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arXiv:0911.5327v1 [astro-ph.HE] 27 Nov 2009
Detection of Gamma-Ray Emission from the Starburst Galaxies
M82 and NGC 253 with the Large Area Telescope on Fermi
A. A. Abdo2,3, M. Ackermann4, M. Ajello4, W. B. Atwood5, M. Axelsson6,7, L. Baldini8,
J. Ballet9, G. Barbiellini10,11, D. Bastieri12,13, K. Bechtol4, R. Bellazzini8, B. Berenji4,
E. D. Bloom4, E. Bonamente14,15, A. W. Borgland4, J. Bregeon8, A. Brez8, M. Brigida16,17,
P. Bruel18, T. H. Burnett19 , G. A. Caliandro16,17, R. A. Cameron4, P. A. Caraveo20,
J. M. Casandjian9, E. Cavazzuti21, C. Cecchi14,15,¨
O. C¸ elik22,23,24, E. Charles4,
A. Chekhtman2,25, C. C. Cheung22, J. Chiang4, S. Ciprini14,15, R. Claus4,
J. Cohen-Tanugi26, J. Conrad27,7,28,29, C. D. Dermer2, A. de Angelis30, F. de Palma16,17,
S. W. Digel4, E. do Couto e Silva4, P. S. Drell4, A. Drlica-Wagner4, R. Dubois4,
D. Dumora31,32, C. Farnier26, C. Favuzzi16,17, S. J. Fegan18, W. B. Focke4, L. Foschini33,
M. Frailis30, Y. Fukazawa34, S. Funk4, P. Fusco16,17, F. Gargano17, D. Gasparrini21 ,
N. Gehrels22,35, S. Germani14,15, B. Giebels18 , N. Giglietto16,17, F. Giordano16,17,
T. Glanzman4, G. Godfrey4, I. A. Grenier9, M.-H. Grondin31,32, J. E. Grove2,
L. Guillemot31,32, S. Guiriec36, Y. Hanabata34, A. K. Harding22, M. Hayashida4, E. Hays22,
R. E. Hughes37, G. J´ohannesson4, A. S. Johnson4, R. P. Johnson5, W. N. Johnson2,
T. Kamae4, H. Katagiri34, J. Kataoka38,39, N. Kawai38,40, M. Kerr19, J. Kn¨odlseder41,
M. L. Kocian4, M. Kuss8, J. Lande4, L. Latronico8, M. Lemoine-Goumard31,32,
F. Longo10,11, F. Loparco16,17 , B. Lott31,32, M. N. Lovellette2, P. Lubrano14,15,
G. M. Madejski4, A. Makeev2,25, M. N. Mazziotta17, W. McConville22,35, J. E. McEnery22,
C. Meurer27,7, P. F. Michelson4, W. Mitthumsiri4, T. Mizuno34, A. A. Moiseev23,35,
C. Monte16,17, M. E. Monzani4, A. Morselli42, I. V. Moskalenko4, S. Murgia4,
T. Nakamori38, P. L. Nolan4, J. P. Norris43 , E. Nuss26, T. Ohsugi34, N. Omodei8,
E. Orlando44, J. F. Ormes43 , M. Ozaki45, D. Paneque4, J. H. Panetta4, D. Parent31,32,
V. Pelassa26, M. Pepe14,15, M. Pesce-Rollins8, F. Piron26, T. A. Porter5, S. Rain`o16,17,
R. Rando12,13, M. Razzano8, A. Reimer46,4, O. Reimer46,4, T. Reposeur31,32, S. Ritz5,
A. Y. Rodriguez47, R. W. Romani4, M. Roth19, F. Ryde28,7, H. F.-W. Sadrozinski5,
A. Sander37, P. M. Saz Parkinson5, J. D. Scargle48 , A. Sellerholm27,7, C. Sgr`o8,
M. S. Shaw4, D. A. Smith31,32, P. D. Smith37, G. Spandre8, P. Spinelli16,17,
M. S. Strickman2, A. W. Strong44, D. J. Suson49, H. Takahashi34, T. Tanaka4,
J. B. Thayer4, J. G. Thayer4, D. J. Thompson22 , L. Tibaldo12,9,13, O. Tibolla50,
D. F. Torres51,47 , G. Tosti14,15, A. Tramacere4,52, Y. Uchiyama45,4, T. L. Usher4,
V. Vasileiou22,23,24, N. Vilchez41, V. Vitale42,53 , A. P. Waite4, P. Wang4, B. L. Winer37,
K. S. Wood2, T. Ylinen28,54,7, M. Ziegler5
– 2 –
1Corresponding authors: K. Bechtol, bechtol@stanford.edu; C. D. Dermer, charles.dermer@nrl.navy.mil;
A.Y.Rodriguez, arodrig@aliga.ieec.uab.es; O. Reimer, Olaf.Reimer@uibk.ac.at; D. F. Torres, dtor-
res@ieec.uab.es.
2Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
3National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001,
USA
4W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmol-
ogy, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA
94305, USA
5Santa Cruz Institute for Particle Physics, Department of Physics and Department of Astronomy and
Astrophysics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA
6Department of Astronomy, Stockholm University, SE-106 91 Stockholm, Sweden
7The Oskar Klein Centre for Cosmo Particle Physics, AlbaNova, SE-106 91 Stockholm, Sweden
8Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy
9Laboratoire AIM, CEA-IRFU/CNRS/Universit´e Paris Diderot, Service d’Astrophysique, CEA Saclay,
91191 Gif sur Yvette, France
10Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy
11Dipartimento di Fisica, Universit`a di Trieste, I-34127 Trieste, Italy
12Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy
13Dipartimento di Fisica “G. Galilei”, Universit`a di Padova, I-35131 Padova, Italy
14Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy
15Dipartimento di Fisica, Universit`a degli Studi di Perugia, I-06123 Perugia, Italy
16Dipartimento di Fisica “M. Merlin” dell’Universit`a e del Politecnico di Bari, I-70126 Bari, Italy
17Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy
18Laboratoire Leprince-Ringuet, ´
Ecole polytechnique, CNRS/IN2P3, Palaiseau, France
19Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
20INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy
21Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy
22NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
23Center for Research and Exploration in Space Science and Technology (CRESST), NASA Goddard Space
Flight Center, Greenbelt, MD 20771, USA
24University of Maryland, Baltimore County, Baltimore, MD 21250, USA
25George Mason University, Fairfax, VA 22030, USA
– 3 –
26Laboratoire de Physique Th´eorique et Astroparticules, Universit´e Montpellier 2, CNRS/IN2P3, Mont-
pellier, France
27Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
28Department of Physics, Royal Institute of Technology (KTH), AlbaNova, SE-106 91 Stockholm, Sweden
29Royal Swedish Academy of Sciences Research Fellow, funded by a grant from the K. A. Wallenberg
Foundation
30Dipartimento di Fisica, Universit`a di Udine and Istituto Nazionale di Fisica Nucleare, Sezione di Trieste,
Gruppo Collegato di Udine, I-33100 Udine, Italy
31Universit´e de Bordeaux, Centre d’´
Etudes Nucl´eaires Bordeaux Gradignan, UMR 5797, Gradignan, 33175,
France
32CNRS/IN2P3, Centre d’ ´
Etudes Nucl´eaires Bordeaux Gradignan, UMR 5797, Gradignan, 33175, France
33INAF Osservatorio Astronomico di Brera, I-23807 Merate, Italy
34Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
35University of Maryland, College Park, MD 20742, USA
36University of Alabama in Huntsville, Huntsville, AL 35899, USA
37Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University,
Columbus, OH 43210, USA
38Department of Physics, Tokyo Institute of Technology, Meguro City, Tokyo 152-8551, Japan
39Waseda University, 1-104 Totsukamachi, Shinjuku-ku, Tokyo, 169-8050, Japan
40Cosmic Radiation Laboratory, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama
351-0198, Japan
41Centre d’´
Etude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F-30128 Toulouse Cedex 4, France
42Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor Vergata”, I-00133 Roma, Italy
43Department of Physics and Astronomy, University of Denver, Denver, CO 80208, USA
44Max-Planck Institut f¨ur extraterrestrische Physik, 85748 Garching, Germany
45Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510,
Japan
46Institut f¨ur Astro- und Teilchenphysik and Institut f¨ur Theoretische Physik, Leopold-Franzens-
Universit¨at Innsbruck, A-6020 Innsbruck, Austria
47Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain
48Space Sciences Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
49Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA
50Max-Planck-Institut f¨ur Kernphysik, D-69029 Heidelberg, Germany
– 4 –
ABSTRACT
We report the detection of high-energy γ-ray emission from two starburst
galaxies using data obtained with the Large Area Telescope on board the Fermi
Gamma-ray Space Telescope. Steady point-like emission above 200 MeV has
been detected at significance levels of 6.8σand 4.8σrespectively, from sources
positionally coincident with locations of the starburst galaxies M82 and NGC 253.
