# {\it HST}/COS Observations of the Warm Ionized Gaseous Halo of NGC 891

## TL;DR

This study uses HST/COS observations to analyze the metallicity and composition of the warm ionized gaseous halo of NGC 891, revealing insights into galaxy accretion, feedback, and the halo's multi-component structure.

## Contribution

First detailed UV spectroscopic analysis of NGC 891's gaseous halo, measuring metallicity and gas properties, and identifying potential accretion and feedback processes.

## Key findings

- Warm ionized gas is about 5 times more massive than cold H I gas.
- Metallicity of volatile elements is about half solar, with some elements showing lower metallicity.
- Detection of low metallicity gas suggests cold mode accretion or tidal disruption.

## Abstract

The metallicity of galactic gaseous halos provides insights into accretion and feedback of galaxies. The nearby edge-on galaxy NGC 891 has a multi-component gaseous halo and a background AGN (LQAC 035+042 003) projected 5 kpc above the disk near the minor axis. Against the UV continuum of this AGN, we detect lines from 13 ions associated with NGC 891 in new {\it HST}/COS spectra. Most of the absorption is from the warm ionized gas with $\log T=4.22\pm0.04$, $\log n_{\rm H}=-1.26\pm0.51$, and $\log N_{\rm H}=20.81\pm 0.20$. The metallicity of volatile elements (i.e., C, N, and S) is about half solar ($\rm[X/H] \approx -0.3\pm 0.3$), while Mg, Fe, and Ni show lower metallicities of $\rm[X/H]\approx-0.9$. The absorption system shows the depletion pattern seen for warm Galactic diffuse clouds, which is consistent with a mixture of ejected solar metallicity disk gases and the hot X-ray emitting halo ($Z=0.1-0.2Z_\odot$). The warm ionized gases are about 5 times more massive than the cold \ion{H}{1} emitting gases around the galactic center, which might lead to accretion with a mean rate of $10^2~M_\odot\rm~yr^{-1}$ for a period of time. We also detect low metallicity ($\approx 0.1~Z_\odot$) gases toward LQAC 035+042 003 at $110\rm~km~s^{-1}$ (a high velocity cloud) and toward another sight line (3C 66A; 108 kpc projected from NGC 891) at $30\rm~km~s^{-1}$. This low metallicity material could be the cold mode accretion from IGM or the tidal disruption of satellites in the NGC 891 halo.