HUBBLE SPACE TELESCOPE STIS SPECTROSCOPY OF THE Lyα 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 Lyα 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 Lyα absorption systems toward the nucleus of NGC 1275 with columns of N(HⅠ) ～ 10~(12)-10~(14) cm~(-2). These columns would not have been detected in the 21 cm line but are easily detected in the Lyα 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 NGC 1275 [feature 8 contains N(HⅠ) ～ 3 x l0~(12) cm~(-2)]. This implies that the large columns detected in the 21 cm line toward 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 toward the extended Lyα in A426. We do not detect Lyα absorption toward the extended Lyα emission in A1795 and A2597 with upper limits N(HⅠ) ～ 1013 cm~(-2) for optically thin absorbers. Our data constrain the covering factor of any high column density gas [N(HⅠ) > 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 ～ 100 kpc scales in the ICM suggest 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.