Gas Accretion Traced in Absorption in Galaxy Spectroscopy

TL;DR

This paper reviews confirmed detections of gas inflow into galaxies via redshifted absorption lines, highlighting the challenges and potential of future high-resolution, spatially-resolved spectroscopy to better understand galaxy accretion processes.

Contribution

It provides a comprehensive review of observational evidence for gas accretion through absorption line studies and discusses future prospects with improved spectral resolution and spatial data.

Findings

Redshifted absorption lines indicate gas inflow in galaxies.

Evidence of ongoing accretion in >5% of star-forming galaxies at z~0.5-1.

Future high-resolution spectroscopy can improve understanding of inflow morphology.

Abstract

The positive velocity shift of absorption transitions tracing diffuse material observed in a galaxy spectrum is an unambiguous signature of gas flow toward the host system. Spectroscopy probing, e.g., NaI D resonance lines in the rest-frame optical or MgII and FeII in the near-ultraviolet is in principle sensitive to the infall of cool material at temperatures ~ 100-10,000 K anywhere along the line of sight to a galaxy's stellar component. However, secure detections of this redshifted absorption signature have proved challenging to obtain due to the ubiquity of cool gas outflows giving rise to blueshifted absorption along the same sightlines. In this chapter, we review the bona fide detections of this phenomenon. Analysis of NaI D line profiles has revealed numerous instances of redshifted absorption observed toward early-type and/or AGN-host galaxies, while spectroscopy of MgII and FeII has provided evidence for ongoing gas accretion onto >5% of luminous, star-forming galaxies at z ~ 0.5-1. We then discuss the potentially ground-breaking benefits of future efforts to improve the spectral resolution of such studies, and to leverage spatially-resolved spectroscopy for new constraints on inflowing gas morphology.