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

Max-Planck-Institut fur Astronomie
Konigstuhl 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, Konigstuhl 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)