Bulgeless galaxies in DESI data keep black holes on the usual stellar-mass track

Merger histories cannot be observed directly, so the study rests on a proxy ^[9]. A prominent bulge is usually evidence of past major mergers. A galaxy without one has probably had few. If its black hole still has the mass expected for its stellar mass, something other than mergers fed it ^[9].

In the textbook version of coevolution, black hole mass tracks three properties of the host: bulge mass, total stellar mass, and the spread of stellar velocities ^[5]. This design is informative because it pulls the first two apart. In the bulgeless galaxies, black hole mass follows total stellar mass ^[4] and does not follow bulge mass ^[13]. If mergers built bulge and black hole together, the bulge should be the better predictor. According to the phys.org account, the split the team found is the opposite of what a merger-driven growth model predicts ^[13].

The comparison is smaller than the headline sample. The paper, in Monthly Notices of the Royal Astronomical Society ^[3], starts from 2,435 galaxies ^[1]. The two groups actually compared hold 546 and 240 galaxies, or 786 in all ^[1]. That leaves 1,649 outside both groups ^[2], and the phys.org report does not say how they were classified or whether they entered any comparison.

What was observed is present-day mass, in black holes that are feeding now ^[1][12]. The growth history comes from a model ^[14]. So the claim that bulgeless galaxies grow black holes "just as effectively" combines a measurement with a reconstruction.

Bars and spiral arms are common in this sample. About two-thirds of the galaxies have bars and the vast majority show spiral arms, and both features are known to move gas toward a galaxy's centre fast enough to feed a growing black hole ^[16]. Simulations have pointed the same way, crediting most black hole growth over cosmic time to these gradual inflows ^[8]. Until now that claim was hard to test against real galaxies.

The authors attach the condition to their own conclusion. "if we assume bulgeless galaxies have experienced very little to no major merger activity, then major mergers are not required to grow the vast majority of the BHs to their observed masses within a reasonable time," the team wrote in the paper ^[15].

I think the condition is stated at the right strength. On this evidence, mergers are not needed to give these black holes normal masses. Whether mergers matter elsewhere is a separate question, and the result is only as clean as the bulge proxy. The bulge-injection check guards against the obvious failure, a faint bulge misread as none ^[11].

The result bears on two findings from the James Webb Space Telescope: black holes that look too big for their high-redshift hosts, and far more early disk galaxies than expected ^[6]. Disks are fragile, and major mergers tend to destroy them ^[7]. The DESI galaxies give an observed route to a normal-mass black hole that leaves the disk intact. Whether that route ran fast enough in the early universe needs its own test.