The growth of the central region by acquisition of counter-rotating gas in star-forming galaxies

TL;DR

This study provides observational evidence that external gas acquisition in blue star-forming galaxies can lead to central growth, with counter-rotating gas fueling nuclear star formation, a process previously observed mainly in red galaxies.

Contribution

It is the first systematic study showing that blue, star-forming galaxies also acquire external gas, leading to central growth and star formation, expanding understanding of galaxy evolution mechanisms.

Findings

Approximately 2% of blue galaxies are counter-rotators.

Counter-rotating blue galaxies have younger central stellar populations.

External gas acquisition drives nuclear star formation in blue galaxies.

Abstract

Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 489 galaxies). The central regions of blue counter-rotators show younger stellar populations and more intense, ongoing star formation than their outer parts, indicating ongoing growth of the central regions. The result offers observational evidence that the acquisition of external gas in blue galaxies is possible; the interaction with pre-existing gas funnels the gas into nuclear regions (< 1 kpc) to form new stars.