The Sightline to Q2343-BX415: Clues to Galaxy Formation in a Quasar Environment

TL;DR

This study investigates a proximate DLA near quasar Q2343-BX415, revealing complex gas dynamics, potential outflows from the host galaxy, and detailed elemental abundances, providing insights into galaxy formation environments.

Contribution

First detailed analysis of a proximate DLA at quasar redshift, revealing gas kinematics, ionization states, and metallicity patterns relevant to galaxy formation.

Findings

Detected two sets of absorption components with different velocities.

Estimated the DLA's distance from the quasar to be 8 or 37 kpc.

Measured metallicity at about 1/5 solar, with normal element abundance patterns.

Abstract

(Abridged) We have discovered a strong DLA coincident in redshift with the faint QSO Q2343-BX415 (R = 20.2, z_em = 2.57393). Follow-up observations at intermediate spectral resolution reveal that the metal lines associated with this 'proximate' DLA consist of two sets of absorption components. One set is moving towards the quasar with velocities of ~ 150-600 km/s; this gas is highly ionized and does not fully cover the continuum source, suggesting that it is physically close to the active nucleus. The other, which accounts for most of the neutral gas, is blueshifted relative to the QSO, with the strongest component at ~ -160 km/s. We consider the possibility that the PDLA arises in the outflowing interstellar medium of the host galaxy of Q2343-BX415, an interpretation supported by strong C IV and N V absorption at nearby velocities, and by the intense radiation field longward of the Lyman limit implied by the high C II*/H I ratio. If Q2343-BX415 is the main source of these UV photons, then the PDLA is located at either ~ 8 or ~ 37 kpc from the active nucleus. Alternatively, the absorber may be a foreground star-forming galaxy unrelated to the quasar and coincidentally at the same redshift, but our deep imaging and follow-up spectroscopy of the field of Q2343-BX415 has not yet produced a likely candidate. We measure the abundances of 14 elements in the PDLA, finding an overall metallicity of ~ 1/5 solar and a normal pattern of relative element abundances for this metallicity. Thus, in this PDLA there is no evidence for the super-solar metallicities that have been claimed for some proximate, high ionization, systems.