The total fluxes of the sources are consistent with γ-ray emission originating from
the interaction of cosmic rays with local interstellar gas and radiation fields and
constitute evidence for a link between massive star formation and γ-ray emission
in star-forming galaxies.
Subject headings: Galaxies: individual (M82, NGC 253) — gamma rays: obser-
vations — cosmic rays — radiation mechanisms: non-thermal
1. Introduction
Cosmic rays are believed to be accelerated by supernova remnant shocks that are formed
when a star explodes (Ginzburg & Syravotskii 1964; Hayakawa 1969). Observations of γrays
from supernova remnants in the Milky Way would apparently offer the best opportunity to
identify the sources of cosmic rays, but cosmic-ray diffusion throughout the Galaxy results in
a bright γ-ray glow, making it difficult to attribute γrays to cosmic-ray electrons, protons or
ions accelerated by Galactic supernova remnants. Direct evidence for the sources of cosmic
rays is therefore still lacking.
The supernova remnant paradigm for cosmic-ray origin can also be tested by measuring
the γ-ray emission from star-forming galaxies. Starburst galaxies, in particular, should have
larger γ-ray intensities compared to the Milky Way due to their increased star-formation rates
and greater amounts of gas and dust that reprocess light into the IR, and, with photons, serve
as targets for γ-ray production by cosmic ray electrons and ions. If the γ-ray production rate
is sufficiently increased, star-forming galaxies will be detectable by the current generation of
51Instituci´o Catalana de Recerca i Estudis Avan¸cats, Barcelona, Spain
52Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy
53Dipartimento di Fisica, Universit`a di Roma “Tor Vergata”, I-00133 Roma, Italy
54School of Pure and Applied Natural Sciences, University of Kalmar, SE-391 82 Kalmar, Sweden
– 5 –
instruments, as early estimates (e.g., V¨olk et al. 1989, 1996; Aky¨uz et al. 1991; Paglione et
al. 1996) and recent detailed models (e.g., Domingo-Santamar´ıa & Torres 2005; Persic et al.
2008; de Cea del Pozo et al. 2009; Rephaeli et al. 2009; Lacki et al. 2009) predict.
Here we report the detection of the starburst galaxies M82 and NGC 253 in high-energy
γrays from observations with the Large Area Telescope (LAT) on board the Fermi Gamma-
ray Space Telescope. A description of the analysis of the observations is given in Section
2. In Section 3, the measured spectra and fluxes are compared with predictions based on
theories of cosmic-ray origin from supernovae in star-forming galaxies.
2. Observations and Analysis
The LAT is a pair-conversion telescope with a precision tracker and calorimeter, a seg-
mented anti-coincidence detector (ACD) which covers the tracker array, and a programmable
trigger and data acquisition system. Incoming γrays convert into electron-positron pairs
while traversing the LAT. The directions of primary γrays are reconstructed using informa-
tion provided by the tracker subsystem while the energies are measured via the calorimeter
subsystem. The ACD subsystem vetoes the great majority of cosmic rays that trigger the
LAT. The energy range of the LAT spans from 20 MeV to >300 GeV with an angular
resolution of approximately 5.1◦at 100 MeV and narrowing to about 0.14◦at 10 GeV1. Full
details of the instrument, onboard and ground data processing, and other mission-oriented
support are given in Atwood et al. (2009).
