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The haloes and environments of nearby galaxies (HERON) – III. A 45 kpc
spiral structure in the GLSB galaxy UGC 4599
Aleksandr V. Mosenkov,1,2★R. Michael Rich,3Michael Fusco,4Julia Kennefick,4,5David Thilker,6
Alexander Marchuk,2,13 Noah Brosch,7Michael West,8Michael Gregg,9Francis Longstaff,10
Andreas J. Koch-Hansen,11 Shameer Abdeen, 12 and William Roque1
1Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84602, USA
2Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, Pulkovskoye Chaussee 65/1, St Petersburg 196140, Russia
3Dept. of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
4Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA
5Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA
6Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
7The Wise Observatory and the Raymond and Beverly Sackler School of Physics and Astronomy, The Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
8Lowell Observatory, Flagstaff, AZ 86001, USA
9Department of Physics, University of California, Davis, CA 95616, USA
10University of California, Los Angeles, CA 90024, USA
11Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12, Heidelberg 69120, Germany
12Dept. of Physics and Astronomy, Georgia Southern University, GA 30458, USA
13Saint Petersburg State University, Universitetskij pr. 28, St. Petersburg 198504, Russia
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We use a 0.7-m telescope in the framework of the Halos and Environments of Nearby Galaxies (HERON) survey to probe
low surface brightness structures in nearby galaxies. One of our targets, UGC 4599, is usually classified as an early-type galaxy
surrounded by a blue ring making it a potential Hoag’s Object analog. Prior photometric studies of UGC 4599 were focused on
its bright core and the blue ring. However, the HERON survey allows us to study its faint extended regions. With an eight hour
integration, we detect an extremely faint outer disk with an extrapolated central surface brightness of 𝜇0,d(𝑟)=25.5mag arcsec−2
down to 31 magarcsec−2and a scale length of 15 kpc. We identify two distinct spiral arms of pitch angle ∼6◦surrounding the
ring. The spiral arms are detected out to ∼45 kpc in radius and the faint disk continues to ∼70 kpc. These features are also
seen in the GALEX FUV and NUV bands, in a deep 𝑢-band image from the 4.3m Lowell Discovery Telescope (which reveals
inner spiral structure emerging from the core), and in Hi. We compare this galaxy to ordinary spiral and elliptical galaxies, giant
low surface brightness (GLSB) galaxies, and Hoag’s Object itself using several standard galaxy scaling relations. We conclude
that the pseudobulge and disk properties of UGC 4599 significantly differ from those of Hoag’s Object and of normal galaxies,
pointing toward a GLSB galaxy nature and filamentary accretion of gas to generate its outer disk.
Key words: Galaxies: spiral - evolution - formation - photometry - structure
1 INTRODUCTION
UGC 4599 is a nearby galaxy (often classified as S0) with an unusual
ring morphology similar to that observed in Hoag’s Object (see
Table 1for the general properties of this galaxy). Finkelman & Brosch
(2011) identify it as the nearest target of its kind, and, therefore,
a promising candidate to explore the class of Hoag-type galaxies.
Hoag’s Object (Hoag 1950) is characterized by a red central spherical
body (an elliptical galaxy or a bulge) surrounded by a distinctive
blue star-forming ring. Hoag’s Object was also found to contain an
★E-mail: aleksandr_mosenkov@byu.edu
Hiring 1.5 times more extended than the star-forming ring (Brosch
et al. 2013).
Formation of ring galaxies may go through a variety of possi-
ble pathways. These include secular evolution through bar-related
resonances (Buta & Combes 1996), gas-rich accretion from other
galaxies (see e.g., Schweizer et al. 1983;Sil’chenko & Moiseev
2020, and references therein), gas accretion from the intergalactic
medium via cosmological filaments (Macciò et al. 2006), wet minor
mergers (Buta & Combes 1996;Reshetnikov & Sotnikova 1997), and
collisions of two or more galaxies (Lynds & Toomre 1976;Apple-
ton & Struck-Marcell 1996). In particular, polar-ring galaxies can be
formed by orthogonal or at least significantly non-coplanar galac-
©2022 The Authors
arXiv:2308.09093v1 [astro-ph.GA] 17 Aug 2023
2A. Mosenkov et al.
tic collisions (Schweizer et al. 1983;Bournaud & Combes 2003).
Roughly 0.5% of nearby S0 galaxies are shown to contain polar rings
(Whitmore et al. 1990,Moiseev et al. 2011).
In their detailed study of UGC 4599, Finkelman & Brosch (2011)
decomposed UGC 4599 into a reddish de Vaucouleurs core, a blue
ring (with some evidence of spiral structure), and detected an ex-
tended Hidisk. The imaging of both Finkelman & Brosch (2011)
and Gutiérrez et al. (2011) reveals signatures of an outer low-surface
brightness (LSB), star forming disk. Therefore, we decided to select
this target for deep LSB imaging with the 0.7m Jeanne Rich tele-
scope as one of the earliest targets of the Haloes and Environments
of Nearby galaxies (HERON) survey (see Rich et al. 2019).
As ground-based telescopes develop in observing capabilities,
both through the use of single small-aperture telescopes (see e.g.,
Martínez-Delgado et al. 2010;Rich et al. 2019) and arrays (Abra-
ham & van Dokkum 2014;Spitler et al. 2019), the identification
and characterization of LSB objects has become more common. For
example, the use of very deep photometric observations resulted in
the discovery of many exceedingly diffuse and ultra diffuse galaxies
(van Dokkum et al. 2015;Koda et al. 2015;van Dokkum et al. 2016,
2018).
Giant LSB (GLSB) galaxies also require long exposures to study
their outskirts. They host very faint and large star-forming disks up to
250 kpc in diameter (see e.g., Boissier et al. 2016) which are charac-
terized by massive dark matter halos (Das 2013), high hydrogen gas
surface densities (Bothun et al. 1987), and generally low metallicities
(Liang et al. 2010). Their extended disks have extrapolated central
surface brightness values much lower than the Freeman (1970) value
of 𝜇0,d(𝐵)=21.65 mag arcsec−2for disk galaxies. GLSB galaxies
usually have a high surface brightness bulge (Saburova et al. 2021),
loosely wound spiral arms (Bothun et al. 1997), and generally lack
bars. However, in GLSB galaxies with a bulge-dominated central
component, tighter spiral arms may be present (Das 2013). GLSB
galaxies also sometimes demonstrate ring structures, as in the case
of UGC 6614 (Das 2013).
The formation histories of GLSB galaxies are not well understood
but include catastrophic collisional scenarios, involving (1) merging
(Saburova et al. 2018;Zhu et al. 2023), (2) head-on collisions with a
massive galaxy leading to the collisional formation of a ring galaxy
(Mapelli et al. 2008), (3) the accretion of gas-rich low-mass galaxies
(Peñarrubia et al. 2006;Hagen et al. 2016), or (4) the accretion
of the cooling hot halo gas stimulated by the merger of intruding
galaxies (Zhu et al. 2018). Also, non-catastrophic solutions have
been proposed: (1) isolated secularly evolved models (Noguchi 2001;
Boissier et al. 2016), (2) the presence of an unusual shallow and
extended dark matter halo (Kasparova et al. 2014) which can lead
to the formation of a giant LSB disk; (3) or the cold accretion of
gas from the intergalactic medium (Lelli et al. 2010;Saburova et al.
2019). These scenarios have been recently investigated by Saburova
et al. (2021) for a sample of seven well-known GLSB galaxies. They
conclude that the GLSB galaxies represent a heterogeneous class of
giant galaxies with LSB disks. This suggests that their formation
can involve various aforementioned mechanisms, among which the
external supply of material to form the giant LSB disk may be more
common.
The aim of this paper is to explore the structural properties of
UGC 4599 in great detail using deep imaging and compare its struc-
ture with ordinary spiral and elliptical galaxies, Hoag’s Object, and
with GLSB galaxies employing standard galaxy scaling relations.
This comparison aims to propose a plausible scenario for the forma-
tion of the peculiar structure of UGC 4599. We confirm recent results
by Sil’chenko et al. (2023) and identify UGC4599 as a GLSB galaxy
Table 1. General properties of UGC4599.
RA (J2000) 08h47m41s(1)
Dec. (J2000) +13d25m09s(1)
Stellar heliocentric velocity (kms−1) 2071 (2)
Distance (Mpc) 32.0 (3)
Scale (kpc arcsec −1) 0.156 (3)
Type (R)SA0 (1)
Major diameter (arcmin) 2.1 (1)
𝑚𝑟(mag) 13.47 (4)
𝑀𝑟(mag) -19.06 (4)
𝑔−𝑟0.67 (4)
References: (1) NASA/IPAC Extragalactic Data base (NED); (2) Durbala
et al. (2020); (3) Haynes et al. (2018); (4) PhotoObj Sloan Digital Sky Survey
(SDSS, Ahumada et al. 2020) table. Apparent and absolute magnitudes
in the 𝑟band and the 𝑔−𝑟color were corrected for Galactic extinction
using Schlafly & Finkbeiner (2011) and K-correction using the K-corrections
calculator (Chilingarian et al. 2010;Chilingarian & Zolotukhin 2012).
with a high surface brightness central component surrounded by a
blue star-forming ring and hosting a LSB disk with barely detectable
embedded spiral structure. Using supplementary observations in the
𝑢waveband from the 4.3m Lowell Discovery Telescope, we also re-
veal a dim spiral structure within the galaxy core which excludes the
possibility that the central body of UGC 4599 is an elliptical galaxy,
unlike Hoag’s Object.
