Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant

M. Ackermann, M. Ajello, L. Baldini, J. Ballet, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, E. D. Bloom, E. Bonamente, A. W. Borgland, T. J. Brandt, J. Bregeon, A. Brez, M. Brigida, P. Bruel, R. Buehler, S. Buson, G. A. CaliandroR. A. Cameron, P. A. Caraveo, J. M. Casandjian, C. Cecchi, E. Charles, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, C. D. Dermer, F. De Palma, S. W. Digel, P. S. Drell, R. Dubois, C. Favuzzi, E. C. Ferrara, W. B. Focke, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, S. Germani, N. Giglietto, F. Giordano, M. Giroletti, T. Glanzman, G. Godfrey, I. A. Grenier, S. Guiriec, D. Hadasch, Y. Hanabata, A. K. Harding, K. Hayashi, M. Hayashida, R. E. Hughes, R. Itoh, G. J́ohannesson, A. S. Johnson, W. N. Johnson, T. Kamae, H. Katagiri, Jun Kataoka, J. Kn̈odlseder, M. Kuss, J. Lande, L. Latronico, S. H. Lee, M. Llena Garde, F. Longo, F. Loparco, M. N. Lovellette, P. Lubrano, A. Makeev, P. Martin, M. N. Mazziotta, J. E. McEnery, J. Mehault, P. F. Michelson, T. Mizuno, C. Monte, M. E. Monzani, A. Morselli, I. V. Moskalenko, S. Murgia, M. Naumann-Godo, S. Nishino, P. L. Nolan, J. P. Norris, E. Nuss, T. Ohsugi, A. Okumura, N. Omodei, E. Orlando, J. F. Ormes, M. Ozaki, D. Parent, V. Pelassa, M. Pepe, M. Pesce-Rollins, F. Piron, T. A. Porter, S. Raiǹo, R. Rando, M. Razzano, A. Reimer, O. Reimer, J. Ripken, T. Sada, H. F W Sadrozinski, C. Sgr̀o, E. J. Siskind, G. Spandre, P. Spinelli, M. S. Strickman, A. W. Strong, D. J. Suson, H. Takahashi, T. Takahashi, T. Tanaka, J. B. Thayer, D. J. Thompson, L. Tibaldo, D. F. Torres, A. Tramacere, Y. Uchiyama, T. Uehara, T. L. Usher, J. Vandenbroucke, V. Vasileiou, N. Vilchez, V. Vitale, A. E. Vladimirov, A. P. Waite, P. Wang, K. S. Wood, Z. Yang, M. Ziegler

    Research output: Contribution to journalArticle

    27 Citations (Scopus)

    Abstract

    We report an analysis of the interstellar γ-ray emission in the third Galactic quadrant measured by the Fermi Large Area Telescope. The window encompassing the Galactic plane from longitude 210° to 250° has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray (CR) densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the γ-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the H i column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the CR spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a CR propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (2.08±0.11)×1020 cm -2(K km s-1)-1 in the Local arm clouds and is not significantly sensitive to the choice of Hi spin temperature. No significant variations are found for clouds in the interarm region.

    Original languageEnglish
    JournalAstrophysical Journal
    Volume726
    Issue number2
    DOIs
    Publication statusPublished - 2011 Jan 10

    Fingerprint

    quadrants
    cosmic ray
    cosmic rays
    rays
    emissivity
    gradients
    galaxies
    spin temperature
    supernova remnants
    calibrating
    longitude
    radial distribution
    optical thickness
    optical depth
    halos
    telescopes
    radii
    propagation
    augmentation
    atoms

