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Astronomers from South Africa have conducted multi-band observations of a nearby dwarf irregular galaxy known as UGCA 320. Results of the observational campaign, recently on the arXiv preprint server, yield important insights into the nature of this galaxy.

Located some 19.7 million light years away in the constellation Virgo, UGCA 320, also known as DDO 161 or HIPASS J1303-17b, is a gas-rich, optically blue, dwarf irregular galaxy that belongs to a nearby, relatively isolated group of dwarf galaxies. The galaxy has a stellar mass of around 93 million solar masses, a systematic velocity of approximately 740 km/s, and its star-formation rate (SFR) is estimated to be less than 0.02 solar masses per year.

Given that still very little is known about the nature and properties of UGCA 320, a team of astronomers led by Adebusola B. Alabi of the North-West University in South Africa decided to take a closer look at this galaxy, focusing mainly on its stellar populations and ionized gas. For this purpose, they employed the Hubble Space Telescope (HST), the Southern African Large Telescope (SALT) and the Very Large Telescope (VLT).

"Here, we combine multi-band HST imaging data with deep long-slit SALT/RSS and integral-field VLT/MUSE spectral data to study the stellar and ionized gas components of UGCA 320," the researchers write.

The new observations found that the radial color profile of UGCA 320 is flat and very blue at all radii, with the difference in magnitude between the visual (V) and infrared (I) spectral bands of about 0.1 mag. The blue color suggests that UGCA 320 is dominated by the population of very young and/or metal-poor stars.

Additionally, it was found that the stellar disk of UGCA 320 is dominated by young stars, mostly with light-weighted ages younger than one billion years. It turned out that while the integrated stellar light from UGCA 320 is dominated by very young stars with a median age of about 120 million years, a substantial population of old stars (with light-weighted stellar age of over 10 billion years) is also present in the galaxy.

The study detected an abundance of ionized gas in UGCA 320, which, according to the astronomers, is powered mostly by recent star formation. The regions in UGCA 320 with the highest ionized gas intensities appear to be clumpy and are predominantly seen above the galaxy's major axis (in the north-east direction). However, the less intense, more diffuse tails of ionized gas are observed below the major axis, extending in the south-west direction.

The collected data indicate rotation in both the stellar and ionized gas components, with both rotating in the same sense and having comparable velocity dispersion. However, within the innermost region of the galaxy, the rotation amplitude of the stars rises steeply to about 30 km/s, more than that of the ionized gas component.

The researchers predict that it will take UGCA 320 some 49.5 billion years to exhaust its entire gas reservoir, assuming star formation continues at the current SFR and that all of the cold gas can migrate towards the disk.

The observations also identified a sharp transition in the kinematic properties of UGCA 320's stellar and ionized gas disks at a radius of about 1,000 light years and distortions in the outer disk region. The authors of the paper concluded that these features point to a recent tidal interaction of UGCA 320, most likely with its close neighbor—the galaxy UGCA 319.