Central star formation in double-peak gas rich radio galaxies

Abstract

The respective contributions of gas accretion, galaxy interactions, and mergers to the mass assembly of galaxies, their morphological transformations, and the changes in their molecular gas content and star formation activity are still not fully understood. Galaxies with two kinematic components which are manifested as a double-peak (DP) in their emission lines, have been identified in a recent work to play a major role in the morphological transformation towards larger bulges. Notably, star-forming DP galaxies display a central star formation enhancement and are thought to be associated to a sequence of recent minor mergers. In order to probe merger induced star formation mechanisms, we conducted observations of the molecular gas content of star-forming DP galaxies in the upper part of the main sequence (MS) of star formation with the IRAM 30m telescope. In combination with existing molecular gas observations from the literature, we gathered a sample of 41 such galaxies. We succeed to fit the same kinematic parameters to the optical ionised and molecular gas emission lines for 24 (59 %) galaxies. We find a central star formation enhancement which is most likely the result of a galaxy merger or galaxy interactions which is indicated by an excess of gas extinction found in the centre. This star formation is traced by radio continuum emissions of 150 MHz, 1.4 GHz and 3 GHz, which are all three linearly correlated with the CO luminosity described by the same slope. By comparing the measured star formation efficiency and the molecular gas mass fraction with the expected values of the MS, we find a significantly larger amount of molecular gas in the present DP galaxies and larger depletion times. We discard a scenario of large scale instabilities driving gas into the centre and find no direct link between the measured kinematic signatures and inclination. This leads us to conclude that the observed DP galaxies are mostly the result of a recent merger that funnelled molecular gas towards the centre, triggering efficient star formation there.