013) of an ap-
parently isolated galaxy with a significantly warped gaseous
disk that also shows clear evidence of the ongoing tidal dis-
ruption of a dwarf companion. Moreover, the appearance of
the enormous great-circle arc feature that has no UV or HI
counterpart is consistent with tidal debris resulting from on-
going satellite accretion in the ΛCDM based models of Bul-
lock & Johnston (2005), Johnston et al. (2008), and Cooper et
12 T. S. CHONIS ET AL.
al. (2010).
The sky-projected geometry of the stream provided by our
images gives some insight to its possible origin. The large
loop-like feature around M63 appears to belong to the great-
circle type stellar tidal streams (like the tidal stream of the
Sagittarius dwarf and that around NGC 5907). From the anal-
ysis of ∼1500 accretion events in 11 stellar halos that are
hierarchically constructed in a ΛCDM universe, Johnston et
al. (2008) find that unbound great-circle morphologies (i.e.,
those with no remaining bound core of particles) arise from
accretion events typically beginning 6-10 Gyr ago on near
circular orbits. These simulations also find that this is the
predominant morphological type associated with still-bound
satellites which arise from more recent events (i.e., within the
last 6 Gyr) on mildly eccentric orbits. Given the measured
surface brightness of µR≈26.1 mag arcsec−2for the M63
stream, the accretion event considered here would fall near
the brightest of those in the Johnston et al. (2008) models, in-
dicating that the progenitor satellite may have been accreted
in the last several Gyr and may still be bound. A discussion
of the prospects for a remaining still-bound satellite core is
given in the following subsection.
Our observations of M63 are consistent with counts of sub-
structure in cosmological simulations for the surface bright-
ness limit of our pilot survey (Bullock & Johnston 2005; John-
ston et al. 2008; Cooper et al. 2010; Martínez-Delgado et al.
2010). Johnston et al. (2008) conclude that finding a sin-
gle satellite clearly in the process of disruption with debris
spread around the host galaxy should not be surprising in a
survey with our sensitivity, but finding many satellites in that
state around a single galaxy would be unusual. Yet, given the
existence of extremely faint features (such as feature d) that
are below our reasonable measurement limit (i.e., the derived
uncertainties are ∼0.5−1.0 mag arcsec−2) and at least 1 mag
arcsec−2dimmer than feature a, for example, it is possible that
we have observed two such accretion events that occurred at
different epochs.
From the presented data, only feature acan convincingly be
determined to be a part of a coherent stellar stream. However,
our data also feature a number of other faint light substruc-
tures around M63, as listed in the previous section. While we
have no conclusive evidence that any of these other features
are additional components of the same coherent stellar stream,
there is reason to believe that they may be “side-effects” re-
sulting from the same accretion event’s influence on the M63
parent system. These possibilities are discussed below in §4.5.
4.2. Location and Fate of the Progenitor Dwarf Galaxy
Our images do not provide any obvious insight on the cur-
rent position or final fate of the progenitor dwarf galaxy of this
stream. The field of the G-NMS image includes a satellite,
UGC 8313 (see Figure 7), a galaxy of type SB(s)c (de Vau-
couleurs et al. 1991) and another member of the M51 galaxy
group (Fouque et al. 1992) at a projected distance of ∼50 kpc
to the northwest. Although our deep image reveals some hint
of a possible faint stellar warp on the northern edge of UGC
8313, the galaxy’s position and lack of any tidal debris in
its vicinity make it very unlikely that this satellite is related
to the main stream to the northeast. As mentioned in §3.2,
the nearby “condensation" UGCA 342 is also likely not to be
a viable candidate for the missing progenitor satellite given
its apparent alignment with the XUV disk discovered by the
GALEX satellite (Thilker et al. 2007) and its extremely dif-
ferent B−Rcolor as compared to the stellar stream (see Table
3).The surviving satellite is likely indiscernible inside the
stream due to a currently small size and especially low sur-
face brightness, which decreases monotonically once stellar
material begins to be stripped from the progenitor dwarf in
the parent’s tidal field (see §5 of Martínez-Delgado et al. 2008
for a discussion on this issue). As in our previously stud-
ied tidal streams (Martínez-Delgado et al. 2008, 2009), a sur-
viving satellite could alternatively be hidden behind or super-
posed on the spiral disk. Because of M63’s lower inclination
as compared to these examples, the probability of this possi-
bility is increased. Although the isophotes of Figure 5 show
no obvious corresponding light over-density, there is a small
area of especially red B−Rcolor to the north of the masked
portion of M63’s disk near where feature ais lost in the outer
disk light, east of feature c(see Figure 6). Although the data
we have at hand cannot confirm nor refute it, this redder patch
could potentially be the low surface brightness remains of a
surviving dwarf galaxy core that is nearly washed out by the
faint disk light due to its projected proximity and current low
surface brightness. By a similar reasoning, the remains could
be hidden in feature b, which also has a red B−Rcolor and is
located close to the outer disk in projection.
