The Heavy Metal Survey: Star Formation Constraints and Dynamical Masses of 21 Massive Quiescent Galaxies at z = 1.3–2.3

Abstract

In this paper, we present the Heavy Metal Survey, which obtained ultradeep medium-resolution spectra of 21 massive quiescent galaxies at 1.3 < z < 2.3 with Keck/LRIS and MOSFIRE. With integration times of up to 16 hr per band per galaxy, we observe numerous Balmer and metal absorption lines in atmospheric windows. We successfully derive spectroscopic redshifts for all 21 galaxies, and for 19 we also measure stellar velocity dispersions (σ v ), ages, and elemental abundances, as detailed in an accompanying paper. Except for one emission-line active galactic nucleus, all galaxies are confirmed as quiescent through their faint or absent Hα emission and evolved stellar spectra. For most galaxies exhibiting faint Hα, elevated [N ii]/Hα suggests a non-star-forming origin. We calculate dynamical masses (M dyn) by combining σ v with structural parameters obtained from the Hubble Space Telescope COSMOS(-DASH) survey and compare them with stellar masses (M *) derived using spectrophotometric modeling, considering various assumptions. For a fixed initial mass function (IMF), we observe a strong correlation between M dyn/M * and σ v . This correlation may suggest that a varying IMF, with high-σ v galaxies being more bottom heavy, was already in place at z ∼ 2. When implementing the σ v -dependent IMF found in the cores of nearby early-type galaxies and correcting for biases in our stellar mass and size measurements, we find a low scatter in M dyn/M * of 0.14 dex. However, these assumptions result in unphysical stellar masses, which exceed the dynamical masses by 34%. This tension suggests that distant quiescent galaxies do not simply grow inside-out into today’s massive early-type galaxies and the evolution is more complicated.

The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies since z ∼ 2

“…In addition to elemental abundances and stellar ages, the fullspectrum fitting provides measurements of the stellar velocity dispersions. We find that the velocity dispersions of the Heavy Metal galaxies agree with previous measurements of the dynamical mass-stellar mass relation at similar redshifts (e.g., Belli et al 2016), and we explore the implications of the Heavy Metal dynamical masses in the companion survey paper (Kriek et al 2024).…”

“…Systematic uncertainties remain <0.1 dex even at the lowest S/N (8 Å −1 ). We refer to Kriek et al (2024) for tabulated details of the observations and spectral energy distribution fitting results.…”

“…These integration times provide a sufficient S/N for modeling the faint stellar absorption features of individual massive quiescent galaxies. The typical S/N for the z ∼ 1.4 sample is 8 Å -1 and 15 Å -1 in the rest frame for LRIS and MOSFIRE-J, respectively, while the S/N for the z ∼ 2.1 sample is 4 Å -1 and 12 Å -1 for MOSFIRE-J and MOSFIRE-H. For further details on the target selection, observing strategy, data reduction, galaxy sample characteristics, and stellar population properties (i.e., stellar masses), we refer to Kriek et al (2024).…”

“…To that end, we have executed the Keck Heavy Metal Survey, an ultradeep spectroscopic survey with LRIS (Oke et al 1995;Rockosi et al 2010) and MOSFIRE (McLean et al 2010 on the Keck I Telescope (see Kriek et al 2024). This survey collected high signal-to-noise ratio (S/N) spectra of 21 massive quiescent galaxies at 1.4  z  2.2, covering multiple Balmer and metal absorption lines.…”

“…Preliminary results of five Heavy Metal galaxies were presented in Kriek et al (2019) and are updated in the current work. The survey design is presented in the companion survey paper (Kriek et al 2024). In Section 2, we describe our full-spectrum modeling technique.…”