Astronomers have utilized the Very Large Telescope (VLT) in Chile to observe the halo of the nearby giant elliptical galaxy, Centaurus A. Their findings, published on June 1 on the arXiv pre-print server, offer significant insights into the star formation history within this galaxy's halo.
Centaurus A, also cataloged as NGC 5128 or Caldwell 77, was discovered nearly two centuries ago and remains the closest giant elliptical galaxy to Earth, situated approximately 12.4 million light-years away. Known for being one of the nearest radio galaxies, its active galactic nucleus (AGN) has been the subject of extensive research. The galaxy's extended halo and radio lobes span almost 2 degrees of the sky in optical maps. As a post-merger galaxy, understanding its formation history has been a focal point for many studies.
Recently, a team of astronomers led by Sima T. Aghdam from the Institute for Research in Fundamental Sciences in Tehran, Iran, conducted VLT observations of Centaurus A to delve deeper into its origins and properties. They focused on two small fields within the galaxy's halo, aiming to shed light on the star formation history. By examining variables in these fields, the team sought to gain a more comprehensive understanding of the galaxy's past.
The researchers based their methodology on identifying long period variable (LPV) stars, which are indicative of the sibling stellar population and thus, the historical star formation due to their high luminosity and strong variability. They identified 395 LPVs in the northeastern field, referred to as Field 1 (located approximately 61,300 light-years from Centaurus A's center), and 671 LPVs in Field 2, the southern field (about 32,300 light-years from the galaxy's center).
Despite the two fields being roughly 91,000 light-years apart on opposite sides of Centaurus A, the astronomers found similar star formation histories in both areas. Their analysis revealed significant increases in star formation rates around 3.8 billion and 800 million years ago.
The researchers suggest that the recent star formation surge, starting about 800 million years ago, could be linked to a merger event that occurred approximately 1 billion years ago. They noted that the majority of the stars in Centaurus A's halo were formed more than 400 million years ago.
From these observations, the team hypothesizes that Centaurus A may have assimilated a small, gas-rich spiral galaxy, which provided the necessary material for ongoing star formation at its center. Additionally, they propose that the intermediate-age stars may have originated from this accreted galaxy or from a previous merger event.