Invited review to appear in "Gas Accretion onto Galaxies", Astrophysics and Space Science Library, eds. A. J. Fox & R. Dave.

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Kate H. R. Rubin

San Diego State University, Department of Astronomy, San Diego, CA 92182

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., Na I resonance lines in the rest-frame optical or Mg II and Fe II in the near-ultraviolet is in principle sensitive to the infall of cool material at temperatures T ∼ 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 Na I line profiles has revealed numerous instances of redshifted absorption observed toward early-type and/or AGN-host galaxies, while spectroscopy of Mg II and Fe II 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.

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