The Invisible Galaxies That Could Not Hide (Forwarded)

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO
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Contact:
Céline Péroux
ESO, Garching, Germany
Phone: +49 89 3200 6634

For immediate release: 15 February 2006

ESO Science Release 06/06

The Invisible Galaxies That Could Not Hide

Metal-Rich Distant Galaxy Found With ESO's VLT

Astronomers, using the unique capabilities offered by the high-
resolution spectrograph UVES on ESO's Very Large Telescope,
have found a metal-rich hydrogen cloud in the distant universe.
The result may help to solve the missing metal problem and
provides insight on how galaxies form.

"Our discovery shows that significant quantities of metals are
to be found in very remote galaxies that are too faint to be
directly seen", said Céline Péroux (ESO), lead-author of the
paper presenting the results [1].

The astronomers studied the light emitted by a quasar located
9 billion light-years away that is partially absorbed by an
otherwise invisible galaxy sitting 6.3 billion light-years
away along the line of sight.

The analysis of the spectrum shows that this galaxy has four
times more metals than the Sun. This is the first time one
finds such a large amount of 'metals' [2] in a very distant
object. The observations also indicate that the galaxy must
be very dusty.

Almost all of the elements present in the Universe were
formed in stars, which themselves are members of galaxies.
By estimating how many stars formed over the history of the
Universe, it is possible to estimate how much metals should
have been produced. This apparently straightforward reasoning
has however since several years been confronted with an
apparent contradiction: adding up the amount of metals
observable today in distant astronomical objects falls short
of the predicted value. When the contribution of galaxies
now observed at cosmological distances is added to that of
the intergalactic medium, the total amounts for no more than
a tenth of the metals expected.

Studying distant galaxies is however a difficult task. The
further a galaxy, the fainter it is, and the small or
intrinsically faint ones won't be observed. This may introduce
severe biases in the observations as only the largest and most
active galaxies are picked up.

Astronomers therefore came up with other ways to study distant
galaxies: they use quasars, most probably the brightest
distant objects known, as beacons in the Universe.

Interstellar clouds of gas in galaxies, located between the
quasars and us on the same line of sight, absorb parts of
the light emitted by the quasars. The resulting spectrum
consequently presents dark 'valleys' that can be attributed to
well-known elements. Thus, astronomers can measure the amount
of metals present in these galaxies -- that are in effect
invisible -- at various epochs.

"This can best be done by high-resolution spectrographs on
the largest telescopes, such as the Ultra-violet and Visible
Echelle Spectrograph (UVES) on ESO's Kueyen 8.2-m telescope
at the Paranal Observatory," declared Péroux.

Her team studied in detail the spectrum of the quasar SDSS
J1323-0021 that shows clear indications of absorption by a
cloud of hydrogen and metals located between the quasar and
us. From a careful analysis of the spectrum, the astronomers
found this 'system' to be four times richer in zinc than the
Sun. Other metals such as iron appear to have condensed into
dust grains.

"If a large number of such 'invisible' galaxies with high
metal content were to be discovered, they might well alleviate
considerably the missing metals problem", said Péroux.

High resolution images and their captions are available at
http://www.eso.org/outreach/press-re...hot-06-06.html

Notes

[1] The discovery is to appear in a paper published by
Astronomy and Astrophysics ("The Most Metal-Rich Intervening
Quasar Absorber Known", by C. Péroux et al.) and available in
PDF format at
http://www.edpsciences.org/articles/...PRAA200602.pdf
The team is composed of Céline Péroux (ESO), Varsha Kulkarni
and Joe Meiring (University of South Carolina, Columbia, USA),
Roger Ferlet (Institut d'Astrophysique de Paris, France),
Pushpa Khare (Utkal University, India), Jim Lauroesch
(Northwestern University, Evanston, USA), Giovanni Vladilo
(Osservatorio di Trieste, Italy), and Don York (University
of Chicago, USA).

[2] For astronomers, metals are all chemical elements heavier
than helium. The Sun, for example, is made mostly of hydrogen
(73%) and helium (25%), and only 2% of 'metals'.

National contacts for the media:

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