Cold quasars and rethinking the deaths of galaxies

New observations of “cold quasars” suggest that our idea of how galaxies die and the behaviour of supermassive black holes may have to be revised.

Allison Kirkpatrick, assistant professor of physics and astronomy at the University of Kansas, announced her discovery of “cold quasars” — galaxies featuring an abundance of cold gas that still can produce new stars despite having a quasar at the center — a breakthrough finding that overturns assumptions about the maturation of galaxies and may represent a phase of every galaxy’s lifecycle that was unknown until now.

A quasar — “quasi-stellar radio source” — is essentially a supermassive black hole in a feeding frenzy, as gas falling towards it forms an accretion disc before falling towards it.

The violent conditions in such discs and their extreme temperatures lead to the production of massive amounts of electromagnetic energy and luminosities that are often hundreds of times greater than that of a typical galaxy.

An optical blue quasar at a lookback time of 7 billion years (this is not a nearby galaxy). Normally, something like this would not have infrared emission. ( Dark Energy Camera Legacy Survey DR7/NOAO)

Kirkpatrick explains: “All the gas that is accreting on the black hole is being heated and giving off X-rays. The wavelength of light that you give off directly corresponds to how hot you are.

“For example, you and I give off infrared light. But something that’s giving off X-rays is one of the hottest things in the universe.”

She explains that this gas starts accreting onto the black hole and starts moving at relativistic speeds; creating a magnetic field around this it. This magnetic field can become twisted and in the same way that solar flares are created, this results in jets of material travelling up through these magnetic field lines eventually firing away from the black hole.

Kirkpatrick continues: “These jets essentially choke off the gas supply of the galaxy, so no more gas can fall on to the galaxy and form new stars. After a galaxy has stopped forming stars, we say it’s a passive dead galaxy.”

But in Kirkpatrick’s survey, about 10% of galaxies hosting an accreting supermassive black hole had a supply of cold gas remaining after entering this phase — and still made new stars.

Kirkpatrick, who revealed her research at a news briefing, entitled “A New Population of Cold Quasars,” held at the annual meeting of the American Astronomical Society in St. Louis, Missouri, Wednesday, June 12, remarks: “That in itself is surprising.

“This whole population is a whole bunch of different objects. Some of the galaxies have very obvious merger signatures; some of them look a lot like the Milky Way and have very obvious spiral arms. Some of them are very compact. From this diverse population, we then have a further 10% that is really unique and unexpected.”

This latter population are very compact, blue, luminous sources, resembling what we should expect a supermassive black hole to look after it has quenched all of the star-forming material in its host galaxy. Thus, evolving into a passive elliptical galaxy.

Kirkpatrick remarks: “Yet we have found a lot of cold gas in these as well. These are the population that I’m calling cold quasars.”

The KU astrophysicist suspects that these “cold quasars” in her survey represent a brief period yet to be recognized in the end-phases of a galaxy’s lifespan.

She continues: “These galaxies are rare because they’re in a transition phase — we’ve caught them right before star formation in the galaxy is quenched and this transition period should be very short.”

Kirkpatrick and her team first identified the objects of interest in an area of the Sloan Digital Sky Survey — the most detailed digital map of the universe available — dubbed Stripe 82.

They then re-examined this area in the x-ray band of the electromagnetic spectrum with the XMM Newton telescope. Kirkpatrick says: “X-rays are the key signature of growing black holes. From there, we surveyed it with the Herschel Space Telescope, a far infrared telescope, which can detect dust and gas in the host galaxy. We selected the galaxies that we could find in both the x-ray and in the infrared.”

She says her findings give scientists new understanding and detail of how the quenching of star formation in galaxies proceeds — in the process, overturning presumptions about quasars.

This is the dust emission of the same blue-quasar galaxy. It is surprisingly bright — in fact, it’s one of the brightest objects in the field, indicating a lot of dust. Due to the resolution of the telescope, we cannot see what that dust actually looks like. ( Herschel/ESA)

Kirkpatrick elaborates: “We already knew quasars go through a dust-obscured phase. We knew they go through a heavily shrouded phase where dust is surrounding the supermassive black hole. We call that the red quasar phase.

“But now, we’ve found this unique transition regime that we didn’t know before. Before, if you told someone you had found a luminous quasar that had a blue optical colour — but it still had a lot of dust and gas in it, and a lot of star formation — people would say, ‘No, that’s not the way these things should look.’”

In the future, Kirkpatrick hopes to determine if this cold quasar phase happens to a specific class of galaxies or every galaxy.

She says: “We thought the way these things proceed was you have a growing black hole, it’s enshrouded by dust and gas, it begins to blow that material out. Then it becomes a luminous blue object.

“We assumed when it blew out its own gas, it would blow out its host gas as well. But it seems with these objects, that’s not the case. These have blown out their own dust — so we see it as a blue object — but they haven’t yet blown out all of the dust and gas in the host galaxies.”

The research suggests that this is a transition phase — Kirkpatrick hints at a 10 million years span, which, on universal timescales, is a very short period— thus making it hard for astronomers to catch.

Kirkpatrick concludes: “We’re doing what we call a blind survey to find objects we weren’t looking for. And by finding these objects, yes, it could imply that this happens to every galaxy.”

Original research funded by NASA, National Science Foundation. Presented at 234th meeting of the American Astronomical Society