The LAT normally operates in a scanning mode (the ‘sky survey’ mode) that covers
the whole sky every two orbits (i.e., ∼3 hrs). We use data taken in this mode from the
commencement of scientific operations in early-August 2008 to early-July 2009. The data
were prepared using the LAT Science Tools package. Only events satisfying the standard low-
background event selection (the so-called ‘Diffuse’ class events corresponding to the P6V3
instrument response functions described in Rando (2009)) and coming from zenith angles
<105◦(to greatly reduce the contribution by Earth albedo γrays) were used in the present
analysis. To further reduce the effect of Earth albedo backgrounds, time intervals when the
Earth was appreciably in the field of view (specifically, when the center of the field of view
was more than 43◦from the zenith) were also excluded from the analysis.
1Angular resolution is defined here as the 68% containment radius of the LAT point spread function
averaged over the intrument acceptance and including photons which convert in either the thick or thin
layers of the tracker array.
– 6 –
We use all γrays with energy >200 MeV within a 10◦radius region of interest (ROI) of
the optical locations for the galaxies M82 and NGC 253. Detection significance maps for each
ROI are shown in Fig. 1. The background model for each ROI includes all LAT-detected
sources along with components describing the diffuse Galactic and isotropic γ-ray emissions.
Each map shows a bright and isolated γ-ray excess above the background that is consistent
with the location of the nominal (optical) position of the respective starburst galaxy.
The data were analyzed using the LAT Science Tools package (v9r15p2), which is avail-
able from the Fermi Science Support Center, using P6V3 post-launch instrument response
functions (IRFs). These IRFs take into account event pile-up and accidental coincidence ef-
fects in the detector subsystems that were not considered in the definition of the pre-launch
IRFs. We used a maximum likelihood fitting procedure (gtlike) to determine the positions of
the γ-ray sources associated with M82 and NGC 253 (see Table 1). The angular separation
between the best-fit location and the core of each galaxy is 0.05◦for M82 and 0.12◦for
NGC 253. Systematic uncertainties in the positions due to inaccuracies in the point spread
function and telescope alignment are estimated to be less than 0.01◦.
We tested the possibility that the sources are spatially extended by fitting two-dimensional
Gaussian-shaped intensity profiles. The widths and locations of the profiles were adjusted
and refit over the region in an iterative procedure but we found no significant evidence for
source extension in our data. We verified these results using a likelihood fitting procedure
capable of modeling spatially extended γ-ray sources (sourcelike). A comparison between
the point and extended source hypotheses using this method produces negligible changes
in detection significance. From our analysis we set upper limits on the angular sizes of the
emitting regions as 0.18◦for M82 and 0.30◦for NGC 253 at the 95% confidence level assum-
ing a two-dimensional Guassian spatial model parameterized by the 68% surface intensity
containment radius. By comparison, the angular sizes of the galaxies are 0.19◦×0.07◦for
M82 and 0.46◦×0.11◦for NGC 253 as measured in the ultraviolet band (Gil de Paz et al.
2007). The starburst cores of M82 (V¨olk et al. 1996) and NGC 253 (Ulvestad 2000) have
an angular extent <0.01◦and cannot be resolved by the LAT.
Spectral analysis is a separate maximum likelihood calculation for which we have adopted
the point-source hypothesis and best-fit position determined during the localization and ex-
tension fitting step.
Diffuse γ-ray emission from the Milky Way is treated with the Galactic diffuse model
described within the gll iem v02.fit file suitable for analysis with the Science Tool gtlike.
In addition to the spatially structured Galactic diffuse emission, Fermi also observes an
isotropic diffuse component which includes both extragalactic diffuse γ-ray emission and
instrumental background from charged particles triggering the LAT. The isotropic diffuse
– 7 –
emission has been treated with isotropic iem v02.txt. Excepting the sources associated with
M82 and NGC 253, all individual objects detected by Fermi after 11 months of scientific
operations within a 10◦radius of the best-fit position of each galaxy are also included into
the background description of each region as distinct point sources.
We considered alternative associations for the two LAT sources of interest aside from
M82 and NGC 253 in the CRATES catalog of flat-spectrum radio sources (14467 entries,
Healey et al. 2007) and the Candidate Gamma-Ray Blazar Survey catalog, CGRaBS (1625
entries, Healey et al. 2008). Both of these catalogs show high correlation with γ-ray bright
blazars based on multiwavelength observations. However, there are no likely CRATES or
CGRaBs objects within the positional uncertainty of either LAT source. Near NGC 253,
the only source of possible concern is a ∼40 mJy NVSS (Condon et al. 1998) radio source
at 1.4 GHz with unknown spectrum. Such a source would be unusually weak by comparison
with the radio fluxes of LAT blazars.
