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VLT Observes Small and Nearby Galaxies

The Stars in Antlia and NGC 6822

27 lutego 1999

The VLT UT1 and FORS1 has performed observations of many different types of objects during the past months. While much effort has been spent on extremely distant galaxies, some exposures were also made on more nearby systems, including some dwarf galaxies in the Local Group of Galaxies. The Andromeda Galaxy and the Magellanic Clouds, as well as our own Milky Way Galaxy, are the best known members of this group.

Images of the Antlia dwarf galaxy

Antlia is a small, low-surface brightness, dwarf galaxy of spheroidal form that was discovered in 1997. It received this name, because it is seen within the southern constallation Antlia (The Air Pump). It is an outlying member of the Local Group of Galaxies.

From previous work, it appears that the Antlia dwarf galaxy has no bright blue stars that belong to a young stellar population, nor is there any compelling evidence of the presence of an extensive population of stars of intermediate age.

It therefore looks as if this small galaxy is dominated by an old stellar population, similar to what has been found for most other dwarf spheroidal galaxies in the Local Group. However, unlike any of these, Antlia appears to contain a cloud of atomic hydrogen gas in its centre with a total mass of about 8 x 10 5 solar masses. It is thus of particular interest to try to determine the history of star formation in a galaxy that, paradoxically, is relatively gas rich (which ought to incur ongoing formation of stars) and yet seemingly dominated entirely by old stars.

Very high quality images of Antlia were obtained in different colours by FORS1 on VLT UT1 in the period 27 - 31 January 1999. These data were taken around full moon, so they do not show as faint stars as they would under normal FORS operating conditions, but the image quality is exceptional. They represent by far the best images ever obtained of this galaxy, and they include the first deep B-band images.

Spectra of stars in NGC 6822

The VLT also obtained images of another dwarf galaxy in the Local Group, NGC 6822 , as well as spectra of some of its stars. This galaxy is of the "irregular" type and is situated at a distance of about 2 million light-years.

In addition to numerous stars that emit a continuous spectrum, the image also shows diffuse emission from interstellar hydrogen gas in NGC 6822. Moreover, strong H-alpha emission is seen from several stellar-wind bubbles around hot young stars.

It is apparent from the figures that many spectral lines in the supergiant star in NGC 6822 are much weaker than in the Milky Way counterparts, but of similar strength as in those in SMC. This confirms the earlier finding that NGC 6822 has a different chemical composition - a lower content of heavy elements (or "metallicity") - than what is observed in our Galaxy.

A detailed analysis of these spectra will allow to determine the exact chemical composition of this irregular dwarf galaxy. The spectra also demonstrate the potential of the FORS instruments at the VLT to do quantitative spectroscopy of stars in external galaxies.

NGC 6822 may become another key galaxy for the study of stellar evolution and stellar winds from massive stars with low metallicity. NGC 6822 may then also be used as a calibrator for the extragalactic distance scale , by means of the method of the stellar wind momentum - luminosity relationship (WLR) .

Outlook

The observations of the Antlia and NGC 6822 dwarf galaxies in the Local Group of Galaxies provide an example of what the VLT will be able to achieve in the future, in terms of imaging nearby galaxies for studies of the stellar population, and also of spectra of individual stars. This may possibly uncover other differences and similarities that will provide clues about the formation and evolution of the galaxies in the Local Group.

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The first image is a true-colour image of the dwarf galaxy Antlia combined from three exposures with the FORS1 multi-mode instrument at VLT UT1, in the B (blue), V (green) and R (red) optical bands. Many background galaxies are seen in the field. The total exposure times were 2400 sec, 2400 sec and 5400 sec, respectively. The seeing varied between 0.5 and 0.7 arcsec. The field measures about 7 x 7 arcmin 2. North is up and East is left.

The second image is a reproduction of a narrow-band image of the central region of NGC 6822. This image was taken with a filter centered on the H-alpha spectral line of atomic hydrogen. It was obtained by the FORS1 Commissioning Team on October 2, 1998, in the FORS high-resolution imaging mode. The exposure time was 15 min and the atmospheric conditions were good. The size of the stellar images in the field is about 0.5 arcsec and this photo shows more details than earlier ones obtained with ground-based telescopes of this galaxy.

The third and fourth image display a FORS1 spectrum of a blue supergiant in NGC 6822 (in the middle), as compared with those of supergiant stars (upper and lower) of similar atmospheric temperature in the Milky Way Galaxy and in the Small Magellanic Cloud (SMC), another irregular dwarf galaxy in the Local Group. The observations were carried out by the FORS Commissioning Team on  1 October, 1998, using the Multiple Object Spectroscopy (MOS) mode of FORS1. A total of 11 stars in NGC 6822 were observed simultaneously with an exposure time of 30 min. The visual magnitude of supergiant star centered on slit no. 5 is V = 18.11 and the spectral type is in the interval O9.5Ia - B0Ia.

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