X-Ray IGM in the Local Group

Recent observations with the dispersive X-ray spectrometers aboard Chandra and Newton Observatory have begun to probe the properties of the X-ray intergalactic medium (IGM) at small redshifts. Using large quantities (∼950 ksec) of spectroscopic data acquired using the Reflection Grating Spectrometer (RGS) aboard Newton Observatory, we investigated the intervening material toward three low redshift, high Galactic latitude Active Galactic Nuclei (AGNs) with nominally featureless spectra: Mrk 421, PKS 2155-304 and 3C 273. Each spectrum provides clear evidence for what appears to be a local (z ∼ 0), highly ionized absorbing medium betrayed by the O VII 1s–2p resonance transition feature seen at 21.6 A (NOVII ∼ 1016 m-2). Measurements are also made for the Ly α transition of the adjacent ionization state, (O VIII; 18.97 A), which potentially constrains the absorber’s temperature. Finally, in a collisional equilibrium approximation, upper limits to diffuse emission intensities place upper limits on the electron density (n e 140 kpc) and lower limits on its mass (M > 5 x 1010 M⊙). Limits on the absorber’s scale length and its velocity distribution lead us to identify it with the Local Group. Having detected the hot gas in our Local Group in absorption, it should be feasible to detect also the extended structure of other low-mass, spiral-dominated groups of galaxies in absorption, with spectra of similar quality.