Astronomers analyzed 2,435 galaxies containing black holes that are actively accreting matter, using data obtained with the Dark Energy Spectroscopic Instrument (DESI). Among them, they identified 546 bulgeless galaxies — systems with little or no central stellar bulge.
Quiet Growth of Supermassive Black Holes
As reported by phys.org, scientists suggest that galaxies without a central bulge form black holes just as efficiently as galaxies that have such a bulge — surprising evidence that black holes do not require galaxy mergers in order to grow normally.
There is a well-established relationship between the mass of a galaxy’s central black hole and properties of its host galaxy, such as the mass of the galactic bulge, the total stellar mass, and the range of different velocities within the galaxy. These relationships suggest that the black hole and the galaxy evolve together.
Recently, two circumstances have complicated the picture of this co-evolution and its driving forces. First, the James Webb Space Telescope has detected high-redshift black holes that appear too massive for their host galaxies compared with what is predicted by relationships in the local Universe. Second, James Webb has also detected far more high-redshift disk galaxies than expected. Since disks are fragile structures that are usually disrupted during major mergers, their abundance raises questions about how black holes grow in galaxies that do not undergo mergers.
Simulations show that most black hole growth over cosmic time is driven by long-term processes — gradual mechanisms unrelated to mergers, such as gas inflows along bars and spiral arms. However, testing this through observations is difficult because astronomers cannot directly see the complete history of galaxy mergers.
This is how they reach such a conclusion: they study bulgeless galaxies because a prominent bulge itself is usually evidence of past major mergers. If galaxies without bulges show normal black hole growth, this is indirect evidence that long-term growth processes really do occur.
Decomposing the Light Profiles of Galaxies
In this new study, a team led by Sophie M. Jewell of the University of Oxford used a large set of spectroscopic and imaging data from the DESI survey to test how supermassive black holes (SMBHs) grow together with their host galaxies. They first assembled a sample of 2,435 disk galaxies containing black holes that are actively accreting matter and growing.
To determine which of these galaxies had little or no bulge, the team used an automated image-processing algorithm called GALFITM to decompose the light profile of each galaxy into separate components — a central point source, disk, and, where present, a bulge or bar — fitting increasingly complex models until they found the best match. This allowed them to separate 546 genuinely bulgeless galaxies from 240 galaxies with some bulge component. The team tested its bulge-detection method by inserting artificial bulges into real images and checking how reliably their processing pipeline recovered them.
The researchers then estimated black hole masses from the width and brightness of broad hydrogen-alpha emission lines and compared how black hole mass correlated both with total stellar mass and specifically with bulge mass in the populations of bulgeless and bulged galaxies.
Surprisingly, they found that bulgeless galaxies still show a black hole-to-stellar-mass relationship consistent with galaxies that have bulges. This is an important clue that merger-free growth can lead to “normal” co-evolution.
Bars and Spiral Arms Feed Black Holes in Bulgeless Galaxies
When the comparison focused specifically on black hole mass and bulge mass, the situation became more interesting. This was where bulgeless galaxies and galaxies with bulges diverged sharply in their properties — contrary to the predictions of a merger-driven growth model. Most black holes in bulgeless galaxies were too massive compared with their relatively smaller bulges.
When the team modeled how these black holes could have grown over time, the simulation results showed that they could have reached their observed masses without mergers, pointing to stellar bars and spiral arms as likely mechanisms for supplying gas.
The team also found that about two-thirds of the galaxies in their sample have bars, while the vast majority show visible spiral arms. Both of these features are known to channel gas toward the center of a galaxy at rates sufficient to feed a growing black hole.
However, the scientists’ modeling differs from other cosmological simulations, such as Horizon-AGN, which may indicate that the population of supermassive black holes represented in the study was not large enough.
The team notes that space telescopes such as Euclid and Roman should help answer some of these open questions. Their sharper images taken from space will make it possible to detect and identify faint and small bulges, giving astronomers a clearer understanding of whether these bulgeless galaxies actually lack black holes.
