Session 51 - Interstellar Medium II.
Display session, Thursday, January 08
Exhibit Hall,

## [51.07] Measuring Neutral and Warm Ionized Galactic Gas Through X-Ray Absorption

J. S. Arabadjis, J. N. Bregman (U. of Michigan)

One of the main components of interstellar gas is the warm ionized material, with an average value of N_HII/N_HI \simeq 0.37 , although the value in the Solar vicinity is not well known. We can determine N_HII along sight lines out of the Galaxy by measuring the X-ray absorption column toward extragalactic objects, since the soft X-ray opacity depends mainly upon the column of both neutral and warm ionized material (mainly through the sum of the HeI+HeII columns). For the extragalactic absorption targets, we use the isothermal parts of galaxy clusters.

At low 21 cm column densities, (N_HI,21cm = 1 to 5 \times10^20 cm^-2) there is a tight correlation between N_HI,21cm and that determined from X-ray absorption. However, the mean value of N_H,X/N_H,21cm = 0.85 (for H/He = 0.10), which is smaller than 1.0 and therefore unphysical; for the mean Galactic N_HII/N_HI ratio, it would be 1.23. An N_H,X/N_H,21cm of 1.0 can be achieved for a He/H ratio near 0.09, a value inferred from emission line studies of Orion. For any sensible He/H ratios, our work implies that the N_HII/N_HI ratio is well below the mean Galactic value. This shows that N_HII along sight lines from the Sun cannot contribute significantly to the far infrared residual, which Puget et al. (1995) attribute to the primary star formation epoch of galaxies at cosmological distances.