The (black hole mass)-(color) relations for early- and late-type galaxies: red and blue sequences

TL;DR

This study uncovers distinct correlations between supermassive black hole mass and galaxy color for early- and late-type galaxies, revealing different growth channels and evolutionary paths for these galaxy types.

Contribution

It presents the first homogeneous analysis of $M_{\rm BH}$-color relations for a diverse galaxy sample, highlighting differences between early- and late-type galaxies.

Findings

Early-type galaxies form a red sequence in $M_{\rm BH}$-color space.

Late-type galaxies follow a blue sequence with steeper $M_{\rm BH}$-sSFR dependence.

The $M_{\rm BH}$-color relations have comparable scatter to the $M_{\rm BH}$-$\sigma$ relation.

Abstract

[Abridged] Tight correlations between supermassive black hole (SMBH) mass ($M_{\rm BH}$) and the properties of the host galaxy have useful implications for our understanding of the growth of SMBHs and evolution of galaxies. Here, we present newly observed correlations between $M_{\rm BH}$ and the host galaxy total UV$-$ [3.6] color ($\mathcal{C_{\rm UV,tot}}$, Pearson's r = $0.6-0.7$) for a sample of 67 galaxies (20 early-type galaxies and 47 late-type galaxies) with directly measured $M_{\rm BH}$ in the GALEX/S$^{4}$G survey. The colors are carefully measured in a homogeneous manner using the galaxies' FUV, NUV and 3.6 $\micron$ magnitudes and their multi-component structural decompositions in the literature. We find that more massive SMBHs are hosted by (early- and late-type) galaxies with redder colors, but the $M_{\rm BH}- \mathcal{C_{\rm UV,tot}}$ relations for the two morphological types have slopes that differ at $\sim 2 \sigma$ level. Early-type galaxies define a red sequence in the $M_{\rm BH}- \mathcal{C_{\rm UV,tot}}$ diagrams, while late-type galaxies trace a blue sequence. Within the assumption that the specific star formation rate of a galaxy (sSFR) is well traced by $L_{\rm UV}/L_{\rm 3.6}$, it follows that the SMBH masses for late-type galaxies exhibit a steeper dependence on sSFR than those for early-type galaxies. The $M_{\rm BH}- \mathcal{C_{\rm UV,tot}}$ and $M_{\rm BH}-L_{\rm 3.6,tot}$ relations for the sample galaxies reveal a comparable level of vertical scatter in the log $M_{\rm BH}$ direction, roughly $5\%-27\%$ more than the vertical scatter of the $M_{\rm BH}-\sigma$ relation. Our $M_{\rm BH}- \mathcal{C_{\rm UV,tot}}$ relations suggest different channels of SMBH growth for early- and late-type galaxies, consistent with their distinct formation and evolution scenarios.