A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z ≲ 2

The assembly of stellar and supermassive black hole (SMBH) mass in elliptical galaxies since z similar to 1 can help to diagnose the origins of locally observed correlations between SMBH mass and stellar mass. We therefore construct three samples of elliptical galaxies, one at z similar to 0 and two at 0.7 less than or similar to z less than or similar to 2.5, and quantify their relative positions in the M (BH)-M (*) plane. Using a Bayesian analysis framework, we find evidence for translational offsets in both stellar mass and SMBH mass between the local sample and both higher-redshift samples. The offsets in stellar mass are small, and consistent with measurement bias, but the offsets in SMBH mass are much larger, reaching a factor of 7 between z similar to 1 and z similar to 0. The magnitude of the SMBH offset may also depend on redshift, reaching a factor of similar to 20 at z similar to 2. The result is robust against variation in the high- and low-redshift samples and changes in the analysis approach. The magnitude and redshift evolution of the offset are challenging to explain in terms of selection and measurement biases. We conclude that either there is a physical mechanism that preferentially grows SMBHs in elliptical galaxies at z less than or similar to 2, or that selection and measurement biases are both underestimated, and depend on redshift.

Automated summary

By constructing three samples of elliptical galaxies at different redshifts and using a Bayesian analysis framework, this study finds evidence for translational offsets in both stellar mass and supermassive black hole (SMBH) mass. The offsets in SMBH mass between different redshifts are larger, and the magnitude and redshift evolution of the offset cannot be explained solely by selection and measurement biases. Therefore, there may be a physical mechanism that preferentially grows SMBHs in elliptical galaxies at lower redshifts or the biases are both underestimated and dependent on redshift.