Near-IR spectroscopy of luminous LoBAL quasars at 1<z<2.5

TL;DR

This study uses near-IR spectroscopy of luminous LoBAL quasars at 1<z<2.5 to investigate their properties and test if they represent an early evolutionary stage of quasars, finding no significant differences from normal quasars.

Contribution

It provides the first robust black hole mass estimates for high-redshift LoBAL quasars and compares their properties to non-BAL quasars, challenging the youth scenario.

Findings

No significant difference in black hole mass or Eddington ratio distributions.

LoBALs have similar UV to mid-IR SEDs to non-BAL quasars, aside from reddening.

LoBALs do not show stronger ionized outflows compared to normal quasars.

Abstract

We present near-IR spectroscopy of 22 luminous low-ionization broad absorption line quasars (LoBAL QSOs) at redshift 1.3<z<2.5, with 12 objects at z~1.5 and 10 at z~2.3. The spectra cover the rest-frame H$\alpha$ and H$\beta$ line regions, allowing us to obtain robust black hole mass estimates based on the broad H$\alpha$ line. We use these data, augmented by a lower-redshift sample from the Sloan Digital Sky Survey, to test the proposed youth scenario for LoBALs, which suggests that LoBALs constitute an early short-lived evolutionary stage of quasar activity, by probing for any difference in their masses, Eddington ratios, or rest-frame optical spectroscopic properties compared to normal quasars. In addition, we construct the UV to mid-IR spectral energy distributions (SEDs) for the LoBAL sample and a matched non-BAL quasar sample. We do not find any statistically significant difference between LoBAL QSOs and non-BAL QSOs in their black hole mass or Eddington ratio distributions. The mean UV to mid-IR SED of the LoBAL QSOs is consistent with non-BAL QSOs, apart from their stronger reddening. At z>1 there is no clear difference in their optical emission line properties. We do not see particularly weak [OIII] or strong FeII emission. The LoBAL QSOs do not show a stronger prevalence of ionized gas outflows as traced by the [OIII] line, compared to normal QSOs of similar luminosity. We conclude that the optical-MIR properties of LoBAL QSOs are consistent with the general quasar population and do not support them to constitute a special phase of active galactic nucleus evolution.