We organize the paper as follows. Sections 2and 3describe our
observations and image processing. Section 4details our photometric
analysis of the galaxy at different wavelengths including investigation
of its spiral structure. In Section 5, we discuss the structural properties
of UGC 4599 compared to other galaxies using famous galaxy scaling
relations and propose realistic formation scenarios of this galaxy.
Section 6summarizes our conclusions. Throughout the paper, all
magnitudes and surface brightnesses are given in the AB magnitude
system.
2 OBSERVATIONS
Deep optical observations of UGC 4599 were obtained in Jan-
uary 2011, with 0.7-m Jeanne Rich Telescope Centurion 28 lo-
cated near Frazier Park, CA, northwest of Los Angeles. The de-
tector at f/3.2 prime focus provides a 2-3 arcsec resolution. On a
moonless night, the zenith sky brightness at the site is typically
∼21.6−22.1mag arcsec−2. A sister telescope of identical design
was implemented later at the Wise Observatory in Israel and is de-
scribed in Brosch et al. (2015).
In the HERON survey, we make use of the Luminance (L) band
filter that allows us to reduce the night sky background and artificial
light pollution. Using this Luminance filter, we are able to obtain
very deep images of local galaxies and explore their LSB structure
with the aid of a relatively small-aperture telescope.
By employing the wide 4000 −7000Å Luminance filter for
UGC 4599 with an 8 hour integration, we rich a surface brightness
limit of 30.2 mag arcsec−2calibrated to the Sloan Digital Sky Sur-
vey (SDSS) 𝑟filter (as a 3𝜎level in square boxes of 10′′ ×10′′).The
transformation to SDSS has a negligible color term and is straight-
forward using calibration stars in the field (see Rich et al. 2019). For
extended surface brightness regions, it is possible to detect structures
marginally fainter than the 31 magarcsec−2limit by averaging in-
tensities in grouped pixels (see Sect. 4.1). The average point spread
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 3
Figure 1. Images of UGC4599 in the HERON Luminance filter converted to the SDSS 𝑟band (top left) and in the LDT 𝑢band (top right). The bottom panels
show the supplementary GALEX FUV (bottom left) and GALEX NUV (bottom right) images. The colorbars decode the surface brightnesses in magnitudes per
square arcsec depicted in the different images. Stars and other background and foreground objects are masked out. The masked west ellipse in the NUV band
covers an image artifact.
function (PSF) FWHM for our stacked image is 1.9 arcsec at a pixel
size of 1.67 arcsec.
Supplementary observations of UGC 4599 were conducted on the
night of November 29, 2016, with the 4.3-m Lowell Discovery Tele-
scope (LDT), formerly the Discovery Channel Telescope (DCT).
This observatory is located forty miles southeast of Flagstaff, AZ. To
acquire images, we used the Large Monolithic Imager (LMI) with a
single 6144×6160 e2v CCD. All images were binned 2×2, so that the
pixel scale is 0.24 arcsec and the field of view is 12.3×12.3arcmin.
We took five individual exposures, each of 1200s duration, using
the SDSS 𝑢filter. To remove the effects of bad pixels and artifacts,
as well as to improve flat fielding, the exposures were obtained with
dithering. Average seeing was poor (FWHM=1.7 arcsec) due to some
thin cirrus contamination. To obtain co-add images, we used the same
standard data reduction techniques as for the HERON data. The depth
of the final stacked image occurred to be 30.1 mag arcsec−2(3𝜎in
10′′ ×10′′ boxes, the 𝑢band).
We also use supplementary observations from the GALEXsatellite
in the FUV band centered at 1450Å (the AIS survey with a total
exposure of 217s) and NUV band centered at 2300Å (the MIS survey
with a total exposure of 2720s) using the last publicly available data
release1. The advantage of using UV observations is that they have
less contamination from foreground stars than in the optical. We also
make use of the Infrared Array Camera (IRAC, Fazio et al. 2004)
1http://galex.stsci.edu/gr6
MNRAS 000,1–16 (2022)
4A. Mosenkov et al.
observations at 3.6 𝜇m (331s of duration) from the Spitzer Space
Telescope (Werner et al. 2004) using the Spitzer Heritage Archive2.
Finally, we use VLA observations from Dowell (2010) for mapping
the Hidistribution in UGC 4599.
3 IMAGE PROCESSING
Initial image reduction was carried out for both the HERON and LDT
observations of UGC 4599 using the standard IRAF routines (bias
subtraction, correction for the dark current, flat fielding, etc.).
For each image, we estimated the sky background around the
galaxy within an elliptical annulus of radius 500 arcsec (this radius
was chosen based on the extent of the galaxy after a preliminary sky
subtraction for all the images) and a width of 30 arcsec.
Due to the presence of several bright stars in our deep exposures,
some further image preparation was done which is of high impor-
tance for an analysis of the faint outer structure of UGC 4599. Instead
of simple masking, we used the IRAF routine DAOPHOT to subtract
the stars. Although careful star subtraction is a relatively time con-
suming process, it is worthwhile here to retain more of the structure
of the galaxy than while using simple masking techniques. In our
modelling, we take into account the core of the PSF determined for
non-saturated bright stars found in the field near the galaxy and the
wings of the extended PSF built in Rich et al. (2019) (see their fig. 10)
for the HERON survey. For the LDT frame, we used the brightest
saturated star BD+13 1990 near the galaxy ring. However, even a
relatively good star subtraction with DAOPHOT did not allow us to
completely remove the light of the foreground stars, especially those
that are saturated: such stars exhibit diffraction spikes and saturated
cores, so a good PSF subtraction is not possible in this case. Unfortu-
nately, the bright foreground star BD+13 1990 completely obscures
an interesting region of the galaxy that likely contains a large segment
of the spiral structure under study. The GALEX and LDT UV images
are useful for studying this segment, since the aforementioned star is
not as bright in the UV as in the optical.
To completely eliminate all contaminating sources in our 1D fitting
and 2D photometric decompositions (see Sects. 4.1 and 4.2) and to
examine the spiral structure (Sect. 4.3), we created a mask for each
of the images by employing SExtractor (Bertin & Arnouts 1996)
and then manually revised it to mask off objects that do not belong
to the target galaxy. The UV and optical images of UGC4599 with
the masks superimposed are displayed in Fig. 1.
In order to describe the surface brightness distribution of the
galaxy, an intensity map in a more common filter system than Lu-
minance is required. For this, we calculated a conversion from the
instrumental Luminance 𝐿filter magnitude to the SDSS 𝑟magnitude.
We selected 18 non-saturated stars of known SDSS 𝑔and 𝑟magni-
tudes in the field of UGC 4599. Aperture photometry of these stars
was then performed using the IRAF routine DAOPHOT. After that, the
Luminance instrumental magnitudes of these stars were compared
to the SDSS 𝑔and 𝑟magnitudes of the same objects for deriving a
calibration equation to convert pixel counts in our image to values
of 𝑟-mag arcsec−2for each pixel in the image. The 𝑔−𝑟term in
the calibration equation appeared to be very small, especially if we
take into account different kinds of errors. These errors originate
from the photometric errors of the archived magnitudes, the errors
in our instrumental photometry, and the fit errors in the comparison
between our instrumental and the archival magnitudes.
2https://sha.ipac.caltech.edu/
Figure 2. Comparison between the azimuthally averaged profiles created for
the HERON 𝐿-band and SDSS 𝑟-band data.
In Fig. 2, we show a comparison for the azimuthally averaged
profiles created for the HERON image with the above described 𝑟-
band calibration and an 𝑟-band SDSS image of UGC4599. As one
can see, the comparison is very good (Δ𝜇 < 0.08 mag arcsec−2)
within 100 arcseconds from the center and is gets worse (Δ𝜇 <
0.26 mag arcsec−2) outwards where the depth of the SDSS photom-
etry is reaching its limit (26.5mag arcsec−2,3𝜎in 10′′ ×10′′ boxes
in the 𝑟band).
4 STRUCTURAL ANALYSIS OF UGC 4599
4.1 Azimuthally averaged profiles
We use different data sources, described in Sect. 2, to explore the sur-
face brightness distribution in UGC 4599 and trace its general struc-
tural components, the bulge and the disk. In particular, we exploit
the GALEX FUV and NUV observations to study the distribution of
the young (0-200 Myr) stellar population coupled with the deep LDT
𝑢-band image, which is sensitive to young stars with a larger range
of ages. The deep HERON coadded frame maps the distribution for
a mix of the old and young stellar populations, whereas the Spitzer
IRAC 3.6 𝜇m image traces the old stellar population, which makes
up the bulk of the galaxy stellar mass. Finally, we analyze the VLA
Himap to ascertain the distribution of the neutral hydrogen gas in
the galaxy disk.
