Normal galaxies begin to arise from the shadows at high energies, as can be seen with the discovery of high-energy gamma-ray emission from the Andromeda galaxy (M 31) by the Fermi /LAT collaboration. We present a study on the search for high-energy emission around galaxies of the Local Group. No significant detection is found from the objects studied here. Upper limits on the high-energy emission of nearby normal galaxies are derived, and we discuss them in the context of gamma-ray emission from cosmic ray interactions with the local interstellar medium in these galaxies.
Unusual suspects for extragalactic high-energy γray emission begin to arise from the shadows: the Fermi/LAT collaboration recently reported the discovery of high-energy γ-ray from the Andromeda galaxy (M 31), our neighbouring galaxy [1]. More powerful sources, particularly active galactic nuclei (AGNs) with relativistic jets pointing close to the line of sight, the so-called blazars, represent the major part of extragalactic sources detected with Fermi/LAT (see e.g. [2]).
However, weaker sources such as starburst galaxies begin to be revealed as high-energy emitters, both in the GeV and TeV ranges, like M 82 [3,4] and NGC 253 [3,5]. Starburst, Seyfert and normal galaxies have long been thought to marginally contribute to the extragalactic diffuse γ-ray emission (see e.g. [6]), however recent studies tend to re-evaluate this contribution to be largely significant, if not dominant, compared to the emission from blazars (see e.g. [7]). We present here a study on the search for highenergy γ-ray emission from normal, nearby galaxies from the Local Group, in order to investigate any signature from cosmic rays interactions with the interstellar medium in the host galaxy, as opposed to the emission from AGNs.
High-energy emission from some galaxies of the Local Group has been reported for the small Magellanic cloud (SMC, [8]), the large Magellanic cloud (LMC, [9]), M 31 [1], the starbursts M 82 and NGC 253 [3], and Cen A [10,11]. We study here the major galaxies from the Local Group for which no high-energy emission has been reported so far: M 81, M 83, IC 342, Maffei 1, Maffei 2, and M 94.
A Fermi/LAT data analysis is performed for the sources in our sample using the public data from August 4, 2008 to January 1, 2011. We use the unbinned likelihood analysis [12] from the publicly available Science Tools version v9r18p6. Data from the diffuse class events are selected, using the P6 V3 instrumental response functions, in the 200 MeV-200 GeV energy range. The isotropic isotropic iem v02 model is used to account for both the extragalactic diffuse emission and residual instrumental background, while the model gll iem v02 accounts for the contribution from the Galactic diffuse emission. The region of interest is taken from a circular region of 10 • of radius around the nominal positions of the sources in our sample, while all the neighbouring sources up to 15 • from the 11 months point source catalogue (1FGL, [13]) are accounted for in the modelled reconstruction of the sources. The gtlike tool is used to assess the detection level of the studied objects, for all of which the Test Statistics (TS) are found to be below 25, resulting in no significant detection. Upper limits at 2σ confidence level are thus derived on their flux in the 200 MeV-200 GeV energy range. No specific energy spectra are assumed to derive these limits, and the eConf C110509 FIG. 1: γ-ray luminosity, or the upper limit at 2σ confidence level on the luminosity, of the different sources in the sample and other known high-energy emitting starburst and normal galaxies, given against the supernova rate times the total gas mass in these objects. The red square points show the expectations on the luminosity from the model of [17], accounting for the uncertainties on RSN and Mgas.
corresponding photon indices are left free to vary in the models. Table I presents a summary of our results for M 81, M 83, IC 342, Maffei 1, Maffei 2, and M 94, as well as previous results obtained on NGC 1068 and NGC 4945 [14] and on the Milky Way, LMC, SMC, M 31, M 33, M 82 and NGC 253 (see [15] and references therein). More details on specific sources from the sample can be found in [15].
Interactions between cosmic rays accelerated in stellar objects (e.g. supernovaeremnants) and the ambient interstellar medium in starburst and normal galaxies are expected to generate high-energy γ-ray emission, through pion decay for hadronic cosmic rays as well as inverse Compton and bremsstrahlung radiations from leptonic cosmic rays (see e.g. [16]). [17] suggested that the supernova rate R SN and the total gas mass M gas in a given galaxy could be a proxy for the expected γ-ray luminosity, through the relationship:
(1) scaled with the supernova rate in the Milky Way R MW SN and the distance of the considered galaxy d.
Figure 1 shows the γ-ray luminosity measured with Fermi/LAT for the known high-energy emitting starburst and normal galaxies, against R SN ×M gas , as well as the upper limits on the fluxes for the objects studied here which are not detected. The luminosities ex-eConf C110509 pected from the model of [17] are also depicted in red, accounting for the observational uncertainties on R SN and M gas for the different objects. For comparison, we also added in this plot the γ-ray luminosity for the Seyfert 2 galaxy NGC 1068 recently detected at high energies [14]. NGC 1068 also harbours a starburst region in its central part, but it was argued in [14] that its high-energy em
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