Alternatively, it is possible that the progenitor satellite is
totally disrupted at the present time. Such a scenario would
imply an old, multiply wrapped stellar stream since total dis-
ruption is unlikely on a single passage of a satellite on a
near circular orbit (e.g., see Figure 3 of Peñarrubia et al.
2010b). However, as discussed in §4.4, our data do not pro-
vide any conclusive evidence for a long, multiply wrapped
stellar stream thanks in large part to the difficulty of observ-
ing faint features in close vicinity to the moderately inclined
outer disk of M63.
4.3. Mass and Age of the Stellar Stream
The observed morphology of feature asuggests a great-
circle, rosette-like stellar tidal stream (where the satellite and
its corresponding debris are in a near circular orbit). Thus,
we can give illustrative estimates of the mass of the disrupted
satellite galaxy as well as the time since disruption using the
analytical formalism of Johnston et al. (2001). This procedure
was adopted by Johnston et al. (2001) for the stream around
NGC 5907 and by Wehner & Gallagher (2005) for the stream
around NGC 3310. Equations 12 and 13 of Johnston et al.
2001 for mass and age,15 respectively, are valid for disrup-
tion times of t<3TΨ= 6πR/vcirc, where TΨis the orbital pe-
riod of the satellite and Ris the orbit’s typical radius. This
time is ∼3.0 Gyr for the system in consideration here (using
a rough estimate for the stream radius R= 29 kpc from fea-
ture a’s projected maximum distance and vcirc ≈180 km s−1
at Rfrom Battaglia et al. 2006). To estimate the mass, we use
the stream width w= 3.3 kpc (see §3.1), which as noted does
not appear to significantly vary along its length. This may be
due to the contamination of faint light from the galaxy, which
also inhibits our ability to determine the pericentric distance
of the orbit RP. However, the lack of variability in wmay
suggest that the time since the satellite’s disruption is small
(i.e., t<1.5TΨfrom Johnston et al. 2001, §2.2). Since the
projected view of the galaxy and the stream does not yield an
15 Note that age in this context is the time since disruption, not the age of
the stellar population. Our observations of B−Ralone cannot constrain the
stellar population of the tidal stream.
THE TIDAL STREAM OF MESSIER 63 13
obvious measure of RP, we simply assume an orbit with small
eccentricity such that RP≈R(this is not a bad approxima-
tion for streams having the great-circle morphology). Using
this information and Equation 12 of Johnston et al. (2001), we
estimate the mass of the progenitor satellite to be ∼3.5×108
M⊙. This is of the same order as the significant progenitor
satellites which assemble stellar halos from 1 <z<7 in the
simulations of Cooper et al. (2010) and of the same order as
the total mass of several Local Group dwarf spheroidals, such
as NGC 147, NGC 185, and the Fornax Dwarf (Mateo 1998).
From Equation 13 of Johnston et al. (2001), we can see that
the estimate of the time since disruption depends linearly on
Ψ, the angular length of the stream. As will be discussed
in the following section, the true value of Ψis not conclu-
sive from our data. Therefore, we parameterize the stream’s
angular length as Ψ= 2πη, where ηis thus the total num-
ber of wraps the stream makes around M63. Using the ra-
dius of a circular orbit with the same energy as the true orbit
Rcirc = (RA+RP)/2≈R(where RAis the apocenter distance),
we estimate that t≈1.8ηGyr. While large uncertainty ex-
ists in these illustrative calculations, a reasonable value of η
shows that the results obtained using the analytical approach
of Johnston et al. (2001) and the results of the more recent
modeling of Johnston et al. (2008) are consistent with an ac-
cretion event having occurred in the last few Gyr.