We searched for flux variability for each γ-ray source by creating monthly flux histories of
the total photon flux >400 MeV arriving from within a circular regions 1◦in radius centered
on the Fermi-determined locations. No flaring events are observed and the χ2goodness-of-fit
test is consistent with constant flux for each source (reduced χ2= 0.80 and 1.03 for M82 and
NGC 253, respectively, each with 9 degrees of freedom). Lack of variability is in accord with
the cosmic-ray origin hypothesis where most of the emission derives from diffuse cosmic-ray
interactions, though mild variability of γrays and radio emission (Kronberg et al. 2000,
Brunthaler et al. 2009b) might still occur if M82 or NGC 253 had a recent supernova. Large
amplitude γ-ray variability on short timescales would rule out a cosmic-ray origin of the γ
radiation.
Table 1 summarizes the results of the analyses of M82 and NGC 253. The overall
detection significance is 6.8σfor M82 and 4.8σfor NGC 253. Note that the significance level
for these moderately hard spectrum sources is based on the number of high energy photons
compared to the expected background, whereas the flux uncertainty is based on the number
of such photons, which is not large, and systematic effects. The integral photon fluxes over
100 MeV are calculated by extrapolation of the fitted spectral models.
3. Interpretation
With the nearest luminous starburst galaxies, M82 and NGC 253, detected by the Fermi
Gamma-ray Space Telescope, we can test long-standing predictions based on the cosmic-ray
paradigm that diffuse γ-ray emission from star-forming galaxies is produced via cosmic-ray
– 8 –
interactions. The distance to M82 is 3.63 ±0.34 Mpc (Freedman et al. 1994), and distance
estimates to NGC 253 range from 2.5 Mpc (Turner & Ho 1985, Mauersberger et al. 1996)
to 3.9±0.37 Mpc (Karachentsev et al. 2003). Vigorous star formation is observed within
the central several hundred parsecs of these galaxies. Estimates of the supernova explosion
(SN) rate vary from ≈0.08 – 0.3 yr−1in M82, to ≈0.1 – 0.3 yr−1in NGC 253, compared to
the supernova rate of ≈0.02 yr−1in the Milky Way. Recent studies of M82 find 7 ×108M⊙
in atomic H I gas and 1.8×109M⊙in H2gas (Casasola et al. 2004). The central region of
NGC 253 contains a bar of molecular gas with an estimated mass of 4.8×108M⊙(Canzian
et al. 1988), and its total gas content is ≈60% of the Milky Way’s (Boomsma et al. 2005;
Houghton et al. 1997; Brunthaler et al. 2009b), reflecting active star formation taking place
in these relatively small galaxies.
Table 2 gives adopted values of distance d, supernova rate RS N , total gas mass MGas,
γ-ray flux F(>100 MeV), and γ-ray luminosities for M82 and NGC 253, alongside those
of the Large Magellanic Cloud (LMC) and the Milky Way. The 100 MeV to 5 GeV γ-ray
luminosity of M82 and NGC 253 is ≈1040 erg s−1, compared to ≈3×1039 erg s−1for the
Milky Way, and ≈4.1×1038 erg s−1for the LMC. These galaxies lack active central nuclei
and so require a different origin for their γ-ray fluxes than from galaxies with supermassive
black-hole jets. The γrays from our Galaxy and the LMC arise predominantly from cosmic
rays interacting with interstellar gas and radiation fields. The starburst galaxies M82 and
NGC 253, though having less gas than the Milky Way, have a factor 2 – 4 greater γ-ray
luminosity, suggesting a connection between active star formation and enhanced cosmic-ray
energy densities in star-forming galaxies.
