A Uniformly Selected Sample of Low-mass Black Holes in Seyfert 1 Galaxies

TL;DR

This study systematically identifies low-mass black holes in Seyfert 1 galaxies using SDSS data, revealing a diverse range of accretion rates and host galaxy properties, and expanding the known sample of such black holes.

Contribution

It introduces an automated, uniform selection method for low-mass black holes in AGNs, increasing the sample size and diversity compared to previous studies.

Findings

Identified 309 low-mass black holes, with 61 newly discovered.

Found many low-mass black holes accreting at low Eddington ratios.

Host galaxies resemble typical star-forming Sbc spirals.

Abstract

We have conducted a systematic search of low-mass black holes (BHs) in active galactic nuclei (AGNs) with broad Halpha emission lines, aiming at building a homogeneous sample that is more complete than previous ones for fainter, less highly accreting sources. For this purpose, we developed a set of elaborate, automated selection procedures and applied it uniformly to the Fourth Data Release of the Sloan Digital Sky Survey. Special attention is given to AGN--galaxy spectral decomposition and emission-line deblending. We define a sample of 309 type 1 AGNs with BH masses in the range $8 \times 10^4$--$2 \times 10^6$ \msun (with a median of $1.2 \times 10^6$ solar mass), using the virial mass estimator based on the broad Halpha line. About half of our sample of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered here for the first time. Our new sample picks up more AGNs with low accretion rates: the Eddington ratios of the present sample range from $<~0.01$ to ~1, with 30% below 0.1. This suggests that a significant fraction of low-mass BHs in the local Universe are accreting at low rates. The host galaxies of the low-mass BHs have luminosities similar to those of $L^*$ field galaxies, optical colors of Sbc spirals, and stellar spectral features consistent with a continuous star formation history with a mean stellar age of less than 1 Gyr.