Limits on Hot Galactic Halo Gas from X-ray Absorption Lines
Yangsen Yao (MIT) , M. Nowak (MIT), Q. D. Wang (UMass), N. Schulz (MIT), C. Canizares (MIT)
Many theoretical calculations and numerical calculations for galaxy formation predict the existence of large-scale (>20 kpc) hot gaseous halos around individual galaxies. However, such halos are yet to be discovered observationally. We here present a differential study of searching for such a halo around our Galaxy. We compared the highly ionized X-ray absorption lines observed in sight-line of a Galactic source 4U 1957+11 (distance D>10 kpc) with those observed toward extragalactic sources LMC X-3 (D 50 kpc) and Mrk 421. We found that all the line absorptions observed in the extragalactic sources can be attributed to the hot gas in a thick Galactic disk, as traced by the absorption lines in the spectra of 4U 1957+11. We then obtained a firm upper limit of the absorption due to the putative Galactic halo and estimated the halo mass and metallicity. These results provide a direct observational test of galaxy formation theories.