Quasars quash star formation in Active Galactic Nuclei (Forwarded)

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Paul Westoby
Astrophysics Research Institute
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Ref.: PN 08/29 (NAM 20)

Quasars quash star formation in Active Galactic Nuclei

An ambitious study of active and inactive galaxies has given new insights
into the complex interaction between super-massive black holes at the
heart of Active Galactic Nuclei (AGN) and star formation in the
surrounding galaxy. Results will be presented in a talk by Paul Westoby
on Friday 4th April at the RAS National Astronomy Meeting in Belfast.

Along with colleagues, Carole Mundell and Ivan Baldry from the
Astrophysics Research Institute of Liverpool John Moores University,
Westoby studied the properties of light from 360,000 galaxies in the local
Universe to understand the relationship between accreting black holes, the
birth of stars in galaxy centres and the evolution of the galaxies as a
whole.

The study finds that gas ejected during the quasar stage of AGN snuffs out
star formation, leaving the host galaxies to evolve passively. The study
also reveals a strong link between galaxy mergers and the formation of
super-massive black holes in AGN, but shows that if the environment
becomes too crowded with galaxies, then the likelihood of firing up a
supermassive black hole becomes suppressed.

Scientists believe that all AGN go through a quasar phase, where the
radiation emitted from the growing accretion disc around the central black
hole becomes so bright that it outshines its entire host galaxy. Today,
most massive galaxies are thought to contain a dormant super-massive black
hole at their heart, a legacy of this earlier phase of powerful quasar
activity, but for reasons unknown, some of these local black holes have
been reignited.

The Liverpool team concentrated on these local AGN, which can be studied
in more detail than their more distant quasar cousins, and, by comparing
the properties of a large number of galaxies, the team addressed a key
question: do galaxies that host AGN represent an adolescent or transition
phase of galaxy evolution?

"The starlight from the host galaxy can tell us much about how the galaxy
has evolved," said Westoby. "Galaxies can be grouped into two simple
colour families: the blue sequence, which are young, hotbeds of
star-formation and the red sequence, which are massive, cool and passively
evolving." Westoby continued, "Scientists have thought for some time that
AGN host galaxies might be a stepping stone between the two families and
therefore represent a critical point in the lifetime of a galaxy, but our
study has been able to rule this out."

Instead the AGNs identified by the team lay in galaxies that showed a
clear overlap with red sequence galaxies. This suggests that the
star-forming days for AGN host galaxies have a distinct cut-off point and
that the post-quasar local AGNs are no longer generating new stars. This
conclusion is reinforced by the team's findings that the majority of local
AGNs are linked with "classical bulges", round balls of stars formed
during violent mergers of gas-rich galaxies early on in the Universe's
history, rather than "pseudo bulges", disc-only galaxies that have not
undergone a major merger since their formation. This implies that the
formation of the super-massive black hole that drives the AGN is linked to
the evolution of the bulge, rather than the galaxy as a whole.

Finally, the team identified an intriguing population of galaxies that
have an active population of young stars together with an actively
accreting black hole, so-called composite galaxies. These masquerade as a
transition population, and lie in the region predicted for galaxies
experiencing AGN feedback -- the process by which material ejected by the
AGN has a direct impact on the evolution of the surrounding galaxy.
However, Westoby and colleagues find feedback an unlikely explanation for
the observed properties of these galaxies and suggest that feedback may
only be important during the quasar phase and not in weaker, nearby AGN.

IMAGES

Images can be found at:
http://cas.sdss.org/astro/en/tools/c....001&dec=1.891
http://cas.sdss.org/astro/en/tools/c...546&dec=-0.222

Figure Caption: "Multicolour SDSS optical images of NGC5806 and NGC5750,
nearby spiral galaxies with active nuclei similar to those being studied
by Westoby and his collaborators. Image credit: The Sloan Digital Sky
Survey."

NOTES FOR EDITORS

ACTIVE GALACTIC NUCLEI (AGN)

Active Galactic Nuclei are now recognised to be integral to galaxy
formation and evolution.

Examples of AGN include quasars in the early Universe and nearby Seyfert
galaxies, both thought to be powered by the release of gravitational
potential energy from material being accreted by a central super-massive
black hole.

The era of greatest quasar activity seems to coincide with turbulent
dynamics at the epoch of galaxy formation in the young, gas-rich Universe.
The black holes believed to be at the heart of most galaxies are then a
legacy of this violent era.

The sample was selected from the galaxy catalogue of Sloan Digital Sky
Survey (SDSS) Fourth Data Release

SLOAN DIGITAL SKY SURVEY (SDSS)

The Sloan Digital Sky Survey, when completed, will provide detailed
optical images covering more than a quarter of the sky, and a
3-dimensional map of about a million galaxies and quasars.

The galaxy catalogue of the fourth data release was issued in June 2005
and contains data on 520 738 galaxies over 6670 square degrees.

Related publication:
"Are Galaxies with AGN a Transition Population", Westoby, Mundell &
Baldry, 2007, Monthly Notices of the Royal Astronomical Society, 382, 1541