Quashing a suspected selection bias in galaxy samples having dynamically measured supermassive black holes

Abstract

ABSTRACT Local early-type galaxies (ETGs) with directly measured black hole masses, Mbh, have been reported to represent a biased sample relative to the ETG population at large. Such galaxies with Spitzer Space Telescope imaging have been purported to possess velocity dispersions, σ, at least ∼0.1 dex larger for a given galaxy stellar mass, M*,gal, than is typically observed among thousands of ETGs imaged by the Sloan Digital Sky Survey (SDSS). This apparent offset led Shankar et al. to reduce the normalisation of the observed Mbh ∝ σ5 relation by at least ∼0.5 dex to give their ‘intrinsic relations’, including σ-based modifications to the observed Mbh–M*,gal relation. These modifications were based on the untested assumption that the stellar masses had been derived consistently between the two samples. Here, we provide the necessary check using galaxies common to the Spitzer Survey of Stellar Structure in Galaxies (S4G) and the SDSS. We find that the stellar masses of galaxies with and without directly measured black hole masses had appeared offset from each other due to the use of inconsistent stellar mass-to-light ratios, Υ*, for the optical and infrared data. We briefly discuss the ‘intrinsic relations’ and why some of these will at times appear to have had partial success when applied to data based on similarly inconsistent values ofΥ*. Finally, we reiterate the importance of the υ (lower-case Υ) term, which we previously introduced into the Mbh–M* relations to help avoid Υ*-related mismatches.