Scott M. Croom talks with
ScienceWatch.com and answers a few questions about
this month's Fast Moving Front in the field of Space
Science. The author has also sent along an image of
Article: The 2dF QSO Redshift Survey - XIV.
Structure and evolution from the two-point correlation
SM;Boyle, BJ;Shanks, T;Smith, RJ;Miller, L;Outram,
PJ;Loaring, NS;Hoyle, F;da Angela, A
Journal: MON NOTIC ROY ASTRON SOC, 356 (2): 415-438 JAN 11
Addresses: Anglo Australian Observ, POB 296, Epping, NSW
Anglo Australian Observ, Epping, NSW 1710, Australia.
Australia Telescope Natl Facil, Epping, NSW 1710,
Univ Durham, Dept Phys, Durham DH1 3LE, England.
Liverpool John Moores Univ, Astrophys Res Inst, Birkenhead
CH41 1, Merseyside, England.
(addresses have been truncated)
Why do you think your paper is highly
We now know that all massive galaxies contain super-massive
black holes—a million to a billion times the
mass of the sun. There is a growing realization that these super-massive
black holes play a critical role in the formation and growth of
When gas is funneled down towards a black hole, it forms an accretion disk.
The gas in the disk is heated via turbulent processes and generates massive
amounts of radiation, as well as relativistic jets. Both the radiation and
jets can couple to the material in the host galaxy, strongly influencing
spatial distribution of 2QZ quasars in our two
survey regions, which are 5x75 degree regions on
the sky, one in each of the north and south
galactic caps. Our galaxy is at the centre of the
plot, and the points are color coded with more
distant (higher redshift) quasars becoming redder.
Because the survey volume is so large the
distribution of quasars appears largely uniform,
but statistical clustering analysis shows very
significant clustering on small
In a high accretion rate mode, these black holes can be seen as quasars,
the most luminous sources in the Universe. Exactly how the high accretion
rates required for a quasar are achieved is not clear, but it is likely
that the merging of galaxies could cause the loss of angular momentum in
the gas required to fuel quasars.
The formation of galaxies, and the impact that feedback from an active
super-massive black hole contains, is currently one of the hottest topics
in cosmology. There is a large amount of research attempting to model these
processes. Our measurements of the clustering and evolution of quasars are
primary inputs that guide and constrain these models.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
The main result of our paper was a new discovery based on a substantial new
dataset. It is the size of this dataset that enabled these new insights
into the physics of quasars.
Would you summarize the significance of your paper
in layman's terms?
By measuring the clustering of quasars, our work has allowed us to estimate
the typical mass of quasar hosts over a wide range of cosmic time—to
a time when the Universe was only 20% of its current age.
It turns out that quasars exist in the same mass hosts at all epochs, which
is the mass typical of small galaxy groups. This mass scale is the optimal
one for galaxy merging, adding substantial weight to the merger hypothesis
for quasar triggering.
Our clustering results are also able to directly demonstrate that quasars
cannot be long-lived on cosmological timescales. There must be many
successive generations of quasars triggered in different galaxies.
How did you become involved in this research and
were any particular problems encountered along the way?
I began my Ph.D. studies at the University of Durham, working on the 2dF
Quasar Redshift Survey (2QZ). This involved target selection and setting
the key science goals of the project, as well as preliminary observations.
The biggest challenge was the massive scale of the project, which targeted
over 20,000 quasars, a factor of 20 more objects than the previous largest
survey. We used approximately 300 nights of telescope time on the
Anglo-Australian Telescope over a period of six years.
The robotic 2-degree Field (2dF) instrument on this telescope enabled the
quantum leap in survey size, by allowing us to observe 400 objects at a
time. With a sample of this size, most measurements are dominated by
systematic rather than statistical errors, so many of the most difficult
stages of the analysis involved quantifying these.
Where do you see your research leading in the
We are currently expanding this work by extending it to an earlier cosmic
time. The 2QZ reached to the epoch when quasars were most active,
approximately 10-11 billion years ago.
However, at earlier epochs the number of quasars is seen to decline. This
decline has only been seen in the most luminous objects, and we are
targeting a much deeper survey of faint distant quasars. The aim here is to
understand how the quasar population is built up from the earliest epochs.
Do you foresee any social or political implications
for your research?
Cosmology is all about finding our place in the Universe and understanding
our origins. Because of this, there continues to be a high level of
interest among the general public. Young and old alike have a natural urge
to understand their origins on many different levels.
Scott Croom, Ph.D.
Sydney Institute for Astronomy (SIfA)
School of Physics
University of Sydney
KEYWORDS: ACTIVE GALACTIC NUCLEI; DIGITAL SKY SURVEY;
SUPERMASSIVE BLACK-HOLES; LYMAN-BREAK GALAXIES; DARK-MATTER HALOES;
COSMOLOGICAL CONSTANT; LUMINOSITY DEPENDENCE; SPACE DISTORTIONS; POWER
SPECTRUM; HOST GALAXIES.