Cosmic Origins Spectrograph Observations of Warm Intervening Gas Towards 3C263

TL;DR

This study uses HST/COS observations to analyze a multi-phase warm gas absorber at redshift 0.326, revealing both photoionized and collisionally ionized components, and highlighting the presence of warm gas in the WHIM or galaxy halos.

Contribution

First detailed analysis of a multi-phase warm gas absorber combining HST/COS and FUSE data, identifying collisionally ionized and photoionized components in the same system.

Findings

Detection of multiple ion species indicating multi-phase gas.

Identification of collisionally ionized gas at T = 5.2 x 10^5 K.

Warm gas has a total hydrogen column density 2 dex higher than photoionized gas.

Abstract

We present HST/COS high S/N observations of the z = 0.32566 multi-phase absorber towards 3C263. The COS data shows absorption from H I, O VI, C III, N III, Si III and C II. The Ne VIII in this absorber is detected in the FUSE spectrum. The low and intermediate ions are kinematically aligned with each other and H I and display narrow line widths of 6 km/s. The O VI lines are kinematically offset by 12 km/s from the low ions and are a factor of four broader. All metal ions except O VI and Ne VIII are consistent with an origin in gas photoionized by the extragalactic background radiation. The bulk of the observed H I is also traced by this photoionized medium. The carbon abundance in this gas phase is near-solar. The O VI and Ne VIII favor an origin in collisionally ionized gas at T = 5.2 x 10^5 K. The H I absorption associated with this warm absorber is a BLA marginally detected in the COS spectrum. This warm gas phase has total hydrogen column density of N(H) ~ 3 x 10^19 which is 2 dex higher than what is traced by the photoionized gas. Simultaneous detection of O VI, Ne VIII and BLAs in an absorber can be a strong diagnostic of gas with temperature in the range of 10^5 - 10^6 K corresponding to the warm phase of the WHIM or shock-heated gas in the extended halos of galaxies.