Galaxy color correlates with mass of supermassive black hole

Nearly every galaxy seems to house a supermassive black hole (SMBH). But how those central black holes relate to galactic processes such as star-formation events remains murky. Astronomers have taken steps by correlating SMBH masses with a handful of galactic properties, including stellar density, stellar velocity dispersion, and the mass and luminosity of the bulge at a galaxy’s center.

Now researchers led by Bililign Dullo and Alexandre Bouquin of the Complutense University of Madrid have explored a correlation between SMBH mass and galaxy color. They’ve found that redder galaxies tend to host more massive SMBHs and bluer galaxies less massive ones, although the slope of the curve depends on the galaxies’ morphology. The correlation could make it easier to estimate unknown SMBH masses and shed light on how SMBHs grow in different galaxies.

Galaxies are often classified into one of two morphologies: early types, such as ellipticals, which are thought to form violently via mergers; and late types, such as spirals, that likely form via gentler processes. (The labels are relics from when astronomers mistakenly believed that early-type galaxies evolved into late-type ones.) Dullo and his colleagues examined 20 early-type and 47 late-type galaxies whose SMBH masses have been determined via measuring the motion of surrounding stars and gas. For each galaxy, the researchers used colors determined by near-IR and UV observations from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) and the Galaxy Evolution Explorer (GALEX).

The researchers found that redder galaxies, which are typically older with few young blue stars, host more massive SMBHs. Digging deeper, they identified separate mass–color relationships for early- and late-type galaxies. As shown in the graph, the mass of SMBHs in early-type, elliptical galaxies (red markers) increases more quickly with the redness of the galaxy than does SMBH mass in late-type, spiral galaxies (blue markers). The curve for late-type galaxies is also shifted toward the blue part of the mass–color sequence as compared with that for early-type galaxies, which tend to be redder and older.

The researchers made predictions for 11 other galaxies with previously measured SMBH masses; their mass estimates were within an order of magnitude of the existing measurements. They then estimated the currently unknown masses of SMBHs in more than a thousand different galaxies observed by GALEX and S⁴G.

In addition to enabling SMBH mass estimates based solely on a galaxy’s morphology and color, the work by Dullo and colleagues offers clues as to how those black holes and their parent galaxies coevolve. Astronomers suspect that SMBHs in early-type galaxies grow abruptly via mergers, like their host galaxies, whereas late-type SMBHs might grow more slowly, feeding on infalling gas. The researchers say the two distinct mass–color sequences support that idea. (B. T. Dullo et al., Astrophys. J. 898, 83, 2020 .)