Fermi-LAT observations of the diffuse γ-ray emission

Implications for cosmic rays and the interstellar medium

M. Ackermann, M. Ajello, W. B. Atwood, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, T. J. Brandt, J. Bregeon, M. Brigida, P. Bruel, R. Buehler, S. Buson & 127 others G. A. Caliandro, R. A. Cameron, P. A. Caraveo, E. Cavazzuti, C. Cecchi, E. Charles, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, S. Cutini, A. De Angelis, F. De Palma, C. D. Dermer, S. W. Digel, E. Do Couto E Silva, P. S. Drell, A. Drlica-Wagner, L. Falletti, C. Favuzzi, S. J. Fegan, E. C. Ferrara, W. B. Focke, P. Fortin, Y. Fukazawa, S. Funk, P. Fusco, D. Gaggero, F. Gargano, S. Germani, N. Giglietto, F. Giordano, M. Giroletti, T. Glanzman, G. Godfrey, J. E. Grove, S. Guiriec, M. Gustafsson, D. Hadasch, Y. Hanabata, A. K. Harding, M. Hayashida, E. Hays, D. Horan, X. Hou, R. E. Hughes, G. Jóhannesson, A. S. Johnson, R. P. Johnson, T. Kamae, H. Katagiri, Jun Kataoka, J. Knödlseder, M. Kuss, J. Lande, L. Latronico, S. H. Lee, M. Lemoine-Goumard, F. Longo, F. Loparco, B. Lott, M. N. Lovellette, P. Lubrano, M. N. Mazziotta, J. E. McEnery, P. F. Michelson, W. Mitthumsiri, T. Mizuno, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, M. Naumann-Godo, J. P. Norris, E. Nuss, T. Ohsugi, A. Okumura, N. Omodei, E. Orlando, J. F. Ormes, D. Paneque, J. H. Panetta, D. Parent, M. Pesce-Rollins, M. Pierbattista, F. Piron, G. Pivato, T. A. Porter, S. Rainò, R. Rando, M. Razzano, S. Razzaque, A. Reimer, O. Reimer, H. F W Sadrozinski, C. Sgrò, E. J. Siskind, G. Spandre, P. Spinelli, A. W. Strong, D. J. Suson, H. Takahashi, T. Tanaka, J. G. Thayer, J. B. Thayer, D. J. Thompson, L. Tibaldo, M. Tinivella, D. F. Torres, G. Tosti, E. Troja, T. L. Usher, J. Vandenbroucke, V. Vasileiou, G. Vianello, V. Vitale, A. P. Waite, P. Wang, B. L. Winer, K. S. Wood, M. Wood, Z. Yang, M. Ziegler, S. Zimmer

    Research output: Contribution to journalArticle

    390 Citations (Scopus)

    Abstract

    The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.

    Original languageEnglish
    Article number3
    JournalAstrophysical Journal
    Volume750
    Issue number1
    DOIs
    Publication statusPublished - 2012 May 1

    Fingerprint

    cosmic ray
    cosmic rays
    rays
    telescopes
    interstellar gas
    galaxies
    gas
    radiation distribution
    point sources
    point source
    sky
    halos
    astrophysics
    interstellar radiation
    likelihood ratio
    gas density
    longitude
    subtraction
    polar regions
    catalogs

    Keywords

    • cosmic rays
    • dust, extinction
    • gamma rays: general
    • gamma rays: ISM
    • ISM: general
    • radiation mechanisms: non-thermal

    ASJC Scopus subject areas

    • Space and Planetary Science
    • Astronomy and Astrophysics

    Cite this

    Ackermann, M., Ajello, M., Atwood, W. B., Baldini, L., Ballet, J., Barbiellini, G., ... Zimmer, S. (2012). Fermi-LAT observations of the diffuse γ-ray emission: Implications for cosmic rays and the interstellar medium. Astrophysical Journal, 750(1), [3]. https://doi.org/10.1088/0004-637X/750/1/3

