A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Astronomy and Astrophysics » Astronomy Misc
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

The Colour of the Young Universe (Forwarded)



 
 
Thread Tools Display Modes
  #1  
Old December 19th 03, 06:48 PM
Andrew Yee
external usenet poster
 
Posts: n/a
Default The Colour of the Young Universe (Forwarded)

ESO Education and Public Relations Dept.

Contacts:

Gregory Rudnick
Max-Planck-Institut für Astrophysik
Garching, Germany
Phone: +49-89-30000-2246
Email:

Marijn Franx
Leidse Sterrewacht
Leiden, Netherlands
Phone: +31 71 527 5870
Email:


----------------------------------------------------------------------------
Text with all links and the photos are available on the ESO Website at URL:
http://www.eso.org/outreach/press-re.../pr-34-03.html
----------------------------------------------------------------------------

For immediate release: 19 December 2003

ESO Press Release 34/03

The Colour of the Young Universe

VLT study gives insight on the evolution of the star formation rate

Summary

An international team of astronomers [1] has determined the colour
of the Universe when it was very young. While the Universe is now
kind of beige, it was much bluer in the distant past, at a time
when it was only 2,500 million years old.

This is the outcome of an extensive and thorough analysis of more
than 300 galaxies seen within a small southern sky area, the
so-called Hubble Deep Field South. The main goal of this advanced
study was to understand how the stellar content of the Universe
was assembled and has changed over time.

Dutch astronomer Marijn Franx, a team member from the Leiden
Observatory (The Netherlands), explains: "The blue colour of the
early Universe is caused by the predominantly blue light from
young stars in the galaxies. The redder colour of the Universe
today is caused by the relatively larger number of older, redder
stars."

The team leader, Gregory Rudnick from the Max-Planck Institut für
Astrophysics (Garching, Germany) adds: "Since the total amount of
light in the Universe in the past was about the same as today and
a young blue star emits much more light than an old red star,
there must have been significantly fewer stars in the young
Universe than there is now. Our new findings imply that the
majority of stars in the Universe were formed comparatively
late, not so long before our Sun was born, at a moment when the
Universe was around 7,000 million years old."

These new results are based on unique data collected during more
than 100 hours of observations with the ISAAC multi-mode instrument
at ESO's Very Large Telescope (VLT), as part of a major research
project, the Faint InfraRed Extragalactic Survey (FIRES). The
distances to the galaxies were estimated from their brightness
in different optical near-infrared wavelength bands.

PR Photo 34/03: The Evolving Colour of the Universe.

Observing the early Universe

It is now well known that the Sun was formed some 4.5 billion years
ago. But when did most of the other stars in our home Galaxy form?
And what about stars in other galaxies? These are some of the key
questions in present-day astronomy, but they can only be answered
by means of observations with the world's largest telescopes.

One way to address these issues is to observe the very young
Universe directly -- by looking back in time. For this, astronomers
take advantage of the fact that light emitted by very distant
galaxies travels a long time before reaching us. Thus, when
astronomers look at such remote objects, they see them as they
appeared long ago.

Those remote galaxies are extremely faint, however, and these
observations are therefore technically difficult. Another
complication is that, due to the expansion of the Universe, light
from those galaxies is shifted towards longer wavelengths [2],
out of the optical wavelength range and into the infrared
region.

In order to study those early galaxies in some detail, astronomers
must therefore use the largest ground-based telescopes, collecting
their faint light during very long exposures. In addition they
must use infrared-sensitive detectors.

Telescopes as giant eyes

The "Hubble Deep Field South (HDF-S)" is a very small portion of
the sky in the southern constellation Tucanae ("the Toucan"). It
was selected for very detailed studies with the Hubble Space
Telescope (HST) and other powerful telescopes. Optical images of
this field obtained by the HST represent a total exposure time
of 140 hours. Many ground-based telescopes have also obtained
images and spectra of objects in this sky area, in particular
the ESO telescopes in Chile.

A sky area of 2.5 x 2.5 arcmin2 in the direction of HDF-S was
observed in the context of a thorough study (the Faint InfraRed
Extragalactic Survey; FIRES, see ESO PR 23/02). It is slightly
larger than the field covered by the WFPC2 camera on the HST,
but still 100 times smaller than the area subtended by the full
moon.

Whenever this field was visible from the ESO Paranal Observatory
and the atmospheric conditions were optimal, ESO astronomers
pointed the 8.2-m VLT ANTU telescope in this direction, taking
near-infrared images with the ISAAC multi-mode instrument.
Altogether, the field was observed for more than 100 hours and
the resulting images (see ESO PR 23/02), are the deepest
ground-based views in the near-infrared Js- and H-bands. The
Ks-band image is the deepest ever obtained of any sky field in
this spectral band, whether from the ground or from space.

These unique data provide an exceptional view and have now
allowed unprecedented studies of the galaxy population in the
young Universe. Indeed, because of the exceptional seeing
conditions at Paranal, the data obtained with the VLT have an
excellent image sharpness (a "seeing" of 0.48 arcsec) and can
be combined with the HST optical data with almost no loss of
quality.

A bluer colour

ESO PR Photo 34/03

Captions: PR Photo 34a/03 shows a set of three-colour images of
intrinsically bright galaxies in the Hubble Deep Field South.
The galaxies are arranged horizontally by the age of the
Universe when the light left each object. For reference, the
Universe is now 13.7 billion years old. The colours of the
galaxies have had the effect of redshift removed [2]. That is,
the colours indicate the amount of light which is emitted at
a given rest-frame wavelength, as observed by someone at the
same redshift as each galaxy. These colours provide information
about the ages of stars in the galaxies, where redder colours
indicate older stars. At the bottom is shown how the mean colour
of bright galaxies changes as the Universe gets older. The
reddening in colour with time is due to the increasing mean age
of the stars, cf. the text.