As an alternative to the dynamical approach above, we can
also estimate the mass of the progenitor satellite by measur-
ing the stream’s Rband luminosity density ΣRand estimating
a mass-to-light ratio M/LR. This method was used to esti-
mate the mass of the progenitor dwarf of the NGC 5907 stellar
stream (Martínez-Delgado et al. 2008) and to measure surface
mass density profiles of spiral galaxies by Bakos et al. (2008).
Since we can clearly make a distinction of only feature afrom
the outer faint disk light of M63, we estimate ΣRusing its av-
erage value of µR= 26.1 mag arcsec−2. With the absolute R
magnitude of the sun16 MR⊙= 4.46, we can arrive at the well-
known expression:
ΣR= 100.4(26.03−µR)LR⊙
pc2,(1)
yielding 0.94+0.09
−0.08 LR⊙pc−2. Using the result of the ellipse
fitting of feature ain §3.1, we estimate the projected area A
subtended by the stream as:
A=ηπqa+w
22−a−w
22.(2)
This results in A= 223ηkpc2. The total Rband luminosity is
then ΣRA= (2.1±0.2)η×108LR⊙. Bell & de Jong (2001)
have used stellar population synthesis models to predict the
dependence of M/Lon broadband colors. Using Table 1 of
that work (i.e., aR= 0.820 with a 0.15 dex reduction for a
Kroupa IMF and bR= 0.851) and our estimation of the stream
B−Rcolor, we estimate that M/LR= 2.0+1.0
−0.7. This yields a
total mass for the stream of (4±2)η×108M⊙.
4.4. Length of the Tidal Stream
Our deep observations of M63 only clearly reveal a single
coherent arc (feature a) having an angular length of Ψ≈π,
which yields a lower limit of η&0.5. This implies that t&0.9
Gyr. However, one can immediately see that η≈1 allows the
16 See: http://mips.as.arizona.edu/∼cnaw/sun.html
two mass determinations from the previous section to agree
quite well at ∼4×108M⊙.
In §3.1, we pointed out a break in the surface brightness dis-
tribution of the otherwise coherent arc. Recent work by Yoon
et al. (2011) has shown that such breaks in cold stellar streams
can be caused by a direct impact with another massive body.
Given the scale of this accretion event and the angular size of
the gap (∼10◦projected), the impacting body would probably
need to be &108M⊙(see Figure 5 of Yoon et al. 2011 which
compares the streams of Palomar 5 and the Sagittarius dwarf).
Since subhalos of ∼108M⊙are thought to contain stars (e.g.,
Greif et al. 2008), the impacting body could be another ac-
creted dwarf galaxy (or the remnants thereof; here, perhaps
the dwarf associated with feature d). Alternatively, gaps in
stellar streams can occur naturally for satellites on fairly ellip-
tical orbits. This is because mass-loss occurs variably about
the orbit with most occurring abruptly near perigalacticon, as
illustrated in Figure 3 of Peñarrubia et al. (2010b). We can-
not rule out this possibility for explaining the stream’s gap
because as previously mentioned, the projection of the stream
on the sky plane is currently unknown. Thus, we cannot de-
termine the eccentricity of the orbit using our images without
additional kinematic data.
Besides the above interesting possibilities, the dim break
in the coherent stream could provide a clue for estimating
Ψ. For example, the break could simply be a gap between
the ends of the leading and trailing arms of the tidal stream
as the first orbital wrap since disruption is being completed.
This interpretation seems plausible given that if the interac-
tion began recently enough to have formed only a single loop
at the present time, we might expect to detect the remaining
progenitor satellite in the stream since it would be midway
between the leading and trailing streamers (Johnston et al.
2001). Given the position of the gap, the remaining core of
the satellite might then be hidden behind or superposed on the
disk (thus evading clear detection; see §4.2). Such a scenario
could also explain the systematic red color gradient observed
on the east side of the outskirts of M63 (see Figure 6): given
a single wrap, the redder B−Rcolors on the east side of the
galaxy could result from the stream being above the disk plane
in the line of sight with the opposite explaining the lack of red
B−Rcolor indices on the west side. If this is the case, η≈1
and t≈1.8 Gyr.
Johnston et al. (2001) addressed the difficulty of observing
faint features in the outskirts of spiral galaxies that are not
seen edge-on. Since M63 has iM63 ≈53.3◦, it is far from edge-
on and faint features at a height of up to ∼20 kpc above the
galaxy plane can become difficult to discern. Thus, there is no
clear evidence for η > 1 because our data are not conclusive
about whether other faint features (such as b,c,e, or f) are a
part of the stellar stream or are instead native to M63 itself.