    Keywords

    • Cosmic rays
    • Gamma rays: ISM
    • ISM: general

    ASJC Scopus subject areas

    • Space and Planetary Science
    • Astronomy and Astrophysics

    Cite this

    Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant. / Ackermann, M.; Ajello, M.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Berenji, B.; Bloom, E. D.; Bonamente, E.; Borgland, A. W.; Brandt, T. J.; Bregeon, J.; Brez, A.; Brigida, M.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Caraveo, P. A.; Casandjian, J. M.; Cecchi, C.; Charles, E.; Chekhtman, A.; Chiang, J.; Ciprini, S.; Claus, R.; Cohen-Tanugi, J.; Conrad, J.; Dermer, C. D.; De Palma, F.; Digel, S. W.; Drell, P. S.; Dubois, R.; Favuzzi, C.; Ferrara, E. C.; Focke, W. B.; Fukazawa, Y.; Funk, S.; Fusco, P.; Gargano, F.; Germani, S.; Giglietto, N.; Giordano, F.; Giroletti, M.; Glanzman, T.; Godfrey, G.; Grenier, I. A.; Guiriec, S.; Hadasch, D.; Hanabata, Y.; Harding, A. K.; Hayashi, K.; Hayashida, M.; Hughes, R. E.; Itoh, R.; J́ohannesson, G.; Johnson, A. S.; Johnson, W. N.; Kamae, T.; Katagiri, H.; Kataoka, Jun; Kn̈odlseder, J.; Kuss, M.; Lande, J.; Latronico, L.; Lee, S. H.; Garde, M. Llena; Longo, F.; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Makeev, A.; Martin, P.; Mazziotta, M. N.; McEnery, J. E.; Mehault, J.; Michelson, P. F.; Mizuno, T.; Monte, C.; Monzani, M. E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Naumann-Godo, M.; Nishino, S.; Nolan, P. L.; Norris, J. P.; Nuss, E.; Ohsugi, T.; Okumura, A.; Omodei, N.; Orlando, E.; Ormes, J. F.; Ozaki, M.; Parent, D.; Pelassa, V.; Pepe, M.; Pesce-Rollins, M.; Piron, F.; Porter, T. A.; Raiǹo, S.; Rando, R.; Razzano, M.; Reimer, A.; Reimer, O.; Ripken, J.; Sada, T.; Sadrozinski, H. F W; Sgr̀o, C.; Siskind, E. J.; Spandre, G.; Spinelli, P.; Strickman, M. S.; Strong, A. W.; Suson, D. J.; Takahashi, H.; Takahashi, T.; Tanaka, T.; Thayer, J. B.; Thompson, D. J.; Tibaldo, L.; Torres, D. F.; Tramacere, A.; Uchiyama, Y.; Uehara, T.; Usher, T. L.; Vandenbroucke, J.; Vasileiou, V.; Vilchez, N.; Vitale, V.; Vladimirov, A. E.; Waite, A. P.; Wang, P.; Wood, K. S.; Yang, Z.; Ziegler, M.

    In: Astrophysical Journal, Vol. 726, No. 2, 10.01.2011.