In this section, we carry out azimuthally averaged profile fitting of
the aforementioned data sets, whereas a detailed 2D decomposition
of the galaxy is presented in Sect. 4.2. To facilitate a general galaxy
structural analysis, 1D fitting is preferable to 2D fittingdue to the low
signal-to-noise ratio at the periphery of the galaxy where the outer
disk dominates the galaxy profile.
To properly model the galaxy profiles, we take into account the
PSF for each galaxy image used. For GALEX, the PSF kernels in
the NUV and FUV wavebands were taken from Aniano et al. (2011).
For the LDT 𝑢-band image, bright, non-saturated stars far from the
galaxy were fitted with a Moffat function, and the PSF wings were ap-
proximated using the bright saturated star BD+13 1990. An extended
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 5
IRAC 3.6𝜇m PSF was retrieved from the IRSA website3. For VLA
Hi, we used a synthesized beam size of 21.1 arcsec by 14.4 arcsec
with a position angle of 8.2◦. All PSF images were then azimuthally
averaged to be taken into account in our 1D fitting of the galaxy
profiles.
In all the wavebands, except for FUV and 21 cm, a significant
intensity peak is present coinciding with the center of the galaxy
core. Therefore, in these bands, we use a ‘bulge+disk’ model to ac-
curately fit the overall 1D galaxy profile, whereas for the GALEX
NUV and VLA Hiprofiles we use a single-disk profile excluding an
inner region within 75 arcsec. In all the cases, the disk profile was ap-
proximated using an exponential function (Patterson 1940;Freeman
1970), whereas the core (if bright enough) was fitted with a Sérsic
function (Sérsic 1963,1968). In all the profiles, the star-forming ring
from 𝑅r,in =30 arcsec to 𝑅r,out =75 arcsec (where it dominates
the surface brightness distribution) was masked off. Unfortunately,
the IRAC 3.6𝜇m field is too small to cover the entire galaxy, so only
an inner part of the faint disk within 𝑅∼150 arcsec is showcased.
This, however, is sufficient to retrieve the parameters of the general
distribution for the old stellar population in UGC4599. The created
azimuthally averaged profiles, along with the superimposed models,
are presented in Fig. 3. The parameters of the disk for each of the
bands under consideration are listed in Table 2.
We point out that although, to the first approximation, the surface
brightness distribution beyond 𝑟≈75 arcsec can be fitted with a
single exponential profile relatively well, some deviations from this
fit are apparent in Fig. 3, especially beyond 120 arcsec. Based on the
scheme proposed in Erwin et al. (2005), Pohlen & Trujillo (2006),
and Erwin et al. (2008), Gutiérrez et al. (2011) classify their 𝐵-band
profile of UGC 4599 as Type III-d: the outer profile is bending up
(“antitruncation”) and it is part of the galaxy disk through the ex-
tended spiral structure. They determine a break of the disk profile at
𝑟=121 arcsec, beyond which the gradient of the surface brightness
profile slightly changes and can be fitted with a different slope and
intercept. However, as can be seen in Fig. 1, the surface brightness
profile of the inner disk (dominating within 75 < 𝑟 < 120 arcsec) is
significantly affected by the spiral arms wound out from the ring. As
we show in Sect. 4.3, the average pitch angle of the spiral structure is
fairly small, so at smaller radii from the ring, the light from the spiral
arms make a significant contribution to the galaxy luminosity profile
and may cause it to appear “higher” over the outer disk profile extrap-
olated into the inner disk region. Therefore, a simple ‘bulge+disk’
decomposition is beneficial because it allows one to determine the
general parameters of the main galaxy components and compare
them to those of other galaxies (see Sect. 5.1) for which a similar
simple decomposition has been performed, without going into detail
of their structure. In addition, we note that the outer disk extends out
to ∼350 arcsec and dominates across a five times larger range of radii
than the inner disk. Obviously, it greatly affects the parameters of the
general disk provided in Table 2. At the same time, the profile of the
inner disk with the smaller errorbars also non-negligibly contributes
to the fitting, and, as a result, we measure an ‘average’ galaxy disk.
As can be seen from Table 2, the disk scale length steadily de-
creases with wavelength, so the stellar disk at 3.6 𝜇m is 3 times
shorter than the FUV disk (the 3.6 𝜇m profile, however, only traces
the inner galaxy disk). This trend is well-known in the literature and
is interpreted by the inside-out formation of the stellar disk (see e.g.,
Muñoz-Mateos et al. 2007, and references therein). In Fig. 4, we com-
3https://irsa.ipac.caltech.edu/data/SPITZER/docs/irac/
calibrationfiles/psfprf/
Table 2. Best-fit parameters of the disk and bulge listed for different wave-
lengths. The central and effective surface brightnesses have been corrected
for Galactic foreground extinction (using Schlafly & Finkbeiner 2011) and
K-correction (Chilingarian et al. 2010;Chilingarian & Zolotukhin 2012).
The Sérsic index 𝑛=2.4(found for the HERON profile) is kept fixed for all
models.
Data 𝜇0,dℎ 𝜇e,b𝑟e,b
(mag arcsec−2) (kpc) (mag arcsec−2) (kpc)
GALEX FUV 28.00 18.71 — —
GALEX NUV 26.75 11.56 28.47 7.02
LDT 𝑢25.70 10.89 24.45 2.34
HERON 𝑟24.35 10.60 21.77 1.82
IRAC 3.6 22.76 6.13 20.37 1.36
VLA Hi0.53∗25.05 — —
Notes: ∗The average Hisurface density in 𝑀⊙pc−2estimated within a
circular isophote of 29 mag arcsec−2in the GALEX NUV band.
pare this trend with that for regular spiral galaxies from Casasola et al.
(2017): the decrease of the scale length for normal spirals appears
to be less dramatic (ℎFUV ∼1.7ℎ3.6) than for UGC 4599. Also, due
to the faint nature of the stellar disk in UGC4599, the scale length
of the galaxy in the visible appears to be 1.5-4 times larger than its
optical radius, whereas for normal disks the ℎ/𝑟25 ratio is close to
1/4. The extended UV emission, coupled with the enormous Hidisk
(∼50 kpc in radius, see also Grossi et al. 2009), qualify UGC 4599 as
an extended ultraviolet disk (XUV-disk) galaxy (Thilker et al. 2007).
As shown by Casasola et al. (2017), for regular spiral galaxies the
scale length of the gas profile is roughly equal to the scale length
in the FUV band, so ℎΣgas/𝑟25 =0.40 ±0.07. For UGC 4599, the
scale length of the neutral hydrogen gas is 1.3 of the FUV scale
length. Since the gas mass in LSB galaxies is dominated by atomic
gas (Wyder et al. 2009 and see a recent study by Galaz et al. 2022
for the CO mass in Malin 1), we can assume that the Higas density
profile demonstrates the general distribution of gas in UGC4599.
Note also that the observed inner depression in the Hidisk is typical
of spiral galaxies (Wang et al. 2014).
Note also that, similar to the disk scale length, the effective radius
of the bulge in UGC 4599 drops with wavelength, whereas the bulge
effective radius in other galaxies usually increases from blue to red
colors (Möllenhoff 2004).
4.2 2D photometric decomposition of UGC 4599
To fully characterize the structure of UGC 4599, we produce a photo-
metric model of the galaxy utilizing its entire 2D image. This is done
through the use of the IMFIT code (Erwin 2015), which fits para-
metric models to point and extended sources in astronomical images.
The program supports building simulated galaxies from scratch and
fitting the structural parameters of existing galaxies from images.
Here we make use of IMFIT to not only obtain the parameters of the
structural components for UGC4599, but also to amplify the visible
spiral structure in the residuals as shown in Sect. 4.3.
Creating an IMFIT photometric model for UGC 4599 implies de-
composing it into several structural components. We point out that
the core of the galaxy is more complex than wasassumed in our sim-
ple 1D modeling in Sect. 4.1. Fig. 5clearly demonstrates that within a
radius of 𝑅pb ∼5arcsec, our 1D models cannot adequately describe
MNRAS 000,1–16 (2022)
6A. Mosenkov et al.
Figure 3. Profiles of UGC 4599 with superimposed models: blue dashed lines correspond to the disk, magenta dashed lines – to the bulge, and red lines depict
the total model.
Figure 4. Dependence of the disk scale length normalized by the optical
radius from the HyperLeda database (Makarov et al. 2014) on wavelength.
