The massive black holes at the centers of galaxies capture the imagination of astronomers and the broader public alike.  This was evident in the recent excitement of the first-ever images of supermassive black holes in both the galaxy M87 and our own Milky Way.  Astronomers have known for a while that every massive galaxy like our Milky Way seems to have a central black hole.  What has only been more recently discovered is that smaller galaxies, known as dwarf galaxies, also contain these giant black holes.  These galaxies have 10 to 100 times less mass in stars than our own Galaxy, and their black holes are also at least 10 to 100 times less massive than our own black hole.  In more massive galaxies the black holes can be very "active," emitting lots of energy in x-rays, when gas falls into the black hole.  In the smaller black holes in dwarf galaxies, much less energy is emitted, making them hard to detect.  There is lots of evidence that the energy emitted by massive black holes in massive galaxies has shaped the galaxy and how many stars form.  What is not known is if the smaller black holes in dwarf galaxies can have these effects, too.

This project tried to answer that question by simulating hundreds of dwarf galaxies with and without black holes.  The simulations start soon after the Big Bang, and black holes form in the first billion years of the (simulated) Universe.  They are allowed to grow until the present day, i.e., over 13 billion years of galaxy evolution is simulated for study.  

One of the key goals of this project was to understand how the presence of black holes in dwarf galaxies changes them.  We discovered that galaxies that are more compact in the early Universe are able to grow bigger black holes.  However, the energy emitted by the bigger black holes as they grow (by consuming gas) eventually causes star formation to become harder, because the black holes destroy the cold, dense gas that stars are born in. Those galaxies that started more compact had become the least compact in stars by the present-day Universe. 

In some cases, the massive black holes in dwarf galaxies released so much energy that star formation stopped altogether.  Astronomers refer to this as "quenching."  We compared our quenched galaxies to observational data, and found we have more than expected.  This is an important clue that our modeling of black holes may still need updates.  Nevertheless, we are able to make a strong conclusion: if a dwarf galaxy is found to be far from other galaxies and is quenched, energy from a black hole is responsible for shutting off the star formation.

We also compared the energy emitted in x-rays by our simulated black holes to observed black holes.  We found good agreement, but we also found that the majority of the black holes (75%) in our simulated dwarf galaxies had such low x-ray emission that they would be undetectable currently.  This allows us to make predictions for future observations, shaping future x-ray telescope design and science goals.  In this way, the research advances the scientific leadership of the United States and safeguards our investment in future telescopes. 

Finally, this project also supported education and training of future scientists and engineers across all academic levels, with the overall goal to diversify the US technical workforce. First, a graduate student lead all of the science discussed above, but this project also supported a first-year undergraduate seminar, with 17 first-year Rutgers undergraduates participating in Fall 2020, the majority of whom were students of color.  The students were introduced to coding in python, participated in cohort building events (online game nights), conducted small research projects in teams of two, and presented their research to the department on the final day of class. The seminar allowed the students to develop a mentoring relationships with members of the Physics & Astronomy department, and introduced the students to basic research tools to get them involved in original research. Additionally, a Rutgers graduate student served as a secondary instructor for the seminar, gaining experience in mentoring and teaching.  The research projects were representative of a wide range of physics (astrophysics, nuclear physics, biophysics, condensed matter physics), and projects were contributed/mentored by grad students, postdocs, and faculty across the different groups. Thus, grad students and postdocs also gained mentoring experience.  Mentoring and participation in research have been shown to increase the retention of underrepresented students in science.

Last Modified: 08/30/2022
Modified by: Alyson Brooks