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

Local early-type galaxies (ETGs) with directly measured black hole masses, M-bh, 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, sigma, at least similar to 0.1 dex larger for a given galaxy stellar mass, M-*,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 M-bh proportional to sigma(5) relation by at least similar to 0.5 dex to give their 'intrinsic relations', including sigma-based modifications to the observed M-bh-M-*,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 (S(4)G) 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, Upsilon(*), 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 of Upsilon(*). Finally, we reiterate the importance of the v (lower-case Upsilon) term, which we previously introduced into the M-bh-M-* relations to help avoid Upsilon(*)-related mismatches.

Automated summary

The use of inconsistent stellar mass-to-light ratios results in offset between galaxies with directly measured black hole masses and those without. It is important to consider the accuracy of stellar mass-to-light ratios to avoid such discrepancies.