The purple crosses depict UGC 4599 (see Table 2), whereas the black filled
circles correspond to the best-fit results for a sample of 18 nearby face-on
spiral galaxies from Casasola et al. (2017) (see their table 7).
the galaxy profiles suggesting the presence of another, unresolved
component in the galaxy core. Therefore, we decided to add this
component in our 2D model, along with a Sérsic bulge (which dom-
inates at 𝑅pb < 𝑟 < 𝑅r,in) and a broken-exponential disk dominating
at 𝑟 > 𝑅r,out with the break radius 𝑅br . Since the spiral arms are
very faint and narrow, we mask them out and neglect their presence
in our decomposition. We note, however, that we attempted to model
the galaxy with an additional two-armed spiral component but our
models never converged because the spiral structure in this galaxy is
rather complex (see Sect. 4.3) and faint, so its weight in the models
is too small to be reliably fitted. Also, we mask off the star-forming
ring within 𝑅in < 𝑟 < 𝑅r,out to make our model simpler and better
convergeable.
We carried out 2D photometric decomposition for both the
HERON and IRAC 3.6 𝜇m images to demonstrate the adequate-
ness of our models in describing the galaxy structure. For the IRAC
Figure 5. Azimuthally averaged profiles of UGC 4599 in different wavebands.
Dashed lines show regions where the next component begin to dominate.
image, we used a single exponential model because its field of view
does not capture the outer disk.
The results of our fitting are presented in Table 3and illustrated
in Fig. 6. Our models suggest that the nucleus of the galaxy is in-
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 7
deed characterized by an unresolved source: in the HERON we used
a PSF, whereas for the IRAC model we employed a Sérsic func-
tion, but the effective radius appeared to be smaller than FWHM/2
(FWHM3.6𝜇m=2.1arcsec), so this component cannot be resolved
at 3.6 𝜇m as well. The galaxy might host an active galactic nucleus
(AGN), but its spectrum does not contain the characteristic AGN
features. Therefore, we speculate that the nuclear component (within
a radius of ∼200 pc) may be a nuclear star cluster or any other com-
pact stellar component possibly related to the inner spiral structure
which seems to be emerging from the innermost resolved region of
a few kpc (see Sect. 4.3).
The other inner component, which makes up the bulk of the core’s
luminosity, is a disk-like structure, possibly a pseudobulge. This
assumption is confirmed by 1) the exponential behaviour of the inner
galaxy profile outside of 𝑅pb ∼5arcsec and 2) the inner spiral
structure which is detailed in Sect. 4.3.Sil’chenko et al. (2023)
also identify a pseudobulge in their 𝑟-band profile of UGC4599
with the central surface brightness 𝜇0,b(𝑟)=20.5mag arcsec −2
and 𝑟e,b=14.8arcsec versus our 𝜇0,b(𝑟)=20.3mag arcsec−2and
𝑟e,b=10.7arcsec for the HERON model.
Finally, the inner disk with the central surface brightness 𝜇0,d=
23.2mag arcsec−2and the scale length ℎin =35.9arcsec is in a
relatively good agreement with the results from Sil’chenko et al.
(2023), 𝜇0,d=23.6mag arcsec−2and ℎin =49.1arcsec, taking into
account that our HERON model consists of the additional unresolved
central component and the outer disk. The outer part of the broken-
exponential profile has a very large radial scale ℎout ≈15 kpc and low
extrapolated central surface brightness 𝜇0,dout ≈25.5mag arcsec−2.
Both values of 𝜇0,dand 𝜇0,dout, coupled with the very large disk
extent, suggest that UGC 4599 should be classified as a GLSB galaxy,
and the dim inner and outer spiral structure changes its morphology
from S0 to S.
4.3 Analysis of the spiral structure in UGC 4599
The faint spiral structure of UGC 4599 is clearly visible in sev-
eral passbands (Fig. 1). In this section, we exploit the LDT 𝑢-band,
HERON and Hiimages to investigate the inner and outer parts of the
spiral structure in great detail. The residuals of the HERON model in
Fig. 7strongly emphasize the spiral arms of the galaxy winding out
from the star-forming ring. The illustration of the likely location of
the inner part of the spiral structure, which extends inward inside of
the ring, is helpful in confirming the pitch angle measurement of the
galaxy.
In all UV and optical 1D profiles in Fig. 3, the ring and spiral arms
are well-seen as bumps above the exponential disk model. Gutiérrez
et al. (2011), based on their deep 𝐵-band image, also obtained the
outer disk profile with faint spiral structure out to ∼200 arcsec.
In our UV and optical images, we see extended spiral structure in
all directions emerging from the tightly wound spiral ring out to
∼290 arcsec.
While present in the GALEX images, the LDT image of UGC 4599
in the 𝑢band displays the faint structure of the galaxy in great de-
tail: the LDT resolution is 1.7 arcsec versus 4.0arcsec for GALEX
FUV and 5.6arcsec for GALEX NUV. The zoomed-in inner part
of the 𝑢-band image in Fig. 8clearly shows the inner spiral struc-
ture (supported by the HERON image, which has a poorer resolution
and a much larger pixel scale). The conclusion, made in Finkelman &
Brosch (2011) that the core of UGC 4599 is surrounded by a detached
ring, is thus not confirmed. In the LDT and HERON images, we can
see one spiral arm emerging from the nucleus. The estimated bright-
Table 3. Results of the IMFIT fitting for UGC 4599 for the HERON and IRAC
3.6 𝜇m image. In parentheses we list the IMFIT functions for describing each
galaxy component.
Component Parameter HERON IRAC Units
𝑟band 3.6 𝜇m
1. Nucleus 𝑀n-17.51 -18.36 mag
(PSF): 𝑓n0.107 0.140
2. Pseudobulge 𝑛1 1
(Sérsic)𝜇e,b22.07 20.53 mag arcsec−2
𝑟e,b1.67 1.36 kpc
𝑀b−18.28 -19.34 mag
𝑓b0.219 0.342
3. Disk 𝜇0,d23.16 22.71 mag arcsec−2
((Broken) ℎin 5.60 7.36 kpc
Exponential): ℎout 15.03 — kpc
𝑅br 18.98 — kpc
𝑀d−19.50 −20.14 mag
𝑓d0.674 0.518
Notes: 𝑛is the Sérsic index, 𝜇e,bis the effective surface brightness
of the Sérsic component at the effective radius 𝑟e,b;𝜇0,dis the disk
central surface brightness and ℎin and ℎout are the inner and outer disk
scale lengths, respectively. 𝑀and 𝑓denote the absolute magnitude and
the fraction of the component’s luminosity, respectively. The surface
brightnesses and absolute magnitudes have been corrected for Galactic
extinction using Schlafly & Finkbeiner (2011) and K-correction using
the K-correction calculator (Chilingarian et al. 2010;Chilingarian &
Zolotukhin 2012).
ness of this inner spiral structure after subtracting the azimuthally
averaged model, obtained in Sect. 4.1, is in a range between 27 and
25 mag arcsec −2in the 𝑢passband. It is important to note that the
shape of the inner spiral arm changes with decreasing distance to the
center: it becomes less round and less wound.
In addition to the UV and optical images, the column density
mapping of Hifrom the VLA confirms the presence of the two-
armed spiral structure in Hi, as described below.
To retrieve the parameters of the spiral arms in UGC 4599, we
employ a method based on an accurate analysis of photometric cuts
perpendicular to the arm direction in a galaxy (Savchenko et al. 2020;
Mosenkov et al. 2020). Each slice, averaged along some segment
of the spiral arm where the curvature of the arm does not change
significantly, is fitted (taking into account PSF smearing) with an
asymmetric Gaussian function with the ‘inward’ 𝑤1and ‘outward’
𝑤2half-widths plus some linear trend arising due to the presence
of the dominant structural component(s) — the bulge, the ring, and
the disk (see the detailed description of the method in Savchenko
et al. 2020). This allows us to measure the arm width, asymmetry,
pitch angle, and the variations of these parameters with radius. With
the aid of this method, we are able to determine the characteristics
of the spiral structure in UGC 4599 notwithstanding its low surface
brightness nature.
The arms are mainly traced based on the IMFIT residual image
shown in Fig. 7. We also measure clearly visible arcs/parts of the
spiral arms in the 𝑢-band image assuming that the spiral structure
behaves in a similar way in the UV and in the optical. Although
most segments of the spiral structure are seen well in Fig. 7, there
are several places where the spiral arm(s) cannot be continued or the
MNRAS 000,1–16 (2022)
8A. Mosenkov et al.
Figure 6. Azimuthally-averaged profile of UGC 4599 for the HERON (left) and IRAC 3.6 𝜇m (right) images with the corresponding IMFIT models superimposed
(see Table 3). Spiral structure and the inner ring was masked in preprocessing before this fit.
Figure 7. Residual HERON image of UGC4599: (Observed Image-
Model)/Observed Image. Dark color displays positive residuals and highlights
the spiral structure.
resolution of our images is insufficient to see details. Such places also
include the brightest foreground star, which unfortunately outshines
the segment where one arm separates into two. There are several
places where the arms are located very close to each other, especially
in the inner part of the galaxy, so it is difficult to discern the spiral
pattern from the ring and the bulge. Using the same approach, we
also trace the arms in the Hiimage.
