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Astronomers have found new evidence that a nearby dwarf galaxy may have recently collided with a neighbor, altering its shape and motion. The study, led by Adebusola Alabi of North-West University in South Africa, reveals that UGCA 320, located about 6 million light-years away, shows clear signs of gravitational disruption likely caused by an encounter with the nearby dwarf galaxy UGCA 319.
Researchers built a detailed picture of UGCA 320 using data from the Hubble Space Telescope, the Southern African Large Telescope, and the Very Large Telescope. The findings, published in Monthly Notices of the Royal Astronomical Society, suggest that the galaxy’s uneven structure and motion are the result of a slow-moving galactic interaction.
UGCA 320 is a dwarf irregular galaxy rich in gas and stars of different ages. Its strikingly blue color, measured at a V-I value of 0.1 magnitudes, points to a young population of stars no older than 120 million years, mixed with ancient stars over 10 billion years old.
The galaxy spans about 2,000 light-years across, and its stars rotate at speeds up to 40 kilometers per second (double the speed of its ionized gas). Both move in the same direction but are slightly misaligned, suggesting the stars and gas no longer share the same plane.
Velocity maps reveal an unusual distortion roughly 1,000 light-years from the center, where stellar rotation slows and becomes irregular. Using a technique known as kinemetry, Alabi’s team found that stars in the galaxy’s core rotate at a 250-degree angle offset from the outer disk.
UGCA 319, located about 100,000 light-years from UGCA 320, appears to be the likely partner in this interaction. The two galaxies move at a relative speed of about 12 km/s. Observations from the MHONGOOSE survey show that UGCA 320’s neutral hydrogen gas extends unevenly, hinting at a tidal pull from its neighbor. However, no gas bridge connects them yet, suggesting the encounter is still in its early phase.
Despite its large gas reserves, UGCA 320 is forming new stars very slowly. Its star formation rate is about 0.025 solar masses per year, enough to sustain growth over billions of years but far below that of more active galaxies. Researchers say the gas remains too cool and diffuse to collapse efficiently into stars, a common trait among dwarf galaxies.
Chemical studies show that both stars and gas in UGCA 320 contain low amounts of heavy elements, between 15% and 30% of the Sun’s levels. The uniform metallicity across the disk suggests that the galaxy has evolved slowly without strong bursts of star formation to enrich its material.
Scientists study galaxies like UGCA 320 because dwarf galaxies played a major role in shaping the early universe. About 13 billion years ago, small, star-forming galaxies emitted intense ultraviolet radiation that helped clear hydrogen gas left over from the Big Bang. Systems like UGCA 320 and UGCA 319 serve as modern analogues, showing how smaller galaxies interact and evolve over time.
The pair, along with a third dwarf named LEDA 886203, forms an isolated group free from the influence of larger galaxies like the Milky Way. This isolation makes them a rare opportunity to study dwarf-galaxy interactions without external interference.
Alabi’s team plans to conduct follow-up observations of UGCA 319 to look for matching signs of disruption and to study a faint tail extending from UGCA 320’s northwest edge. These studies could confirm whether the galaxies are actively exchanging material.
Researchers say UGCA 320’s quiet collision helps explain why some dwarf galaxies stay faint while others grow brighter after encounters. In a universe shaped by mergers, even the smallest interactions can leave lasting marks on the galaxies we see today.
Source: Stars and ionized gas in UGCA 320: a nearby gas-rich, dwarf Irregular galaxy
