The Black Hole Mass-Color Relations for Early- and Late-type Galaxies: Red and Blue Sequences

Dullo, Bililign T.; Bouquin, Alexandre Y. K.; Gil de Paz, Armando; Knapen, Johan H.; Gorgas, Javier
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The Astrophysical Journal

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The tight correlations between supermassive black hole (SMBH) mass (MBH) and the properties of the host galaxy have useful implications for our understanding of the growth of SMBHs and of the evolution of galaxies. Here, we present newly observed correlations between MBH and the host galaxy total UV-[3.6] color ( ${{ \mathcal C }}_{\mathrm{UV},\mathrm{tot}}$ , 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 MBH in the Galaxy Evolution Explorer/S4G survey. The colors are carefully measured in a homogeneous manner using the far-UV, near-UV, and 3.6 μm magnitudes of the galaxies and their multicomponent structural decompositions in the literature. We find that more massive SMBHs are hosted by (early- and late-type) galaxies with redder colors, but the ${M}_{\mathrm{BH}}-{{ \mathcal C }}_{\mathrm{UV},\mathrm{tot}}$ relations for the two morphological types have slopes that differ at ∼2σ level. Early-type galaxies define a red sequence in the ${M}_{\mathrm{BH}}-{{ \mathcal C }}_{\mathrm{UV},\mathrm{tot}}$ 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 LUV/L3.6, it follows that the SMBH masses for late-type galaxies exhibit a steeper dependence on sSFR than those for early-type galaxies. The ${M}_{\mathrm{BH}}-{{ \mathcal C }}_{\mathrm{UV},\mathrm{tot}}$ and MBH - L3.6,tot relations for the sample galaxies reveal a comparable level of vertical scatter in the log MBH direction, approximately 5%-27% more than the vertical scatter of the MBH - σ relation. Our ${M}_{\mathrm{BH}}-{{ \mathcal C }}_{\mathrm{UV},\mathrm{tot}}$ relations suggest different channels of SMBH growth for early- and late-type galaxies, consistent with their distinct formation and evolution scenarios. These new relations offer the prospect of estimating SMBH masses reliably using the galaxy color alone. Furthermore, we show that they are capable of estimating intermediate black hole masses in low-mass early- and late-type galaxies.
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