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Title: Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?

We report the Fermi Large Area Telescope (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 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 $$0.1\mbox{--}100\,\mathrm{GeV}$$, presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of $$2.0\times {10}^{-12}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}\,$$ on the $$0.1\mbox{--}100\,\mathrm{GeV}$$ energy flux (95% confidence level). This revised upper limit 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 $$10\sigma $$. Its spectrum is consistent with a power law with photon index $${\rm{\Gamma }}=2.4\pm {0.1}_{\mathrm{stat}+\mathrm{syst}}$$ and a $$0.1\mbox{--}100\,\mathrm{GeV}$$ energy flux of $$(5.6\pm {0.6}_{\mathrm{stat}+\mathrm{syst}})\times {10}^{-12}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$$. M31 is detected to be extended with a $$4\sigma $$ significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0fdg4 and no offset from the center of the galaxy, 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 of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. In conclusion, 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.
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Publication Date:
Grant/Contract Number:
AC02-76SF00515
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 836; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE
Contributing Orgs:
Fermi-LAT Collaboration
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; galaxies: Local Group—gamma rays; galaxy—cosmic rays: general
OSTI Identifier:
1355736

Ackermann, M., Ajello, M., Albert, A., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bellazzini, R., Bissaldi, E., Bloom, E. D., Bonino, R., Bottacini, E., Brandt, T. J., Bregeon, J., Bruel, P., Buehler, R., Cameron, R. A., Caputo, R., Caragiulo, M., Caraveo, P. A., Cavazzuti, E., Cecchi, C., Charles, E., Chekhtman, A., Chiaro, G., Ciprini, S., Costanza, F., Cutini, S., D’Ammando, F., Palma, F. de, Desiante, R., Digel, S. W., Lalla, N. Di, Mauro, M. Di, Venere, L. Di, Favuzzi, C., Funk, S., Fusco, P., Gargano, F., Giglietto, N., Giordano, F., Giroletti, M., Glanzman, T., Green, D., Grenier, I. A., Guillemot, L., Guiriec, S., Hayashi, K., Hou, X., Jóhannesson, G., Kamae, T., Knödlseder, J., Kong, A. K. H., Kuss, M., Mura, G. La, Larsson, S., Latronico, L., Li, J., Longo, F., Loparco, F., Lubrano, P., Maldera, S., Malyshev, D., Manfreda, A., Martin, P., Mazziotta, M. N., Michelson, P. F., Mirabal, N., Mitthumsiri, W., Mizuno, T., Monzani, M. E., Morselli, A., Moskalenko, I. V., Negro, M., Nuss, E., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J. F., Paneque, D., Persic, M., Pesce-Rollins, M., Piron, F., Porter, T. A., Principe, G., Rainò, S., Rando, R., Razzano, M., Reimer, O., Sánchez-Conde, M., Sgrò, C., Simone, D., Siskind, E. J., Spada, F., Spandre, G., Spinelli, P., Tanaka, K., Tibaldo, L., Torres, D. F., Troja, E., Uchiyama, Y., Wang, J. C., Wood, K. S., Wood, M., Zaharijas, G., and Zhou, M.. Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?. United States: N. p., Web. doi:10.3847/1538-4357/aa5c3d.
Ackermann, M., Ajello, M., Albert, A., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bellazzini, R., Bissaldi, E., Bloom, E. D., Bonino, R., Bottacini, E., Brandt, T. J., Bregeon, J., Bruel, P., Buehler, R., Cameron, R. A., Caputo, R., Caragiulo, M., Caraveo, P. A., Cavazzuti, E., Cecchi, C., Charles, E., Chekhtman, A., Chiaro, G., Ciprini, S., Costanza, F., Cutini, S., D’Ammando, F., Palma, F. de, Desiante, R., Digel, S. W., Lalla, N. Di, Mauro, M. Di, Venere, L. Di, Favuzzi, C., Funk, S., Fusco, P., Gargano, F., Giglietto, N., Giordano, F., Giroletti, M., Glanzman, T., Green, D., Grenier, I. A., Guillemot, L., Guiriec, S., Hayashi, K., Hou, X., Jóhannesson, G., Kamae, T., Knödlseder, J., Kong, A. K. H., Kuss, M., Mura, G. La, Larsson, S., Latronico, L., Li, J., Longo, F., Loparco, F., Lubrano, P., Maldera, S., Malyshev, D., Manfreda, A., Martin, P., Mazziotta, M. N., Michelson, P. F., Mirabal, N., Mitthumsiri, W., Mizuno, T., Monzani, M. E., Morselli, A., Moskalenko, I. V., Negro, M., Nuss, E., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J. F., Paneque, D., Persic, M., Pesce-Rollins, M., Piron, F., Porter, T. A., Principe, G., Rainò, S., Rando, R., Razzano, M., Reimer, O., Sánchez-Conde, M., Sgrò, C., Simone, D., Siskind, E. J., Spada, F., Spandre, G., Spinelli, P., Tanaka, K., Tibaldo, L., Torres, D. F., Troja, E., Uchiyama, Y., Wang, J. C., Wood, K. S., Wood, M., Zaharijas, G., & Zhou, M.. Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?. United States. doi:10.3847/1538-4357/aa5c3d.
