Supermassive black holes millions to billions times the mass of our Sun lie at the heart of most, maybe all large galaxies. Some of these power brilliantly luminous, rapidly growing objects called active galactic nuclei that gather and condense enormous quantities of dust, gas and stars.
Because astronomers had seen these objects primarily in the oldest, most massive galaxies that glow with the red light of aging stars, many thought active galactic nuclei might help to bring an end to the formation of new stars, though the evidence was always circumstantial.
That idea has now been overturned by a new survey of the sky that found active galactic nuclei in all kinds and sizes of galaxies, including young, blue, star-making factories.
“The misconception was simply due to observational biases in the data,” said Alison Coil, assistant professor of physics at the University of California, San Diego and an author of the new report, which will be published in The Astrophysical Journal.
“Before this study, people found active galactic nuclei predominantly at the centers of the most massive galaxies, which are also the oldest and are making no new stars,” said James Aird, a postdoc at the University of California, San Diego’s Center for Astrophysics and Space Sciences, who led the study.
Black holes, such as those at the centers of active galactic nuclei, can’t be observed directly as not even light escapes their gravitational field. But as material swirls toward the event horizon, before it’s sucked into the void, it releases intense radiation across the electromagnetic spectrum, including visible light. Of these, X-rays are often the brightest as they can penetrate the dust and gas that sometimes obscures other wavelengths.
“When we take into account variations in the strength of the X-ray signal, which can be relatively weak even from extremely fast-growing black holes, we find them over a whole range of galaxies,” Aird said
He searched the sky for X-rays from active galactic nuclei using two orbiting telescopes, the XMM-Newton and the Chandra X-ray Observatory, and compared those signals to a large-scale survey of about 100,000 galaxies that mapped their colors and distances.
Coil led that survey, called PRIMUS, along with colleagues now at New York University and the Harvard College Observatory. Using the twin Magellan telescopes at Las Campanas Observatory in Chile, they detected the faint light of faraway galaxies.
They measured both the color of each galaxy and how much the spectrum of that light had shifted as the galaxies receded in our expanding universe – an estimate of their distance from Earth. Because distances in space reach back in time, they’ve captured nearly two-thirds of the history of the universe in particular segments of the sky.
Galaxies can be distinguished by the color of their light. Younger galaxies glow with the bluish light of young stars. As starmaking ceases, and stars burn through their fuel, the color of their light shifts toward red.
In a sample of about 25,000 of the galaxies from the PRIMUS survey, Aird found 264 X-ray signals emanating from galaxies of every kind: massive and smaller, old elliptical red galaxies and younger blue spirals. They’re everywhere.
So as suspects in the quenching of star formation, active galactic nuclei have been exonerated. And because the astronomers saw similar signals stretching far back into time, they conclude that the physical processes that trigger and fuel active galactic nuclei haven’t changed much in the last half of the universe’s existence.
Yet starmaking has ceased in many galaxies, probably when they ran out of gas, though it’s not clear how that happens. The interstellar gas could all be used up, turned into stars, but Coil studies another possibility: fierce galactic winds that have been seen blowing gas and dust from so-called starburst galaxies.
The source of those winds, and their influence on the evolution of galaxies, is one of Coil’s main areas of current investigation.
Alison Coil is an Alfred P. Sloan Foundation Fellow. The National Science Foundation and NASA provided funding for the PRIMUS survey.
Dr. Coil will carry out three complementary research projects to tackle several key outstanding questions in galaxy evolution. Her goal is to uncover the physical processes behind the dramatic evolution observed in galaxies and AGN in the latter half of cosmic history, focusing specifically on the build-up of stellar mass in galaxies, the role of environment on AGN accretion, and the prevalence and importance of outflowing galactic winds. Her work will primarily use data from the PRIsm MUlti-object Survey (PRIMUS), the largest faint galaxy spectroscopic redshift survey taken to date, of which she is a co-I. She and her team will lead projects to measure the evolution in the galaxy stellar mass function, the clustering properties of AGN, and study the physical properties of outflowing galactic winds in distant galaxies.
The broader impacts of this proposal range from increasing student enrollment in the general astrophysics survey course at UCSD to increasing the representation of women in physics at UCSD and beyond. Dr. Coil has created a multi-tiered approach targeting women at several levels in the academic pipeline, focusing on middle school and graduate students, as well as postdoctoral researchers. She will expand the Women in Physics group she recently created at UCSD to use mentoring to both support and enhance the experiences of current graduate student and postdoctoral women in the physics department, as well as to recruit more women to the program. She will also engage female graduate students in outreach aimed at middle school girls, through the use of hands-on physics demos. This proposal will also train and mentor graduate students and a postdoctoral scholar in the research activities described above.
"What is unique about this particular galaxy is that it is forming stars so rapidly with such a tiny supply of gas," said Aleksandar Diamond-Stanic, a fellow at the University of California's Southern California Center for Galaxy Evolution who helped make the discovery. A team of nine astrophysicists recently reported the finding in Astrophysical Journal Letters.
The team of astronomers estimated the amount of gas in the galaxy using the IRAM Plateau de Bure Interferometer, a telescope in the French Alps that detects a light signal associated with hydrogen gas, the fuel of stars. Images from the Hubble Space Telescope show gas concentrated in a zone just a few hundred light years across, yet that gas is condensing and igniting new stars at a rate hundreds of times that of our own Milky Way galaxy.
The distant galaxy, 6 billion light years away, initially popped out of an image captured by a satellite-based NASA instrument called WISE, for Wide-field Infrared Survey Explorer. The image revealed infrared light, an indication of star formation, pouring out of the galaxy.
That rate of star formation combined with the estimate of available fuel indicates an efficiency close to the theoretical maximum, called the Eddington limit.
"This galaxy is like a highly tuned sports car, converting gas to stars at the most efficient rate thought to be possible," said Jim Geach, an astrophysicist at McGill University who led the study.
"We've caught it just before it runs out of gas," adds Diamond-Stanic, a member the research group led by Alison Coil, a physics professor at UC San Diego who also co-authored the report. This rate of star birth is so ferocious that most of the galaxy's gas will be gone in just a few tens of millions of years, a brief episode in the course of its evolution.
That's why they think no galaxy quite like this one has ever been seen before. Once star formation abates, the team expects the galaxy to mature into a steadier state: an ordinary reddish, elliptical galaxy.