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* 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).
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