Old Galaxies in the Young Universe: VLT Unravels New Population of Very Old Massive Galaxies (Forwarded)

From: Andrew Yee (ayee_at_nova.astro.utoronto.ca)
Date: 08/05/04 

Date: Wed, 04 Aug 2004 21:14:59 -0400

ESO Education and Public Relations Dept.

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Contacts

Andrea Cimatti
INAF - Osservatorio Astrofisico di Arcetri
Firenze, Italy
Phone: +39-055-2752-297
Mobile: +39-347-4686338
Email: cimatti@arcetri.astro.it

Alvio Renzini
European Southern Observatory
Garching, Germany
Phone: +49 89 3200 6413
Email: arenzini@eso.org

ESO Press Release 17/04

Under embargo until July 7, 19:00 CEST

Old Galaxies in the Young Universe

Very Large Telescope Unravels New Population of Very Old
Massive Galaxies [1]

   Summary

   Current theories of the formation of galaxies are based
   on the hierarchical merging of smaller entities into
   larger and larger structures, starting from about the size of a
   stellar globular cluster and ending with clusters of galaxies.
   According to this scenario, it is assumed that no massive galaxies
   existed in the young universe.

   However, this view may now have to be revised. Using the multi-
   mode FORS2 instrument on the Very Large Telescope at Paranal,
   a team of Italian astronomers [2] have identified four remote
   galaxies, several times more massive than the Milky Way galaxy,
   or as massive as the heaviest galaxies in the present-day
   universe. Those galaxies must have formed when the Universe
   was only about 2,000 million years old, that is some 12,000 million
   years ago.

   The newly discovered objects may be members of a population of
   old massive galaxies undetected until now.

   The existence of such systems shows that the build-up of massive
   elliptical galaxies was much faster in the early Universe than
   expected from current theory.

   PR Photo 21a/04: Small Part of the K20 Field Showing the z=1.9
                                 Elliptical Galaxy (ACS/HST).
   PR Photo 21b/04: Averaged Spectrum of Old Galaxies
                                (FORS2/VLT).

Hierarchical merging

Galaxies are like islands in the Universe, made of stars as well as
dust and gas clouds. They come in different sizes and shapes.
Astronomers generally distinguish between spiral galaxies -- like
our own Milky Way, NGC 1232 or the famous Andromeda galaxy --
and elliptical galaxies, the latter mostly containing old stars and
having very little dust or gas. Some galaxies are intermediate
between spirals and ellipticals and are referred to as lenticular or
spheroidal galaxies.

Galaxies are not only distinct in shape, they also vary in size: some
may be as "light" as a stellar globular cluster in our Milky Way (i.e.
they contain about the equivalent of a few million Suns) while
others may be more massive than a million million Suns. Presently,
more than half of the stars in the Universe are located in massive
spheroidal galaxies.

One of the main open questions of modern astrophysics and
cosmology is how and when galaxies formed and evolved starting
from the primordial gas that filled the early Universe. In the most
popular current theory, galaxies in the local Universe are the result
of a relatively slow process where small and less massive galaxies
merge to gradually build up bigger and more massive galaxies. In
this scenario, dubbed "hierarchical merging", the young Universe
was populated by small galaxies with little mass, whereas the
present Universe contains large, old and massive galaxies -- the
very last to form in the final stage of a slow assembling process.

If this scenario were true, then one should not be able to find
massive elliptical galaxies in the young universe. Or, in other
words, due to the finite speed of light, there should be no such
massive galaxies very far from us. And indeed, until now no old
elliptical galaxy was known beyond a radio-galaxy at redshift 1.55
[3] that was discovered almost ten years ago.

The K20 survey

   ESO PR Photo 21a/04 ESO PR Photo 21a/04
   Part of the K20 Field, centred on the z=1.9 galaxy (ACS/HST)

   Caption: ESO PR Photo 21a/04 shows a small region in the K20
   field surveyed by the astronomers. This region is centred on the
   newly discovered z=1.9 redshift galaxy. The image is based on
   frames acquired by the Advanced Camera for Surveys (ACS) on
   the Hubble Space Telescope in the framework of the GOODS
   Public HST Treasury Program (P.I. M. Giavalisco, STScI,
   Baltimore, USA). They show the real colours of the galaxies. The
   four old massive spheroidal galaxies discovered in this survey
   appear very red compared to the other faint galaxies. (Image
   courtesy of Piero Rosati and Bob Fosbury, ESO Garching).

