We report an analysis of the interstellar $\gamma$-ray emission in the third Galactic quadrant measured by the {Fermi} Large Area Telescope. The window encompassing the Galactic plane from longitude $210\arcdeg$ to $250\arcdeg$ has kinematically well-defined segments of the Local and the Perseus arms, suitable to study the cosmic-ray densities across the outer Galaxy. We measure no large gradient with Galactocentric distance of the $\gamma$-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 \HI\ column densities. No significant variations are found in the interstellar spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray 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 cosmic-ray 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, $X_{\rm CO} = N({\rm H_{2}})/W_{\rm CO}$, is found to be $(2.08 \pm 0.11) \times 10^{20} {\rm cm^{-2} (K km s^{-1})^{-1}}$ 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.
Knowledge of the distribution of cosmic-ray (CR) densities within our Galaxy is a key to understanding their origin and propagation. Highenergy CRs interact with the gas in the interstellar medium (ISM) or the interstellar radiation field, and produce γ-rays via nucleon-nucleon interac-tions, electron Bremsstrahlung and inverse Compton (IC) scattering. Since the ISM is transparent to these γ-rays, we can probe CRs in the local ISM, beyond direct measurements performed in the solar system, as well as in remote locations of the Galaxy. Although much effort has been made since the COS-B era (e.g., Strong et al. 1988;Strong & Mattox 1996), the results have been limited by the angular resolution, effective area and energy coverage of the instruments. The advent of the Fermi Gamma-ray Space Telescope enables studying the spectral and spatial distribution of diffuse γ-rays and CRs with unprecedented sensitivity.
Here we report an analysis of diffuse γ-ray emission observed in the third Galactic quadrant. The window with Galactic longitude 210 • ≤ l ≤ 250 • and latitude -15 • ≤ b ≤ +20 • hosts kinematically well-defined segments of the Local and the Perseus spiral arms and is one of the best regions to study the CR density distribution across the outer Galaxy. The region has been already studied by Digel et al. (2001) using EGRET data. The improved sensitivity and angular resolution of the Fermi LAT (Large Area Telescope; Atwood et al. 2009) and recent developments in the study of the ISM allow us to examine the CR spectra and density distribution with better accuracy. We exclude from the analysis the region of the Monoceros R2 giant molecular cloud and the Southern Filament of the Orion-Monoceros complex (e.g., Wilson et al. 2005), in l ≤ 222 • and b ≤ -6 • , because 1) star forming activity and possible high magnetic fields suggested by the filamentary structure (e.g., Morris, Montani, & Thaddeus 1980;Maddalena et al. 1986) could indicate a special CR environment, and 2) an OB association in Monoceros R2 may hamper the determination of ISM densities from dust tracers (see § 2.1.2 for details).
Study of the X CO conversion factor which transforms the integrated intensity of the 2.6 mm line of carbon monoxide, W CO , into the molecular hydrogen column density, N (H 2 ), is also possible since the region contains well-known molecular complexes. In the Local arm we find the molecular clouds associated with Canis Major OB 1, NGC 2348and NGC 2632(Mel’nik & Efremov 1995;Kaltcheva & Hilditch 2000).
At a few kpc from the Solar System, in the interarm, lower-density region located between the Local and Perseus arms, we find Maddalena’s cloud (Maddalena & Thaddeus 1985), a giant molecular cloud remarkable for its lack of star formation, and the cloud associated with Canis Major OB 2 (Kaltcheva & Hilditch 2000).
This study complements the Fermi LAT study of the Cassiopeia and Cepheus region in the second quadrant reported by Abdo et al. (2010a). The paper is organized as follows. We describe the model preparation in § 2 and the γ-ray observations, data selection and the analysis procedure in § 3. The results are presented in § 4, where we also discuss the emissivity profile measured for the atomic gas and we compare it with predictions by a CR propagation model. A summary of the study is given in § 5.
In order to derive the γ-ray emissivities associated with the different components of the ISM we need to determine the interstellar gas column densities separately for each region and gas phase. For atomic hydrogen we used the Leiden/Argentine/Bonn Galactic H I survey by Kalberla et al. (2005). In order to turn the H I line intensities into N (H I) column densities, a uniform spin temperature T S = 125 K has often been adopted in previous studies. We will consider this option to directly compare our results with the former EGRET analysis of the same region (Digel et al. 2001) and other studies of the Galactic diffuse emission by the LAT (Abdo et al. 2009a(Abdo et al. , 2010a)). Recent H I absorption studies (Dickey et al. 2009), however, point to larger average spin temperatures in the outer Galaxy, so we have tried different choices of T S to evaluate how the optical depth correction affects the results. We will find that the emissivity per H I atom and the inferred CR density is affected by up to ∼ 50% in the Perseus arm, and will take this uncertainty into account in the discussion.
The integrated intensities of the 2.6 mm line of CO, W CO , have been derived from the composite survey by Dame, Hartmann, & Thaddeus (2001). The data have been filtered with the moment-masking technique in order to reduce the noise while keeping the resolution of the original data.
Figure 1 shows the velocity-longitude profile of H I emission in our region of interest (ROI). The preparation of maps accounting for the different Galactic structures present along the line of sight is similar to that described in detail in Abdo et al. (2010a)
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