We examine several possible correlations between total gas mass, supernova rate, and
γ-ray luminosity of these four galaxies as illustrated in Fig. 2 (cf. Pavlidou & Fields 2001,
for local group galaxies). In the left-hand panel, we find a poor correlation between γ-
ray luminosity and gas mass, and a weak linear correlation between γ-ray luminosity and
supernova rate. Models that attribute the γrays to cosmic-ray processes depend both on
enhanced cosmic-ray intensities, which depends on the supernova rate, and large quantities
of target gas, suggesting that the γ-ray luminosity is proportional to the product of the
total supernova rate and gas mass, as shown in the right-hand panel of Fig. 2. Note that
while the detection of galaxies in this sample is flux-limited, the measured gas masses and
supernova rates for all galaxies are not, so that the dependence of γ-ray luminosity on these
parameters reflect underlying physical relationships rather than sensitivity effects. Although
the sample size is small, this result argues in favor of a scaling of γ-ray luminosity according
to expectations from the hypothesis that the emission is produced by cosmic-ray interactions.
Evaluation of the dependence of γ-ray luminosity on galaxy properties is complicated,
– 9 –
however, by star formation rates that depend on location in the galaxy. Radio and infrared
observations reveal that the starburst activity in M82 and NGC 253 takes place in a relatively
small central region, radius ∼200 pc for both M82 (V¨olk et al. 1996) and NGC 253 (Ulvestad
2000), so that the distribution of the cosmic rays in the galaxies is probably not uniform. In
cases where γ-ray emission can be resolved, as for the Milky Way, this can be seen directly
(Dragicevich et al. 1999). For instance, γ-ray emission from the LMC is mostly produced in
the star-forming region 30 Doradus, and does not simply trace star formation and total gas
mass (Abdo et al. 2009a).
Theoretical predictions, despite using different assumptions and treating the processes
with varying levels of detail, are largely consistent with the detected integral flux of M82
(e.g. V¨olk et al. 1989; Ak¨uz et al. 1991; Persic et al. 2008; de Cea et al. 2009) and NGC
253 (e.g. Paglione et al. 1996; Domingo-Santamar´ıa & Torres 2005; Persic et al. 2008). Fig.
3 shows the predicted and observed spectra. In the case of NGC 253, the predicted photon
flux (>100 MeV) is 2.3×10−8photons cm−2s−1(Domingo-Santamar´ıa & Torres 2005) and
2×10−8photons cm−2s−1(Persic et al. 2008). For M82, the predicted photon flux (>100
MeV) is between 2.6×10−8and 8.3×10−9photons cm−2s−1(de Cea et al. 2009) due to
systematic uncertainties in model parameters; and ≈10−8photons cm−2s−1(Persic et al.
2008). Furthermore, extrapolation of the best-fit power-law spectral model at GeV energies
provides a smooth connection to flux densities of M82 reported at TeV energies (Acciari et
al. 2009). Although not highly constraining due to the faintness of M82 in the GeV band,
the fitted spectrum suggests that a single physical emission mechanism dominates from GeV
to TeV energies. The relationship between the GeV and TeV emission for NGC 253 is less
clear given the current data. Also, note that the inner starburst region of NGC 253 has
about a factor of 3 less radio flux than that of M82 at 1.4 GHz, consistent with the galaxy
being less luminous in γrays (M82, Klein et al. 1988; NGC 253, Carilli 1996).
The star-forming galaxy contribution to the extragalactic γ-ray background (EGB) can
be estimated by writing the EGB intensity as ǫIsf
ǫ
∼
=RHζρbLγ/4π, where the Hubble radius
RH∼
=4200 Mpc for a Hubble constant of 71 km s−1Mpc−1,ζ∼3 – 10 is a cosmological
factor accounting for more active star formation at redshift z&1, and ρ=ρ3/(1000 Mpc3)
is the local space density of normal and star-forming galaxies. The factor b∼
=0.4 corrects
for the intensity at 100 MeV given the >100 MeV luminosity. Writing Lγ= 1040L40 erg s−1
gives ǫIsf
ǫ
∼
=3.5×10−10bζρ3L40 erg cm−2s−1sr−1. For L∗galaxies like the Milky Way, ρ3∼
=3
– 10, and for starburst galaxies like M82 and NGC 253, ρ3is an order of magnitude smaller
(e.g., Scoville 1992). At 100 MeV, a diffuse intensity of ǫIEGB
ǫ(100 MeV) ∼
=2.4×10−9erg
cm−2s−1sr−1was measured with EGRET (Sreekumar et al. 1998), similar to the Fermi
value at 100 MeV (Abdo et al. 2009b). Inserting values for Lγfrom Table 2, one finds
that star-forming and starburst galaxies could make a significant, &10% contribution to the
– 10 –
EGB at 100 MeV, as previously suggested (Pavlidou & Fields 2002; Thompson et al. 2007).