    Fermi-LAT observations of the diffuse γ-ray emission : Implications for cosmic rays and the interstellar medium. / Ackermann, M.; Ajello, M.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Berenji, B.; Blandford, R. D.; Bloom, E. D.; Bonamente, E.; Borgland, A. W.; Brandt, T. J.; Bregeon, J.; Brigida, M.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Caraveo, P. A.; Cavazzuti, E.; Cecchi, C.; Charles, E.; Chekhtman, A.; Chiang, J.; Ciprini, S.; Claus, R.; Cohen-Tanugi, J.; Conrad, J.; Cutini, S.; De Angelis, A.; De Palma, F.; Dermer, C. D.; Digel, S. W.; Do Couto E Silva, E.; Drell, P. S.; Drlica-Wagner, A.; Falletti, L.; Favuzzi, C.; Fegan, S. J.; Ferrara, E. C.; Focke, W. B.; Fortin, P.; Fukazawa, Y.; Funk, S.; Fusco, P.; Gaggero, D.; Gargano, F.; Germani, S.; Giglietto, N.; Giordano, F.; Giroletti, M.; Glanzman, T.; Godfrey, G.; Grove, J. E.; Guiriec, S.; Gustafsson, M.; Hadasch, D.; Hanabata, Y.; Harding, A. K.; Hayashida, M.; Hays, E.; Horan, D.; Hou, X.; Hughes, R. E.; Jóhannesson, G.; Johnson, A. S.; Johnson, R. P.; Kamae, T.; Katagiri, H.; Kataoka, Jun; Knödlseder, J.; Kuss, M.; Lande, J.; Latronico, L.; Lee, S. H.; Lemoine-Goumard, M.; Longo, F.; Loparco, F.; Lott, B.; Lovellette, M. N.; Lubrano, P.; Mazziotta, M. N.; McEnery, J. E.; Michelson, P. F.; Mitthumsiri, W.; Mizuno, T.; Monte, C.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Naumann-Godo, M.; Norris, J. P.; Nuss, E.; Ohsugi, T.; Okumura, A.; Omodei, N.; Orlando, E.; Ormes, J. F.; Paneque, D.; Panetta, J. H.; Parent, D.; Pesce-Rollins, M.; Pierbattista, M.; Piron, F.; Pivato, G.; Porter, T. A.; Rainò, S.; Rando, R.; Razzano, M.; Razzaque, S.; Reimer, A.; Reimer, O.; Sadrozinski, H. F W; Sgrò, C.; Siskind, E. J.; Spandre, G.; Spinelli, P.; Strong, A. W.; Suson, D. J.; Takahashi, H.; Tanaka, T.; Thayer, J. G.; Thayer, J. B.; Thompson, D. J.; Tibaldo, L.; Tinivella, M.; Torres, D. F.; Tosti, G.; Troja, E.; Usher, T. L.; Vandenbroucke, J.; Vasileiou, V.; Vianello, G.; Vitale, V.; Waite, A. P.; Wang, P.; Winer, B. L.; Wood, K. S.; Wood, M.; Yang, Z.; Ziegler, M.; Zimmer, S.

    In: Astrophysical Journal, Vol. 750, No. 1, 3, 01.05.2012.

