Witnessing the star formation quenching in L* ellipticals

Abstract

We study the evolution of L∗ elliptical galaxies in the colour–magnitude diagram in terms of their star formation history and environment, in an attempt to learn about their quenching process. We have visually extracted 1109 L∗ galaxies from a sample of 36 500 galaxies that were spectroscopically selected from Stripe82 of the Sloan Digital Sky Survey (SDSS). From this sample, we have selected 51 ellipticals based on their surface-brightness profile being well-fitted by a single Se´rsic profile with Se´rsic indices 3 < n < 6. Our sample consists of 12 blue-cloud L∗ ellipticals (BLEs), 11 green-valley L∗ ellipticals (GLEs), and 28 red-sequence L∗ ellipticals (RLEs). We find that most of the RLEs and GLEs have been quenched only recently, or are still forming stars, based on their [O III] and H α emission, while the BLEs are forming stars vigorously. The star formation in BLEs is found to be extended over the galaxy and not confined to their central region. In about 40 per cent of the L∗ ellipticals (10 BLEs, 4 GLEs, and 5 RLEs), star formation quenching seems to have started only recently, based on the lower [O III] emission compared to the [O II] and H α emission, at a given metallicity. We also find that the galaxy colour is correlated with the cosmic-web environment, with the BLEs tending to reside in lower density regions, the RLEs preferring denser, clustered regions, and the GLEs found in either. One possible scenario is that as the star-forming ellipticals migrate into the clusters, their star formation is suffocated by the hot intracluster medium.