John S. Arabadjis

Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption

We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through their continuous absorption of extragalactic X-ray spectra at |b|>25°. For NH, 21 cm 5×1020 cm-2, which occurs at our lower latitudes, the X-ray absorption column NH, x is nearly double NH, 21 cm. This excess column cannot be due to the warm ionized component even if He were entirely He I, so it must be due to a molecular component. This result implies that for lines of sight out of the plane with |b|~30°, molecular gas is common, with a column density comparable to NH, 21 cm. This work bears upon the far-infrared background, since a warm ionized component, anticorrelated with NH, 21 cm, might produce such a background. Not only is such an anticorrelation absent, but if the dust is destroyed in the warm ionized gas the far-infrared background may be slightly larger than that deduced by Puget and coworkers.

On the Extreme-Ultraviolet Emission from Galaxy Clusters

An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of these systems, with important physical implications. We have reexamined this feature in the five clusters for which it has been discussed, using the most recent X-ray absorption cross sections, X-ray data processing techniques, and a consistent set of H I data. For the Virgo cluster, we find that the spectrum can be fitted with a single-temperature thermal plasma and an X-ray absorption column that is not significantly different than the Galactic H I column. The results for Abell 1367, Abell 1656 (Coma), Abell 1795, and Abell 2199 are similar in that the difference between the X-ray absorption column and the Galactic H I column is less than 3 σ for He/H=0.09, and for He/H=0.10 only one cluster location leads to a Galactic H I column more than 3 σ above the X-ray absorption column (Coma, with one location with a 3.6 σ difference). We conclude that there is no strong evidence for the extremely soft X-ray excess in galaxy clusters.

Mapping the Galactic Halo. III. Simulated Observations of Tidal Streams

We have simulated the evolution of tidal debris in the Galactic halo in order to guide our ongoing survey to determine the fraction of halo mass accreted via satellite infall. Contrary to naive expectations that the satellite debris will produce a single narrow velocity peak on a smooth distribution, there are many different signatures of substructure, including multiple peaks and broad but asymmetrical velocity distributions. Observations of the simulations show that there is a high probability of detecting the presence of tidal debris with a pencil-beam survey of 100 deg2. In the limiting case of a single 107 M⊙ satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg2 field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. There is also surprisingly little dependence of the detection probabilities on the time since the satellite became tidally disrupted, or on the initial orbit of the satellite, except for the time spent in the survey volume.

On the Internal Absorption of Galaxy Clusters

A study of the cores of galaxy clusters with the Einstein Solid State Spectrometer (SSS) indicated the presence of absorbing material corresponding to 1012 M☉ of cold cluster gas, possibly resulting from cooling flows. Since this amount of cold gas is not confirmed by observations at other wavelengths, we examined whether this excess absorption is present in the ROSAT PSPC observations of 20 bright galaxy clusters. For of the clusters, successful spectral fits were obtained with absorption due only to the Galaxy, and therefore no extra absorption is needed within the clusters, in disagreement with the results from the Einstein SSS data for some of the same clusters. For of the clusters, none of our spectral fits was acceptable, suggesting a more complicated cluster medium than the two-temperature and cooling-flow models considered here. However, even for these clusters, substantial excess absorption is not indicated.