By means of our method, we successfully traced the spiral structure
Figure 8. The inner region of UGC 4599 as shown by the LDT 𝑢-band image
minus an azimuthally-averaged model. The inner spiral (mostly shown in
dark), traced by multiple star-forming regions, winds up toward the center.
from the inner region out to a radius of 45 kpc. The resulting spiral
structure, mapped individually for the LDT 𝑢-band, HERON, and Hi
data, is presented in Fig. 9. It is easy to see that although the masks
of the spiral arms, highlighted by the stars and gas, look similar in
general, they are slightly different in details. This fact can be due to
the different resolution of the images used and the slightly different
locations of the star-forming regions in the optical and UV for the
spirals and the atomic gas.
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 9
As was noted earlier, Fig. 8demonstrates the presence of one inner
arm emerging from the pseudobulge and winding out to form the ring.
The inner spiral arm, depicted by blue color in Figs. 9and 10, has
a pitch angle of 7.0◦±0.4◦. In the place blanketed by the bright
star, we find that the arm splits into two distinct outer arms. The fact
that the galaxy has two outer spiral arms is also confirmed by the Hi
image, which clearly displays the two arm ends at the periphery of
the galaxy.
The peaks in the polar coordinates presented in Fig. 10 also demon-
strate a complex behavior of the spiral arms. We see that the HIand
optical data coincide well across the azimuthal angle in most of the
places which strengthens our results. Fitting the entire data, we de-
rive a pitch angle of approximately 4.5◦for both distributions. At the
same time, we can see in Fig. 10 that one of the split arms, marked
by the red color, acts differently after the split (bifurcation) point.
This arm demonstrates a different slope, agreed between the Hiand
stellar data, resulting in a twice as large pitch angle of 8.2◦. This can
be additional evidence in favor of the two-armed model. The average
pitch angle between the two spiral arms is 𝑃=6.4◦, which we will
use for comparison with other spiral galaxies in Sect. 5.2.
5 DISCUSSION
We now proceed to a quantitative comparison of UGC 4599 to other
galaxies to elucidate its nature and discuss various scenarios of its
formation.
5.1 Galaxy scaling relations
It is interesting to see how UGC4599 compares with other galaxies
(including other GLSB galaxies) on standard galaxy scaling relations.
For that, we also performed a photometric decomposition of Hoag’s
Object (see Appendix A) to demonstrate that UGC 4599 and Hoag’s
Object are not only different morphologically (UGC 4599 is a GLSB,
ring spiral galaxy, whereas Hoag’s Object is an elliptical galaxy
surrounded by a star-forming ring), but also have different locations
on the scaling relations.
For comparison purposes, we display the results of both 1D and 2D
photometric decomposition for the HERON data detailed in Sect. 4.
In Fig. 11, plots aand b, we show the location of UGC4599 on
the size–luminosity and Kormendy relations for bulges and elliptical
galaxies. We use two samples of galaxies: regular early- and late-type
galaxies from Gadotti (2009) and GLSB galaxies from Saburova et al.
(2021). As one can see, the bulge of UGC 4599 strongly deviatesfrom
pseudo- and classical bulges, as well as from elliptical galaxies: it
has an effective radius typical of classical bulges, but a very low
surface brightness, more typical of pseudobulges. Hoag’s Object,
however, belongs to the locus of elliptical galaxies and does not
deviate from the general trends. In contrast to UGC4599, the bulges
in the displayed GLSB galaxies are, on average, more luminous and
have a larger effective radius than the bulges in regular spiral galaxies,
deviating from the Kormendy relation for the classical bulges, but
following the general trend on the size-luminosity relation.
In Fig. 11c, we depict the fundamental plane, which links the
three global parameters of elliptical galaxies (Djorgovski & Davis
1987) and bulges (Falcón-Barroso et al. 2002): the central velocity
dispersion (in km/s), the effective radius (in kpc), and the mean
surface brightness within the effective radius (in mag arcsec−2). As
one can see, the pseudobulge of UGC 4599 from our 2D model
strongly deviates from the general trend, whereas Hoag’s Object and
GLSB galaxies generally follow the fundamental plane.
Figure 9. Smoothed masks for the spiral structure in UGC4599, based on
the photometric cuts made perpendicular to the tangent along the spiral arms.
The underlying image in the upper panel displays the HERON data residue
after the model subtraction; the bottom image shows the Hidata. The color
curves outline different spiral arms. The blue arc corresponds to the LDT
𝑢-band data.
Finally, on the 𝜇0,d–ℎrelation (Fig. 11,d), the disk of UGC 4599
appears to be extremely faint and large, so it is an obvious outlier
from the general trend for the regular disks, whereas the other GLSB
galaxies follow this relation but at much lower surface brightnesses
and disk scale lengths. Hoag’s Object is not present in the plot be-
cause it does not have a stellar disk.
Interestingly, the position of UGC 4599 on the baryonic Tully-
Fisher relation (Tully & Fisher 1977;Sprayberry et al. 1995;Mc-
Gaugh & Schombert 2015), which connects the total mass of stars
and gas with the maximum galaxy rotation velocity, follows the
general trend for disk galaxies obtained in Lelli et al. (2019), with
log 𝑀bar/𝑀⊙=10.49 (see Sect. 5.3) and 𝑉rot =170.4km/s (com-
MNRAS 000,1–16 (2022)
10 A. Mosenkov et al.
Figure 10. Pitch angle estimation for the spiral arms in UGC4599 in the log-
polar coordinate system based on the LDT 𝑢-band (inner region), HERON
𝑟-band, and Hidata. Red symbols depict the split spiral arm, as shown with
the same color in Fig. 9. Blue symbols depict the inner spiral arm traced in the
LDT 𝑢-band image. The grey line represents the best linear fit to the HERON
data with a pitch angle of 4.5◦.
puted using the 𝑊20 =180.4km/s emission line width taken from
Dowell 2010 and inclination angle𝑖=27◦from HyperLeda). This is
another evidence that UGC 4599 is not affected by strong starbursts or
mergers and its disk is not overheated. Moreover, this galaxy should
likely be superthin, according to, for example, Bizyaev et al. (2017).
In general, we can conclude that UGC4599 is a very unusual
galaxy considering the galaxy scaling relations. The parameters of
its bulge and disk differ from those in regular spiral galaxies. The
core of Hoag’s Object, however, is consistent with a typical elliptical
galaxy and does not deviate from the standard scaling relations for
the ellipticals. Our sample of GLSB galaxies is too small to make a
fair comparison of UGC 4599 with these galaxies, but on all scaling
relations under consideration this GLSB galaxy has a very different
location.
5.2 Spiral structure in UGC4599
We can also try to characterize UGC4599 in the context of the spiral
structure’s properties as compared to those in other spiral galaxies.
The spiral arms of UGC 4599 are more tightly wound than, on aver-
age, observed in other spirals (for example, consider the distribution
presented in figure 11 in Savchenko et al. 2020), although there is
a tendency for early-type spirals to exhibit lower pitch angles than
measured in late-type spirals (see figure 10 in Yu & Ho 2020). One
of the interesting details of the spiral structure in UGC 4599 is the
large maximum azimuthal angle (the number of turns of the spiral
arms) — almost 10𝜋rad that is not typical of regular spiral galaxies
(Savchenko et al. 2020).
We can also compare another interesting parameter of spiral struc-
ture, the width 𝑤=𝑤1+𝑤2of the spiral arms and how it changes
with galactocentric radius 𝑟in the form 𝑤∝𝑎 𝑟. On average, for both
the Hiand HERON images, the width of the spiral arms increases
with radius, similar to what was measured for most of the galaxies in
Savchenko et al. (2020), but with a relatively small slope of 𝑎=0.03
compared to the mean 0.14 for the Savchenko et al. (2020) sample.
The width of the spiral arms for the Hidata are, on average, 1.5
times larger than in the optical, and there is no significant difference
between the widths of the two outer arms after the bifurcation point.
In contrast, the HERON image clearly shows that the arm, which is
colour-coded with red in Fig. 9, is thinner than the other outer arm.
This difference amounts, on average, to 40% according to our anal-
ysis. The inner spiral is approximately twice as narrow as the outer
arms in both the Hiand optical images. Normalized by the optical ra-
dius 𝑟25, the average width of the spirals in the optical (𝑤1+𝑤2)/𝑟25
is close to 0.1, which is a typical value for the sample considered in
Savchenko et al. (2020) (see their figure 16 and figure 5 in Mosenkov
et al. 2020). The width asymmetry 𝐴=(𝑤2−𝑤1)/𝑤2of the arms
in UGC 4599 is close to zero (𝐴=0.03 for the Hiand 0.04 for the
HERON data) which indicates that, on average, the inward side 𝑤1
of the arms in UGC 4599 is approximately the same as the outward
side 𝑤2. As shown in Savchenko et al. (2020), most spiral galaxies
have a positive average width asymmetry 𝐴 > 0.1−0.2(see their
figure 18) consistent with the Modal Density Wave Theory (Lin et al.
