NGC 1705 in the Infrared

NGC 1705: WFPC2 (color) and NICMOS J-band (black/white) image of the central star region. Image Credit: M. Tosi and L. Origlia (INAF, Osservatorio Astronomico di Bologna.), A. Aloisi and M. Sirianni (JHU), L. Greggio (INAF, Osservatorio Astronomico di Padova), M. Clampin, C. Leitherer and A. Nota (STScI) Place mouse on image to view NICMOS observation

A straightforward way to grasp some hints on the evolution of a galaxy is to resolve it into single stars. The above image pair shows the core of NGC 1705 in both optical as well as near-infrared wavelengths taken with Hubble. The massive central star cluster, which appears as a blur of white in the optical image is sharper in the near-infrared image. This near-IR filter allows light to pass through near 1100 Angstroms (110 nanometers). It lets through starlight but gas and dust surrounding these stars does not show through. In addition, the J band is more sensitive to redder stars, thus it detects them more easily. The optical image was taken with Hubble's Wide Field Planetary Camera 2 also known as WFPC2. The near-IR image was photographed with the Near Infrared Camera and Multi-Object Spectrometer or NICMOS onboard Hubble.

More on what these images tell astronomers:

Thanks to the predictions of stellar evolutionary theory, astronomers are able to date stars based on their colors, related to a certain temperature: in general blue hot stars are younger, while red cool stars are older. Complicated simulations can further be used to infer the so-called star-formation history, namely the major epochs of the stellar production and the rate of this activity. In addition, sophisticated chemical evolution models can be subsequently applied in order to reproduce the observed chemical properties of gas and stars based on the previously inferred star-formation history.

The nearby dwarf irregular galaxy NGC 1705 is truly an ideal laboratory where to conduct this type of investigation. The central region of NGC 1705 is dominated by a gigantic unresolved stellar cluster. Both young (a few Myr) and old (more than 5 Gyr and probably up to 13.5 Gyr) stars are clearly present in the galaxy, with the former strongly concentrated toward the galactic center and the latter more spread out. Moreover, simulations indicate that the galaxy has experienced an almost continuous star formation at a non-exceptional level over a large fraction of the universe age.

Only recently (about 10-15 Myr ago) a conspicuous star-formation activity has taken place, giving birth to many of the young field stars as well as to the central super star cluster. This burst is probably associated to the galactic outflow observed with other techniques in different spectral ranges, i.e. emission from warm (10,000 K) ionized gas as traced by H alpha, or X-ray emission from very hot (1,000,000 K) gas. It has been widely demonstrated that large-scale outflows are triggered by a large number of recently born young massive stars that quickly die exploding as supernovae. These so-called galactic winds can abandon a dwarf galaxy due to its shallow gravitational potential, with the final result of ejecting metal-enriched gas into the empty intergalactic space.

NGC 1705 WFPC2 mosaic courtesy of P. Montegriffo (INAF, Osservatorio Astronomico di Bologna)

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Other Dwarf Irregular Galaxies:
Our closest neighbors: The Large and Small Magellanic Clouds