Active Galactic Nuclei in the Green Valley at z$\sim$0.7

TL;DR

This study investigates the relationship between active galactic nuclei (AGN) and star formation in massive galaxies at z~0.7, finding a high AGN fraction in green valley galaxies but no definitive link to star formation quenching.

Contribution

It provides a detailed analysis of AGN prevalence and star formation activity in green valley galaxies at z~0.7 using combined optical and NIR spectroscopy, with a focus on the role of AGN in galaxy quenching.

Findings

89% of green valley or below galaxies host AGN

AGN host galaxies are 0.37 dex below the star-forming main sequence

No conclusive evidence linking AGN activity to star formation suppression

Abstract

We present NIR spectroscopy using MMT/MMIRS for a sample of twenty-nine massive galaxies ($\mathrm{log\ M_* / M_{\odot} \gtrsim10}$) at $\mathrm{z\sim0.7}$ with optical spectroscopy from the LEGA-C survey. Having both optical and NIR spectroscopy at this redshift allows us to measure the full suite of rest-optical strong emission lines, enabling the study of ionization sources and the rest-optical selection of active galactic nuclei (AGN), as well as the measurement of dust-corrected $\mathrm{H\alpha}$-based SFRs. We find that eleven out of twenty-nine galaxies host AGN. We infer the nonparametric star formation histories with the SED fitting code \texttt{Prospector} and classify galaxies as star-forming, green valley, or quiescent based on their most recent sSFRs. We explore the connection between AGN activity and suppressed star formation and find that $89\pm15\%$ of galaxies in the green valley or below host AGN, while only $15\%\pm8\%$ of galaxies above the green valley host AGN. We construct the star-forming main sequence (SFMS) and find that the AGN host galaxies are 0.37 dex below the SFMS while galaxies without detectable AGN are consistent with being on the SFMS. However, when compared to a bootstrapped mass-matched sample, the SFRs of our sample of AGN host galaxies are consistent with the full LEGA-C sample. Based on this mass-matched analysis, we cannot rule out that this suppression of star formation is driven by other processes associated with the higher mass of the AGN sample. We therefore cannot link the presence of AGN activity to the quenching of star formation.