(ORDO NEWS) — Using the Subaru Telescope, an international team of astronomers have made deep multi-stripe photometric observations of the dwarf irregular galaxy known as NGC 6822.
At about 1.63 million light-years away, NGC 6822 (also known as Barnard’s Galaxy) is a dwarf irregular galaxy in the Local Group. It is one of the closest galaxies to the Milky Way, but lies outside its virial radius. The galaxy is about 7,000 light-years across and hosts a spatially extended system of globular clusters (GCs).
NGC 6822 is located in such a way that it can be observed from both the northern and southern hemispheres. This modest distance allows for multi-band studies from optical to near-infrared and mid-infrared.
Moreover, the average metal-intermediate chemistry and the identification of solid young, intermediate, and old stellar indicators, as well as the apparent isolation, make NGC 6822 a very interesting laboratory for stellar pulsation and evolution.
That’s why a team of astronomers led by Maria Tantalo from the Tor Vergata University of Rome in Italy has performed an accurate multi-stripe photometry of NGC 6822 using the Hyper-Supreme-Cam (HSC) camera on the Maunakea Subaru telescope.
The HSC dataset was supplemented with multiband images collected with the wideband MegaPrime device on the Canada-France-Hawaii Telescope (CFHT), the dark energy camera (DECam) on the Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) and the wideband camera mounted on the Isaac Newton Telescope (INT) on La Palma.
As a result, the Tantalo team obtained a set of photometric data covering an area of two square degrees at the center of NGC 6822 with three different photometric ranges. In total, they performed about 40 million photometric measurements of objects across the entire visibility zone.
The resulting catalog includes more than 1 million stars that have at least one measurement in two different photometric ranges. This makes it the broadest and most homogeneous set of photometric data ever collected for nearby dwarf irregular galaxies, with the exception of the Magellanic Clouds.
The study found that young, intermediate, and old stars in NGC 6822 show different radial distributions. The old stellar population is distributed spherically and extends over radial distances greater than previously thought (about 1 degree). As for the young cluster, it shows a well-defined band and a disk-like distribution that is off-center compared to the old one.
In addition, the study showed that the carbon-rich stars in NGC 6822 are more concentrated in the center and have structural parameters similar to both young and old stellar tracers.
The astronomers also estimated the average population ratio between carbon stars and M-type stars, which turned out to be 0.67. This suggests that the stars of the asymptotic giant branch of the galaxy have an average iron content of approximately -1.25.
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