    Research output: Contribution to journalArticle

    Ackermann, M, Ajello, M, Atwood, WB, Baldini, L, Ballet, J, Barbiellini, G, Bastieri, D, Bechtol, K, Bellazzini, R, Berenji, B, Blandford, RD, Bloom, ED, Bonamente, E, Borgland, AW, Brandt, TJ, Bregeon, J, Brigida, M, Bruel, P, Buehler, R, Buson, S, Caliandro, GA, Cameron, RA, Caraveo, PA, Cavazzuti, E, Cecchi, C, Charles, E, Chekhtman, A, Chiang, J, Ciprini, S, Claus, R, Cohen-Tanugi, J, Conrad, J, Cutini, S, De Angelis, A, De Palma, F, Dermer, CD, Digel, SW, Do Couto E Silva, E, Drell, PS, Drlica-Wagner, A, Falletti, L, Favuzzi, C, Fegan, SJ, Ferrara, EC, Focke, WB, Fortin, P, Fukazawa, Y, Funk, S, Fusco, P, Gaggero, D, Gargano, F, Germani, S, Giglietto, N, Giordano, F, Giroletti, M, Glanzman, T, Godfrey, G, Grove, JE, Guiriec, S, Gustafsson, M, Hadasch, D, Hanabata, Y, Harding, AK, Hayashida, M, Hays, E, Horan, D, Hou, X, Hughes, RE, Jóhannesson, G, Johnson, AS, Johnson, RP, Kamae, T, Katagiri, H, Kataoka, J, Knödlseder, J, Kuss, M, Lande, J, Latronico, L, Lee, SH, Lemoine-Goumard, M, Longo, F, Loparco, F, Lott, B, Lovellette, MN, Lubrano, P, Mazziotta, MN, McEnery, JE, Michelson, PF, Mitthumsiri, W, Mizuno, T, Monte, C, Monzani, ME, Morselli, A, Moskalenko, IV, Murgia, S, Naumann-Godo, M, Norris, JP, Nuss, E, Ohsugi, T, Okumura, A, Omodei, N, Orlando, E, Ormes, JF, Paneque, D, Panetta, JH, Parent, D, Pesce-Rollins, M, Pierbattista, M, Piron, F, Pivato, G, Porter, TA, Rainò, S, Rando, R, Razzano, M, Razzaque, S, Reimer, A, Reimer, O, Sadrozinski, HFW, Sgrò, C, Siskind, EJ, Spandre, G, Spinelli, P, Strong, AW, Suson, DJ, Takahashi, H, Tanaka, T, Thayer, JG, Thayer, JB, Thompson, DJ, Tibaldo, L, Tinivella, M, Torres, DF, Tosti, G, Troja, E, Usher, TL, Vandenbroucke, J, Vasileiou, V, Vianello, G, Vitale, V, Waite, AP, Wang, P, Winer, BL, Wood, KS, Wood, M, Yang, Z, Ziegler, M & Zimmer, S 2012, 'Fermi-LAT observations of the diffuse γ-ray emission: Implications for cosmic rays and the interstellar medium', Astrophysical Journal, vol. 750, no. 1, 3. https://doi.org/10.1088/0004-637X/750/1/3
    Ackermann, M. ; Ajello, M. ; Atwood, W. B. ; Baldini, L. ; Ballet, J. ; Barbiellini, G. ; Bastieri, D. ; Bechtol, K. ; Bellazzini, R. ; Berenji, B. ; Blandford, R. D. ; Bloom, E. D. ; Bonamente, E. ; Borgland, A. W. ; Brandt, T. J. ; Bregeon, J. ; Brigida, M. ; Bruel, P. ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; Caraveo, P. A. ; Cavazzuti, E. ; Cecchi, C. ; Charles, E. ; Chekhtman, A. ; Chiang, J. ; Ciprini, S. ; Claus, R. ; Cohen-Tanugi, J. ; Conrad, J. ; Cutini, S. ; De Angelis, A. ; De Palma, F. ; Dermer, C. D. ; Digel, S. W. ; Do Couto E Silva, E. ; Drell, P. S. ; Drlica-Wagner, A. ; Falletti, L. ; Favuzzi, C. ; Fegan, S. J. ; Ferrara, E. C. ; Focke, W. B. ; Fortin, P. ; Fukazawa, Y. ; Funk, S. ; Fusco, P. ; Gaggero, D. ; Gargano, F. ; Germani, S. ; Giglietto, N. ; Giordano, F. ; Giroletti, M. ; Glanzman, T. ; Godfrey, G. ; Grove, J. E. ; Guiriec, S. ; Gustafsson, M. ; Hadasch, D. ; Hanabata, Y. ; Harding, A. K. ; Hayashida, M. ; Hays, E. ; Horan, D. ; Hou, X. ; Hughes, R. E. ; Jóhannesson, G. ; Johnson, A. S. ; Johnson, R. P. ; Kamae, T. ; Katagiri, H. ; Kataoka, Jun ; Knödlseder, J. ; Kuss, M. ; Lande, J. ; Latronico, L. ; Lee, S. H. ; Lemoine-Goumard, M. ; Longo, F. ; Loparco, F. ; Lott, B. ; Lovellette, M. N. ; Lubrano, P. ; Mazziotta, M. N. ; McEnery, J. E. ; Michelson, P. F. ; Mitthumsiri, W. ; Mizuno, T. ; Monte, C. ; Monzani, M. E. ; Morselli, A. ; Moskalenko, I. V. ; Murgia, S. ; Naumann-Godo, M. ; Norris, J. P. ; Nuss, E. ; Ohsugi, T. ; Okumura, A. ; Omodei, N. ; Orlando, E. ; Ormes, J. F. ; Paneque, D. ; Panetta, J. H. ; Parent, D. ; Pesce-Rollins, M. ; Pierbattista, M. ; Piron, F. ; Pivato, G. ; Porter, T. A. ; Rainò, S. ; Rando, R. ; Razzano, M. ; Razzaque, S. ; Reimer, A. ; Reimer, O. ; Sadrozinski, H. F