Wednesday, July 24, 2013

1307.6195 (A. Gupta et al.)

Probing the anisotropy of the Milky Way gaseous halo: Sight-lines toward Mrk 421 and PKS2155-304    [PDF]

A. Gupta, S. Mathur, M. Galeazzi, Y. Krongold
The halo of the Milky Way contains a large reservoir of warm-hot gas that contains a large fraction of the missing baryons from the Galaxy. The average physical properties of this circumgalactic medium(CGM) are determined by combining average absorption and emission measurements along several extragalactic sightlines. However, there is a wide distribution of \ovii column density of through the Galactic halo,and the halo emission measure also shows a similarly wide distribution. This clearly shows that the Galactic warm-hot gaseous halo is anisotropic. We present Suzaku observations of fields close to two sightlines along which we have \ovii absorption measurements with Chandra. The column densities along these two sightlines are similar within errors, but we find that the emission measure is lower than the average near the Mrk 421 direction ($0.0025\pm0.0006$cm$^{-6} $pc) and is higher than average close to the PKS2155-304 sightline ($0.0042\pm0.0008 $cm$^{-6} $pc). In the Mrk 421 direction we derive the density of $1.6^{+2.6}_{-0.8} \times 10^{-4}$ cm$^{-3}$ (lower than average) and pathlength of $334^{+685}_{-274}$ kpc (larger than average). On the other hand, in the PKS2155-304 direction we measure the gas density of $3.6^{+4.5}_{-1.8} \times 10^{-4} $cm$^{-3}$ (higher than average) and path-length of $109^{+200}_{-82} $kpc (lower than average). Thus the best fit values of density and pathlength along these sightlines are different by a factor of two. They are, however, consistent with each other within errors because of similar column densities along the two sightlines and large errors on derived parameters.
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