Ackermann, M., Ajello, M., Albert, A., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Bellazzini, R., Bissaldi, E., Bloom, E. D., Bonino, R., Bottacini, E., Brandt, T. J., Bregeon, J., Bruel, P., Buehler, R., Cameron, R. A., Caputo, R., Caragiulo, M., Caraveo, P. A., Cavazzuti, E., Cecchi, C., Charles, E., Chekhtman, A., Chiaro, G., Ciprini, S., Costanza, F., Cutini, S., D’Ammando, F., Palma, F. de, Desiante, R., Digel, S. W., Lalla, N. Di, Mauro, M. Di, Venere, L. Di, Favuzzi, C., Funk, S., Fusco, P., Gargano, F., Giglietto, N., Giordano, F., Giroletti, M., Glanzman, T., Green, D., Grenier, I. A., Guillemot, L., Guiriec, S., Hayashi, K., Hou, X., Jóhannesson, G., Kamae, T., Knödlseder, J., Kong, A. K. H., Kuss, M., Mura, G. La, Larsson, S., Latronico, L., Li, J., Longo, F., Loparco, F., Lubrano, P., Maldera, S., Malyshev, D., Manfreda, A., Martin, P., Mazziotta, M. N., Michelson, P. F., Mirabal, N., Mitthumsiri, W., Mizuno, T., Monzani, M. E., Morselli, A., Moskalenko, I. V., Negro, M., Nuss, E., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J. F., Paneque, D., Persic, M., Pesce-Rollins, M., Piron, F., Porter, T. A., Principe, G., Rainò, S., Rando, R., Razzano, M., Reimer, O., Sánchez-Conde, M., Sgrò, C., Simone, D., Siskind, E. J., Spada, F., Spandre, G., Spinelli, P., Tanaka, K., Tibaldo, L., Torres, D. F., Troja, E., Uchiyama, Y., Wang, J. C., Wood, K. S., Wood, M., Zaharijas, G., and Zhou, M.. 2017. "Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?". United States. doi:10.3847/1538-4357/aa5c3d. https://www.osti.gov/servlets/purl/1355736.
@article{osti_1355736,
title = {Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda?},
author = {Ackermann, M. and Ajello, M. and Albert, A. and Baldini, L. and Ballet, J. and Barbiellini, G. and Bastieri, D. and Bellazzini, R. and Bissaldi, E. and Bloom, E. D. and Bonino, R. and Bottacini, E. and Brandt, T. J. and Bregeon, J. and Bruel, P. and Buehler, R. and Cameron, R. A. and Caputo, R. and Caragiulo, M. and Caraveo, P. A. and Cavazzuti, E. and Cecchi, C. and Charles, E. and Chekhtman, A. and Chiaro, G. and Ciprini, S. and Costanza, F. and Cutini, S. and D’Ammando, F. and Palma, F. de and Desiante, R. and Digel, S. W. and Lalla, N. Di and Mauro, M. Di and Venere, L. Di and Favuzzi, C. and Funk, S. and Fusco, P. and Gargano, F. and Giglietto, N. and Giordano, F. and Giroletti, M. and Glanzman, T. and Green, D. and Grenier, I. A. and Guillemot, L. and Guiriec, S. and Hayashi, K. and Hou, X. and Jóhannesson, G. and Kamae, T. and Knödlseder, J. and Kong, A. K. H. and Kuss, M. and Mura, G. La and Larsson, S. and Latronico, L. and Li, J. and Longo, F. and Loparco, F. and Lubrano, P. and Maldera, S. and Malyshev, D. and Manfreda, A. and Martin, P. and Mazziotta, M. N. and Michelson, P. F. and Mirabal, N. and Mitthumsiri, W. and Mizuno, T. and Monzani, M. E. and Morselli, A. and Moskalenko, I. V. and Negro, M. and Nuss, E. and Ohsugi, T. and Omodei, N. and Orlando, E. and Ormes, J. F. and Paneque, D. and Persic, M. and Pesce-Rollins, M. and Piron, F. and Porter, T. A. and Principe, G. and Rainò, S. and Rando, R. and Razzano, M. and Reimer, O. and Sánchez-Conde, M. and Sgrò, C. and Simone, D. and Siskind, E. J. and Spada, F. and Spandre, G. and Spinelli, P. and Tanaka, K. and Tibaldo, L. and Torres, D. F. and Troja, E. and Uchiyama, Y. and Wang, J. C. and Wood, K. S. and Wood, M. and Zaharijas, G. and Zhou, M.},
abstractNote = {We report the Fermi Large Area Telescope (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 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 $0.1\mbox{--}100\,\mathrm{GeV}$, presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of $2.0\times {10}^{-12}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}\,$ on the $0.1\mbox{--}100\,\mathrm{GeV}$ energy flux (95% confidence level). This revised upper limit 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 $. Its spectrum is consistent with a power law with photon index ${\rm{\Gamma }}=2.4\pm {0.1}_{\mathrm{stat}+\mathrm{syst}}$ and a $0.1\mbox{--}100\,\mathrm{GeV}$ energy flux of $(5.6\pm {0.6}_{\mathrm{stat}+\mathrm{syst}})\times {10}^{-12}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$. M31 is detected to be extended with a $4\sigma $ significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0fdg4 and no offset from the center of the galaxy, 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 of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. In conclusion, 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.},
doi = {10.3847/1538-4357/aa5c3d},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 836,
place = {United States},
year = {2017},
month = {2}
}