VERITAS Observations of M31 (the Andromeda Galaxy)

Gamma-ray haloes can exist around galaxies due to the interaction of escaping galactic cosmic rays with the surrounding gas. We have searched for such a halo around the nearby giant spiral Andromeda galaxy M31 using almost 7 years of Fermi LAT data at energies above 300 MeV. The presence of a diffuse gamma-ray halo with total photon flux 2.6 ± 0.6 × 10 −9 cm −2 s −1 , corresponding to a luminosity (0.3-100 GeV) of (3.2±0.6)×1038 erg s −1 (for a distance of 780 kpc) was found at a 5.3σ confidence level. The halo form does not correspond to the extended baryonic HI disc of M31, as would be expected in hadronic production of gamma photons from cosmic ray interaction, nor it is spherically symmetric, as could be in the case of dark matter annihilation. The best-fit halo template corresponds to two 6-7.5 kpc bubbles symmetrically located perpendicular to the M31 galactic disc, similar to the 'Fermi bubbles' found around the Milky Way centre, which suggests the past activity of the central supermassive black hole or a star-formation burst in M31.

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“…Future observations, including at energies > 100 GeV (Bird et al 2015;Smith et al 2015) would certainly clarify this issue. …”

Section: Discussionmentioning

confidence: 94%

Evidence of Fermi bubbles around M31

Пширков

Vasiliev

Постнов

2016

Monthly Notices of the Royal Astronomical Society: Letters

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“…Prior to Fermi-LAT other pioneering experiments set limits on a tentative signal (Fichtel et al 1975;Pollock et al 1981;Sreekumar et al 1994;Hartman et al 1999), with the first space-based γ-ray observatories dating back to 1962 (Kraushaar & Clark 1962;Kraushaar et al 1972). Note that M31 has not been significantly detected by any ground-based γ-ray telescopes, which are typically sensitive to energies above ∼100 GeV (Abeysekara et al 2014;Funk 2015;Bird 2016;Tinivella 2016).…”

Section: Introductionmentioning

confidence: 99%

Fermi-LAT Observations of γ-Ray Emission toward the Outer Halo of M31

Karwin

Murgia

Campbell

et al. 2019

ApJ

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The Andromeda Galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter (DM) halo which may span up to ∼600 kpc across and comprises ∼90% of the galaxy's total mass. This halo size translates into a large diameter of 42 • on the sky for an M31-Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with γ-ray telescopes. Using 7.6 years of Fermi Large Area Telescope (Fermi-LAT) observations, we make a detailed study of the γ-ray emission between 1-100 GeV towards M31's outer halo, with a total field radius of 60 • centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic ray (CR) propagation code GALPROP to construct specialized interstellar emission models (IEMs) to characterize the foreground γ-ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upwards of ∼120-200 kpc from the center of M31. We discuss plausible interpretations of the excess emission but emphasize that uncertainties in the MW foreground, and in particular, modeling of the H I-related components, have not been fully explored and may impact the results.

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“…Early observations [1][2][3][4] were only able to place upper limits until the galaxy was first detected in gamma-rays using 2 years of Fermi-LAT data at 5.3σ significance, along with some evidence of a spatial extension at the 1.8σ confidence level [5]. It has also been observed by high energy Cherenkov telescopes, though no detection has yet been made at energies above the TeV [9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Exploring a cosmic-ray origin of the multiwavelength emission in M31

2019

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A recent detection of spatially extended gamma-ray emission in the central region of the Andromeda galaxy (M31) has led to several possible explanations being put forth, including dark matter annihilation and millisecond pulsars. Another possibility is that the emission in M31 can be accounted for with a purely astrophysical cosmic-ray (CR) scenario. This scenario would lead to a rich multi-wavelength emission that can, in turn, be used to test it. Relativistic cosmic-ray electrons (CRe) in magnetic fields produce radio emission through synchrotron radiation, while X-rays and gamma rays are produced through inverse Compton scattering. Additionally, collisions of primary cosmic-ray protons (CRp) in the interstellar medium produce charged and neutral pions that then decay into secondary CRe (detectable through radiative processes) and gamma-rays. Here, we explore the viability of a CR origin for multi-wavelength emission in M31, taking into consideration three scenarios: a CR scenario dominated by primary CRe, one dominated by CRp and the resulting secondary CRe and gamma rays from neutral pion decay, and a final case in which both of these components exist simultaneously. We find that the multi-component model is the most promising, and is able to fit the multi-wavelength spectrum for a variety of astrophysical parameters consistent with previous studies of M31 and of cosmic-ray physics. However, the CR power injection implied by our models exceeds the estimated CR power injection from typical astrophysical cosmic-ray sources such as supernovae.

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VERITAS Observations of M31 (the Andromeda Galaxy)

Cited by 9 publications

References 14 publications

Evidence of Fermi bubbles around M31

Evidence of Fermi bubbles around M31

Fermi-LAT Observations of γ-Ray Emission toward the Outer Halo of M31

Exploring a cosmic-ray origin of the multiwavelength emission in M31

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