HST Spectroscopy of Lyα Emission in Low-Redshift Weak Emission-Line Quasars: Probing for Rapid Accretion in Active Galactic Nuclei

The existence of very high-redshift quasars (z > 6) suggests that some supermassive black holes grew very quickly during the first hundreds of millions of years after the Big Bang, through accretion and/or mergers. Over the past ~25 years, puzzling examples of radio-quiet, Type 1 quasars with uncharacteristically weak or absent broad emission lines have been found, which, as described below, may eventually offer some clues to understanding how the first supermassive black holes may have grown via accretion. While these weak emission-line quasars (WLQs) tend to have optical-UV properties typical of normal quasars, they have unusual X-ray properties that suggest their weak emission lines are caused by a column of gas shielding their broad emission line regions from the photoionizing continuum. A leading interpretation connects this shielding gas to the inner edge of a geometrically-thick “slim” accretion disk and its wide-angle outflows. Such an interpretation requires WLQs to be accreting near (or above) the Eddington limit, thereby making WLQs potential laboratories for understanding the accretion physics operating during phases of near/super-Eddington supermassive black hole growth in the early Universe. The original systematic searches for WLQs revealed an N ~ 100 object population of WLQs from optical spectroscopy of high-redshift (z > 3) quasars, defined by a Lyα + N V rest-frame equivalent width (EW) threshold < 15.4 Å. Identification of lower-redshift (z < 3) WLQ candidates, however, has relied primarily on optical spectroscopy of weak broad emission lines at longer rest-frame wavelengths. We present spectroscopy covering the Lyα + N V complex of six candidate low-redshift (0.9 < z < 1.5) WLQs, based on observations with the Hubble Space Telescope. With these new observations expanding existing optical coverage into ultraviolet, we explore unifying the low- and high-z WLQ populations via EW[Lyα+NV]. Two objects in the sample unify well with high-z WLQs, and three others appear consistent with a “bridge” population between WLQs and normal quasars, while the final object is consistent with typical quasars. We discuss how the new constraints on EW[Lya+NV] favor a soft ionizing continuum and are consistent with expectations from the “slim disk” shielding gas model. We conclude by briefly discussing in-progress research to extend the above results to lower-mass active galactic nuclei (M_BH ~ 10⁶ M_sun) to test for slim disks through the combination of Chandra X-ray data with new observations from the Very Large Array.