    Research output: Contribution to journalArticle

    Ackermann, M, Ajello, M, Baldini, L, Ballet, J, Barbiellini, G, Bastieri, D, Bechtol, K, Bellazzini, R, Berenji, B, Bloom, ED, Bonamente, E, Borgland, AW, Brandt, TJ, Bregeon, J, Brez, A, Brigida, M, Bruel, P, Buehler, R, Buson, S, Caliandro, GA, Cameron, RA, Caraveo, PA, Casandjian, JM, Cecchi, C, Charles, E, Chekhtman, A, Chiang, J, Ciprini, S, Claus, R, Cohen-Tanugi, J, Conrad, J, Dermer, CD, De Palma, F, Digel, SW, Drell, PS, Dubois, R, Favuzzi, C, Ferrara, EC, Focke, WB, Fukazawa, Y, Funk, S, Fusco, P, Gargano, F, Germani, S, Giglietto, N, Giordano, F, Giroletti, M, Glanzman, T, Godfrey, G, Grenier, IA, Guiriec, S, Hadasch, D, Hanabata, Y, Harding, AK, Hayashi, K, Hayashida, M, Hughes, RE, Itoh, R, J́ohannesson, G, Johnson, AS, Johnson, WN, Kamae, T, Katagiri, H, Kataoka, J, Kn̈odlseder, J, Kuss, M, Lande, J, Latronico, L, Lee, SH, Garde, ML, Longo, F, Loparco, F, Lovellette, MN, Lubrano, P, Makeev, A, Martin, P, Mazziotta, MN, McEnery, JE, Mehault, J, Michelson, PF, Mizuno, T, Monte, C, Monzani, ME, Morselli, A, Moskalenko, IV, Murgia, S, Naumann-Godo, M, Nishino, S, Nolan, PL, Norris, JP, Nuss, E, Ohsugi, T, Okumura, A, Omodei, N, Orlando, E, Ormes, JF, Ozaki, M, Parent, D, Pelassa, V, Pepe, M, Pesce-Rollins, M, Piron, F, Porter, TA, Raiǹo, S, Rando, R, Razzano, M, Reimer, A, Reimer, O, Ripken, J, Sada, T, Sadrozinski, HFW, Sgr̀o, C, Siskind, EJ, Spandre, G, Spinelli, P, Strickman, MS, Strong, AW, Suson, DJ, Takahashi, H, Takahashi, T, Tanaka, T, Thayer, JB, Thompson, DJ, Tibaldo, L, Torres, DF, Tramacere, A, Uchiyama, Y, Uehara, T, Usher, TL, Vandenbroucke, J, Vasileiou, V, Vilchez, N, Vitale, V, Vladimirov, AE, Waite, AP, Wang, P, Wood, KS, Yang, Z & Ziegler, M 2011, 'Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant', Astrophysical Journal, vol. 726, no. 2. https://doi.org/10.1088/0004-637X/726/2/81
    Ackermann, M. ; Ajello, M. ; Baldini, L. ; Ballet, J. ; Barbiellini, G. ; Bastieri, D. ; Bechtol, K. ; Bellazzini, R. ; Berenji, B. ; Bloom, E. D. ; Bonamente, E. ; Borgland, A. W. ; Brandt, T. J. ; Bregeon, J. ; Brez, A. ; Brigida, M. ; Bruel, P. ; Buehler, R. ; Buson, S. ; Caliandro, G. A. ; Cameron, R. A. ; Caraveo, P. A. ; Casandjian, J. M. ; Cecchi, C. ; Charles, E. ; Chekhtman, A. ; Chiang, J. ; Ciprini, S. ; Claus, R. ; Cohen-Tanugi, J. ; Conrad, J. ; Dermer, C. D. ; De Palma, F. ; Digel, S. W. ; Drell, P. S. ; Dubois, R. ; Favuzzi, C. ; Ferrara, E. C. ; Focke, W. B. ; Fukazawa, Y. ; Funk, S. ; Fusco, P. ; Gargano, F. ; Germani, S. ; Giglietto, N. ; Giordano, F. ; Giroletti, M. ; Glanzman, T. ; Godfrey, G. ; Grenier, I. A. ; Guiriec, S. ; Hadasch, D. ; Hanabata, Y. ; Harding, A. K. ; Hayashi, K. ; Hayashida, M. ; Hughes, R. E. ; Itoh, R. ; J́ohannesson, G. ; Johnson, A. S. ; Johnson, W. N. ; Kamae, T. ; Katagiri, H. ; Kataoka, Jun ; Kn̈odlseder, J. ; Kuss, M. ; Lande, J. ; Latronico, L. ; Lee, S. H. ; Garde, M. Llena ; Longo, F. ; Loparco, F. ; Lovellette, M. N. ; Lubrano, P. ; Makeev, A. ; Martin, P. ; Mazziotta, M. N. ; McEnery, J. E. ; Mehault, J. ; Michelson, P. F. ; Mizuno, T. ; Monte, C. ; Monzani, M. E. ; Morselli, A. ; Moskalenko, I. V. ; Murgia, S. ; Naumann-Godo, M. ; Nishino, S. ; Nolan, P. L. ; Norris, J. P. ; Nuss, E. ; Ohsugi, T. ; Okumura, A. ; Omodei, N. ; Orlando, E. ; Ormes, J. F. ; Ozaki, M. ; Parent, D. ; Pelassa, V. ; Pepe, M. ; Pesce-Rollins, M. ; Piron, F. ; Porter, T. A. ; Raiǹo, S. ; Rando, R. ; Razzano, M. ; Reimer, A. ; Reimer, O. ; Ripken, J. ; Sada, T. ; Sadrozinski, H. F W ; Sgr̀o, C. ; Siskind, E. J. ; Spandre, G. ; Spinelli, P. ; Strickman, M. S. ; Strong, A. W. ; Suson, D. J. ; Takahashi, H. ; Takahashi, T. ; Tanaka, T. ; Thayer, J. B. ; Thompson, D. J. ; Tibaldo, L. ; Torres, D. F. ; Tramacere, A. ; Uchiyama, Y. ; Uehara, T. ; Usher, T. L. ; Vandenbroucke, J. ; Vasileiou, V. ; Vilchez, N. ; Vitale, V. ; Vladimirov, A. E. ; Waite, A. P. ; Wang, P. ; Wood, K. S. ; Yang, Z. ; Ziegler, M. / Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant. In: Astrophysical Journal. 2011 ; Vol. 726, No. 2.
    @article{9b92d10bfee140b7aa2f32cd22a22ec7,
    title = "Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant",
    abstract = "We report an analysis of the interstellar γ-ray emission in the third Galactic quadrant measured by the Fermi Large Area Telescope. The window encompassing the Galactic plane from longitude 210° to 250° has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray (CR) densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the γ-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the H i column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the CR spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a CR propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (2.08±0.11)×1020 cm -2(K km s-1)-1 in the Local arm clouds and is not significantly sensitive to the choice of Hi spin temperature. No significant variations are found for clouds in the interarm region.",
    keywords = "Cosmic rays, Gamma rays: ISM, ISM: general",
    author = "M. Ackermann and M. Ajello and L. Baldini and J. Ballet and G. Barbiellini and D. Bastieri and K. Bechtol and R. Bellazzini and B. Berenji and Bloom, {E. D.} and E. Bonamente and Borgland, {A. W.} and Brandt, {T. J.} and J. Bregeon and A. Brez 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 Casandjian, {J. M.} 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 Dermer, {C. D.} and {De Palma}, F. and Digel, {S. W.} and Drell, {P. S.} and R. Dubois and C. Favuzzi and Ferrara, {E. C.} and Focke, {W. B.} and Y. Fukazawa and S. Funk and P. Fusco and F. Gargano and S. Germani and N. Giglietto and F. Giordano and M. Giroletti and T. Glanzman and G. Godfrey and Grenier, {I. A.} and S. Guiriec and D. Hadasch and Y. Hanabata and Harding, {A. K.} and K. Hayashi and M. Hayashida and Hughes, {R. E.} and R. Itoh and G. J́ohannesson and Johnson, {A. S.} and Johnson, {W. N.} and T. Kamae and H. Katagiri and Jun Kataoka and J. Kn̈odlseder and M. Kuss and J. Lande and L. Latronico and Lee, {S. H.} and Garde, {M. Llena} and F. Longo and F. Loparco and Lovellette, {M. N.} and P. Lubrano and A. Makeev and P. Martin and Mazziotta, {M. N.} and McEnery, {J. E.} and J. Mehault and Michelson, {P. F.} 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 S. Nishino and Nolan, {P. L.} and Norris, {J. P.} and E. Nuss and T. Ohsugi and A. Okumura and N. Omodei and E. Orlando and Ormes, {J. F.} and M. Ozaki and D. Parent and V. Pelassa and M. Pepe and M. Pesce-Rollins and F. Piron and Porter, {T. A.} and S. Raiǹo and R. Rando and M. Razzano and A. Reimer and O. Reimer and J. Ripken and T. Sada and Sadrozinski, {H. F W} and C. Sgr̀o and Siskind, {E. J.} and G. Spandre and P. Spinelli and Strickman, {M. S.} and Strong, {A. W.} and Suson, {D. J.} and H. Takahashi and T. Takahashi and T. Tanaka and Thayer, {J. B.} and Thompson, {D. J.} and L. Tibaldo and Torres, {D. F.} and A. Tramacere and Y. Uchiyama and T. Uehara and Usher, {T. L.} and J. Vandenbroucke and V. Vasileiou and N. Vilchez and V. Vitale and Vladimirov, {A. E.} and Waite, {A. P.} and P. Wang and Wood, {K. S.} and Z. Yang and M. Ziegler",
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    TY - JOUR

