Browsing by Subject "GAMMA-RAY EMISSION"

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  • Ade, P. A. R.; Keihänen, E.; Kurki-Suonio, H.; Lähteenmäki, A.; Suur-Uski, A-S.; Valiviita, J.; Planck Collaboration (2015)
    Planck data when combined with ancillary data provide a unique opportunity to separate the diffuse emission components of the inner Galaxy. The purpose of the paper is to elucidate the morphology of the various emission components in the strong star-formation region lying inside the solar radius and to clarify the relationship between the various components. The region of the Galactic plane covered is 1 = 300 degrees -> 0 degrees -> 60 degrees where star-formation is highest and the emission is strong enough to make meaningful component separation. The latitude widths in this longitude range lie between 1 and 2, which correspond to FWHM z-widths of 100-200 pc at a typical distance of 6 kpc. The four emission components studied here are synchrotron, free-free, anomalous microwave emission (AME), and thermal (vibrational) dust emission. These components are identified by constructing spectral energy distributions (SEDs) at positions along the Galactic plane using the wide frequency coverage of Planck (28.4-857 GHz) in combination with low-frequency radio data at 0.408-2.3 GHz plus WMAP data at 23-94 GHz, along with far-infrared (FIR) data from COBE-DIRBE and IRAS. The free-free component is determined from radio recombination line (RRL) data. AME is found to be comparable in brightness to the free-free emission on the Galactic plane in the frequency range 20-40 GHz with a width in latitude similar to that of the thermal dust; it comprises 45 +/- 1% of the total 28.4 GHz emission in the longitude range 1 = 300 degrees -> 0 degrees -> 60 degrees. The free-free component is the narrowest, reflecting the fact that it is produced by current star-formation as traced by the narrow distribution of OB stars. It is the dominant emission on the plane between 60 and 100 GHz. RRLs from this ionized gas are used to assess its distance, leading to a free-free z-width of FWHM approximate to 100 pc. The narrow synchrotron component has a low-frequency brightness spectral index beta(synch) approximate to -2.7 that is similar to the broad synchrotron component indicating that they are both populated by the cosmic ray electrons of the same spectral index. The width of this narrow synchrotron component is significantly larger than that of the other three components, suggesting that it is generated in an assembly of older supernova remnants that have expanded to sizes of order 150 pc in 3 x 10(5) yr; pulsars of a similar age have a similar spread in latitude. The thermal dust is identified in the SEDs with average parameters of T-dust = 20.4 +/- 0.4 K, beta(FIR) = 1.94 +/- 0.03 (>353 GHz), and beta(mm) = 1.67 +/- 0.02 (
  • Ade, P. A. R.; Juvela, M.; Keihänen, E.; Kurki-Suonio, H.; Suur-Uski, A. -S.; Valiviita, J.; Planck Collaboration; Fermi LAT Collaboration (2015)
    The nearby Chamaeleon clouds have been observed in gamma rays by the Fermi Large Area Telescope (LAT) and in thermal dust emission by Planck and IRAS. Cosmic rays and large dust grains, if smoothly mixed with gas, can jointly serve with the Hi and (CO)-C-12 radio data to (i) map the hydrogen column densities, N-H, in the different gas phases, in particular at the dark neutral medium (DNM) transition between the H I-bright and CO-bright media; (ii) constrain the CO-to-H-2 conversion factor, X-CO; and (iii) probe the dust properties per gas nucleon in each phase and map their spatial variations across the clouds. We have separated clouds at local, intermediate, and Galactic velocities in H i and (1)2CO line emission to model in parallel the gamma-ray intensity recorded between 0.4 and 100 GeV; the dust optical depth at 353 GHz, tau(353); the thermal radiance of the large grains; and an estimate of the dust extinction, A(VQ), empirically corrected for the starlight intensity. The dust and gamma-models have been coupled to account for the DNM gas. The consistent gamma-emissivity spectra recorded in the different phases confirm that the GeV-TeV cosmic rays probed by the LAT uniformly permeate all gas phases up to the (CO)-C-12 cores. The dust and cosmic rays both reveal large amounts of DNM gas, with comparable spatial distributions and twice as much mass as in the CO-bright clouds. We give constraints on the H I-DNM-CO transitions for five separate clouds. CO-dark H-2 dominates the molecular columns up to A(V) similar or equal to 0.9 and its mass often exceeds the one-third of the molecular mass expected by theory. The corrected A(VQ) extinction largely provides the best fit to the total gas traced by the gamma rays. Nevertheless, we find evidence for a marked rise in A(VQ)/N-H with increasing N-H and molecular fraction, and with decreasing dust temperature. The rise in tau(353)/N-H is even steeper. We observe variations of lesser amplitude and orderliness for the specific power of the grains, except for a coherent decline by half in the CO cores. This combined information suggests grain evolution. We provide average values for the dust properties per gas nucleon in the different phases. The gamma rays and dust radiance yield consistent X-CO estimates near 0.7 x 10(20) cm(-2) K-1 km(-1) s. The A(VQ) and tau(353) tracers yield biased values because of the large rise in grain opacity in the CO clouds. These results clarify a recurrent disparity in the gamma-versus dust calibration of X-CO, but they confirm the factor of 2 difference found between the X-CO estimates in nearby clouds and in the neighbouring spiral arms.