This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers analyzed 2,435 galaxies hosting actively feeding black holes using data from the Dark Energy Spectroscopic Instrument (DESI). Among them, they identified 546 "bulgeless" galaxies—systems with little or no central stellar bulge.
Their paper, published in Monthly Notices of the Royal Astronomical Society, suggests that galaxies without a central bulge grow black holes just as effectively as galaxies that have one—a surprising sign that black holes don't need galaxy mergers to grow normally. There is a well-established relationship between a galaxy's central black hole mass and properties of its host galaxy, such as the bulge mass, total mass in stars and the spread of different velocities within the galaxy. These relationships suggest that the galaxy's black hole and the galaxy evolve together.
Two things have recently complicated the picture of this coevolution and what drives it. First, the James Webb Space Telescope (JWST) found black holes at high redshift that appear "too big" for their host galaxies compared to what relationships in the local universe predict. Second, JWST also found far more disk galaxies at high redshift than expected.
Since disks are fragile structures that major mergers tend to destroy, their abundance raises the question of how black holes grow in galaxies that aren't undergoing mergers. Simulations suggest most black hole growth over cosmic time is driven by secular processes—gradual, nonmerger mechanisms like gas inflows along bars and spiral arms—rather than mergers. But testing this observationally is hard because astronomers cannot directly "see" a galaxy's full merger history.
So here is how they infer it: They study bulgeless galaxies, since a prominent bulge is itself usually evidence of past major mergers. If bulgeless galaxies show normal black hole growth, that's indirect evidence that secular growth works. In this new study, a team led by Sophie M.
Jewell of the University of Oxford used a large spectroscopic and imaging data set from the DESI survey to test how supermassive black holes (SMBHs) grow alongside their host galaxies. They first put together a sample of 2,435 disk galaxies hosting actively feeding and growing black holes. To pin down which of these galaxies had little or no bulge, the team used an automated imaging pipeline called GALFITM to break each galaxy's light profile into separate components—a central point source, a disk and, where present, a bulge or bar—fitting increasingly complex models until the best match was found.
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