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 ( \sim 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 \sim 0 ) , highly ionized absorbing medium betrayed by the \ion O7 1s–2p resonance transition feature seen at 21.6Å ( N _ { OVII } \sim 10 ^ { 16 } cm ^ { -2 } ) .
Measurements are also made for the Ly \alpha transition of the adjacent ionization state , ( \ion O8 ; 18.97Å ) , 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 } < 2 \times 10 ^ { -4 } cm ^ { -3 } ) , lower limits on the scale length of the absorber ( L > 140 kpc ) and lower limits on its mass ( M > 5 \times 10 ^ { 10 } M _ { \odot } ) .
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 .