Observations with the Fermi Gamma-ray Space Telescope provide evidence that GeV
emission has been detected from the starburst galaxy M82, and weaker though still significant
evidence for detection of NGC 253. The Fermi LAT detections of these galaxies at GeV
energies, together with the recent discovery of >700 GeV γrays from M82 with VERITAS
(Acciari et al. 2009) and >220 GeV γrays from NGC 253 with H.E.S.S. (Acero et al. 2009),
introduce a new class of γ-ray sources to γ-ray astronomy. Unlike γ-ray emitting blazars and
radio galaxies powered by supermassive black holes, the evidence presented here supports
a cosmic-ray origin for γ-ray production in starburst galaxies. Fermi observations over the
upcoming years will improve our knowledge of spectra, variability properties, and number of
γ-ray bright starburst galaxies, which will also constitute important targets for observations
with planned large Cherenkov telescope observatories CTA and AGIS2.
The Fermi LAT Collaboration acknowledges generous ongoing support from a number
of agencies and institutes that have supported both the development and the operation of the
LAT as well as scientific data analysis. These include the National Aeronautics and Space
Administration and the Department of Energy in the United States, the Commissariat `a
l’Energie Atomique and the Centre National de la Recherche Scientifique / Institut National
de Physique Nucl´eaire et de Physique des Particules in France, the Agenzia Spaziale Italiana
and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture,
Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization
(KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallen-
berg Foundation, the Swedish Research Council and the Swedish National Space Board in
Sweden.
Additional support for science analysis during the operations phase is gratefully acknowl-
edged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d’ ´
Etudes
Spatiales in France.
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This preprint was prepared with the AAS L
A
T
E
X macros v5.2.
– 14 –
Right Ascension (deg.)
Declination (deg.)
68
70
72
M82
0
10
20
30
40
50
145150155
Right Ascension (deg.)
Declination (deg.)
−28
−26
−24
NGC 253
Background 1
Background 2
0
5
10
15
20
25
30
35
40
101214
Fig. 1.— Test statistic maps obtained from photons above 200 MeV showing the celestial
regions (6◦by 6◦) around M82 and NGC 253. Aside from the source associated with each
galaxy, all other Fermi-detected sources within a 10◦radius of the best-fit position have been
included in the background model as well as components describing the diffuse Galactic and
isotropic γ-ray emissions. Black triangles denote the positions of M82 and NGC 253 at
optical wavelengths; gray lines indicate the 0.68, 0.95, and 0.99 confidence level contours
on the position of the observed γ-ray excess; green squares show the positions of individual
background sources. The color scale indicates the point-source test statistic value at each
location on the sky, proportional to the logarithm of the likelihood ratio between a γ-ray
point-source hypothesis (L1) versus the null hypothesis of pure background (L0); T S ≡
2(ln L1−ln L0) (Mattox et al. 1996).
Table 1: Results of maximum likelihood analyses (gtlike) of M82 and NGC 253.
RAaDecara
95 F(>100 MeV)bphoton indexbsignificancec
(deg) (deg) (deg) (10−8ph cm−2s−1)
M82 149.06 69.64 0.11 1.6±0.5stat ±0.3sys 2.2±0.2stat ±0.05sys 6.8
NGC 253 11.79 -25.21 0.14 0.6±0.4stat ±0.4sys 1.95±0.4stat ±0.05sys 4.8
aSource localization results (J2000) with r95 corresponding to the 95% confidence error radius
around the best-fit position.
bParameters of power-law spectral models fitted to the data: integrated photon flux >100
MeV and photon index.
cDetection significance of each source.
– 15 –
Table 2: Properties of γ-ray galaxies lacking active central nuclei.
