Starburst galaxies, which are characterised by a high rate of star formation, are found to form stars differently across cosmic time.

In the past, these galaxies formed new stars evenly throughout their entire structures, whereas in the present universe, star formation has become centrally concentrated.

In a recent study published in the Monthly Notices of the Royal Astronomical Society, PhD student Marcie Mun at the Australian National University and the MAGPI survey team have captured a distinct characteristic of starbursting galaxies unique to nearby galaxies.

This was made possible with the MAGPI survey, which targets a population of galaxies from 3 – 4 billion years ago with the Multi-Unit Spectroscopic Explorer instrument on the Very Large Telescope of the European Southern Observatory. This period sits just between today and cosmic noon (about 10 billion years ago), where the latter is known to host a plethora of highly star-forming and gas-rich galaxies.

Images of example MAGPI galaxies, where from left to right, each row shows the white-light image, segmentation map identifying the pixels that belong to the galaxy, stellar mass surface density map, star formation rate surface density map, and the ratio of the measured vs. expected star formation rate surface density for the corresponding stellar mass surface density (higher values indicating higher than expected star formation activity). From top to bottom are example galaxies classified as starburst (pink), main sequence (blue; regular star-forming galaxies), green valley (green; galaxies in the process of quenching), and quenched (red).

One of the notable strengths of the MAGPI survey is its high spatial resolution. This allowed the team to spatially resolve the stars and ionised gas within the galaxies at a comparable resolution to that of nearby galaxies probed by other integral field spectroscopy surveys.

Taking advantage of the epoch and resolution, Marcie bridges the local universe and cosmic noon to find an evolution in the formation of stars within starburst galaxies. Galaxies at and beyond 3 – 4 billion years ago have formed stars evenly from the centre to the outskirts, likely due to higher gas accretion rates providing sufficient star-forming fuel across the entire galaxy. However, nearby galaxies show centrally concentrated star formation, hinting towards phenomena such as galaxy-galaxy interactions funnelling gas towards the galaxy centre, leading to a nuclear starburst.

On the other hand, passive or quenching galaxies seem to stay constant in their mannerisms over a long time. Star formation is ceasing in these galaxies starting in the centre, suggesting feedback due to the supermassive black hole in the centre as the main culprit. When switched on, black holes are renowned for their ability to blow out cold gas from the galaxy’s centre, which may begin quenching the galaxy from the inner regions to the outskirts – a phenomenon known as inside-out quenching.