Cosmic Origins Spectrograph Detection of Ne VIII Tracing Warm - Hot Gas Towards PKS 0405-123

TL;DR

This study reports the detection of Ne VIII in a low-redshift galaxy environment, revealing a multiphase, collisionally ionized warm-hot gas component that traces the galaxy's hot halo or WHIM, contributing to understanding baryon distribution.

Contribution

First detection of Ne VIII in this system using HST/COS, demonstrating the presence of warm-hot gas in galaxy halos and the utility of UV spectroscopy for baryon census.

Findings

Ne VIII detected with high significance in both lines of the doublet.

Collisionally ionized gas at T ~ 5 x 10^5 K with large baryonic column density.

Gas metallicity estimated at [Ne/H] ~ -0.6 dex.

Abstract

We report on the detection of Ne VIII in the HST/Cosmic Origins Spectrograph spectrum of the intervening absorption system at z = 0.495096 towards PKS 0405-123. The high S/N COS spectrum also covers absorption from H I, C III, O III, O IV and O VI associated with this multiphase system. The Ne VIII is detected with high significance in both lines of the doublet, with integrated column densities of log Na(Ne VIII 770) = 13.96 +/- 0.06 dex and log Na(Ne VIII 780) = 14.08 +/- 0.07 dex. We find the origin of Ne VIII consistent with collisionally ionized gas at T ~ 5 x 10^5 K with a large baryonic column density of N(H) ~ 10^{19} - 10^{20} cm^-2. The metallicity in the Ne VIII gas phase is estimated to be [Ne/H] ~ -0.6 +/- 0.3 dex. The intermediate ions such as C III, O III, O IV and H I are consistent with photoionization in lower ionization gas at T ~ 10,000 K. The O V and O VI in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at 180 km/s systematic velocity and 110 kpc projected separation from a M_R = -19.6 galaxy of extended morphology. The collisionally ionized gas at T ~ 5 x 10^5 K points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of "missing baryons" in the low-z universe through the detection of Ne VIII systems.