1969): inside of the co-rotation radius, the shock is formed behind the
spiral arm and, thus, 𝑤2should be larger than 𝑤1(Grosbøl & Dottori
2012). Therefore, we can suggest that this mechanism may be less
important for inducing the extremely faint spiral arms in UGC 4599.
A correlation between the mass of galactic supermassive black
holes and spiral galaxy pitch angle has been obtained in many studies
(Seigar et al. 2008;Berrier et al. 2013;Davis et al. 2017,2015). This
so-called M-P relation is advantageous for estimating the SMBH
mass, especially when a well defined velocity dispersion for the
galaxy is absent or when other methods are not available. The M-P
relation has also been shown to have less scatter than other methods
when applied to spiral galaxies (Davis et al. 2017). However, Díaz-
García et al. (2019) did not find any strong correlation between mean
pitch angle and SMBH mass (estimated from central stellar velocity
dispersion) for nearby spiral galaxies from the Spitzer Survey of
Stellar Structure in Galaxies (S4G, Sheth et al. 2010).
SMBH masses, calculated for UGC4599 using different scaling
relations, are tabulated in Table 4. Using equation (8) from Davis
et al. (2017) (if we assume that the M-P relation holds true for
UGC 4599), we arrive at an estimated SMBH mass for this galaxy of
log(𝑀/𝑀⊙)=8.48 ±0.16, corresponding to a black hole mass
of 3.01 ×108𝑀⊙. This places UGC 4599 at the very high end
of the SMBH mass distribution for spiral galaxies and the lower
end for elliptical galaxies (for late-type SMBH mass functions, see
Davis et al. 2014; for a thorough review of black hole mass func-
tions see Kelly & Merloni 2012). The 𝑀BH −𝑛log quadratic re-
lation (Graham & Driver 2007) yields, however, a three orders of
magnitude less-massive SMBH (3.20 ×105𝑀⊙) assuming an ex-
ponential pseudobulge, while the 𝑀BH −𝜎relation (Sahu et al.
2019) gives a mass of 1.13 ×106𝑀⊙(adopting the dispersion ve-
locity value 86.67 ±2.78 km/sfrom SDSS DR12). Graham (2007)
summarizes and compares work done by multiple authors on black
hole mass spheroid luminosity scaling relations. Since we have de-
rived the properties of the galaxy components using an 𝑟-band cali-
brated image, we use the log(𝑀BH/𝑀⊙)=(−0.38 ±0.04)( 𝑀𝑅+21)
+(8.12 ±0.08)relation from Graham (2007), employing the 𝑅-
band bulge absolute magnitude -19.34 mag (see Table 3) and find
𝑀BH =1.2×107𝑀⊙.
As one can see, these estimates of the SMBH mass for UGC4599
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 11
Figure 11. a) The effective radius vs. bulge luminosity relation for the samples of galaxies from Gadotti (2009) and GLSB galaxies from Saburova et al. (2021).
b) Kormendy relation for the same samples as in a).c) The fundamental plane, with coefficients taken from Saulder et al. (2013) for de Vaucouleurs’ models of
elliptical galaxies (the scatter plot is adopted from their study, see their fig. B.51). The renormalised surface brightness 𝐼0is related to the mean effective surface
brightness ⟨𝜇e,b⟩through log 𝐼0=−⟨ 𝜇e,b⟩/2.5.d) 𝜇0,d–ℎrelation for the stellar disks from Gadotti (2009). The structural parameters for the GLSB galaxies
are taken from the following studies: Malin 1 (Pickering et al. 1997), Malin 2 (Kasparova et al. 2014), NGC7589 (Saburova et al. 2021), UGC 1378 (Saburova
et al. 2019), UGC 1382 (Hagen et al. 2016), UGC 1922 (Saburova et al. 2018), and UGC 6614 (Saburova et al. 2021).
are totally inconsistent among themselves which also proves the pe-
culiar nature of this galaxy.
5.3 Star Formation Rate of UGC 4599
The NUV band image from GALEX allows us to infer the star forma-
tion rate of the ‘disk+spirals’ in UGC4599 as compared to the SFR
in the entire galaxy. We do not use the FUV-band image because its
photometric depth is poorer than that of the NUV-band image. Also,
it is of great importance to compare UGC 4599 with other galaxies,
for example, with galaxies from the ALFALFA-SDSS Catalog (Dur-
bala et al. 2020), which present stellar masses, star formation rates,
and Himasses estimated using the same recipes as in this paper.
The total stellar mass of UGC 4599 was calculated using the
Spitzer 3.6 𝜇m data by means of conversion to stellar mass via the
𝑀∗/𝐿value of Eskew et al. (2012). The SFR and the SFR surface
density were computed using the GALEX NUV flux and an SFR
calibration from Kennicutt & Evans (2012) (their table 1, see also
Murphy et al. 2011;Hao et al. 2011):
MNRAS 000,1–16 (2022)
12 A. Mosenkov et al.
Table 4. Black hole mass estimates derived from various scaling relations
Relation Reference log(𝑀BH /𝑀⊙)𝑀BH(𝑀⊙)
𝑀BH −𝑃[1] 8.48 ±0.16 3.01 ×108
𝑀BH −𝜎[2] 6.05 ±0.14 1.13 ×106
𝑀BH −𝑛[3] 5.51 ±0.30 3.20 ×105
𝑀BH −𝐿[4] 7.09 ±0.14 1.20 ×107
Notes:
References: [1] = Davis et al. (2017), [2] = Sahu et al. (2019), [3] = Graham
& Driver (2007), [4] = Graham (2007). We used the log-quadratic 𝑀BH −𝑛
relation (7) from Graham (2007) employing the Sérsic index 𝑛=1(see
Table 2). For the 𝑀BH −𝐿relation we used the log(𝑀BH/𝑀⊙)=( −0.38 ±
0.04) ( 𝑀𝑅+21) + (8.12 ±0.08)𝑅-band bulge absolute magnitude scaling
relation.
Table 5. Additional characteristics of UGC 4599 estimated in the present
study.
Quantity Galaxy Disk
log 𝑀∗(𝑀⊙) 10.31 10.04
log SFR ( 𝑀⊙yr−1) -0.64 -0.75
log SFR29 (𝑀⊙yr−1) -0.83 -0.92
log ΣSFR (𝑀⊙yr−1kpc−2) -4.65 -4.72
log ΣSFR,29 (𝑀⊙yr−1kpc−2) -3.80 -3.89
log 𝑀HI (𝑀⊙) 10.03 —
log ΣHI (𝑀⊙pc−2) 0.12 —
log SFRNUV =log 𝜈𝐿 𝜈−43.17 ,(1)
where 𝜈𝐿 𝜈is the NUV spectral energy density uncorrected for
the 22 𝜇m emission. This correction was not done since the galaxy
has no detection in the WISE W4 waveband and there are no Spitzer
MIPS observations. This method assumes a constant star formation
rate, solar metallicity, and Salpeter (1955) stellar initial mass function
(IMF). We do not account for dust reddening or extinction since i)
UGC 4599 is near to face-on, 2) as shown by Wyder et al. (2009), the
ratio of far-infrared to UV flux for low surface brightness galaxies
is significantly less than unity, implying low internal UV attenuation
and dust content.
We estimate the SFR and the surface density of the SFR within
both a circular aperture with radius 350 arcsec (the extent of the
galaxy in the UV and optical images under study) and an isophote of
29 mag arcsec −2in the NUV band to compare our results with those
from Wyder et al. (2009) for 19 low surface brightness galaxies. We
also provide the total mass and the surface density of the neutral
hydrogen gas based on the VLA map. For computing both the stellar
mass and SFR of the disk, we use the disk parameters retrieved in
Sect. 4.1. The measured quantities are listed in Table 5.
We note here that although the SFR surface density is highest in
the pseudobulge and lowest in the spiral arms, 77% of the total star
formation of the galaxy is attributed to the faint but extended disk
and the embedded spiral structure. The central body with the ring
contribute roughly 23%. This can be explained by the fact that the
diffuse star formation in the outer disk and spiral arms cover a larger
area than the core and the ring and, thus, make a greater contribution
to the total SFR. We can draw the same conclusion for the stellar
mass: despite the very low luminosity of the disk at 3.6 𝜇m, the
disk-to-total mass ratio 0.52 is rather high because of the large extent
of the disk.
After describing the star formation in UGC 4599, we turn to com-
paring it to other LSB galaxies in terms of SFR, average ΣSFR, and
the gas mass and surface densities. The comparison with 18 LSB
galaxies from Wyder et al. (2009) shows that UGC 4599 has a very
typical SFR, ΣSFR, and ΣHI . The location of this galaxy in the SFR
surface density vs. gas surface density is in good agreement with
with the position of other LSB galaxies from Wyder et al. (2009) and
GLSB galaxies from Saburova et al. (2021) (see their figure 16).
