Quasars quash star formation in Active Galactic Nuclei (Forwarded)

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CONTACTS

Paul Westoby
Astrophysics Research Institute
Liverpool John Moores University
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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