4.5. Effect on the Parent Galaxy System
It is interesting to consider in what ways a minor merger
such as the one we have observed might affect the parent
galaxy system since such events should be common. Our
view of the ongoing disruption of a satellite galaxy around
M63 could hint at the inside-out formation of the stellar halo.
As can be seen through our photometry (Table 3), the stellar
halo’s color is fairly red and quite similar to the color of the
stellar stream, indicating that it consists of a mix of stars. As
shown by recent simulations by Purcell et al. (2010), accreted
stars from the disrupted dwarf can occupy the same outer re-
14 T. S. CHONIS ET AL.
gions as the thick envelope of stars ejected from the outer disk
as a result of the interaction. While our data hint at a system-
atic color gradient towards redder B−Rcolors near the stream
(see Figure 6), the simulations imply that it should not be sur-
prising to find a similar color between the stream and outer
regions of M63.
Such a merger could also explain some of the other low
surface brightness features we observe around M63, such as
features b,c,e, and f. The simulations of Purcell et al. (2009,
2010), which model 1:10 satellite-to-host mass ratio minor
mergers with a variety of orbital inclinations, show that large
groups of stars can be ejected from the stellar disk to large
heights (&30 kpc) in an azimuthally asymmetric manner. As
seen in the simulations, such stellar disturbances in the outer
disk could be an explanation for the large “wing”-like fea-
tures mentioned. We note that our surface photometry shows
that all of the listed photometric features’ B−Rcolors (except
for b) are fully consistent with being the same color as the
outer disk as measured at the µR=23.5 mag arcsec−2isophote,
which supports this claim. Especially convincing are features
cand fgiven their small photometric uncertainties. After sev-
eral Gyr, such structures likely settle into a thick disk compo-
nent.
As has already been mentioned, M63 is the most recent of
several galaxies (e.g., NGC 5097 and NGC 4013) discovered
to have an extremely warped HI disk in the presence of a re-
cent merger event. These examples counter the argument of
Sancisi (1976) who dismiss the tidal origin of such warps due
to the assumption of galaxy isolation. The interaction may
also affect the HI disk in other ways. As pointed out in §3.1,
there are two enormous voids (each &15 kpc in size) in the HI
distribution, spaced roughly 180◦apart and located roughly
where the tidal stream intersects the disk. Simulations by
Bekki & Chiba (2006) and Kannan et al. (2011) have shown
that an impact of a dark subhalo of 108M⊙on the HI disk
can create holes on kpc scales. These simulations require the
halos to contain some modest gas fraction to supply a dissipa-
tional force to create a void in the gas, since gravity alone can
cause HI over-densities due to gravitational focusing (Kan-
nan et al. 2011). Given that the stellar stream appears to have
no gas associated with it, and the B−Rcolor and estimated
mass are consistent with gas-poor dwarf spheroidals and S0
galaxies, it would seem unlikely that there would be enough
gas in the progenitor satellite galaxy to produce HI voids of
such large scale upon impact. Additionally, the cited simula-
tions produce rings of high density gas around the void, which
induces star formation. We do not observe enhanced star for-
mation around the voids either in our deep optical images or
in the GALEX data. Finally, in the time it would take the
satellite to complete half of an orbit, the rotation of the HI
disk would have likely rotated the voids out of alignment with
the stream such that the collision points on the disk would not
be located in such a 180◦symmetric fashion.
As an alternative explanation, companions as little as 1%
of the mass of the parent galaxy have been theorized to drive
tidally induced spiral structure (Byrd & Howard 1992; Mihos
& Hernquist 1994). As such, the ongoing minor merger stud-
ied in this work could possibly be an explanation for the un-
derlying Kband spiral structure in M63 observed by Thornley
(1996). Similarly, the extension of these tidally driven spi-
ral arms into the outer disk could induce instabilities in the
gaseous disk at high radii giving rise to Type 1 XUV disk
features outside of where traditional star formation occurs.
As simulated by Bush et al. (2008), this scenario is plausi-
ble because inner spiral structure propagates into the gaseous
HI outer disk. Indeed, such spiral over-densities of gas can
be seen in the HI map (Figure 9; Battaglia et al. 2006) cor-
responding to the location of XUV emission detected both in
our deep optical images (e.g., features gand h) and in GALEX
imaging (Figure 10). The propagation of the spiral density
waves at high radii in the HI disk could also be responsible
for clearing out the large-scale voids.
