CHARACTERIZING THE CIRCUMGALACTIC MEDIUM OF NEARBY GALAXIES WITH HST/COS AND HST/STIS ABSORPTION-LINE SPECTROSCOPY*

摘要

The circumgalactic medium (CGM) of late-type galaxies is characterized using UV spectroscopy of 11 targeted QSO/galaxy pairs at z ≤ 0.02 with the Hubble Space Telescope Cosmic Origins Spectrograph (COS) and ~60 serendipitous absorber/galaxy pairs at z ≤ 0.2 with the Space Telescope Imaging Spectrograph. CGM warm cloud properties are derived, including volume filling factors of 3%-5%, cloud sizes of 0.1-30?kpc, masses of 10-108 M ☉, and metallicities of ~0.1-1 Z ☉. Almost all warm CGM clouds within 0.5 R vir are metal-bearing and many have velocities consistent with being bound, "galactic fountain" clouds. For galaxies with L 0.1 L*, the total mass in these warm CGM clouds approaches 1010 M ☉, ~10%-15% of the total baryons in massive spirals and comparable to the baryons in their parent galaxy disks. This leaves 50% of massive spiral-galaxy baryons "missing." Dwarfs (0.1 L*) have smaller area covering factors and warm CGM masses (≤5% baryon fraction), suggesting that many of their warm clouds escape. Constant warm cloud internal pressures as a function of impact parameter (P/k ~ 10 cm–3 K) support the inference that previous COS detections of broad, shallow O VI and Lyα absorptions are of an extensive (~400-600?kpc), hot (T ≈ 106?K), intra-cloud gas which is very massive (≥1011 M ☉). While the warm CGM clouds cannot account for all the "missing baryons" in spirals, the hot intra-group gas can, and could account for ~20% of the cosmic baryon census at z ~ 0 if this hot gas is ubiquitous among spiral groups.