Quasars Help Trace Ancestors of Giant Elliptical Galaxies

By using the Hubble Space Telescope (HST) as a ‘time machine’, astronomers
from the Universities of Edinburgh and Oxford in the UK and the Space
Telescope Science Institute (STScI) in Baltimore in the USA have been able
to trace back the history of massive elliptical galaxies. They have found
that galaxies of this kind, which still exist today, were already luminous
families of stars about 10 billion years ago when the universe was only one
third its present age. Their latest observations suggest that these galaxies
were still in the process of formation at that time, converting large
quantities of gas into brilliant blue stars. Studying quasars, and the
galaxies that host them, has been the key to this piece of cosmic
archaeology. The results will be presented at the National Astronomy
Meeting in Bristol on Tuesday 9 April by Dr Marek Kukula of Edinburgh
University.

Quasars are amongst the most luminous objects known and very strong evidence
suggests that a quasar is powered by material being sucked onto a
supermassive black hole at the centre of a galaxy. Even forty years after
their discovery, these objects seem extraordinary: less than one light-day
across yet outshining by a thousand times or more an entire surrounding
galaxy spanning thousands of light years and consisting of tens of billions
of stars.

In the present-day universe quasars are extremely rare but they were much
more common at earlier epochs. Their numbers peak between redshifts of 2 and
3, when the universe was roughly a third of its present age. However,
astronomers now know that dormant black holes of the necessary mass to power
quasars (roughly a billion times the mass of the Sun) are found in the
centres of many nearby massive elliptical galaxies. Such massive black holes
have been found nowhere else in the local universe and this suggests that
the quasars seen at high redshifts are in the ancestors of today’s massive
ellipticals. For the past 5 years, the Edinburgh/Oxford/STScI team has been
using the HST to study galaxies hosting quasars at a range of distances from
the local universe out to a redshift of about 2, corresponding to a distance
of around 10 billion light years.

“The extraordinary luminosity of quasars poses a particular problem when we
wish to observe the galaxies in which they lie,” says Marek Kukula. “The
central quasar nucleus is often so bright that it completely swamps the
starlight from the surrounding galaxy and for many years it proved
impossible to obtain reliable information about quasar hosts. HST has
changed all that, allowing us to separate the halo of starlight from the
glare of the central quasar.”

The team started by observing some of the nearest quasars, at redshifts of
around 0.2, and found, that with very few exceptions, their host galaxies
are massive, luminous ellipticals – exactly what they expected. Then to
understand how these massive elliptical galaxies formed and evolved, they
turned their attention to more distant quasars. Visible light emitted by a
quasar at a redshift of 2 is received as near infrared radiation, at a
wavelengths of several microns. So to make a proper comparison with visible
images of nearby quasars, astronomers need near-infrared images of the
remote ones.

For very distant host galaxies, it is far more difficult to separate the
starlight from the light of the central quasar. However, detailed analysis
of the team’s high-redshift HST images shows that luminous galaxies are
present around the quasars.

“This is an exciting result,” says Marek Kukula, “since it implies that
today’s massive elliptical galaxies were already luminous stellar systems at
a redshift of 2, when the universe was only a third of its current age.
However, our infrared images alone cannot tell us is how close these
high-redshift host galaxies are to becoming fully-fledged massive
ellipticals like the ones we see around us in the local universe. To find
out whether the hosts already contain a large mass of mature stars, or
whether they are still relatively small galaxies with a high proportion of
young, but very luminous stars we need to determine their colours. Young
stars will be much bluer than an older, more established stellar population.
Initial results from our most recent HST observations seem to indicate that
the host galaxies are emitting more ultraviolet light than would be expected
if they are made up purely of mature stars. This suggests that in these very
distant quasars we really are seeing the massive elliptical galaxies in the
process of formation.”

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Issued by: RAS Press Officers

Dr Jacqueline Mitton
Phone: +44 (0)1223-564914    Fax:    +44 (0)1223-572892
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Peter Bond
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E-mail: PeterRBond@cs.com    Mobile phone: 07711-213486

National Astronomy Meeting Press Room phones:
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CONTACT FOR THIS RELEASE
Dr Marek J. Kukula, Institute for Astronomy, University of Edinburgh, Royal
Observatory EH9 3HJ
mjk@roe.ac.uk  phone: +44 (0)131 668 8357   fax: +44 (0)131 668 8416