Thilker et al. (2007) looked for a link between interactions
and XUV disks. While their statistical tests comparing the
mean perturbation parameter f(Varela et al. 2004) for the en-
tire sample to that for galaxies hosting a Type 1 XUV disk
morphology showed no significant link, those authors do ad-
mit some limitations. In particular, the fparameter does not
take into account undetected low surface brightness objects
(or in this case, debris or faint stellar streams). Thilker et al.
(2007) also find that 75% of Type 1 XUV disk objects show
morphological evidence (including HI warps) of an interac-
tion, merger, or external perturbation and suggest that inter-
actions may still be a viable way to drive star formation in
the outer disk. M63 was classified as relatively isolated in
that work, and in light of the ongoing interaction presented
here, this may require re-evaluation. Indeed, other galaxies
around which stellar tidal streams have been discovered also
display Type 1 XUV disks. In particular, M83 shows XUV
disk emission also indicating star formation at extremely high
radii (Gil de Paz et al. 2007b), has a value of findicating
isolation (Thilker et al. 2007), and yet displays at least one
great-circle like tidal stream indicative of an ongoing interac-
tion with a satellite (Malin & Hadley 1997). Exploration of a
larger sample of galaxies showing previously undetected evi-
dence of a disrupted dwarf satellite that is also common to the
sample of Thilker et al. (2007) is necessary to further explore
the possible link between minor mergers and XUV disks.
With the increasing number of discoveries of stellar halo
substructure and streams around external galaxies (which will
likely grow quickly thanks to a new systematic survey for
such features; Martínez-Delgado et al. 2010), we are learning
that satellite accretion is an essential part of galaxy evolution.
The near future will likely bring new hypotheses about the
impact of such events on the evolution of spiral galaxies,
including disk warps or extended star formation, which can
be based on reliable statistics rather than speculation based
on a few individual systems. The results of our pilot survey
of stellar tidal streams in nearby galaxies (Martínez-Delgado
et al. 2010), including the deep images of M63 and its tidal
stream analyzed in this work, have shown how the stellar
halos of spiral galaxies in the Local Universe still contain a
significant number of relics from their hierarchical, inside-out
formation. This presents us with a unique opportunity to
be witnesses of the latest stages of galaxy evolution and the
ongoing assembly of spiral galaxy halos.
This work is partially supported by the Texas Nor-
man Hackerman Advanced Research Program under grant
003658-0295-2007. S.R.M. appreciates support from NSF
grant AST-0807945. We would like to thank McDonald Ob-
servatory and its staff for supporting the photometric obser-
vations, T. Taylor for useful bits of code which aided in the
analysis, A. Gil de Paz, M. A. Gómez-Flechoso, N. Martin, E.
Bell, H. W. Rix, S. Odewahn, M. Cornell, and K. Gebhardt for
useful scientific discussions, K. Jordi for supplying the ma-
chine readable catalog of stars with matched BVRI and ugriz
THE TIDAL STREAM OF MESSIER 63 15
photometry, and G. Battaglia for providing the electronic for-
mat of the HI map of M63. Additionally, we would like to
acknowledge the anonymous referee for useful comments and
suggestions which have improved this paper.
Funding for the SDSS has been provided by the Alfred P.
Sloan Foundation, the Participating Institutions, the National
Science Foundation, the U.S. Department of Energy, the
National Aeronautics and Space Administration, the Japanese
Monbukagakusho, the Max Planck Society, and the Higher
Education Funding Council for England. The SDSS Web
Site is http://www.sdss.org/. GALEX (Galaxy Evolution
Explorer) is a NASA Small Explorer, launched in 2003
April. We acknowledge NASA’s support for construction,
operation, and science analysis for the GALEX mission,
developed in cooperation with the Centre National d‘Etudes
Spatiales of France and the Korean Ministry of Science and
Technology. This research has made use of the NASA/IPAC
Extragalactic Database (NED) and the NASA/IPAC Infrared
Science Archive, which is operated by the Jet Propulsion
Laboratory, California Institute of Technology, under contract
with the National Aeronautics and Space Administration.
We acknowledge the use of the HYPERLEDA database
(http://leda.univ-lyon1.fr).
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