Big-Picture Research links Growth of Galaxies, Black Holes

Newswise — Astronomy could be described as a form of time travel. Astronomers use large telescopes to peer into periods of cosmic history to see what galaxies looked like before. While giant lenses offer snapshots of deep space and time, presenting billions of enormous collections of stars, gas and dust, scientists still don’t know if the growth of a single galaxy and that of its inner supermassive black hole occurred at the same time or one after the other.

This is where the research of University of California San Diego’s Alison Coil and her colleagues James Aird (University of Leicester, UK) and Antonis Georgakakis (National Observatory of Athens) comes into view. The scientists recently published research findings in the Monthly Notices of the Royal Astronomical Society to reveal how supermassive black holes are growing at the center of galaxies, and how that growth relates to the growth of galaxies themselves.

Coil, a professor in UC San Diego’s Department of Physics and at the Center for Astrophysics and Space Sciences (CASS), explained that the research indicates a close connection between the growth of star-forming galaxies and the growth of the black holes at their centers.

According to Coil and Aird, usually galactic black holes—regions of space with intense gravitational fields from which no matter or radiation can escape—are silent unseen giants. But when they are growing, accreting as an astrophysicist would say, they shine with light from the hot gas and dust that swirls around them, and are seen with telescopes across a range of colorful wavelengths. These lighted centers—sometimes brighter than the entire galaxy—are referred to as Active Galactic Nuclei (AGN).

“We find a correlation that indicates that both star formation and AGN have a common origin, likely being related to the overall amount of gas in such galaxies,” explained Coil, adding that the details of this relationship have previously been elusive due to the rarity of the periods of black hole growth.

In their research, Coil, Aird and Georgakakis set out to identify galaxies with AGN and figure out how rapidly both the galaxies and the AGN were growing. First, they examined a sample of four patches of sky using NASA’s Hubble Space Telescope and the European Southern Observatory’s VISTA telescope to identify galaxies within those patches. These appeared as the color red—the more distant the galaxy, the redder it appears (due to an effect known as “redshift”). Next, they picked out the galaxies with AGN using NASA’s Chandra X-ray imaging—the most sensitive X-ray technology available. By overlaying the data, the researchers identified which galaxies from within their sample had AGN and which did not. Consequently, they were able to quantify how the fraction of galaxies with an AGN depended on the star formation rate of the galaxy—how rapidly it was forming new stars.

“Essentially AGN, right at the centers of galaxies, will flicker on and off over millions of years—a short period of time for a galaxy, but far too long for us to observe directly,” explained Aird. “This flickering has made it difficult to link the overall properties of galaxies and their AGN, requiring this careful study of a very large sample of galaxies.”

The researchers also separated out galaxies with lower star formation rates that look different to the normal star-forming population and found a surprising result.

“When we separately considered galaxies with very low star formation rates we found a much higher fraction had AGN then we would have expected,” noted Aird. “These results show that a wider variety of mechanisms can bring matter into the centers of galaxies and fuel AGN—it’s not just related to the amount of gas in a galaxy.”

Aird described the research results as “big picture” scientific findings in that they address questions about the existence of the universe, why it looks the way it does and why the properties of galaxies may be connected to how they grow.

This research was supported by an STFC Ernest Rutherford Fellowship (grant code: ST/P004172/1), the THALES project (383549), jointly funded by the European Union and the Greek Government in the framework of the program “Education and lifelong learning.” The work is based in part on observations taken by the CANDELS MultiCycle Treasury Program and the 3D-HST Treasury Program (GO 12177 and 12328) with the NASA/ESA HST, operated by the Association of Universities for Research in Astronomy, Inc., (NASA contract NAS5-26555).