W ; Sgrò, C. ; Siskind, E. J. ; Spandre, G. ; Spinelli, P. ; Strong, A. W. ; Suson, D. J. ; Takahashi, H. ; Tanaka, T. ; Thayer, J. G. ; Thayer, J. B. ; Thompson, D. J. ; Tibaldo, L. ; Tinivella, M. ; Torres, D. F. ; Tosti, G. ; Troja, E. ; Usher, T. L. ; Vandenbroucke, J. ; Vasileiou, V. ; Vianello, G. ; Vitale, V. ; Waite, A. P. ; Wang, P. ; Winer, B. L. ; Wood, K. S. ; Wood, M. ; Yang, Z. ; Ziegler, M. ; Zimmer, S. / Fermi-LAT observations of the diffuse γ-ray emission : Implications for cosmic rays and the interstellar medium. In: Astrophysical Journal. 2012 ; Vol. 750, No. 1.
    @article{25579be29ec8491e9f8cf2821539ed20,
    title = "Fermi-LAT observations of the diffuse γ-ray emission: Implications for cosmic rays and the interstellar medium",
    abstract = "The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.",
    keywords = "cosmic rays, dust, extinction, gamma rays: general, gamma rays: ISM, ISM: general, radiation mechanisms: non-thermal",
    author = "M. Ackermann and M. Ajello and Atwood, {W. B.} and L. Baldini and J. Ballet and G. Barbiellini and D. Bastieri and K. Bechtol and R. Bellazzini and B. Berenji and Blandford, {R. D.} and Bloom, {E. D.} and E. Bonamente and Borgland, {A. W.} and Brandt, {T. J.} and J. Bregeon and M. Brigida and P. Bruel and R. Buehler and S. Buson and Caliandro, {G. A.} and Cameron, {R. A.} and Caraveo, {P. A.} and E. Cavazzuti and C. Cecchi and E. Charles and A. Chekhtman and J. Chiang and S. Ciprini and R. Claus and J. Cohen-Tanugi and J. Conrad and S. Cutini and {De Angelis}, A. and {De Palma}, F. and Dermer, {C. D.} and Digel, {S. W.} and {Do Couto E Silva}, E. and Drell, {P. S.} and A. Drlica-Wagner and L. Falletti and C. Favuzzi and Fegan, {S. J.} and Ferrara, {E. C.} and Focke, {W. B.} and P. Fortin and Y. Fukazawa and S. Funk and P. Fusco and D. Gaggero and F. Gargano and S. Germani and N. Giglietto and F. Giordano and M. Giroletti and T. Glanzman and G. Godfrey and Grove, {J. E.} and S. Guiriec and M. Gustafsson and D. Hadasch and Y. Hanabata and Harding, {A. K.} and M. Hayashida and E. Hays and D. Horan and X. Hou and Hughes, {R. E.} and G. J{\'o}hannesson and Johnson, {A. S.} and Johnson, {R. P.} and T. Kamae and H. Katagiri and Jun Kataoka and J. Kn{\"o}dlseder and M. Kuss and J. Lande and L. Latronico and Lee, {S. H.} and M. Lemoine-Goumard and F. Longo and F. Loparco and B. Lott and Lovellette, {M. N.} and P. Lubrano and Mazziotta, {M. N.} and McEnery, {J. E.} and Michelson, {P. F.} and W. Mitthumsiri and T. Mizuno and C. Monte and Monzani, {M. E.} and A. Morselli and Moskalenko, {I. V.} and S. Murgia and M. Naumann-Godo and Norris, {J. P.} and E. Nuss and T. Ohsugi and A. Okumura and N. Omodei and E. Orlando and Ormes, {J. F.} and D. Paneque and Panetta, {J. H.} and D. Parent and M. Pesce-Rollins and M. Pierbattista and F. Piron and G. Pivato and Porter, {T. A.} and S. Rain{\`o} and R. Rando and M. Razzano and S. Razzaque and A. Reimer and O. Reimer and Sadrozinski, {H. F W} and C. Sgr{\`o} and Siskind, {E. J.} and G. Spandre and P. Spinelli and Strong, {A. W.} and Suson, {D. J.} and H. Takahashi and T. Tanaka and Thayer, {J. G.} and Thayer, {J. B.} and Thompson, {D. J.} and L. Tibaldo and M. Tinivella and Torres, {D. F.} and G. Tosti and E. Troja and Usher, {T. L.} and J. Vandenbroucke and V. Vasileiou and G. Vianello and V. Vitale and Waite, {A. P.} and P. Wang and Winer, {B. L.} and Wood, {K. S.} and M. Wood and Z. Yang and M. Ziegler and S. Zimmer",
    year = "2012",
    month = "5",
    day = "1",
    doi = "10.1088/0004-637X/750/1/3",
    language = "English",
    volume = "750",
    journal = "Astrophysical Journal",
    issn = "0004-637X",
    publisher = "IOP Publishing Ltd.",
    number = "1",

