A High Signal-to-Noise HST Spectrum Toward J1009+0713: Precise Absorption Measurements in the CGM of Two Galaxies

Abstract

High signal-to-noise spectra toward background quasars are crucial for uncovering weak absorption in the circumgalactic medium (CGM) of intervening galaxies, such as the diagnostic lines of N V that provide insight to the ionization process of warm gas but typically have low equivalent widths. We present a new spectrum from the Hubble Space Telescope with a signal-to-noise ratio of $\sim20-35$ toward the quasar SDSS J1009+0713 and analyze absorption systems in the CGM of two $L^\star$ galaxies close to the line of sight. We identify additional absorption in the CGM of these galaxies that was not reported by the previous lower signal-to-noise spectrum, as well as Milky Way absorbers and quasar outflows from J1009+0713. We measure $\log (N_\mathrm{NV}/N_\mathrm{OVI})\sim-1.1$ for two CGM absorbers, inconsistent with gas in collisional ionization equilibrium and consistent with a radiatively cooling bulk flow of $\sim50-150$ km s$^{-1}$, which could be produced by galactic winds. These column density ratios are also consistent with those found for other $L^\star$ galaxies and for some gas in the Milky Way's halo. We place upper limits of $\log (N_\mathrm{NV}/N_\mathrm{OVI})<-1.8$ to $-1.2$ for other O VI absorbers in the same halos, which suggests that O VI is produced by different processes in different parts of the CGM, even within the same galactic halo. Together with the kinematically different structure of high- and low-ionization lines, these results indicate there are many components to a single galaxy's gaseous halo. We find the redshift number density of Ly-$α$ forest absorbers and broad Ly-$α$ absorbers are consistent with expectations at this redshift.