Strong Galaxy-wide Star Formation in the Distant Universe (Forwarded)

Max-Planck-Institut für Astronomie
Königstuhl 17
D-69117 Heidelberg
Germany

Contact:
Dr. Helmut Dannerbauer
Tel.: 00 49 6221 528 354

Dr. Jakob Staude
Tel.: 00 49 6221 528 229

2008-01-23

Strong Galaxy-wide Star Formation in the Distant Universe

An international team of astronomers from France, Germany, the USA and
India has observed for the first time the cool molecular gas in ordinary
massive galaxies in the young, distant universe. The scientists discovered
much more of it than being observed in galaxies in the local universe.
This gas is the building material for stars still nowadays born in normal,
undisturbed and not active galaxies in our local universe. The
observations have been made with the millimeter interferometer located at
the Plateau de Bure (France). The Institute for Radio Astronomy in the
Millimeter regime (IRAM) in Grenoble is operating this telescope.

This finding indicates that massive galaxies built major fractions of
their stars at lower rates and over longer timescales and not than thought
before during explosive events being triggered through the collisions of
two or more galaxies. This result opens new major possibilities for
understanding galaxy formation in the young, distant universe shortly
after the big bang.

In galaxies of our cosmological neighbourhood we are observing how star
formation is currently going on. Here star formation is observed to take
place in two fundamental modes. In more or less undisturbed, ordinary
spiral galaxies -- like our Milky Way -- new stars are formed primarily in
the spiral arms of the disk-like structure. These are the sites which
contain most of the building material for star formation -- molecular
hydrogen gas. Currently, a few solar masses per year of newborn stars are
produced in our Milky Way.

When spiral galaxies merge due to a close encounter, a more efficient
process is observed in which stars are formed at much higher rates but at
limited time scales up to a few hundred million years. During major galaxy
collisions, the molecular gas is efficiently compressed toward the center
of the merging system, reaching densities that are much higher than in
spiral galaxies. This can enhance in the central regions the rate of star
formation up to several hundred solar masses per year, or more, in these
objects, which are thought to very rapidly consume their gas. Astronomers
call the resulting galaxies Ultra-Luminous InfraRed Galaxies.

But what is the dominating mode of star formation in the young universe?
As observations showed, collisions of galaxies in the distant Universe are
thought to be much more common than nowadays due to the higher spatial
density of galaxies (since then because of the cosmic expansion the galaxy
density went gradually down); and even ordinary massive galaxies in the
distant Universe were forming stars at prodigious rates. This provided
widespread support to the idea that the predominant star formation mode in
the distant Universe was the collisions-like, high efficient ULIRG-phase.
Observations of molecular gas properties in distant galaxies, limited to
the brightest and rarest systems (the ULIRGS), had so far confirmed this
picture.

Using high sensitivity newly refurbished detectors for radio waves in the
millimeter range at the Plateau de Bure Interferometer[*], the authors
of this here presented work have been able for the first time to measure
the molecular gas content of ordinary and representative galaxies in the
distant Universe. The team, including Helmut Dannerbauer from the
Max-Planck-Institute for Astronomy in Heidelberg (Germany), observed two
massive, disk-like galaxies which emitted their light 4.3 billion years
after the big bang and succeeded in detecting both sources.

The findings of this project, described in a paper published in
Astrophysical Journal Letters on January 20th 2008, have profound
consequences for understanding the processes regulating massive galaxy
formation in the distant Universe. Both galaxies are found to host giant
gas reservoirs, but with a star formation mode that closely resembles the
one observed in local spiral galaxies -- a phenomenon observed here for
the first time: The ordinary distant massive galaxies behave like
scaled-up version of the Milky Way galaxy, with proportionally larger
molecular gas reservoir and star formation activity, but with overall
similar efficiency in forming stars.

This discovery is helping astronomers to shed light into the way galaxy
gradually built up their stellar component. It suggests that mergers
between galaxies are not the major channel for star formation in distant
galaxies. The large gas reservoirs newly observed could maintain star
formation in these galaxies over hundreds of million years, ten times
longer than in the extreme sources that were known before, implying that a
large fraction of the stars in massive galaxies is formed relatively
slowly. These observations also help to explain the clumpy-like appearance
of distant galaxies, because the large gas reservoirs are prone to
fragmentation due to gravitational instabilities.

These new observations have also shown that ordinary galaxies in the
distant Universe, 10-100 times more common than the extreme sources (e.g.
the ULIRGS) so far studied, are now within reach of direct observations of
their molecular gas content with current.

Original publication:
Emanuele Daddi(1), Helmut Dannerbauer(2), David Elbaz(1), Mark
Dickinson(3), Glenn Morrison(4,5), Daniel Stern(6), S. Ravindranath(7),

Vigorous Star Formation with low Efficiency in Massive Disk Galaxies at z
= 1.5,
The Astrophysical Journal Letters, January 20th, 2008.

(1) Service d'Astrophysique, CEA Saclay, Orme des Merisiers, 91191
Gif-sur-Yvette Cedex, France
(2) MPIA, Königstuhl 17, D-69117 Heidelberg, Germany
(3) NOAO, 950 N. Cherry Ave., Tucson, AZ, 85719
(4) Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
(5) Canada-France-Hawaii Telescope, Kamuela, HI, 96743, USA
(6) Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109
(7) IUCAA, Pune University Campus, Pune 411007, Maharashtra, India
[*] The Plateau de Bure Interferometer is managed by IRAM. IRAM is
supported by the Max-Planck-Society, INSU/CNRS (France), and IGN (Spain).

IMAGE CAPTIONS:

[Fig. 1:
http://www.mpia.de/Public/Aktuelles/...080123_1gr.jpg
(464KB)]
Massive distant galaxies resemble our Milky Way galaxies in their disk
like structure, but have much larger gas content and much higher activity
of star formation (artist view, MPIA / ESO / ESA / NASA / HST).

[Fig. 2:
http://www.mpia.de/Public/Aktuelles/...3/080123_2.jpg (24KB)]
Hubble Space Telescope images centered on the two distant massive galaxies
for which the molecular gas content (white contours) has been measured for
the first time.
(Pictu E. Daddi / CEA-France / HST / NASA)