In Fig. 12, we consider two correlations between the SFR and the
stellar mass (left plot) and the SFR and the neutral gas mass (right
plot). As can be seen, UGC 4599, Hoag’s object, and the GLSB
galaxies are obvious outliers from these relations, implying that these
galaxies have a significantly lower SFR than regular galaxies with
the same stellar or gas mass.
5.4 Formation scenarios of UGC 4599
The formation of this intriguing galaxy may be explained by several
scenarios, discussed in detail in Finkelman & Brosch (2011) and
Sil’chenko et al. (2023). Here, we only list most plausible of them, in
line with our analysis. The collisional ring scenario, major merging,
minor accretion events, and the external accretion of material from
a donor galaxy have been largely discarded because they are not
supported by observations.
Firstly, the formation of the star-forming ring in UGC 4599 may
be related to secular evolution through bar-related resonances (Buta
& Combes 1996). Our 𝑢-band image in Fig. 8showcases at least one
inner spiral arm with a larger pitch angle than in the outer structure,
winding up toward the nucleus. Also, in contrast to the conclusion
in Finkelman & Brosch (2011) that the bulge in UGC4599 is de
Vaucouleurs, we find that it is exponential (or close to it), so it should
be classified as a pseudobulge (Fisher & Drory 2008;Gadotti 2009
and references therein). Pseudobulges hold clues to the formation
of bars and nuclear structures (nuclear bars, nuclear rings, nuclear
spirals, and other nuclear substructures) through secular processes in
the disks and bulges (Kormendy & Kennicutt 2004). Therefore, the
morphology of UGC 4599 is probably governed by secular evolution.
Perhaps, high-resolution observations will reveal nuclear substruc-
tures which are smeared out in our ground-based images.
Secondly, the existence of the star-forming ring, the extremely faint
XUV-disk, and even more extended and similar (in mass) hydrogen
disk suggest an external origin of the gas through the cold accre-
tion from the intergalactic medium. This is a likely scenario because
most galaxies in the group where UGC 4599 resides are gas-rich and
blue, with star-forming or starburst spectra (e.g. CGCG 061-011 or
MCG+02-22-006). Also, the time for accumulating the estimated
amount of gas in UGC 4599 (the accretion rate depends on the halo
mass) is consistent with the estimate on the age of UGC4599 (see
discussion in Finkelman & Brosch 2011). Most importantly, this
scenario is supported by recent results from Sil’chenko et al. (2023)
who used long-slit spectroscopy and other data to explore the stel-
lar kinematics in the central body of UGC 4599 and different strong
emission-line flux ratios in the ring. Surprisingly, the gas metallicity
in the ring appeared to be unusually low as compared to the major-
ity of the outer star-forming rings in S0 galaxies with nearly solar
metallicities (Sil’chenko et al. 2019;Proshina et al. 2020). There-
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 13
Figure 12. SFR as a function of stellar mass (left) and as a function of mass of neutral hydrogen (right). Grey dots depict galaxies from the ALFALFA-SDSS
Galaxy Catalog (Durbala et al. 2020) where the SFR was estimated using GALEX NUV photometry and the stellar mass was calculated using the method
proposed by McGaugh & Schombert (2015).
fore, based on our deep photometric analysis and recent results by
Sil’chenko et al. (2023), we conclude that the ring and the XUV
disk of UGC 4599 are likely the result of gas accretion from a cos-
mological filament. Interestingly, Hoag’s Object is believed to be
formed through the same mechanism (Finkelman et al. 2011;Brosch
et al. 2013), but instead of a giant LSB disk with an extended spiral
structure only a star-forming ring around the elliptical galaxy was
produced. In contrast to UGC 4599, Hoag’s Object generally follows
the main galaxy scaling relations.
6 CONCLUSIONS
This research more closely examines the structure of the GLSB
galaxy UGC 4599 consisting of a reddish core, blue star-forming
ring, and an extremely faint outer disk with embedded two-armed
spiral structure. We have used several data sources from the UV to
NIR, including our own deep observations in the 𝑢band on the 4.3m
Lowell Discovery Telescope and in the wide Luminance filter using
the 0.7m Jeanne Rich telescope in the framework of the HERON sur-
vey reaching a surface brightness limit of 31 mag arcsec−2. We have
also analyzed a VLA Himap of UGC 4599 taken from the literature.
We report that the two-armed structure manifests itself in both
UV and deep optical observations and extends out to 45 kpc. This
structure is also well separated in the Hidisk. As such, UGC4599
demonstrates many of the characteristics of XUV disks (Thilker et al.
2007;Salim & Rich 2010), which can be detected not only in UV
observations, but also in deep optical integrations.
The mean pitch angle for the two-armed outer spiral structure in
UGC 4599 is estimated to be 𝑃=6.4◦±1.5◦. The spiral structure
of this galaxy resembles the one in other GLSB galaxies: it exhibits
relatively narrow spiral arms, has a small pitch angle, and winds
multiple times about the galaxy center. The spiral arms are extremely
faint (∼25 −27 mag arcsec−2in the HERON image) and become
apparent only in deep UV and optical images. Based on our deep 𝑢-
band image, we trace the inner spiral structure within the galaxy core,
in contrast to Finkelman & Brosch (2011) who claim that the ring
is completely detached from the central component. The inner spiral
arm has a larger pitch angle than the overall outer spiral structure and
may point to the existence of a bar in the near past. The parameters of
the stellar spiral structure and the one traced by the neutral hydrogen
are, on average, well consistent but show some differences at smaller
scales.
Weunder took a detailed investigation of the structure of UGC 4599
to characterize how this galaxy compares to ordinary spiral galaxies
and other GLSB galaxies. We fitted a single exponential disk to az-
imuthally averaged profiles obtained in different wavelengths, from
the UV to 3.6 𝜇m. The disk scale length decreases with wavelength
and differs by a factor of three for the young (GALEX FUV) and old
(Spitzer IRAC 3.6 𝜇m) stellar populations. The gaseous disk has the
largest extent with a disk scale length of 25 kpc. Our accurate 2D
decomposition of the HERON image reveals an unresolved nuclear
component, an exponential pseudobulge with a low effective surface
brightness (𝜇e,b(𝑟)=22.07 mag arcsec−2) but a large effective ra-
dius (𝑟e,b=1.67 kpc). The outer stellar disk has an extremely low
central surface brightness (𝜇0,d(𝑟)=25.5mag arcsec −2) and a very
large scale length (ℎ=15.0kpc), comparable to the disk parameters
in GLSB galaxies.
The comparison of UGC 4599 with regular early- and late-type
galaxies, as well as with other GLSB galaxies and Hoag’s Object
(Fig. 11), on the Kormendy relation, bulge size–luminosity relation,
the fundamental plane for bulges and elliptical galaxies, and the disk
surface brightness–scale length relation shows that on each of these
relations UGC 4599 significantly deviates from other galaxies, in-
cluding the selected GLSB galaxies. Hoag’s Object, on the contrary,
follows the general trends for the elliptical galaxies.
Finally, we measured the SFR of the entire galaxy and its XUV
disk. A majority of the SFR occurs in the outer disk and embedded
spiral arms (77%) rather than in the pseudobulge and the ring (23%),
whereas the total SFR is 0.229𝑀⊙yr−1which is typical of average-
sized LSB galaxies but slightly lower than in GLSB galaxies.
Based on our analysis and previous studies of UGC 4599, we
conclude that cold accretion of gas through a cosmological filament
is the most favoured mechanism for the formation of the observed
extended, LSB outer disk with embedded two-armed spiral structure.
The star-forming ring within the inner disk may have formed thanks
to the same source of gas via secular evolution.
MNRAS 000,1–16 (2022)
14 A. Mosenkov et al.
ACKNOWLEDGEMENTS
We thank Olga Sil’chenko for her comments, which allowed us to
improve the quality of the publication.
Alexander Marchuk and Aleksandr Mosenkov acknowledge finan-
cial support from the Russian Science Foundation (grant no. 22-22-
00483).
These results made use of the 4.3-meter Lowell Discovery Tele-
scope (LDT), formerly the Discovery Channel Telescope (DCT).
Lowell is a private, non-profit institution dedicated to astrophysi-
cal research and public appreciation of astronomy and operates the
LDT in partnership with Boston University, the University of Mary-
land, the University of Toledo, Northern Arizona University and Yale
University. The Large Monolithic Imager was built by Lowell Ob-
servatory using funds provided by the National Science Foundation
(AST-1005313). This research was funded in part by NASA through
the Arkansas Space Grant Consortium. The authors acknowledge
logistical support from the Polaris Observatory Association in the
operations of the Jeanne Rich 0.7m telescope. We would like to
thank Jayce Dowell and Liese van Zee for their input and directing
us to Neutral Hydrogen data.
