In Search of the Largest Velocity Dispersion Galaxies

TL;DR

This study investigates galaxies with exceptionally high velocity dispersions, revealing potential overestimations in black hole masses or a change in the black hole-bulge relationship at the high-mass end.

Contribution

It provides new measurements of galaxy velocity dispersions using high-quality spectra, challenging existing black hole mass estimates and the universality of the black hole-bulge relation.

Findings

Maximum velocity dispersion observed is 444 km/s.

Black hole masses in QSOs may be overestimated at high masses.

Velocity dispersion growth levels off in the brightest elliptical galaxies.

Abstract

We present Hobby-Eberly Telescope (HET) observations for galaxies at redshift z < 0.3 from the Sloan Digital Sky Survey (SDSS) showing large velocity dispersions while appearing to be single galaxies in HST images. The high signal-to-noise HET spectra provide more definitive velocity dispersions. The maximum velocity dispersion we find is 444 km/s. Emission-line widths in QSOs indicate that black holes can exist with masses exceeding 5 billion solar masses, implying velocity dispersions greater than 500 km/s by the local black hole mass - velocity dispersion relationship. This suggests either that QSO black hole masses are overestimated or that the black hole - bulge relationship changes at high black hole mass. The latter option is consistent with evidence that the increase in velocity dispersion with luminosity levels off for the brightest elliptical galaxies.