Suspects in the quenching of star formation exonerated

Active galactic nuclei (AGN) – produced by matter swept violently into the vortex around a supermassive black hole that may have billions of times as much mass as our Sun – can put on some of the most spectacular fireworks in the universe, over periods as long as 100 million years.

Astrophysicists have sometimes speculated that AGN may be so energetically active that they diminish or even extinguish formation of new stars in the host galaxy.

Recently published research, making use of the PRIMUS faint galaxy spectroscopic redshift survey, suggests that the speculations are wrong, and that AGN can be found even in galaxies in which very active star formation is occurring. The research gives some answers to the larger question of what special characteristics, if any, the host galaxies of AGN may have.

Suspects in the quenching of star formation exonerated

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.

The research involved a search for AGN that was done in the X-ray portion of the spectrum, using the XMM-Newton and the Chandra satellite telescopes. For AGN that were found, researchers examined spectroscopic data for their host galaxies, obtained in the PRIMUS survey of 120,000 faint galaxies out to redshift z=1 (almost 8 billion light years distant). This data provided the color and distance (and hence approximate age) of the galaxies. Galaxies in which stars are rapidly forming are more blue, while galaxies consisting mainly of older stars are more red. (Because younger stars tend to be blue while older ones are red.) The most distant galaxies in the survey, at z=1, are seen as they were less than about 5 billions after they formed.

In a subset of 25,000 galaxies of the survey, there were 264 X-ray signals indicative of galaxies containing an AGN. The result was that AGN were found in all types of galaxies: large and small, ellipticals and spirals, red and blue. There was no evidence that the presence of AGN correlated with star-forming activity. And this was true as much for distant as for nearby galaxies, even though the most distant (hence younger) galaxies tend to have the highest rates of star formation. AGN are also more frequent among the younger, more distant galaxies.

This isn’t really all that surprising, since both star formation and AGN activity depend on the presence in the galaxies of large quantities of interstellar gas that hasn’t yet condensed into stars. Galaxies do tend to use up the available uncondensed gas as they age. This may be simply because the gas all winds up in stars, or because large numbers of very hot young stars themselves drive a lot of gas out of galaxies.

Further reading:

PRIMUS: The dependence of AGN accretion on host stellar mass and color

The PRIsm MUlti-Object Survey (PRIMUS) I: Survey Overview and Characteristics

PRIMUS: PRIsm MUlti-object Survey


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