Using data from the REsolved Spectroscopy Of a Local VolumE (RESOLVE) and the Environmental COntext Catalog (ECO) surveys, astronomers have found evidence of the basic building blocks of supermassive black holes in gas-rich dwarf galaxies.
Growing, massive black holes reside in dwarf galaxies, where their radiation competes with the light of abundant young stars. Image credit: NASA / ESA / Hubble / M. Polimera.
As a giant spiral galaxy, the Milky Way is believed to have been built up from mergers of many smaller dwarf galaxies.
Each dwarf that falls in may bring with it a central massive black hole, tens or hundreds of thousands of times the mass of our Sun, potentially destined to be swallowed by the Milky Way’s central supermassive black hole.
But how often dwarf galaxies contain a massive black hole is unknown, leaving a key gap in understanding how black holes and galaxies evolve together.
“Black holes are a fascinating topic. But there’s still the lingering question: where do such supermassive black holes come from? Our work is a teeny step closer to answering that question,” said first author Mugdha Polimera, an astronomer in the Department of Physics and Astronomy at the University of North Carolina at Chapel Hill.
Polimera and colleagues used ultraviolet and radio data from RESOLVE and ECO surveys to assess the presence of these growing black holes.
Most astronomical surveys select samples that favor big and bright galaxies, but these surveys are complete inventories of large volumes of the present-day Universe in which dwarf galaxies are abundant.
The astronomers realized that spectroscopic data used to assess the presence of a growing black hole would often be ambiguous in the same specific way for dwarf galaxies. These galaxies were typically thrown out of surveys and the ambiguity was ignored.
The researchers suspected that taking into account two typical properties of dwarf galaxies — their mostly primordial elemental composition (mainly hydrogen and helium) and their high rate of forming new stars — might resolve the ambiguity in favor of the presence of a growing black hole.
Theoretical simulations that confirmed this suspicion: the observed ambiguity is exactly what the simulations predict for a primordial composition, highly star-forming dwarf galaxy containing a growing massive black hole.
The final step in the research involved the search for galaxies in the surveys that exactly matched the criteria — resulting in the finding that growing, massive black holes are more common in dwarf galaxies than previously thought.
“We all got nervous. The first question to my mind was: have we missed a way in which extreme star formation alone could explain these galaxies? The answer was a resounding no,” Polimera said.
“We’re left with this result that is shocking,” added co-author Sheila Kannappan, also from the Department of Physics and Astronomy at the University of North Carolina at Chapel Hill.
The team’s paper was published in the Astrophysical Journal.
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Mugdha S. Polimera et al. 2022. RESOLVE and ECO: Finding Low-metallicity z ∼ 0 Dwarf AGN Candidates Using Optimized Emission-line Diagnostics. ApJ 931, 44; doi: 10.3847/1538-4357/ac6595
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