Understanding our neighbourhood in the universe (Forwarded)

ESA News
http://www.esa.int

17 December 2007

Understanding our neighbourhood in the universe

With its powerful detectors, Integral has performed the most-sensitive
all-sky survey ever, finding expected clumpy areas at large scales in our
local universe. Scientists working with ground-based telescopes have found
the same local clumps, while looking for sources of cosmic showers.

Integral performed the survey in the hard X-ray band of the electromagnetic
spectrum. Although it wasn't the first such survey, Integral's strength lies
in the fact that it is unbiased towards sources that are shrouded by dust or
gas, invisible at optical and soft X-ray wavelengths.

An interesting correlation has been found in data collected with the Auger
telescopes in Argentina. Such a correlation with optical data is opening a
new door in the exploration of the local universe.

Looking around, our neighbouring local universe is fairly homogeneous.
However, a lot of 'clumps' or structures are found at smaller scales, less
than 100 megaparsecs or about 326 million light-years.

Scientists believe that as the early universe expanded and grew, small
perturbations introduced later formed structures confined to smaller spatial
scales, leaving the universe homogeneous on scales larger than 100-200
megaparsecs (about 326 million to 652 million light-years). Integral has
confirmed just this.

The catalogue produced with the survey includes more than a hundred
extragalactic objects, mainly Active Galactic Nuclei (AGNs), located at
distances up to 200-300 megaparsecs (about 652 to 980 million light-years).
Thanks to the hard X-ray selection, among these sources, the survey has
included many dust-enshrouded AGNs.

The survey found galaxies, clusters, superclusters, voids and even the
highest known mass clumps (of mass greater than that of million of billions
Suns put together) ever-observed: including the nearby Virgo cluster and the
more distant Great Attractor and Perseus-Pisces supercluster.

Nearly every galaxy in the local Universe is believed to have a central,
supermassive black hole and some of these black holes are observed to be
AGNs. AGNs are supermassive black holes that are devouring large amounts of
matter -- excellent candidates for the production of extremely energetic
particles.

There is evidence that AGN activity is strongly related to the evolution of
galaxies. This is why it is believed that the density of AGNs in a given
region should be proportional to that of ordinary galaxies. This would make
it possible to use AGNs as standard references to reveal clumps of matter in
the local universe.

A group of astronomers at the Space Research Institute (IKI, Moscow, Russia)
and Max-Planck Institute for Astrophysics (Garching, Germany) led by Prof.
Rashid Sunyaev used the Integral hard X-ray all-sky survey to find out what
the distribution of nearby AGNs is like. The map they obtained clearly
showed the known, nearby clumps of matter.

Using a completely different method of observation, scientists using the
Auger shower telescopes in Argentina were observing Ultra-High Energy Cosmic
Rays (UHECRs). These particles are striking the Earth atmosphere and create
secondary particles which are detected at the Earth surface.

This was found by correlating the directions of a few UHECRs with a
catalogue of AGNs based on optical measurements. Such catalogues are biased
against dust-enshrouded AGNs in contrast with the catalogue of AGNs detected
by Integral in the hard X-ray band. Combining Integral data with that from
ground-based observations could open a new door to explore the local
Universe with instruments not previously available to astronomers.

Notes for editors:

The results presented in this article make reference to the paper published
in the Astronomy & Astrophysics journal: 'INTEGRAL/IBIS all-sky survey in
hard X-rays', by R. Krivonos, M. Revnivtsev, A. Lutovinov, S. Sazonov, E.
Churazov, and R. Sunyaev (2007, A&A 475, 775).

For more information:

Christoph Winkler, ESA Integral Project Scientist
Email: Christoph.Winkler @ esa.int

[NOTE: Images supporting this release are available at
http://www.esa.int/esaSC/SEM2K4KV3AF_index_1.html ]