This research has made use of the NASA/IPAC Infrared
Science Archive (IRSA; http://irsa.ipac.caltech.edu/
frontpage/), and the NASA/IPAC Extragalactic Database (NED;
https://ned.ipac.caltech.edu/), both of which are operated
by the Jet Propulsion Laboratory, California Institute of Technology,
under contract with the National Aeronautics and Space Admin-
istration. This research has made use of the HyperLEDA database
(http://leda.univ-lyon1.fr/;Makarov et al. 2014). This work
is based in part on observations made with the Spitzer Space Tele-
scope, which is operated by the Jet Propulsion Laboratory, California
Institute of Technology under a contract with NASA.
This project used data obtained with the Dark Energy Camera
(DECam), which was constructed by the Dark Energy Survey (DES)
collaboration. Funding for the DES Projects has been provided by the
U.S. Department of Energy, the U.S. National Science Foundation,
the Ministry of Science and Education of Spain, the Science and
Technology Facilities Council of the United Kingdom, the Higher
Education Funding Council for England, the National Center for
Supercomputing Applications at the University of Illinois at Urbana-
Champaign, the Kavli Institute of Cosmological Physics at the Uni-
versity of Chicago, Center for Cosmology and Astro-Particle Physics
at the Ohio State University, the Mitchell Institute for Fundamental
Physics and Astronomy at Texas A&M University, Financiadora de
Estudos e Projetos, Fundacao Carlos Chagas Filho de Amparo, Fi-
nanciadora de Estudos e Projetos, Fundacao Carlos Chagas Filho
de Amparo a Pesquisa do Estado do Rio de Janeiro, Conselho Na-
cional de Desenvolvimento Cientifico e Tecnologico and the Minis-
terio da Ciencia, Tecnologia e Inovacao, the Deutsche Forschungs-
gemeinschaft and the Collaborating Institutions in the Dark Energy
Survey. The Collaborating Institutions are Argonne National Labo-
ratory, the University of California at Santa Cruz, the University of
Cambridge, Centro de Investigaciones Energeticas, Medioambien-
tales y Tecnologicas-Madrid, the University of Chicago, University
College London, the DES-Brazil Consortium, the University of Ed-
inburgh, the Eidgenossische Technische Hochschule (ETH) Zurich,
Fermi National Accelerator Laboratory, the University of Illinois at
Urbana-Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC),
the Institut de Fisica d’Altes Energies, Lawrence Berkeley National
Laboratory, the Ludwig-Maximilians Universitat Munchen and the
associated Excellence Cluster Universe, the University of Michigan,
the National Optical Astronomy Observatory, the University of Not-
tingham, the Ohio State University, the University of Pennsylvania,
the University of Portsmouth, SLAC National Accelerator Labora-
tory, Stanford University, the University of Sussex, and Texas A&M
University.
The Legacy Surveys consist of three individual and complemen-
tary projects: the Dark Energy Camera Legacy Survey (DECaLS;
NOAO Proposal ID # 2014B-0404; PIs: David Schlegel and Arjun
Dey), the Beijing-Arizona Sky Survey (BASS; NOAO Proposal ID #
2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band
Legacy Survey (MzLS; NOAO Proposal ID # 2016A-0453; PI: Ar-
jun Dey). DECaLS, BASS and MzLS together include data obtained,
respectively, at the Blanco telescope, Cerro Tololo Inter-American
Observatory, National Optical Astronomy Observatory (NOAO); the
Bok telescope, Steward Observatory, University of Arizona; and
the Mayall telescope, Kitt Peak National Observatory, NOAO. The
Legacy Surveys project is honored to be permitted to conduct as-
tronomical research on Iolkam Du’ag (Kitt Peak), a mountain with
particular significance to the Tohono O’odham Nation.
NOAO is operated by the Association of Universities for Research
in Astronomy (AURA) under a cooperative agreement with the Na-
tional Science Foundation.
BASS is a key project of the Telescope Access Program (TAP),
which has been funded by the National Astronomical Observatories
of China, the Chinese Academy of Sciences (the Strategic Prior-
ity Research Program "The Emergence of Cosmological Structures"
Grant # XDB09000000), and the Special Fund for Astronomy from
the Ministry of Finance. The BASS is also supported by the Exter-
nal Cooperation Program of Chinese Academy of Sciences (Grant
# 114A11KYSB20160057), and Chinese National Natural Science
Foundation (Grant # 11433005).
The Legacy Survey team makes use of data products from the
Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE),
which is a project of the Jet Propulsion Laboratory/California Insti-
tute of Technology. NEOWISE is funded by the National Aeronautics
and Space Administration.
The Legacy Surveys imaging of the DESI footprint is supported
by the Director, Office of Science, Office of High Energy Physics
of the U.S. Department of Energy under Contract No. DE-AC02-
05CH1123, by the National Energy Research Scientific Comput-
ing Center, a DOE Office of Science User Facility under the same
contract; and by the U.S. National Science Foundation, Division of
Astronomical Sciences under Contract No. AST-0950945 to NOAO.
This research has made use of GALEX data obtained from the
Mikulski Archive for Space Telescopes (MAST); support for MAST
for non-HST data is provided by the NASA Office of Space Science
via grant NNX09AF08G and by other grants and contracts (MAST
is maintained by STScI, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract
NAS5-26555).
This research has made use of Karl G. Jansky Very Large Ar-
ray (VLA) data which were kindly provided by Jayce Dowell. The
National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Asso-
ciated Universities, Inc.
This research made use of the “K-corrections calculator” service
available at http://kcor.sai.msu.ru/
DATA AVAILABILITY
The data underlying this article will be shared on reasonable request
to the corresponding author.
MNRAS 000,1–16 (2022)
Spiral structure of UGC 4599 15
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MNRAS 000,1–16 (2022)
16 A. Mosenkov et al.
Table A1. Results of the IMFIT modeling for Hoag’s Object. Under each
component’s name we provide the corresponding IMFIT function used.
Component Parameter Value Units
1. Bulge: 𝑛4.1
(Sérsic)𝜇e,b21.23 mag arcsec−2
𝑟e,b2.22 kpc
𝑀b−20.61 mag
𝑓b0.599
2. Ring: 𝜇r25.45 mag arcsec−2
(GaussianRing)𝑅r26.89 kpc
𝜎r3.32 kpc
𝑀r−20.17 mag
𝑓r0.401
Notes: 𝑛is the Sérsic index, 𝜇e,bis the effective radius of the Sérsic com-
ponent at the effective radius 𝑟e,b;𝜇ris the maximum surface brightness of
the elliptical Gaussian ring centered at 𝑅rwith the 𝜎rparameter controlling
the width of the ring. 𝑀and 𝑓denote the absolute magnitude and the frac-
tion of the component’s luminosity, respectively. The surface brightnesses
and absolute magnitudes have been corrected for Galactic extinction using
Schlafly & Finkbeiner (2011) and K-correction using the K-correction cal-
culator (Chilingarian et al. 2010;Chilingarian & Zolotukhin 2012).
van Dokkum P., et al., 2016, ApJ,828, L6
van Dokkum P., et al., 2018, Nature,555, 629
APPENDIX A: 2D PHOTOMETRIC DECOMPOSITION OF
HOAG’S OBJECT
We exploited the DESI Legacy Imaging Surveys (Dey et al. 2019) for
fitting the structure of Hoag’s Object in the 𝑟band. A cut-out with the
galaxy was retrieved from the Legacy viewer, along with a coadd PSF
FITS file. We corrected for the flat background and masked off all
other background and foreground sources. To fully take into account
the scattered light from the PSF, we created an extended PSF image,
with the core described by the retrieved coadd PSF image (within
𝑟 < 8arcsec) and the outer par t of the profile (the wings,𝑟≥8arcsec)
using a fixed power law4𝑟−2out to 𝑟=100 arcsec.
The decomposition was carried out into Sérsic and Gaussian ring
components using the IMFIT code. The results of the fitting are listed
in Table A1. The Figs. A1 and A2 demonstrate a 1D azimuthally
averaged profile and 2D image/model/residual images, respectively.
As one can see, the profile is extending down to 31 magarcsec−2
and is described by the model fairly well. We see no signs of a stellar
disk or a halo. The Gaussian ring adequately recovers the bump in
the 1D profile, which corresponds to the tightly wound spiral arms in
the galaxy image. The parameters of the Sérsic profile are consistent
with Finkelman et al. (2011) who fitted a 𝐵-band surface brightness
profile with a two-component ‘Sérsic + exponential disk’ model:
𝑛=3.9±0.2(versus our 4.1), 𝑟e,b=2.8±0.1arcsec (versus our
2.5arcsec), and 𝜇e,b(𝐵)=22.6±0.5mag arcsec −2(versus our
21.23 mag arcsec−2in the 𝑟band). The 𝑟-band deep image and, most
importantly, the residual image do not reveal any (LSB) tidal features
suggesting that the galaxy is in a quasi-equilibrium state.
4See https://www.legacysurvey.org/dr9/psf/ for details.
Figure A1. IMFIT fit of Hoag’s Object using the 𝑟-band image taken from
the DESI Legacy Imaging Surveys. The components include a central bulge
and a star forming ring.
This paper has been typeset from a T
EX/L
A
T
EX file prepared by the author.
MNRAS 000,1–16 (2022)
