AGN and Star-Formation Properties of Inside-out Assembled Galaxy Candidates at z<0.1

TL;DR

This study analyzes a large sample of local galaxies to understand their assembly history, star formation activity, and AGN presence, revealing that many are in transition from star-forming to quiescent states with ongoing AGN activity.

Contribution

It identifies and characterizes inside-out assembled galaxy candidates at low redshift, linking their assembly history with star formation quenching and AGN activity, which is a novel insight.

Findings

Majority are classified as AGN or composite in BPT diagram.

Most are below the main sequence, in green valley or red sequence.

AGN candidates have lower SFRs but retain spiral morphology.

Abstract

We study a sample of 48127 galaxies selected from the SDSS MPA-JHU catalogue, with $\log M_{\star}/M_{\odot} = 10.73 - 11.03$ and $z<0.1$. Local galaxies in this stellar mass range have been shown to have systematically shorter assembly times within their inner regions ($<0.5~R_{50}$) when compared to that of the galaxy as a whole, contrary to lower or higher mass galaxies which show consistent assembly times at all radii. Hence, we refer to these galaxies as Inside-Out Assembled Galaxy (IOAG) candidates. We find that the majority of IOAG candidates with well-detected emission lines are classified as either AGN (40%) or composite (40%) in the BPT diagram. We also find that the majority of our sources are located below the main sequence of star formation, and within the green valley or red sequence. Most BPT-classified star-forming IOAG candidates have spiral morphologies and are in the main sequence, whereas Seyfert 2 and composites have mostly spiral morphologies but quiescent star formation rates (SFRs). We argue that a high fraction of IOAG candidates seem to be in the process of quenching, moving from the blue cloud to the red sequence. Those classified as AGN have systematically lower SFRs than star-forming galaxies suggesting that AGN activity may be related to this quenching. However, the spiral morphology of these galaxies remains in place, suggesting that the central star-formation is suppressed before the morphological transformation occurs.