In order to better understand the formation process of galaxies and
to verify if the hierarchical merging scenario is valid, a team of
Italian and ESO astronomers [2] used ESO's Very Large Telescope
as a "time machine" to do a search for very remote elliptical
galaxies. However, this is not trivial. Distant elliptical galaxies, with
their content of old and red stars, must be very faint objects indeed
at optical wavelengths as the bulk of their light is redshifted into the
infrared part of the spectrum. Remote elliptical galaxies are thus
among the most difficult observational targets even for the largest
telescopes; this is also why the 1.55 redshift record has persisted
for so long.

But this challenge did not stop the researchers. They obtained deep
optical spectroscopy with the multi-mode FORS2 instrument on the
VLT for a sample of 546 faint objects found in a sky area of 52
arcmin2 (or about one tenth of the area of the Full Moon) known as
the K20 field, and which partly overlaps with the GOODS-South
area. Their perseverance paid off and they were rewarded by the
discovery of four old, massive galaxies with redshifts between 1.6
and 1.9. These galaxies are seen when the Universe was only
about 25% of its present age of 13,700 million years.

For one of the galaxies, the K20 team benefited also from the
database of publicly available spectra in the GOODS-South area
taken by the ESO/GOODS team.

A new population of galaxies

   ESO PR Photo 21b/04 ESO PR Photo 21b/04
   Averaged Spectrum of Old Galaxies (FORS2/VLT).

   Caption: ESO PR Photo 21b/04 shows the averaged spectrum
   (blue) of the four newly discovered old massive galaxies
   compared to a set of template spectra. The bottom compares it
   with the spectrum of a star having a surface temperature of 7200
   degrees (green) and 6800 degrees (red), respectively. The upper
   graph makes a comparison with synthetic spectra of simulated
   simple stellar populations with ages of 500, 1100 and 3000 million
   years. This figure demonstrates that the newly found galaxies
   mostly contain old low-mass stars and must have formed between
   1,000 and 2,000 million years earlier than the epoch at which they
   are now seen.

The newly discovered galaxies are thus seen when the Universe
was about 3,500 million years old, i.e. 10,000 million years ago. But
from the spectra taken, it appears that these galaxies contain stars
with ages between 1,000 and 2,000 million years. This implies that
the galaxies must have formed accordingly earlier, and that they
must have essentially completed their assembly at a moment when
the Universe was only 1,500 to 2,500 million years old.

The galaxies appear to have masses in excess of one hundred
thousand million solar masses and they are therefore of sizes
similar to the most massive galaxies in the present-day Universe.
Complementary images taken within the GOODS ("The Great
Observatories Origins Deep Survey") survey by the Hubble Space
Telescope show that these galaxies have structures and shapes
more or less identical to those of the present-day massive elliptical
galaxies.

The new observations have therefore revealed a new population of
very old and massive galaxies.

The existence of such massive and old spheroidal galaxies in the
early Universe shows that the assembly of the present-day massive
elliptical galaxies started much earlier and was much faster than
predicted by the hierarchical merging theory. Says Andrea Cimatti
(INAF, Firenze, Italy), leader of the team: "Our new study now
raises fundamental questions about our understanding and
knowledge of the processes that regulated the genesis and the
evolutionary history of the Universe and its structures."

More information

The research presented in this Press Release appears in the July 8
issue of the research journal Nature ("Old galaxies in the young
Universe" by A. Cimatti et al.).

Notes

[1] This press release is coordinated with the Istituto Nazionale di
Astrofisica (INAF). The Italian version is available at www.inaf.it .

[2] The team is composed of Andrea Cimatti (INAF, Firenze),
Emanuele Daddi and Alvio Renzini (ESO, Germany), Paolo
Cassata, Eros Vanzella and Giulia Rodighiero (Universita di
Padova, Italy), Lucia Pozzetti, Marco Mignoli and Giovanni
Zamorani (INAF, Bologna), Stefano Cristiani (INAF, Trieste), and
Adriano Fontana (INAF, Roma).

[3] In astronomy, the redshift denotes the fraction by which the lines
in the spectrum of an object are shifted towards longer
wavelengths. The observed redshift of a remote galaxy provides an
estimate of its distance.

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