The Astrophysical Journal<\/a>.<\/p>\n\u201cIt\u2019s more than just bookkeeping,\u201d says Fan Zou, a postdoctoral researcher in the UM astronomy department who led the study. \u201cOur study gives clues about how supermassive black holes are born. It also provides crucial hints about how often black hole signatures in dwarf galaxies can be found with new or future telescopes.\u201d<\/p>\n
What\u2019s going on in small galaxies is of particular interest because they\u2019re more reflective of what the universe was like longer ago, Zou says.<\/p>\n
\u201cIt\u2019s important to get an accurate black hole head count in these smaller galaxies,\u201d Zou says. \u201cWith low mass galaxies, we expect that they haven\u2019t changed much since they were born, so they\u2019re kind of like fossils from the early universe. By looking at low-mass galaxies and black holes, we can learn more about what was happening in the early universe.\u201d<\/p>\n
\u201cThe formation of big black holes is expected to be rarer, in the sense that it occurs preferentially in the most massive galaxies being formed, so that would explain why we don\u2019t find black holes in all the smaller galaxies,\u201d says Anil Seth, a coauthor and a professor at University of Utah.<\/p>\n
There are currently two main theories about how supermassive black holes form. One is that they grow from smaller black holes, created when giant stars run out of fuel and collapse. The second idea is that the giant black holes are born big from the collapse of enormous gas clouds so that they have the mass of thousands of suns to begin with. The team\u2019s findings suggest the latter is more likely.<\/p>\n
As material falls onto black holes, it is heated by friction and produces X-rays. Many of the massive galaxies in the study contain bright X-ray sources in their centers, a clear signature of supermassive black holes. But the study\u2019s smaller galaxies\u2014galaxies with masses less than 3 billion suns\u2014usually lacked these unambiguous black hole signals (for comparison, the Milky Way has a mass of around 60 billion suns).<\/p>\n
The researchers considered two possible explanations for this lack of X-ray sources. The first is that the fraction of galaxies containing massive black holes is much lower for these less massive galaxies. The second is that the amount of X-rays produced by matter falling onto these black holes is so faint that Chandra cannot detect it.<\/p>\n
\u201cWe think, based on our analysis of the Chandra data, that there really are fewer black holes in these smaller galaxies than in their larger counterparts,\u201d says Elena Gallo, a coauthor and UM professor of astronomy.<\/p>\n
Gallo and her colleagues were able to consider both possibilities for the lack of X-ray sources in small galaxies in their large Chandra sample. The amount of gas falling onto a black hole determines how bright or faint they are in X-rays. Because smaller black holes are expected to pull in less gas than larger black holes, they should be fainter in X-rays and often not detectable. The researchers confirmed this expectation.<\/p>\n
They also found, however, that this explanation alone could not account for the entire deficit of X-ray sources. That is, there was an additional deficit beyond what was expected. And this additional deficit could be accounted for if many of the low mass galaxies simply don\u2019t have any black holes at their centers.<\/p>\n
\u201cOur analysis, statistically, is able to tell us that the likelihood is much higher that the black holes aren\u2019t there,\u201d Gallo says.<\/p>\n
Gallo and Zou say the Laser Interferometer Space Antenna, a joint project between NASA and the European Space Agency that\u2019s being developed to launch in 2035, will be able to further test their conclusions. With NASA\u2019s funding outlook murky at the moment, though, it remains to be seen how the agency will continue the legacy of large, flagship missions like Chandra, the Hubble Space Telescope, and the JWST.<\/p>\n
\u201cThese machines, these products of human ingenuity, have really given us an understanding of the universe and our place in it,\u201d Gallo says. \u201cThese great missions have delivered enormous knowledge and this study is one tiny piece of that.\u201d<\/p>\n
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