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

TL;DR

This study uses high signal-to-noise HST spectra to analyze the CGM of two galaxies, revealing new absorption features and providing insights into ionization processes and gas dynamics within galactic halos.

Contribution

The paper presents the first high signal-to-noise spectrum of a quasar sightline, uncovering new absorption features and detailed ionization diagnostics in the CGM of two galaxies.

Findings

Detection of additional CGM absorption not seen in lower S/N data

Measured N V to O VI ratios suggest radiatively cooling gas flows

Different processes likely produce O VI in various CGM regions

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-$\alpha$ forest absorbers and broad Ly-$\alpha$ absorbers are consistent with expectations at this redshift.