Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda?

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.

14 pages, 7 figures, 4 tables

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• The Mass Profile of the Galaxy to 80 kpc
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• Millisecond pulsar interpretation of the Galactic center gamma-ray excess
• A study of the population of LMXBs in the bulge of M31
• A Luminous Gamma-ray Binary in the Large Magellanic Cloud
• Discovery of GeV emission from the direction of the luminous infrared galaxy NGC 2146
• A New Determination of the Spectra and Luminosity Function of Gamma-Ray Millisecond Pulsars

Cited by in corpus (47)

• The $\gamma$-ray Emission of Star-Forming Galaxies
• X-ray bubbles in the circumgalactic medium of TNG50 Milky Way- and M31-like galaxies: signposts of supermassive black hole activity
• Gamma rays as probes of cosmic-ray propagation and interactions in galaxies
• Millisecond Pulsars from Accretion Induced Collapse as the Origin of the Galactic Centre Gamma-ray Excess Signal
• Dark Matter Interpretation of the Fermi-LAT Observations Toward the Outer Halo of M31
• Search for $\gamma$-ray emission from dark matter particle interactions from Andromeda and Triangulum Galaxies with the Fermi Large Area Telescope
• Extragalactic gamma-ray background from AGN winds and star-forming galaxies in cosmological galaxy formation models
• Evidence for a high-energy tail in the gamma-ray spectra of globular clusters
• Giant cosmic ray halos around M31 and the Milky Way
• Testing gamma-ray models of blazars in the extragalactic sky
• Evidence of AGN activity in the gamma-ray emission from two starburst galaxies
• A Search for Dark Matter Lines at the Galactic Center with 14 Years of Fermi Data
• Point Sources from Dissipative Dark Matter
• Dark Matter Candidates and Searches
• Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars
• GeV $\gamma$-ray emission from M33 and Arp~299
• Millisecond Pulsars Modify the Radio-SFR Correlation in Quiescent Galaxies
• TeV Dark Matter Searches in the Extragalactic Gamma-Ray Sky
• Sensitivity of the Cherenkov Telescope Array to TeV photon emission from the Large Magellanic Cloud
• Updated constraints on WIMP dark matter annihilation by radio observations of M31
• Gamma-ray image reconstruction of the Andromeda galaxy
• Spatial extension of dark subhalos as seen by Fermi-LAT and implications for WIMP constraints
• The Andromeda Gamma-Ray Excess: Background Systematics of the Millisecond Pulsars and Dark Matter Interpretations
• Search for the Gamma-ray Emission from M33 with Fermi LAT
• A Multi-frequency analysis of possible Dark Matter Contributions to M31 Gamma-Ray Emissions
• Measuring the Mass of Missing Baryons in the Halo of Andromeda Galaxy with Gamma-Ray Observations
• Cosmic-ray transport and gamma-ray emission in M31
• On Dark Matter Explanations of the Gamma-Ray Excesses from the Galactic Center and M31
• On the Gamma-Ray Emission of the Andromeda Galaxy M31
• Prospects of newly detecting nearby star-forming galaxies by the Cherenkov Telescope Array
• Indirect detection prospects for d${}^{\ast }$(2380) dark matter
• Constraining the hadronic properties of star-forming galaxies above $1\mathrm{GeV}$ with 15-years Fermi-LAT data
• CONGRuENTS (COsmic-ray, Neutrino, Gamma-ray and Radio Non-Thermal Spectra). I. A predictive model for galactic non-thermal emission
• Gamma-rays from the circumgalactic medium of M31
• Galactic Traversability: A New Concept for Extragalactic SETI
• Contribution of dark matter annihilation to gamma-ray burst afterglows near massive galaxy centers
• Constraints on the Emission of Gamma Rays from M31 with HAWC
• Identifying the Gamma-ray Emission of the Nearby Galaxy M83
• Prospects for annihilating dark matter from M31 and M33 observations with the Cherenkov Telescope Array
• Cosmic Rays and Non-thermal Emission Induced by Accretion of Cool Gas onto the Galactic Disk
• Nature of M31 gamma-ray halo in relation to dark matter annihilation
• The Contribution From TeV Halos to the Isotropic Gamma-Ray Background
• Theoretical indication of a possible asymmetry in gamma-radiation between Andromeda halo hemispheres due to Compton scattering on electrons from their hypothetical sources in the halo
• A geomagnetic filter for the Fermi-LAT background
• Hunting Star-Forming Galaxies in the Gamma-Ray Domain
• Dips in the sub-TeV gamma-ray light curves from central parts of galaxies due to transiting luminous stars
• On the Possible Asymmetry in Gamma Rays from Andromeda Due to Inverse Compton Scattering of Star Light on Electrons from Dark Matter Annihilation