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Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?
- Source :
- The Astrophysical Journal, The Astrophysical Journal, American Astronomical Society, 2017, 836, pp.208-208. ⟨10.3847/1538-4357/aa5c3d⟩, The Astrophysical Journal, 2017, 836, pp.208. ⟨10.3847/1538-4357/aa5c3d⟩, The astrophysical journal / 1 836(2), 208 (2017). doi:10.3847/1538-4357/aa5c3d
- Publication Year :
- 2017
- Publisher :
- Institute of Physics Publishing, 2017.
-
Abstract
- The Fermi LAT has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using 2 yr of observations with the Fermi LAT, local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit (UL) has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1-100 GeV, presenting detailed morphological and spectral analyses. M33 remains undetected and we computed an UL for it. This revised UL remains consistent with the observed correlation between gamma-ray luminosity and star-formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 sigma and to be extended with 4 sigma. Its spectrum is consistent with a power law. The spatial distribution of the emission is consistent with a uniform disk with a radius of 0.4 deg and no offset from the center of M31, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star-formation activity suggesting that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic Center excess found in Fermi-LAT observations of the MW.<br />Comment: 14 pages, 7 figures, 4 tables
- Subjects :
- Andromeda Galaxy
[SDU.ASTR.CO]Sciences of the Universe [physics]/Astrophysics [astro-ph]/Cosmology and Extra-Galactic Astrophysics [astro-ph.CO]
cosmic rays
gamma rays: galaxies
Local Group
Astronomy and Astrophysics
Space and Planetary Science
Milky Way
Astrophysics::High Energy Astrophysical Phenomena
Astrophysics
Astrophysics::Cosmology and Extragalactic Astrophysics
01 natural sciences
0103 physical sciences
Galaxy formation and evolution
galaxie [gamma rays]
Interacting galaxy
10. No inequality
010303 astronomy & astrophysics
Astrophysics::Galaxy Astrophysics
cosmic ray
Physics
010308 nuclear & particles physics
Astronomy
Astronomy and Astrophysic
Galaxy
Dwarf spheroidal galaxy
cosmic rays – gamma rays: galaxies – Local Group
Globular cluster
Elliptical galaxy
ddc:520
Astrophysics - High Energy Astrophysical Phenomena
Subjects
Details
- Language :
- English
- ISSN :
- 0004637X and 15384357
- Database :
- OpenAIRE
- Journal :
- The Astrophysical Journal, The Astrophysical Journal, American Astronomical Society, 2017, 836, pp.208-208. ⟨10.3847/1538-4357/aa5c3d⟩, The Astrophysical Journal, 2017, 836, pp.208. ⟨10.3847/1538-4357/aa5c3d⟩, The astrophysical journal / 1 836(2), 208 (2017). doi:10.3847/1538-4357/aa5c3d
- Accession number :
- edsair.doi.dedup.....6a41a44f6f949143fde89a325688843c