Dynamical Black Hole Masses of BL Lac Objects from the Sloan Digital Sky Survey

TL;DR

This study measures black hole masses in 71 BL Lac objects using spectral analysis and finds they all host similar black holes around 10^{8.5} solar masses, with host galaxies acting as standard candles.

Contribution

It provides the first large sample measurement of black hole masses in BL Lac objects, showing their masses are uniform and not the main driver of spectral diversity.

Findings

Black hole masses are approximately 10^{8.5} solar masses.

BL Lac host galaxies follow the black hole mass - bulge luminosity relation.

Black hole mass does not significantly influence spectral energy distribution diversity.

Abstract

We measure black hole masses for 71 BL Lac objects from the Sloan Digital Sky Survey with redshifts out to z~0.4. We perform spectral decompositions of their nuclei from their host galaxies and measure their stellar velocity dispersions. Black hole masses are then derived from the black hole mass - stellar velocity dispersion relation. We find BL Lac objects host black holes of similar masses, ~10^{8.5} M_sun, with a dispersion of 0.4 dex, similar to the uncertainties on each black hole measurement. Therefore, all BL Lac objects in our sample have the same indistinguishable black hole mass. These 71 BL Lac objects follow the black hole mass - bulge luminosity relation, and their narrow range of host galaxy luminosities confirm previous claims that BL Lac host galaxies can be treated as standard candles. We conclude that the observed diversity in the shapes of BL Lac object spectral energy distributions is not strongly driven by black hole mass or host galaxy properties.