Big Black Holes in Little Galaxies

Title: The properties of AGN in dwarf galaxies identified via SED fitting

Authors: B. Bichang’a, S. Kaviraj, I. Lazar, R. A. Jackson, S. Das, D. J. B. Smith, A. E. Watkins, G. Martin

First Author’s Institution: Centre for Astrophysics Research, University of Hertfordshire, Hatfield, United Kingdom

Status: published in Monthly Notices of the Royal Astronomical Society [open access]

Little Monsters

Astronomers agree that almost every galaxy contains a supermassive black hole (SMBH) – a black hole at least a million times more massive than the sun – in its center. This includes the Milky Way, whose SMBH is called Sagittarius A* (pronounced “A-star”). Most of the time, these black holes are relatively inactive; however, when they accrete large amounts of material, they can become very luminous and produce jets of radiation and particles. SMBHs in this state are referred to as active galactic nuclei, or AGNs.

AGNs are well-studied in large galaxies, but the author’s of today’s paper focus on AGNs in low-mass galaxies. These dwarf galaxies are characterized by containing only 10-100 million solar masses of material (compare to the ~1 trillion solar masses of the Milky Way), meaning that their SMBHs can make up a significant fraction of their total mass. Researchers are trying to determine what impact this has on the properties of the dwarf galaxies and how this impact differs from those seen in larger galaxies.

Finding AGNs in Dwarfs

The authors begin with a catalog of galaxy observations called COSMOS2020. COSMOS2020 has spectroscopic data for about a million galaxies, ranging from ultraviolet (UV) to infrared (IR). COSMOS 2020 is an improvement over its predecessor (COSMOS2015) with enhanced optical data from Hyper-Suprime Cam and improved astrometric data from Gaia. From this catalog, they select dwarf galaxies that are between ~30 million solar masses and ~3 billion solar masses, and those within a redshift range of 0.1 to 0.3. This leaves a little over 1,000 dwarf galaxies for today’s study.

With this sample, the authors then attempt to fit the spectra of their dwarf galaxies to existing templates. These templates are generated from galaxies in cosmological simulations that account for factors like star formation, stellar metallicity (the concentration of heavy elements in stars), dust effects, and AGN emission. If the authors can’t find a template that matches well with one of the dwarf galaxies in their sample, then that dwarf galaxy is removed. This leaves a final sample of about 500 dwarf galaxies, which may or may not have AGN, to study the impacts of AGNs in these types of galaxies.

Small Impacts for Small Galaxies

Today’s authors analyzed this final sample in a handful of ways. First, they considered the fraction of each galaxy’s total luminosity emitted by the (potential) AGN, shown in Figure 1. The distribution in red highlights a population of dwarf galaxies where the AGN contributes significantly to the galaxy’s total energy output. They find that roughly a third of the dwarfs in their sample are believed to have AGN activity.

Another factor under consideration was the larger environment around the dwarf galaxy. For this, the authors compared the positions of galaxies from the two populations in Figure 1 with respect to larger universal structures (referred to as the cosmic web), shown in Figure 2. They find no significant distinctions between where dwarf galaxies with AGN and those without are located within the cosmic web.

One final major feature is star formation. Stars form when cool gas clouds condense and collapse, but AGNs can impart energy into these gas clouds and heat them up, inhibiting star formation. The authors find that there tends to be marginally less star formation in dwarf galaxies with AGN, but there is no evidence of significant, rapid decreases in star formation associated with AGN activity.

In all, perhaps AGNs do not have has large of an impact on dwarf galaxies as was expected. However, these findings still inform the astronomical community on the relationships between AGN activity and dwarf galaxy properties. This also gives astronomers an opportunity to compare results from simulations of AGN-hosting dwarf galaxies with today’s results to determine if the theoretical effects agree with observations.

Astrobite edited by Will Golay

Featured image credit: Vivienne F. Baldassare, Amy E. Reines, Elena Gallo, Jenny E. Greene. Hubble Space Telescope Imaging of the Active Dwarf Galaxy RGG 118 (Figure 1)

• About the Author

About Brandon Pries

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I've also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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