Active Galactic Nuclei – Revealing Black Hole Growth in Galaxies and the Structure of the Universe

Active galactic nuclei (AGN) are brilliant and enigmatic markers distributed throughout the Universe. AGN host the most powerful energy sources in the Universe and are some of the most distant cosmic objects observed. The central engine of an active galaxy can produce more than 100 times the energy of a large spiral galaxy such as the Milky Way across the spectrum of wavelengths from radio waves to X-rays and sometimes the highest-energy gamma rays. They do have, however, a wide luminosity range of greater than 10⁷. Their distribution can trace large-scale structures in the Universe, revealing not only how galaxies come to be but also how the Universe is organized. LSST observations will make significant contributions to the information we have about these remarkable objects.

Ordinary galaxies such as the Milky Way harbor supermassive black holes (SMBH) at their centers; but while the black hole in a normal galaxy is “starved,” active galaxies are well fed with gas and stars. Their central core emits much more energy than can be explained by their stellar content, and we observe them as AGN. Some can be optically variable over hours (e.g., blazars) or weeks (e.g., quasars) indicating a small accretion disk (around the size of our Solar System in many cases) with material heated to emit prodigious amounts of energy, which in some cases is ejected in jets that are likely aligned with the spin axis of the central black hole. The Milky Way’s central black hole was probably active in the distant past but has now largely settled into hibernation, an example of how galaxies can switch from being active to being normal and vice versa.

SMBH play critical roles in galaxy evolution: bulge mass and velocity dispersion correlations with SMBH mass suggest a close link between the build-up of mass in galaxies and their central SMBH. Researchers believe feedback exists between the central black hole and star formation in galaxies: as the black hole swallows gas and stars, the process generates tremendous luminosity or photon power, heating gas in the vicinity. Radiation pressure generates winds that blow outward clearing out gas and thus decreasing star formation and SMBH feeding. The emission coming from these AGN is a broadband phenomenon, so observations across many wavelengths probe different aspects of the physics of the central engine and in combination can answer the questions about the SMBH’s nature and evolution.

LSST will be instrumental in producing data in the .3 to 1.1 micron range. It will provide a well-defined sample of at least 10 million optically selected AGN – at least an order of magnitude more than we now know. LSST will allow detection of AGN out to redshifts of approximately z=7, observing objects in the first billion years of cosmic time. The exponential growth in the detection of quasars over the last few decades will continue as LSST increases the sample size to likely over 10⁷ quasars. LSST will observe AGN across many observation epochs, through a range of timescales, with multi-color coverage and high photometric accuracy. Over the 10-year survey many visits will allow examination of the variability of AGNs and discovery of rare events such as transient SMBH fueling. LSST will be able to resolve close companion galaxies to AGN letting scientists study how mergers drive quasar activity. LSST will monitor gravitationally lensed AGN and its cadence is well suited to map out wavelength-dependent microlensing light curves of AGN to probe AGN emission regions.

Combining LSST and other surveys’ data to execute multi-wavelength investigations will probe the mysteries in more depth. For example, LSST will enable excellent optical follow-up for AGN in thousands of Chandra and XMM-Newton fields. Scientists will use radio survey data of LSST AGN to quantify in great detail how radio properties depend on luminosity and redshift across a wide part of the luminosity-redshift parameter space. Augmenting LSST photometry with multiwavelength data will make possible unprecedented temporal investigations. For example, AGN that exhibit flares or other unusual temporal behavior in LSST data will trigger alerts for multiwavelength follow-up at other relevant wavelengths.

Understanding AGN requires looking further into why their emission is variable on different timescales. Scientists have proposed several reasons for AGN emission variability including accretion disk instabilities, changes in accretion rate, evolution of relativistic jets, and line-of-sight absorption changes. The observed variability depends on luminosity, wavelength, timescale, and the presence of strong radio jets but correlating the observations with proposed physical causes is complex. The data from LSST will help to unravel the different mechanisms that produce variability on different timescales. Because LSST will expand existing databases enormously, it will improve greatly the categorization of the range and kinds of AGN variability and consequently, understanding of the underlying physics. LSST’s massive number of observations will allow scientists to track variability and to identify rare but revealing events in numbers sufficient for modeling.

LSST will discover SMBH tidal disruption events (when the SMBH rips apart a star), inspiraling of binary supermassive black holes, and perhaps mergers of binary supermassive black holes. LSST should detect about 100-200 tidal disruption events per year enabling scientists to measure event rates as a function of redshift, host galaxy type, and level of nuclear activity and thereby determine what effects these tidal disruptions have on the luminosity function of moderate luminosity active nuclei. Researchers believe that SMBH mergers are components of SMBH growth and galaxy evolution. It is difficult to find SMBH mergers but if the proposed LISA gravitational wave observatory data are combined with LSST data, identification of emission from these events could provide critical information to study the physics of accretion during SMBH mergers and for measurement of redshifts and cosmological parameters.

LSST will observe approximately 4,000 luminous AGN that are gravitationally lensed into multiple images, more than a 10-fold increase over current observations. These will allow scientists to study the wavelength dependent variability of microlensing events. The large number of expected microlensing events at z=1-4 will allow scientists to search for evolution of AGN structure across redshift range, luminosity, and Eddington ratio (intrinsic luminosity over Eddington luminosity, a measure of observed versus maximum potential brightness).

On an even larger scale, studying AGN clustering can provide information about the galaxies that host AGN. Looking at the relationship between AGN clustering and that of ordinary galaxies can indicate how they are related. Examining this clustering will provide information about underlying dark matter clustering in the Universe. The enormous number of AGN found with LSST will cover a very large range of luminosity in each redshift interval. With these data, researchers will be able to determine the clustering and, therefore, bias and host galaxy halo mass over a large range of cosmic time and black hole accretion rate. These studies will reveal more about quasar activity – how merger activity can drive it – and their surrounding halos.

AGN provide spectacular examples of powerful galactic processes in the Universe. They are windows into the physics of supermassive black holes, galaxy evolution and the structure of the Universe. LSST’s contribution to observations, especially when coupled with those of other observatories and surveys, promises to reveal underlying mysteries at galaxy centers and throughout the Universe over time.

Article written by Anna H. Spitz and Niel Brandt