Very Large Telescope Unravels New Population of Very Old Massive
Galaxies

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.

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

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

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.).

The full text of ESO PR 17/04, with two photos and all weblinks, is
available at:
http://www.eso.org/outreach/press-rel/pr-2004/pr-17-04.html
(after expiry of the embargo period).

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.

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