OSCIR Unveils Rich Dust Content in the Second Most Metal Deficient Galaxy Known
October 9, 2002
SBS 0335-052 is a gas-rich dwarf galaxy at a distance of 170 million light-years (52 Mpc). It is the second most metal-poor galaxy known, with a heavy metal content only 1/40th that of the Sun. Such a low metallicity means that this galaxy contains significantly lower quantities of the heavy elements produced by the process of nucleosynthesis in stars. This lack of metals could be because the galaxy is very young. Another possibility is that SBS 0335-052 is an old galaxy that has experienced a very low star formation rate throughout its lifetime.
Canadian Ph. D. student Stephanie Plante (Universite Laval, Quebec) and French astronomer Marc Sauvage (Service d'Astrophysique, Saclay) used OSCIR on Gemini North to image SBS 0335-052 in the mid-infrared at 10.8 and 21 microns. Their images revealed a massive supercluster of young stars with a total mass equivalent to 2 million times the mass of the Sun in the core of the galaxy. It is estimated that the age of this cluster is 5 millions years or less.
The Gemini investigators were able to infer the properties of the dust-hidden cluster by analyzing the properties of the interstellar dust surrounding these stars. This is possible since the dust 'processes' the stellar radiation produced by the stars by absorbing the ultra-violet and optical light from embedded stars and then re-radiating it in the mid-infrared. Interstellar dust particles are small solid particles made of the most common heavy elements like carbon, silicates, ices and other molecules in solid form.
the Gemini observations with those made by the ISOPHOT instrument
aboard the Infrared Satellite Observatory in the far infrared at 60, 65
and 100 microns, Plante and Sauvage have used the software DUSTY to
model both the star cluster and its surrounding dust. The two flux
points at 10.8 and 21 microns derived from the OSCIR images were
crucial in constraining some of the key model parameters which allowed
them to construct a detailed model of the dust properties.
Gemini results are interesting because they indicate that the paucity
of metals did not preclude SBS 0335-052 from forming 105
solar masses of dust. The unresolved N band Gemini image and
calculations using the DUSTY model show that there is enough dust to
hide the massive star cluster from view in the optical. Plante and
Sauvage estimate the amount of extinction toward the cluster to be
equivalent to 30 magnitudes in the V band, which is why the region is
totally undetectable at optical wavelengths. Interestingly, this is
very similar to the amount of extinction that we observe toward the
center of the Milky Way galaxy. The two million solar masses of young
stars are packed in a region not larger than 10 pc (~30 light-years)
The presence of such a high density of
stars implies a high density of radiation. The ultraviolet radiation
emitted by the cluster is intense enough to destroy the smallest dust
molecules (PAH) and even the smallest grains (VSM) present in its near
environment. Only the biggest dust grains (around 1 micron or slightly
less) survive. The Q band (21 micron) observation with OSCIR allowed
the researchers to ascertain that even though there are no PAH emission
features seen, there is some carbonaceous dust present in SBS0335-052's
massive stellar cluster.
Finally, Plante and
Sauvage caution that if dust-enshrouded super-star clusters are
commonly associated with a starbursting environment, the star formation
rate deduced by looking at the rest-frame optical or ultraviolet
portion of the spectrum should be viewed with caution.
Plante and Sauvage have published their work in The Astronomical Journal, October 2002, vol. 124, pp. 1995-2005.