Study: Many small galaxies may lack central black holes ^{CEFR B2}

A study led by researchers at the University of Michigan used NASA’s Chandra X-ray Observatory to analyse X-ray data from more than 1,600 galaxies gathered over 20-plus years. The sample ranged from galaxies more than ten times the mass of the Milky Way down to dwarf galaxies with masses less than a few percent of our home galaxy. The work appears in The Astrophysical Journal.

Results show a clear difference by mass: roughly 30% of dwarf galaxies likely contain supermassive black holes, while over 90% of massive galaxies similar to the Milky Way show central X-ray sources. Many massive systems display bright X-ray emission at their centres, which signals gas heating as it falls onto a black hole. Galaxies with masses below about 3 billion suns usually lacked such unambiguous X-ray signatures; for comparison, the Milky Way’s mass is about 60 billion suns.

The researchers conclude that two effects explain the shortfall of X-ray detections in small galaxies. First, smaller black holes are expected to accrete less gas and therefore be fainter in X-rays, and the data confirm this trend. Second, faintness alone does not account for the full deficit, so a substantial fraction of low-mass galaxies may genuinely lack central black holes. The pattern favours formation models in which the largest black holes begin large, for example from collapsing massive gas clouds, rather than growing only from smaller stellar remnants. Planned observations with the Laser Interferometer Space Antenna in 2035 could provide further tests, although funding uncertainties for large missions may affect follow-up efforts.