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Are Supermassive Black Holes Galactic Regulators?

Artist’s impression of the thick ring of dust that can obscure the energetic processes that occur near the supermassive black hole of an active galactic nuclei. [NASA/SOFIA/Lynette Cook]

This article was originally published in the Summer 2019 (vol. 48, no. 3) issue of Mercury magazine, an ASP members-only quarterly publication.

At the core of every massive galaxy resides a supermassive black hole (SMBH). The majority of these black holes, including the one at the heart of our own Milky Way galaxy, called Sagittarius A*, are relatively dormant cosmic entities. However, some of these objects have voracious appetites. For these select few, which are known as active galactic nuclei (AGN), an abundant supply of gas, dust, and interstellar matter rapidly funnel onto their central cores. As this accreting matter swirls around the heart of the host galaxy, powerful electromagnetic radiation is generated, which may appear as brilliant jets emanating perpendicular to the galaxy’s plane of rotation.

There are outstanding questions regarding the physics of AGN and astronomers are trying to understand how this energetic accretion disk, and its associated outflow of radiation, influence the overall properties of a host galaxy. To tackle this unresolved phenomenon, Robert Maiolino at the Kavli Institute for Cosmology (UK) and Giovanni Cresci at The National Institute for Astrophysics (Italy) considered both theoretical and observational results to elucidate the dynamics of AGN and galactic co-evolution.

They report on X-ray observations which directly map outflows on parsec-scales near the accreting SMBH. Outflows confirmed in this region fiercely emerge from the galaxy’s central engine at velocities near 10 percent the speed of light! Comparatively, as the outflows propagate out to kiloparsec-scales, their presence is inferred through interactions with neighboring matter. For example, massive, high velocity outflows may strike nearby gas and strip them of some of their electrons. These highly ionized species of gas can then be measured in the spectra of galaxies that host an AGN.

AGN models consider the dynamics of these energetic environments and attempt to predict how star formation is affected by this radiation. Several recent models yield results that suggest these outflows can halt star formation, whereby galactic matter necessary to form stars is heated and ultimately removed in the path of the outflows.

However, the statistical observational evidence to support these results is unfortunately sparse. In response, astronomers consider a relatively new technology—Integral Field Unit (IFU) spectroscopy. This technology operates by utilizing a bundle of optical fibers that are assembled into a precise array to conduct observations. These observations can then generate spatially resolved spectra across an entire galaxy. In fact, many observatories, such as the Apache Point Observatory in Sunspot, New Mexico, rely on IFU spectroscopy to determine the local impacts of AGN outflows across a galaxy.

Utilizing this method, astronomers conducted near-infrared observations of a sample of AGN between a redshift range of z ~ 1-3, and determined a spatial anti-correlation between fast outflowing gas (traced by OIII emission) and the presence of star formation (traced by Hα emission) in the central regions of the galaxies in the survey, supporting the results presented in the simulations.

The findings also suggest that the outflows do not necessarily cause a galaxy-wide shutdown of star formation, and that the local suppression of star formation may not occur simultaneously with the outflows. Rather, the researchers propose that the presence of outflows and subsequent suppression of star formation may occur after a delay, on the timescale of a billion years. They reason that the energetic outflows heat the nearby gas to such an extent that the cooling necessary for star formation becomes inhibited as the galaxy evolves. This mechanism is known as “negative feedback” and operates to limit the total mass of a galaxy.

In contrast, some theorists pose a “positive feedback” mechanism, whereby fast AGN outflows may actually stimulate star formation through the compression of molecular clouds (as traced by CO observations at millimeter wavelengths) in the galactic disk, or even directly in the outflows! The basis of this assertion stems from the observed correlation between AGN luminosity, which traces AGN activity, and nuclear star formation rate, which is measured by the density of molecular gas in the central region of a galaxy. While evidence has been proposed for this relation in a z ~ 1.6 quasar and a local AGN (NGC 5643), there is still a lack of statistically robust observational data to support the process.

Ultimately, higher spectral and spatial resolution observations, more accurate models, and a richer sample of AGN at a uniform redshift are required to determine the true nature of AGN feedback—either positive, negative, or a combination of the two. Researchers are eager to uncover this cosmic mystery and the topic continues to be the subject of much debate in the astronomical community!

James Negus is currently pursuing his Ph.D. in astrophysics at the University of Colorado Boulder, and also serves as the ASP's Junior Board Fellow. He analyzes Active Galactic Nuclei and their role in galactic evolution utilizing the Sloan Digital Sky Survey. Read more articles by James.