The astronomers were able to detect unambiguously about 300
galaxies on these images. For each of them, they measured the
distance by determining the redshift [2]. This was done by means
of a newly improved method that is based on the comparison of the
brightness of each object in all the individual spectral bands
with that of a set of nearby galaxies.

In this way, galaxies were found in the field with redshifts as
high as z = 3.2, corresponding to distances around 11,500 million
light-years. In other words, the astronomers were seeing the light
of these very remote galaxies as they were when the Universe was
only about 2.2 billion year old.

The astronomers next determined the amount of light emitted by
each galaxy in such a way that the effects of the redshift were
"removed". That is, they measured the amount of light at
different wavelengths (colours) as it would have been recorded
by an observer near that galaxy. This, of course, only refers
to the light from stars that are not heavily obscured by dust.

Summing up the light emitted at different wavelengths by all
galaxies at a given cosmic epoch, the astronomers could then
also determine the average colour of the Universe (the "cosmic
colour") at that epoch. Moreover, they were able to measure
how that colour has changed, as the Universe became older.

They conclude that the cosmic colour is getting redder with time.
In particular, it was much bluer in the past; now, at the age
of nearly 14,000 million years, the Universe has a kind of beige
colour.

When did stars form?

The change of the cosmic colour with time may be interesting in
itself, but it is also an essential tool for determining how
rapidly stars were assembled in the Universe.

Indeed, while the star-formation in individual galaxies may
have complicated histories, sometimes accelerating into true
"star-bursts", the new observations -- now based on many
galaxies -- show that the "average history" of star-formation
in the Universe is much simpler. This is evident by the
observed, smooth change of the cosmic colour as the Universe
became older.

Using the cosmic colour the astronomers were also able to
determine how the mean age of relatively unobscured stars in
the Universe changed with time. Since the Universe was much
bluer in the past than it is now, they concluded that the
Universe is not producing as many blue (high mass, short-lived)
stars now as it was earlier, while at the same time the red
(low mass, long-lived) stars from earlier generations of star
formation are still present. Blue, massive stars die more
quickly than red, low-mass stars, and therefore as the age of
a group of stars increases, the blue short-lived stars die and
the average colour of the group becomes redder. So did the
Universe as a whole.

This behaviour bears some resemblance with the ageing trend in
modern Western countries where less babies are born than in
the past and people live longer than in the past, with the
total effect that the mean age of the population is rising.

The astronomers determined how many stars had already formed
when the Universe was only about 3,000 million years old. Young
stars (of blue colour) emit more light than older (redder) stars.
However, since there was just about as much light in the young
Universe as there is today -- although the galaxies are now much
redder -- this implies that there were fewer stars in the early
Universe than today. The present study inidcates that there were
ten times fewer stars at that early time than there is now.

Finally, the astronomers found that roughly half of the stars in
the observed galaxies have been formed after the time when the
Universe was about half as old (7,000 million years after the
Big Bang) as it is today (14,000 million years).

Although this result was derived from a study of a very small
sky field, and therefore may not be completely representative
of the Universe as a whole, the present result has been shown
to hold in other sky fields.

More information

The research described in this Press Release will appear in the
December 20 issue of the Astrophysical Journal ("The rest frame
optical luminosity density, color, and stellar mass density of
the Universe from z=0 to 3" by Gregory Rudnick et al.)

Notes

[1]: Members of the team include Gregory Rudnick (MPA Garching,
Germany), Hans-Walter Rix and Ignacio Trujillo (MPIA Heidelberg,
Germany), Marijn Franx, Ivo Labbe, Natascha Foerster Schreiber,
Arjen van de Wel, Paul van der Werf and Lottie van Starkenburg
(Leiden Observatory, The Netherlands), Michael Blanton (New
York University, USA), Emmanuele Daddi and Alan Moorwood (ESO,
Germany) and Pieter van Dokkum (Yale University, USA).

[2]: In astronomy, the "redshift" denotes the factor by which the
lines in the spectrum of an object are shifted towards longer
wavelengths. Since the redshift of a cosmological object
increases with distance, the observed redshift of a remote
galaxy also provides an estimate of its distance.

----------------------------------------------------------------------------
ESO Press Information is available on Receive email notification
the WWW at about important news from ESO -
http://www.eso.org/outreach/press-rel/. subscribe to the
----------------------------------------------------------------------------
(c) ESO Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
----------------------------------------------------------------------------

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Breakthrough in Cosmology Kazmer Ujvarosy Space Shuttle 3 May 22nd 04 09:07 AM
Breakthrough in Cosmology Kazmer Ujvarosy Space Station 0 May 21st 04 08:02 AM
Breakthrough in Cosmology Kazmer Ujvarosy Policy 0 May 21st 04 08:00 AM
Has ESA's XMM-Newton cast doubt over dark energy? (Forwarded) Andrew Yee Astronomy Misc 0 December 12th 03 08:15 PM
Astronomers reveal the first detailed maps of galaxy distributionin the early universe (Forwarded) Andrew Yee Astronomy Misc 0 July 18th 03 12:23 AM


All times are GMT +1. The time now is 03:03 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.