The Evolution of Advanced Merger (U)LIRGs on the Color-Stellar Mass Diagram

TL;DR

This study investigates the evolutionary stages of advanced merger (U)LIRGs by analyzing star formation indicators and colors, revealing evidence of quenching and a transition from starburst to post-starburst phases.

Contribution

It provides new observational evidence of the evolutionary process of (U)LIRGs on the color-stellar mass diagram, highlighting the transition from starburst to quiescence.

Findings

Redder NUV-r colors correlate with lower EW(Hα) and higher Dₙ(4000).

Adv-merger (U)LIRGs near the green valley show signs of fading from starburst activity.

Some (U)LIRGs are at the initial post-starburst stage with ages around 1 Gyr.

Abstract

Based on a sample of 79 local advanced merger (adv-merger) (U)LIRGs, we search for the evidence of quenching process by investigating the distributions of the star formation history indicators (EW(H$\alpha$), EW(H$\delta$$_A$) and D$_n(4000)$) on the NUV-r color-mass and SFR-$M_{\ast}$ diagrams. The distributions of the EW(H$\alpha$) and D$_n(4000)$ on the NUV-r color-mass diagram show clear trends that at a given stellar mass, galaxies with redder NUV-r colors have smaller EW(H$\alpha$) and larger D$_n(4000)$. The reddest adv-merger (U)LIRGs close to the green valley have D$_n(4000)$$>1.4$ mostly. In addition, in the SFR-$M_{\ast}$ diagram, as the SFR decreases, the EW(H$\alpha$) decreases and the D$_n(4000)$ increases, implying that the adv-merger (U)LIRGs on the star formation main sequence have more evolved stellar populations than those above the main sequence. These results indicate that a fraction of the adv-merger (U)LIRGs have already exhibited signs of fading from the starburst phase and that the NUV-r reddest adv-merger (U)LIRGs are likely at the initial stage of post-starbursts with age of $\sim 1$ Gyr, which is consistent with the gas exhausting time-scales. Therefore, our results offer additional support for the fast evolutionary track from the blue cloud to the red sequence.