Balmer Absorption in Iron Low-Ionization Broad Absorption Line Quasars

Karen M. Leighly, Sarah C. Gallagher, Hyunseop Choi, Donald M. Terndrup, Julianna R. Voelker, Gordon T. Richards, Leah K. Morabito

First listed 2025-09-09 | Last updated 2025-09-09

Abstract

While C IV is the most common absorption line in Broad Absorption Line Quasar spectra, Balmer absorption lines are among the rarest. We present analysis of Balmer absorption in a sample of fourteen iron low-ionization BAL quasars (FeLoBALQs); eight are new identifications. We measured velocity offset, width, and apparent optical depth. The partial covering ubiquitous in BAL quasar spectra alters the measured Balmer optical depth ratios; taking that into account, we estimated the true H(n= 2) column density. We found the anticipated correlation between Eddington ratio and outflow speed, but it is weak in this sample because nearly all of the objects have the low outflow speeds characterizing loitering outflow FeLoBAL quasars (H. Choi et al. 2022b), objects that are also found to have low accretion rates (K. M. Leighly et al. 2022; H. Choi et al. 2022a). Measures of dN/dv, the differential column density with respect to the outflow speed, are anticorrelated with the luminosity and Eddington ratio: the strongest absorption is observed at the lowest speeds in the lowest luminosity objects. The absorption line width is correlated with αoi, the Fλ point-to-point slope between 5100A and 3 microns. This parameter is strongly correlated with the Eddington ratio among low-redshift quasars (K. M. Leighly et al. 2024). Balmer absorption lines have been recently found in the spectra of Little Red Dots (LRDs), a class of high-redshift objects discovered by JWST. We note suggestive similarities between LRDs and FeLoBAL quasars in the emission line shape, the presence of steep reddening and a scattered blue continuum, the lack of hot dust emission, and X-ray weakness.

Short digest

Analyzes Balmer absorption in 14 FeLoBAL quasars (8 new), measuring velocities, widths, and apparent optical depths, and correcting for partial covering to infer true H(n=2) columns. Finds only a weak Eddington ratio–outflow speed trend because most objects are low‑speed “loitering” outflows; dN/dv peaks at low velocities and anticorrelates with luminosity and Eddington ratio. Reports a correlation between Balmer-line width and the optical–IR slope αoi, tying absorber kinematics to SED/accretion state. Notes parallels with Little Red Dots—V‑shaped continua, steep reddening with a scattered blue component, weak hot dust, and X‑ray weakness—hinting at shared conditions that elevate the n=2 population.

Key figures to inspect

• Balmer series profiles (Hα/Hβ/Hγ) with partial‑covering fits: inspect Balmer decrement departures to quantify covering fraction and derive the true H(n=2) column density.
• Eddington ratio versus outflow speed scatter plot: verify the concentration of points at |v| < 2000 km s−1 (loitering regime) and the resulting weak correlation.
• dN/dv as a function of velocity, split by luminosity/Eddington bins: check that the strongest absorption occurs at the lowest speeds and in the lowest‑L, lowest‑Edd objects.
• Absorption-line width versus αoi: assess the significance and slope of the correlation linking kinematics to SED shape/accretion rate.
• Rest‑optical/near‑IR SED or spectra comparison: identify steep reddening, scattered blue continuum, and lack of hot‑dust bump; contrast FeLoBALQs with representative LRD spectra.