    }

    TY - JOUR

    T1 - Fermi-LAT observations of the diffuse γ-ray emission

    T2 - Implications for cosmic rays and the interstellar medium

    AU - Ackermann, M.

    AU - Ajello, M.

    AU - Atwood, W. B.

    AU - Baldini, L.

    AU - Ballet, J.

    AU - Barbiellini, G.

    AU - Bastieri, D.

    AU - Bechtol, K.

    AU - Bellazzini, R.

    AU - Berenji, B.

    AU - Blandford, R. D.

    AU - Bloom, E. D.

    AU - Bonamente, E.

    AU - Borgland, A. W.

    AU - Brandt, T. J.

    AU - Bregeon, J.

    AU - Brigida, M.

    AU - Bruel, P.

    AU - Buehler, R.

    AU - Buson, S.

    AU - Caliandro, G. A.

    AU - Cameron, R. A.

    AU - Caraveo, P. A.

    AU - Cavazzuti, E.

    AU - Cecchi, C.

    AU - Charles, E.

    AU - Chekhtman, A.

    AU - Chiang, J.

    AU - Ciprini, S.

    AU - Claus, R.

    AU - Cohen-Tanugi, J.

    AU - Conrad, J.

    AU - Cutini, S.

    AU - De Angelis, A.

    AU - De Palma, F.

    AU - Dermer, C. D.

    AU - Digel, S. W.

    AU - Do Couto E Silva, E.

    AU - Drell, P. S.

    AU - Drlica-Wagner, A.

    AU - Falletti, L.

    AU - Favuzzi, C.

    AU - Fegan, S. J.

    AU - Ferrara, E. C.

    AU - Focke, W. B.

    AU - Fortin, P.

    AU - Fukazawa, Y.

    AU - Funk, S.

    AU - Fusco, P.

    AU - Gaggero, D.

    AU - Gargano, F.

    AU - Germani, S.

    AU - Giglietto, N.

    AU - Giordano, F.

    AU - Giroletti, M.

    AU - Glanzman, T.

    AU - Godfrey, G.

    AU - Grove, J. E.

    AU - Guiriec, S.

    AU - Gustafsson, M.

    AU - Hadasch, D.

    AU - Hanabata, Y.

    AU - Harding, A. K.

    AU - Hayashida, M.

    AU - Hays, E.

    AU - Horan, D.

    AU - Hou, X.

    AU - Hughes, R. E.

    AU - Jóhannesson, G.

    AU - Johnson, A. S.

    AU - Johnson, R. P.

    AU - Kamae, T.

    AU - Katagiri, H.

    AU - Kataoka, Jun

    AU - Knödlseder, J.

    AU - Kuss, M.

    AU - Lande, J.

    AU - Latronico, L.

    AU - Lee, S. H.

    AU - Lemoine-Goumard, M.

    AU - Longo, F.

    AU - Loparco, F.

    AU - Lott, B.

    AU - Lovellette, M. N.

    AU - Lubrano, P.

    AU - Mazziotta, M. N.

    AU - McEnery, J. E.

    AU - Michelson, P. F.

    AU - Mitthumsiri, W.

    AU - Mizuno, T.

    AU - Monte, C.

    AU - Monzani, M. E.

    AU - Morselli, A.

    AU - Moskalenko, I. V.

    AU - Murgia, S.

    AU - Naumann-Godo, M.