    T1 - Constraints on the cosmic-ray density gradient beyond the solar circle from fermi γ-ray observations of the third galactic quadrant

    AU - Ackermann, M.

    AU - Ajello, M.

    AU - Baldini, L.

    AU - Ballet, J.

    AU - Barbiellini, G.

    AU - Bastieri, D.

    AU - Bechtol, K.

    AU - Bellazzini, R.

    AU - Berenji, B.

    AU - Bloom, E. D.

    AU - Bonamente, E.

    AU - Borgland, A. W.

    AU - Brandt, T. J.

    AU - Bregeon, J.

    AU - Brez, A.

    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 - Casandjian, J. M.

    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 - Dermer, C. D.

    AU - De Palma, F.

    AU - Digel, S. W.

    AU - Drell, P. S.

    AU - Dubois, R.

    AU - Favuzzi, C.

    AU - Ferrara, E. C.

    AU - Focke, W. B.

    AU - Fukazawa, Y.

    AU - Funk, S.

    AU - Fusco, P.

    AU - Gargano, F.

    AU - Germani, S.

    AU - Giglietto, N.

    AU - Giordano, F.

    AU - Giroletti, M.

    AU - Glanzman, T.

    AU - Godfrey, G.

    AU - Grenier, I. A.

    AU - Guiriec, S.

    AU - Hadasch, D.

    AU - Hanabata, Y.

    AU - Harding, A. K.

    AU - Hayashi, K.

    AU - Hayashida, M.

    AU - Hughes, R. E.

    AU - Itoh, R.

    AU - J́ohannesson, G.

