Hubble Space Telescope STIS spectroscopy of the Lyman-Alpha emission line in the central dominant galaxies in A426, A1795, and A2597: Constraints on clouds in the intracluster medium

摘要

We report on HST/STIS spectra of the Ly-Alpha emission in the central dominant galaxies in three rich clusters of galaxies. We find evidence for a population of clouds in the intracluster medium. We detect 10 Ly-Alpha absorption systems towards the nucleus of NGC1275 with columns of N(HI) ~10^12 to 10^14 cm^−2. These columns would not have been detected in the 21 cm line, but are easily detected in the Ly-Alpha line. Most of the absorption features are located in the broad wings of the emission line. The detected absorption features are most consistent with associated nuclear absorption systems. There is very little nuclear absorption at the systemic velocity in NGC1275 (Feature 8 contains N(HI) ~3 × 10^12 cm^−2). This implies that the large columns detected in the 21 cm line towards the parsec scale radio source avoid the line of sight to the nucleus. This gas may be located in a circumnuclear disk or torus. We detect at least one and possibly two absorption features towards the extended Ly-Alpha in A426. We do not detect Ly-Alpha absorption towards the extended Ly-Alpha emission in A1795, and A2597 with upper limits N(HI) ~10^13 cm^−2 for optically thin absorbers. Our data constrain the covering factor of any high column density gas (N(HI) u3e 10^15 cm^−2) in the ICM to be less than 25%. Our results suggest that the lack of observed intermediate temperature gas is not explained by obscuration. In addition, the low columns of gas on the ~100 kpc scales in the ICM suggests that (1) the rate at which cold gas accumulates in the ICM on these scales is very low, and (2) the dense nebulae in the central ~10 kpc must have cooled or been deposited in situ.