    AU - Norris, J. P.

    AU - Nuss, E.

    AU - Ohsugi, T.

    AU - Okumura, A.

    AU - Omodei, N.

    AU - Orlando, E.

    AU - Ormes, J. F.

    AU - Paneque, D.

    AU - Panetta, J. H.

    AU - Parent, D.

    AU - Pesce-Rollins, M.

    AU - Pierbattista, M.

    AU - Piron, F.

    AU - Pivato, G.

    AU - Porter, T. A.

    AU - Rainò, S.

    AU - Rando, R.

    AU - Razzano, M.

    AU - Razzaque, S.

    AU - Reimer, A.

    AU - Reimer, O.

    AU - Sadrozinski, H. F W

    AU - Sgrò, C.

    AU - Siskind, E. J.

    AU - Spandre, G.

    AU - Spinelli, P.

    AU - Strong, A. W.

    AU - Suson, D. J.

    AU - Takahashi, H.

    AU - Tanaka, T.

    AU - Thayer, J. G.

    AU - Thayer, J. B.

    AU - Thompson, D. J.

    AU - Tibaldo, L.

    AU - Tinivella, M.

    AU - Torres, D. F.

    AU - Tosti, G.

    AU - Troja, E.

    AU - Usher, T. L.

    AU - Vandenbroucke, J.

    AU - Vasileiou, V.

    AU - Vianello, G.

    AU - Vitale, V.

    AU - Waite, A. P.

    AU - Wang, P.

    AU - Winer, B. L.

    AU - Wood, K. S.

    AU - Wood, M.

    AU - Yang, Z.

    AU - Ziegler, M.

    AU - Zimmer, S.

    PY - 2012/5/1

    Y1 - 2012/5/1

    N2 - The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.

    AB - The γ-ray sky >100 MeV is dominated by the diffuse emissions from interactions of cosmic rays with the interstellar gas and radiation fields of the Milky Way. Observations of these diffuse emissions provide a tool to study cosmic-ray origin and propagation, and the interstellar medium. We present measurements from the first 21 months of the Fermi Large Area Telescope (Fermi-LAT) mission and compare with models of the diffuse γ-ray emission generated using the GALPROP code. The models are fitted to cosmic-ray data and incorporate astrophysical input for the distribution of cosmic-ray sources, interstellar gas, and radiation fields. To assess uncertainties associated with the astrophysical input, a grid of models is created by varying within observational limits the distribution of cosmic-ray sources, the size of the cosmic-ray confinement volume (halo), and the distribution of interstellar gas. An all-sky maximum-likelihood fit is used to determine the X CO factor, the ratio between integrated CO-line intensity and H2 column density, the fluxes and spectra of the γ-ray point sources from the first Fermi-LAT catalog, and the intensity and spectrum of the isotropic background including residual cosmic rays that were misclassified as γ-rays, all of which have some dependency on the assumed diffuse emission model. The models are compared on the basis of their maximum-likelihood ratios as well as spectra, longitude, and latitude profiles. We also provide residual maps for the data following subtraction of the diffuse emission models. The models are consistent with the data at high and intermediate latitudes but underpredict the data in the inner Galaxy for energies above a few GeV. Possible explanations for this discrepancy are discussed, including the contribution by undetected point-source populations and spectral variations of cosmic rays throughout the Galaxy. In the outer Galaxy, we find that the data prefer models with a flatter distribution of cosmic-ray sources, a larger cosmic-ray halo, or greater gas density than is usually assumed. Our results in the outer Galaxy are consistent with other Fermi-LAT studies of this region that used different analysis methods than employed in this paper.

    KW - cosmic rays

    KW - dust, extinction

    KW - gamma rays: general

    KW - gamma rays: ISM

    KW - ISM: general

    KW - radiation mechanisms: non-thermal

    UR - http://www.scopus.com/inward/record.url?scp=84859748659&partnerID=8YFLogxK

    UR - http://www.scopus.com/inward/citedby.url?scp=84859748659&partnerID=8YFLogxK

    U2 - 10.1088/0004-637X/750/1/3

    DO - 10.1088/0004-637X/750/1/3

    M3 - Article

    VL - 750

    JO - Astrophysical Journal

    JF - Astrophysical Journal

    SN - 0004-637X

    IS - 1

    M1 - 3

    ER -