Blowout: AGNs Quenching Star Formation in Dwarf Galaxies

Paper Title: The role of AGN feedback on the evolution of dwarf galaxies from cosmological simulations: SMBHs suppress star formation in low-mass galaxies

Authors: Elena Arjona-Gálvez, Arianna Di Cintio, Robert J. J. Grand

First Author’s Institution: Instituto de Astrofísica de Canarias, Tenerife, Spain

Status: submitted to Astronomy & Astrophysics [open access] (available on arXiv)

Baby Galaxies

Dwarf galaxies are some of the smallest galaxies in the universe, typically hosting masses of about 10-100 million solar masses (compare to a trillion solar masses for a galaxy like the Milky Way). Dwarf galaxies are usually found as satellite galaxies around regular galaxies, including the ~60 dwarf galaxies in the vicinity of the Milky Way. It’s believed that the first galaxies to form in the early universe resembled modern-day dwarf galaxies in terms of mass and size. However, unlike modern dwarfs, these galaxies were not satellites of larger galaxies.

Dwarf galaxies have been studied intensely over the last few decades as astronomers continue to investigate the “problems” associated with our understanding of them, including the “missing satellite” problem, the “cusp-core” or “cuspy halo” problem, and the “too-big-to-fail” problem. Though some of these problems have been at least partially resolved by including more accurate physical models in simulations, astronomers have yet to pinpoint a comprehensive solution to these problems. Today’s authors suggest that accounting for feedback from Active Galactic Nuclei (AGNs) could be a viable way to address some of the outstanding problems with dwarf galaxies.

AGNs in Dwarf Galaxies

The authors of today’s paper ran cosmological simulations that model the formation and development of low-mass galaxies in the early universe, similar in size to modern-day dwarf galaxies. These simulations account for gravity between particles, electromagnetism, gas hydrodynamics, star formation, and supernovae. They seed massive black holes in galaxies that have sufficient mass. These black holes also accrete the surrounding gas, which serves as fuel for AGN feedback. Critically, the authors perform multiple runs that include AGN feedback and multiple corresponding runs that do not. This allows them to test the effects of AGN feedback while keeping everything else the same.

Plot of total stellar mass and mass contained within the center of the dwarf galaxy. Black points correspond to galaxies without AGN feedback, and colored squares represent galaxies with feedback, where color indicates the mass of the AGN. The solid blue and dashed black lines represent estimates from models by other researchers. Galaxies with black holes over a million solar masses show a noticeable decrease in total stellar mass when AGN feedback is included.
Figure 1: plot of the total stellar mass vs. the central galaxy mass for simulated galaxies. Black points represent galaxies from simulations without AGN feedback, and are connected to colored squares representing the corresponding galaxy with AGN feedback included. Color indicates the mass of the central black hole, increasing from blue to pink. The blue solid and black dashed lines represent estimates from models by other researchers. Galaxies with a black hole mass of at least 106 solar masses have noticeably lower amounts of mass in stars, as well as lower amounts of mass in the center of the galaxy. (Figure 3 from today’s paper.)

Figure 1 highlights that dwarf galaxies experiencing AGN feedback tend to have lower stellar masses than their counterparts with no feedback. This is particularly true for galaxies with AGNs between 1 and 10 million solar masses, which is comparable to the mass of the black hole at the center of the Milky Way.

Additionally, it’s worth noting that the AGN feedback can also affect the mass distribution of the galaxy, which is parametrized by the M200 variable shown on the horizontal axis in Figure 1. M200 describes the total amount of mass within a characteristic radius from the center of the galaxy, such that galaxies with higher values have more mass in the center. The author’s simulations show that AGN feedback can also reduce M200, indicating that the feedback is pushing mass from the center of the galaxy away from the center and towards the edges.

Images comparing dwarf galaxy morphologies between simulations run with and without AGN feedback. Halos are numbered according to the mass of the central black hole, with 0 being the most massive.
Figure 2: images comparing the appearance of dwarf galaxies run in simulations with and without AGN feedback. Halos are numbered according to the mass of the central black hole, with 0 being the most massive. Galaxies without AGN feedback (right plots in each pair) tend to show more structure and more stars than galaxies affected by AGN feedback. This is particularly prominent in Halos 0, 6, and 10. Note that this is not observed in all galaxies; see Halos 3 and 4 as counterexamples. (Figure 7 from today’s paper.)

Figure 2 shows several examples of corresponding galaxies between the feedback and no-feedback simulations. A handful of these dwarf galaxies display clear spiral structures in the no-feedback simulations, but this structure is weaker or absent in the simulations with AGN feedback included. This indicates that the AGN feedback is also disrupting the galactic structure, which is consistent with its effects on the mass distribution.

An AGN: It’ll Quench Ya

The physical interpretation behind the results shown above is well understood by astronomers who study galaxy formation and structure. Feedback from the AGN, typically in the form of hot gas, deposits energy into the interstellar medium (ISM), which hosts all of the gas for forming new stars. Star formation happens when this gas cools enough to condense and collapse into a star. However, the AGN feedback is heating up this gas, making it more difficult to form stars. This process is known as “quenching” star formation, and it has been observed in more massive galaxies as well.

In the dwarf galaxy simulations from today’s paper, AGN feedback quenches star formation by about half for galaxies with black holes that are at least a million solar masses. This AGN feedback also reduces the amount of matter in the center of the galaxy, which is consistent with previous work focused on resolving the “cuspy halo” problem. Additionally, AGN feedback can have an effect on the morphology (the shape and appearance) of dwarf galaxies, though there are no features that appear to be present for all dwarfs. The authors note that future work can test how the different versions of simulated AGN feedback can affect the results, as well as study less-massive dwarf galaxies than the ones presented here.

Astrobite edited by Tori Bonidie

Featured image credit: ESO/M. Kornmesser

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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