Galaxy dRSN MGas Fγa4πd2FγaLγa
(Mpc) (yr−1) (109M⊙) (10−8ph cm−2s−1) (1042 ph s−1) (1039 erg s−1)
LMCb0.049 ±0.001 0.005 ±0.002 0.67 ±0.08 26.3 ±4.7 0.074 ±0.013 0.041 ±0.007
Milky Wayc1 0.02 ±0.01 6.5 ±2.0 4.6 ±2.3 5.5 ±2.8 3.2 ±1.6
M82 3.6 ±0.3 0.2 ±0.1 2.5 ±0.7 1.6 ±0.5 25 ±9 13 ±5.0
NGC 253 3.9 ±0.4 0.2 ±0.1 2.5 ±0.6 0.6 ±0.4 11 ±7 7.2 ±4.7
aγ-ray fluxes, Fγ, and luminosities, Lγ, computed in the energy range 100 MeV to 5 GeV.
bLMC: distance measurement by Pietrzynski et al. 2009; supernova rate estimated by Tammann, et al. 1994; mass estimate by
Bruns et al. 2005 (see also Westerlund 1997); γ-ray flux from Abdo et al. 2009a.
cGas mass estimate from Dame 1992; γ-ray flux from the Milky Way as viewed from a distance of 1 Mpc; γ-ray luminosity
estimated using models which take into account pion-decay, inverse Compton, and bremsstrahlung photons produced in both
the Galactic disk and halo (Bloemen et al. 1984, Strong et al. 2000, Pavlidou & Fields 2002).
)
−1
yr
Sun
M
9
( 10
Gas
M×SN rate
−3
10 −2
10 −1
10 1 10
)
−1
erg s
39
−ray Luminosity (100 MeV − 5 GeV) ( 10γ
−2
10
−1
10
1
10
2
10
Gas
M×SN rate
)
−1
SN rate ( yr
−3
10 −2
10 −1
10 1 10
)
−1
erg s
39
−ray Luminosity (100 MeV − 5 GeV) ( 10γ
−2
10
−1
10
1
10
2
10
SN rate
)
Sun
M
9
( 10
Gas
M
−2
10 −1
10 1 10 2
10
)
−1
erg s
39
−ray Luminosity (100 MeV − 5 GeV) ( 10γ
−2
10
−1
10
1
10
2
10
Gas
M
Fig. 2.— Relationship between supernova rate, total gas mass, and total γ-ray luminosity
of four galaxies detected by their diffuse high-enery emission. In order of ascending γ-ray
luminosity, the plotted galaxies are the LMC, Milky Way, NGC 253, and M82. Three panels
are shown to compare different possible correlations with the γ-ray luminosity: total gas mass
(left), supernova rate (center), and product of the total gas mass and supernova rate (right).
This figure is based upon the observed quantities and associated uncertainties presented in
Table 2.
– 16 –
Energy (MeV)
3
10 4
10 5
10 6
10 7
10
)
−1
s
−2
dN/dE (MeV cm
2
E
−8
10
−7
10
−6
10
−5
10
−4
10 M82 VERITAS 2009
Blom et al. 1999
Persic et al. 2008
de Cea et al. 2009
Best−fit Power Law
LAT Data
Energy (MeV)
3
10 4
10 5
10 6
10 7
10
)
−1
s
−2
dN/dE (MeV cm
2
E
−8
10
−7
10
−6
10
−5
10
−4
10 NGC253 =2.0−3.0)ΓH.E.S.S. 2009 (
Paglione et al. 1996
Domingo, Torres 2005
Rephaeli et al. 2009
Best−fit Power Law
LAT Data
Fig. 3.— Spectral energy distributions of M82 and NGC 253. The spectra were obtained
using gtlike, with flux points extracted based upon the parameters presented in Table 1.
Upper limits from the LAT correspond to the 0.68 confidence level. Three flux points in
the TeV energy range are provided by VERITAS observations of M82 (Acciari et al. 2009).
The single very high energy flux point for NGC 253 is computed from the integral photon
flux over 220 GeV reported by the H.E.S.S. collaboration (Acero et al. 2009) and assumes a
power-law spectral model marginalized over photon indices ranging from 2.0 to 3.0. Several
theoretical predictions are plotted for comparison to the observed γ-ray spectra.
