Does circumgalactic OVI trace low-pressure gas beyond the accretion shock? Clues from HI and low-ion absorption, line kinematics, and dust extinction

摘要: Large OVI columns are observed around star-forming, low-redshift ~L* galaxies, with a dependence on impact parameter indicating that most O^5+ particles reside beyond half the halo virial radius (>~100 kpc). In order to constrain nature of gas traced by OVI, we analyze additional observables outer halo, namely HI column ratios 1-10, an absence low-ion absorption, mean differential extinction E(B-V)~10^-3, and linear relation between velocity width. We contrast these observations two physical scenarios: (1) traces high-pressure (~30 cm^-3 K) collisionally-ionized cooling from virially-shocked phase, (2) low-pressure ( ~100 kpc offset energy radiated gas. contrast, scenario can explain all considered single phase in thermal equilibrium, provided baryon overdensity is comparable dark-matter overdensity, enriched >~Z_sun/3 ISM-like dust-to-metal ratio. The implies cool flow mass rate ~5 M_sun yr^-1, star formation central galaxies. line widths consistent shear expected within this flow. predicts bimodality absorption at kpc, due pressure jump across accretion shock.