    AU - Johnson, A. S.

    AU - Johnson, W. N.

    AU - Kamae, T.

    AU - Katagiri, H.

    AU - Kataoka, Jun

    AU - Kn̈odlseder, J.

    AU - Kuss, M.

    AU - Lande, J.

    AU - Latronico, L.

    AU - Lee, S. H.

    AU - Garde, M. Llena

    AU - Longo, F.

    AU - Loparco, F.

    AU - Lovellette, M. N.

    AU - Lubrano, P.

    AU - Makeev, A.

    AU - Martin, P.

    AU - Mazziotta, M. N.

    AU - McEnery, J. E.

    AU - Mehault, J.

    AU - Michelson, P. F.

    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 - Nishino, S.

    AU - Nolan, P. L.

    AU - Norris, J. P.

    AU - Nuss, E.

    AU - Ohsugi, T.

    AU - Okumura, A.

    AU - Omodei, N.

    AU - Orlando, E.

    AU - Ormes, J. F.

    AU - Ozaki, M.

    AU - Parent, D.

    AU - Pelassa, V.

    AU - Pepe, M.

    AU - Pesce-Rollins, M.

    AU - Piron, F.

    AU - Porter, T. A.

    AU - Raiǹo, S.

    AU - Rando, R.

    AU - Razzano, M.

    AU - Reimer, A.

    AU - Reimer, O.

    AU - Ripken, J.

    AU - Sada, T.

    AU - Sadrozinski, H. F W

    AU - Sgr̀o, C.

    AU - Siskind, E. J.

    AU - Spandre, G.

    AU - Spinelli, P.

    AU - Strickman, M. S.

    AU - Strong, A. W.

    AU - Suson, D. J.

    AU - Takahashi, H.

    AU - Takahashi, T.

    AU - Tanaka, T.

    AU - Thayer, J. B.

    AU - Thompson, D. J.

    AU - Tibaldo, L.

    AU - Torres, D. F.

    AU - Tramacere, A.

    AU - Uchiyama, Y.

    AU - Uehara, T.

    AU - Usher, T. L.

    AU - Vandenbroucke, J.

    AU - Vasileiou, V.

    AU - Vilchez, N.

    AU - Vitale, V.

    AU - Vladimirov, A. E.

    AU - Waite, A. P.

    AU - Wang, P.

    AU - Wood, K. S.

    AU - Yang, Z.

    AU - Ziegler, M.

    PY - 2011/1/10

    Y1 - 2011/1/10

    N2 - We report an analysis of the interstellar γ-ray emission in the third Galactic quadrant measured by the Fermi Large Area Telescope. The window encompassing the Galactic plane from longitude 210° to 250° has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray (CR) densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the γ-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the H i column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the CR spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a CR propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (2.08±0.11)×1020 cm -2(K km s-1)-1 in the Local arm clouds and is not significantly sensitive to the choice of Hi spin temperature. No significant variations are found for clouds in the interarm region.

    AB - We report an analysis of the interstellar γ-ray emission in the third Galactic quadrant measured by the Fermi Large Area Telescope. The window encompassing the Galactic plane from longitude 210° to 250° has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray (CR) densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the γ-ray emissivities per interstellar H atom over the regions sampled in this study. The gradient depends, however, on the optical depth correction applied to derive the H i column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the CR spectrum up to the Perseus arm for particles with GeV to tens of GeV energies. The emissivity as a function of Galactocentric radius does not show a large enhancement in the spiral arms with respect to the interarm region. The measured emissivity gradient is flatter than expectations based on a CR propagation model using the radial distribution of supernova remnants and uniform diffusion properties. In this context, observations require a larger halo size and/or a flatter CR source distribution than usually assumed. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (2.08±0.11)×1020 cm -2(K km s-1)-1 in the Local arm clouds and is not significantly sensitive to the choice of Hi spin temperature. No significant variations are found for clouds in the interarm region.

    KW - Cosmic rays

    KW - Gamma rays: ISM

    KW - ISM: general

    UR - http://www.scopus.com/inward/record.url?scp=84891186447&partnerID=8YFLogxK

    UR - http://www.scopus.com/inward/citedby.url?scp=84891186447&partnerID=8YFLogxK

    U2 - 10.1088/0004-637X/726/2/81

    DO - 10.1088/0004-637X/726/2/81

    M3 - Article

    AN - SCOPUS:84891186447

    VL - 726

    JO - Astrophysical Journal

    JF - Astrophysical Journal

    SN - 0004-637X

    IS - 2

    ER -