DeepDive: A deep dive into the physics of the first massive quiescent galaxies in the Universe

TL;DR

This study uses JWST/NIRSpec spectroscopy to analyze the properties and diversity of massive quiescent galaxies at high redshift, revealing their stellar populations, low star formation, and AGN presence, with implications for galaxy evolution.

Contribution

First deep spectroscopic survey of massive quiescent galaxies at z~3-4 using JWST, extending wavelength coverage and analyzing stellar populations and AGN activity.

Findings

Most high-redshift quiescent galaxies have weak 4000A breaks and younger stellar populations.

Detected faint Iron and Magnesium absorption lines in stacked spectra.

Identified AGN activity in some targets through broad Hα lines.

Abstract

We present the DeepDive program, in which we obtained deep ($1-3$ hours) JWST/NIRSpec G235M/F170LP spectra for 10 primary massive ($\log{(M_\star/M_\odot)}=10.8-11.5$) quiescent galaxies at $z\sim3-4$. A novel reduction procedure extends the nominal wavelength coverage of G235M beyond H$\alpha$ and [NII] at $z\sim4$, revealing weak, narrow H$\alpha$ lines indicative of low star formation rates (${\rm SFR}\sim0-5\, M_\odot\, {\rm yr^{-1}}$). Two out of 10 primary targets have broad H$\alpha$ lines, indicating the presence of AGNs. We also conduct an archival search of quiescent galaxies observed with NIRSpec gratings in the DAWN JWST Archive, which provides a statistical context for interpreting the DeepDive targets. This archival search provides a spectroscopic sample of 140 quiescent galaxies spanning $1<z<5$ and covering more than an order of magnitude in stellar mass. We revisit the selection of quiescent galaxies based on rest-frame $UVJ$ colors, specific star formation rates, and the detection of the 4000\r{A} spectral break, finding $\sim90\%$ overlap between these criteria. The sample of a total of 150 quiescent galaxies constructed in this study shows that those at $z\sim3-5$, including the DeepDive targets, typically exhibit weaker 4000\r{A} breaks and bluer colors than their lower-redshift counterparts, indicating generally younger stellar populations. Stacked spectra of sources grouped by the $D_n4000$ index reveal faint Iron and Magnesium absorption line features in the stellar continuum even for the low $D_n4000$ ($D_n4000<1.35$) subsample at high redshift ($z\sim3$). In addition, higher $D_n4000$ subsamples show fainter nebular emission lines. These results demonstrate that medium-resolution NIRSpec spectroscopy is essential for robustly characterizing the diversity and evolution of early quiescent galaxies. All data from this study will be made publicly available.