Implications of line locking for quasar-mode feedback

Norman Murray (CITA)

April 12, 2023

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Abstract: Quasar electromagnetic radiation, winds, and jets have been proposed as sources of feedback (positive and negative) on their host galaxies. The first two are often referred to as quasar-mode feedback; since most quasars do not have powerful jets, winds are currently the leading candidate to quench (halt the star formation of) field galaxies with halo masses greater than 1.e12 or so. The mass, momentum, and energy outflow rates of these winds are hard to determine observationally, but if the physics driving the outflows can be nailed down, the estimates can be firmed up. Quasar spectra are dominated by broad (~5,000 km/s) emission (and less frequently, absorption) lines of Lithium-like ions of H, C, N, O, and Si; the latter 4 ions have strong doublet ground state transition, i.e., two lines separated by 400-1000 km/s. Surveys yielding tens to hundreds of thousands of quasar spectra have shown that about half of quasars exhibit narrow (< 500km/s) blue-shifted (hence outflowing) absorption lines, and about a third show broad (>3000 km/s) absorption lines; the latter are known as BAL quasars. Recent work has demonstrated that, of quasars showing one or more triply ionized carbon (C IV) doublets, two thirds show triplets, i.e., three lines separated by the C IV double splitting of ~500km/s. I will explain how this “line locking” arises, and show that it demonstrates that the outflow, both narrow and broad, are driven almost exclusively by radiation pressure, ruling out significant contributions from both hot shocked gas, and from Blandford-Payne type magnetic driving.