Dust enshrouded star formation
in the cluster of galaxies Abell 1689
Contact : Pierre-Alain
Using the ISOCAM camera working at wavelengths higher than 10 microns, SAp scientists
and their colleagues discovered that galaxies of the cluster Abell 1689 host
star-formation regions which are partially or totally enshrouded in dust.
For galaxies detected at 15 microns, the total star formation rate is on average
ten times higher than that estimated from usual indicators calibrated in the optical.
Compared to those of Abell 1689, galaxies in nearby clusters form stars at
much more moderate rates and in a more 'visible' way. The presence of hidden
star-forming regions in distant clusters, if confirmed, reflects a more general
change in the properties of galaxies with time. Such an evolution is known from
the mid 80s as the Butcher-Oemler effect, after the names of those who
first found it. The researchers of Saclay and their collaborators have for
the first time enlighted it in the infrared.
Stars are born, live and die: this was established since long in the field of stellar
astronomy. For thirty years, astronomers have also observed that, at their own
scales, galaxies are formed, transformed and may be destroyed. If the evolution
of stars mainly results from internal processes - the progressive exhaustion of gas
fueling nuclear reactions - galaxy evolution is above all driven by external events.
Among the hazards of a galaxy life, collisions play a major role. Slow encounters
and mergers between galaxies have a dramatic effect on their morphology; fast
and multiple collisions cause a secular evolution ('galaxy harassment'); chocks
occur in gas clouds within galaxies plunging at high speed in a dense intergalactic
Such environmental effects are aggravated in rich clusters because of their high
densities. The concentration of X-ray emitting hot intracluster medium adds to
the accumulation of galaxies. It follows that galaxies located inside clusters
evolve more rapidly than those outside ("in the field"). Clusters themselves
are dynamical structures which form and evolve gradually. Therefore because
their global characteristics change and in particular because their density
increases, the impact of environmental effects on the properties of
cluster galaxies increases with time.
The Butcher-Oemler effect itself corresponds to an increase in the proportion of
blue cluster galaxies as a function of redshift. Two other evolution effects were
found: an increase of the proportion of spiral galaxies (morphological
Butcher-Oemler effect) and of galaxies with an optical spectrum exhibiting
signs of active or past star formation (spectroscopic Butcher-Oemler effect).
Basically, nearby clusters, i.e. old ones, are mainly made of
lazy elliptical galaxies not willing to form stars, while distant clusters, i.e. young ones,
contain mainly gas-rich active spirals that recently fell in the cluster
and that hence have not yet been too much affected by the cluster environment.
The intra-cluster medium and the galaxy collisions contribute to eventually
slow or even stop the star formation activity in newly arrived galaxies,
that are stripped of their gas reservoir. However specialists in distant
clusters fight on the following question: before suffocating, do galaxies trapped
in the cluster environment exhibit a last starburst that would rapidly exhaust them ?
Indeed, it is straitforward to imagine - and models predict it - that the shock
of their interstellar medium against the intracluster gas creates instabilities
and further causes the collapse of gas clouds and thus star formation. However
the detailed study of field perturbed galaxies indicate that such an activity
often takes place below a dust layer that only X-ray, infrared or
radio telescopes can penetrate.
| Saclay astronomers and their collaborators have probed this dust enshrouded
star formation activity through the infrared window. Using the camera
on board the ISO satellite, which was partly built at Saclay, they mapped at 7 and 15
microns about ten rich cluster of galaxies up to redshifts of 0.9.
At that redshift lie clusters among the most distant and hence young known so far.
Data reduction required several months of work and the development of new
|ISOCAM image at 15 microns of the cluster J1888.16CL
The final infrared maps, such as the one shown here-above, do not look very
nice, compared to the spectacular images produced by the Hubble Space Telescope,
but in their own field, they are the deepest and most precise at our disposal
for the moment. Thanks to the excellent sensibility of ISOCAM detectors, about one
hundred galaxies emitting in the infrared were detected towards each cluster;
this is much more than expected.
Now, one had still to check that these sources really belong to the
cluster. Their identification and characterization required a long and
laborious work. The good astrometric precision of ISOCAM made it easy to
find their counterpart on optical images.
Leur distance a été ensuite déterminée
lors de plusieurs missions d'observations effectuée au Chili et à Hawaii.
These were queried in the archives
of several observatories such as ESO or HST. Distances were then determined
during several observing runs in Chile and Hawaii. The
La Silla ), the VLT (Paranal )
and the CFH (
Mauna Kea ) telescopes were used among others. A multi-slit spectroscopy
technique allowed to obtain the spectra of about thirty objects in a single
exposure. The spectra not only provide a measure of the redshift, an indicator
required to assess the cluster membership of a given source, but also precious
piece of information on the nature of the infrared emitters. Are they perturbed
starburst galaxies, more quiescient spirals or apparently passive ellipticals ?
This study first focussed on the most nearby cluster in the sample, Abell 1689.
The results were recently published in the journal Astronomy
& Astrophysics (*)
The massive and rich cluster Abell 1689 is a well known by cosmologists.
It was the subject of numerous studies and was even
used as a gravitational telescope for observations of distant galaxies.
Indeed it acts as a gravitational lens that magnifies the light of background
sources which appear in deep optical images as arclets.
Some of them are visible in the opposite image.
| NTT image of the cluster of galaxies Abell 1689
| However ISOCAM has mostly detected genuine cluster
galaxies. Selected from infrared observations, they are dusty objects;
mainly spirals, some of them showing signs of a current interaction, as
illustrated in the opposite figure, but also strangely enough a few
elliptical or lenticular galaxies that are believed to contain small
quantities of dust.
Their optical spectra exhibit, for most of them, weak emission lines
of ionized hydrogen and oxygen that are characteristics of a moderate
star formation activity. Others do not exhibit such lines as if they
were sterile. However their high 15 micron flux emitted by dust
heated by young stars clearly indicates that they host star nurseries.
It seems that in Abell 1689, 90% of the star formation activity is hidden
inside dust cocoons.
Optical Hubble Space Telescope images of ISOCAM sources
in the cluster of galaxies Abell 1689
So, do galaxies falling in clusters suffocate after loosing or consuming their
gas reservoir ? A clear answer to this question is still premature. ISOCAM
observations of Abell 1689 have shown that in this cluster galaxies still form
stars at a relative high rate, but in a very discreet manner. The processing
of the ISOCAM data on other clusters that are more distant and a priori
more interesting is still going on.
Contact Pierre-Alain Duc
(*) "Hidden star-formation in the cluster of galaxies Abell
Duc P.-A., Poggianti B.M., Fadda D.,
Elbaz D., Flores H., Chanial P., Franceschini A., Moorwood A. and
Cesarsky C., 2002, A&A 382, 60 (article
in PDF format 840kb)
"An excess of mid-IR luminous galaxies in Abell 1689 ?"
Fadda D., Elbaz D., Duc P.-A., Flores H., Franceschini A. and Cesarsky C., 2000, A&A 361, 827