A Recipe for Growing Black Holes in Small Galaxies

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Title: The Active Fraction of Massive Black Holes in Dwarf Galaxies
Authors: Fabio Pacucci, Mar Mezcua, and John A. Regan
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Accepted to ApJ

Dwarf galaxies, with masses less than 10 billion times the mass of the Sun (M), host massive black holes. Unlike the central black holes in more massive galaxies, those in dwarf galaxies may not have quite become supermassive. This presents an opportunity to study analogs of supermassive black hole “seeds,” giving us a glimpse at the growth of supermassive black holes — a process that is not quite understood yet.

Active galactic nuclei (AGN) get their name from the active accretion of the supermassive black holes that power them, but not all supermassive black holes are “active.” Some, like the one in our own galaxy, are inactive. What causes AGN to shut off or turn on is still a mystery, but there’s no denying their activity makes them easier to observe. In dwarf galaxies, this is especially useful since their central black holes are usually smaller and therefore harder to detect, so many of the observations of massive black holes in dwarf galaxies have been of low-mass AGN. This means that current observations aiming to obtain the occupation fraction — the fraction of dwarf galaxies that have massive black holes, active or inactive — could actually just be measuring the fraction of dwarfs with active massive black holes.

The authors of today’s paper developed a theoretical model to predict the fraction of dwarf galaxies that contain active massive black holes (103 M < M < 107 M), based on galaxy properties and observational constraints. In the model, the fraction of active black holes depends on the number density of the gas in the galaxy and the available angular momentum at its center. However, these two parameters are not necessarily available from observations, so the authors use proxies instead. The stellar mass (the mass of the galaxy that comes from stars) is used as a proxy for the number density of the gas. Instead of angular momentum, the authors use rotational support, a quantity determined from the rotational velocity and velocity dispersion of gas and stars. In the analysis, a black hole is considered active if it meets the criteria for efficient accretion.

Constraining a Physical Model

Fraction of dwarf galaxies with a massive black hole detected in the X-ray versus the stellar mass of the galaxy

Figure 1: The y-axis shows the fraction of dwarf galaxies that have a massive black hole detected in the X-ray. The x-axis shows the mass of stars in the dwarf galaxy. The points with error bars show data from observations in previous works. The blue line shows the expected X-ray detected fraction as a function of stellar mass based on the model developed in this paper. [Pacucci, Mezcua & Regan 2021]

Accreting black holes emit radiation across the electromagnetic spectrum, so they have been detected in dwarf galaxies at many different wavelengths. Some of the most extensive studies have been conducted at X-ray wavelengths, encompassing both wide surveys (which cover lots of targets) and deep observations (which see faint objects). These data provide valuable constraints on the authors’ theoretical model. The authors use the X-ray observations to constrain the emission from star formation and whether the AGN are Compton-thick, or surrounded by enough hydrogen gas to absorb most of the X-rays emitted. These factors affect the detectability of the AGN in observations by outshining or obscuring (respectively) the AGN X-ray emission. By adjusting the model parameters, the authors match their calculated fraction of active massive black holes with detectable X-ray emission to what we actually observe. Their results are shown in Figure 1.

After calibrating their model to match observations in the X-ray, the authors then remove the detectability constraints to calculate the fraction of all active massive black holes, not just those that are detectable in X-rays. The results are shown in Figure 2, along with results from simulations and semi-analytical models. The model presented in today’s paper predicts the active fraction of black holes in dwarf galaxies, since that is more likely what is being measured by current observations, rather than the total occupation fraction computed by previous simulations.

Fraction of dwarf galaxies with a massive black hole vs stellar mass of the galaxy.

Figure 2: Fraction of dwarf galaxies with a massive black hole vs. stellar mass of the galaxy. The blue lines show the results of this paper, the active fraction of massive black holes as a function of mass. The solid and dotted blue lines differ in metallicity of the galaxy. The rest of the curves and colored regions show the occupation fraction of massive black holes in dwarf galaxies from observations in other work. [Pacucci, Mezcua & Regan 2021]

The range of active fractions estimated with this model is 5–22%, which is lower than the occupation fractions from other work. The authors note that this is expected, based on the definitions of the two fractions. Assuming not all massive black holes are active, the active fraction should be lower than the overall fraction of massive black holes found in dwarf galaxies.

The authors explore the effects of the galaxy’s metallicity on the predicted active fraction. Metallicity refers to the types of elements that are present in the gas: if you have mostly hydrogen and only small amounts of elements more massive than helium — what astronomers call metals — you have low metallicity. Lower-metallicity gas leads to more active star formation, which produces a lot of X-rays that can wash out the X-ray emission from the active massive black hole. Accounting for the effects of low metallicity, the calculated active fraction can be higher for this model (blue dotted line in Figure 2) — up to 30% for the most massive dwarf galaxies.

Dwarf galaxies allow us to study possible seeds of supermassive black hole growth, which is necessary to understand black hole growth and galaxy evolution. One way to check if our models of black hole growth are correct is to see if the predicted fraction of active massive black holes in galaxies matches observations. The authors of today’s paper developed a model that predicts the fraction of massive black holes in dwarf galaxies that are active, based on galaxy properties, and compare it to observations. Models like this could be useful to compare with results from upcoming observatories like JWST and Athena, among others, which could actually observe supermassive black hole seeds in the early universe.

Original astrobite edited by Jamie Sullivan.

About the author, Gloria Fonseca Alvarez:

I’m a fourth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole spin from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.