BASS XXXV. The $M_\rm{BH}$-$\sigma_\rm{\star}$ Relation of 105-Month Swift-BAT Type 1 AGNs

TL;DR

This study measures stellar velocity dispersions in 173 local Type 1 AGNs to test black hole mass estimates and the $M_ m{BH}$-$\sigma_ m{ m{ extstar}}$ relation, revealing biases and a shallower relation compared to inactive galaxies.

Contribution

It provides one of the largest samples with both stellar velocity dispersions and virial black hole mass estimates, testing their consistency and implications for SMBH scaling relations.

Findings

High consistency between two independent $\sigma_ m{ extstar}$ measurements.

Systematic underestimation of $M_ m{BH}$ from broad emission lines.

Shallower $M_ m{BH}$-$\sigma_ m{ extstar}$ relation in AGNs compared to inactive galaxies.

Abstract

We present two independent measurements of stellar velocity dispersions ( $\sigma_\rm{\star}$ ) from the Ca\,H+K \& Mg\,\textsc{i} region (3880--5550~\AA) and the Calcium Triplet region (CaT, 8350--8750~\AA) for 173 hard X-ray-selected Type 1 AGNs ($z \leq$ 0.08) from the 105-month Swift-BAT catalog. We construct one of the largest samples of local Type 1 AGNs that have both single-epoch (SE) 'virial' black hole mass ($M_\rm{BH}$) estimates and $\sigma_\rm{\star}$ measurements obtained from high spectral resolution data, allowing us to test the usage of such methods for SMBH studies. We find that the two independent $\sigma_\rm{\star}$ measurements are highly consistent with each other, with an average offset of only $0.002\pm0.001$ dex. Comparing $M_\rm{BH}$ estimates based on broad emission lines and stellar velocity dispersion measurements, we find that the former is systematically lower by $\approx$0.12 dex. Consequently, Eddington ratios estimated through broad-line $M_\rm{BH}$ determinations are similarly biased (but in the opposite way). We argue that the discrepancy is driven by extinction in the broad-line region (BLR). We also find an anti-correlation between the offset from the $M_\rm{BH}$ - $\sigma_\rm{\star}$ relation and the Eddington ratio. Our sample of Type 1 AGNs shows a shallower $M_\rm{BH}$ - $\sigma_\rm{\star}$ relation (with a power law exponent of $\approx$3.5) compared with that of inactive galaxies (with a power-law exponent of $\approx$4.5), confirming earlier results obtained from smaller samples.