Stellar Clusters Forming in the Blue Dwarf Galaxy NGC 5253 (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Leonardo Vanzi
European Southern Observatory, Santiago, Chile
Phone: +56 2 463 3104
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For immediate release: 18 November 2004

ESO Press Photos 31a-c/04

Stellar Clusters Forming in the Blue Dwarf Galaxy NGC 5253

ESO PR Photo 31a/04 ESO PR Photo 31b/04
NGC 5253 - VLT/HST NGC 5253 - VLT K-band
Composite Image

ESO PR Photo 31c/04
NGC 5253 - VLT L-Band

Caption: ESO PR Photo 31a/04 shows a composite colour-coded
image of the small starburst galaxy NGC 5253, at a distance
of 11 million light-years. It is based on one near-infrared
image (in the K-band at wavelength 2.16 μm; here coded red)
obtained with the ESO Very Large Telescope (VLT) and two
images in the visual spectral region (V- (0.55 μm) and
I-bands (0.79 μm), here blue and green, respectively) with
the NASA/ESA Hubble Space Telescope (HST). The image covers
80 x 80 arcseconds on the sky. North is up and East is to
the left. ESO PR Photo 31b/04 shows the VLT ISAAC K-band
image, obtained under excellent seeing conditions (0.4
arcsec). The Field of View (FOV) is 51 x 60 arcseconds.
ESO PR Photo 31c/04 displays an ISAAC image of NGC 5253
obtained in infrared the L-band (3.78 μm), with a very
massive young stellar cluster (see text) as the brightest
object. The FOV is 49 times 64 arcseconds.

Star formation is one of the most basic phenomena in the
Universe. Inside stars, primordial material from the Big Bang
is processed into heavier elements that we observe today. In
the extended atmospheres of certain types of stars, these
elements combine into more complex systems like molecules and
dust grains, the building blocks for new planets, stars and
galaxies and, ultimately, for life. Violent star-forming
processes let otherwise dull galaxies shine in the darkness
of deep space and make them visible to us over large
distances.

Star formation begins with the collapse of the densest parts
of interstellar clouds, regions that are characterized by
comparatively high concentration of molecular gas and dust
like the Orion complex (ESO PR Photo 20/04) and the Galactic
Centre region (ESO Press Release 26/03). Since this gas and
dust are products of earlier star formation, there must have
been an early epoch when they did not yet exist.

But how did the first stars then form? Indeed, to describe
and explain "primordial star formation" -- without molecular
gas and dust -- is a major challenge in modern Astrophysics.

A particular class of relatively small galaxies, known as
"Blue Dwarf Galaxies", possibly provide nearby and
contemporary examples of what may have occurred in the early
Universe during the formation of the first stars. These
galaxies are poor in dust and heavier elements. They contain
interstellar clouds which, in some cases, appear to be quite
similar to those primordial clouds from which the first
stars were formed. And yet, despite the relative lack of the
dust and molecular gas that form the basic ingredients for
star formation as we know it from the Milky Way, those Blue
Dwarf Galaxies sometimes harbour very active star-forming
regions. Thus, by studying those areas, we may hope to better
understand the star-forming processes in the early Universe.

Very active star formation in NGC 5253

NGC 5253 is one of the nearest of the known Blue Dwarf
Galaxies; it is located at a distance of about 11 million
light-years in the direction of the southern constellation
Centaurus. Some time ago a group of European astronomers [1]
decided to take a closer look at this object and to study
star-forming processes in the primordial-like environment
of this galaxy.

True, NGC 5253 does contains some dust and heavier elements,
but significantly less than our own Milky Way galaxy.
However, it is quite extreme as a site of intense star
formation, a profuse "starburst galaxy" in astronomical
terminology, and a prime object for detailed studies of
large-scale star formation.

ESO PR Photo 31a/04 provides an impressive view of NGC 5253.
This composite image is based on a near-infrared exposure
obtained with the multi-mode ISAAC instrument mounted on
the 8.2-m VLT Antu telescope at the ESO Paranal Observatory
(Chile), as well as two images in the optical waveband
obtained from the Hubble Space Telescope data archive
(located at ESO Garching). The VLT image (in the K-band at
wavelength 2.16 μm) is coded red, the HST images are blue
(V-band at 0.55 μm) and green (I-band at 0.79 μm),
respectively.

The enormous light-gathering capability and the fine optical
quality of the VLT made it possible to obtain the very
detailed near-infrared image (cf. PR Photo 31b/04) during
an exposure lasting only 5 min. The excellent atmospheric
conditions of Paranal at the time of the observation (seeing
0.4 arcsec) allow the combination of space- and ground-based
data into a colour photo of this interesting object.

A major dust lane is visible at the western (right) side of
the galaxy, but patches of dust are visible all over, together
with a large number of colourful stars and stellar clusters.
The different colour shades are indicative of the ages of the
objects and the degree of obscuration by interstellar dust.
The near-infrared VLT image penetrates the dust clouds much
better than the optical HST images, and some deeply embedded
objects that are not detected in the optical therefore appear
as red in the combined image.

Measuring the size and infrared brightness of each of these
"hidden" objects, the astronomers were able to distinguish
stars from stellar clusters; they count no less than 115
clusters. It was also possible to derive their ages -- about
50 of them are very young in astronomical terms, less than
20 million years. The distribution of the masses of the
cluster stars ressembles that observed in clusters in other
starburst galaxies, but the large number of young clusters
and stars is extraordinary in a galaxy as small as NGC 5253.

When images are obtained of NGC 5253 at progressively longer
wavelengths, cf. ESO PR Photo 31c/04 which was taken with the
VLT in the L-band (wavelength 3.7 μm), the galaxy looks quite
different. It no longer displays the richness of sources seen
in the K-band image and is now dominated by a single bright
object. By means of a large number of observations in
different wavelength regions, from the optical to the radio,
the astronomers find that this single object emits as much
energy in the infrared part of the spectrum as does the entire
galaxy in the optical region. The amount of energy radiated at
different wavelengths shows that it is a young (a few million
years), very massive (more than one million solar masses)
stellar cluster, embedded in a dense and heavy dust cloud
(more than 100,000 solar masses of dust; the emission seen
in PR Photo 31c/04 comes from this dust).

A view towards the beginnings

These results show that a galaxy as tiny as NGC 5253, almost
100 times smaller than our own Milky Way galaxy, can produce
hundreds of compact stellar clusters. The youngest of these
clusters are still deeply embedded in their natal clouds, but
when observed with infrared-sensitive instruments like ISAAC
at the VLT, they stand out as very bright objects indeed.

The most massive of these clusters holds about one million
solar masses and shines as much as 5000 very bright massive
stars. It may well be very similar to the progenitors in the
early Universe of the old globular clusters we now observe in
large galaxies like the Milky Way. In this sense, NGC 5253
provides us with a direct view towards our own beginnings.

Note

[1] The group consists of Giovanni Cresci (University of
Florence, Italy), Leonardo Vanzi (ESO-Chile) and Marc Sauvage
(CEA/DSN/DAPNIA, Saclay, France). More details about the
present investigation is available in a research paper ("The
Star Cluster population of NGC 5253" by G. Cresci et al.) to
appear soon in the leading research journal Astronomy &
Astrophysics (a preprint is available as astro-ph/0411486).

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