The Typical Massive Quiescent Galaxy at z ∼ 3 is a Post-starburst

Abstract

We have obtained spectroscopic confirmation with Hubble Space Telescope WFC3/G141 of a first sizeable sample of nine quiescent galaxies at 2.4<z<3.3. Their average near-UV/optical rest-frame spectrum is characterized by low attenuation (Av∼0.6 mag) and a strong Balmer break, larger than the 4000Å break, corresponding to a fairly young age of ∼300 Myr. This formally classifies a substantial fraction of classically selected quiescent galaxies at z ∼ 3 as post-starbursts, marking their convergence to the quenching epoch. The rapid spectral evolution with respect to z ∼ 1.5 quiescent galaxies is not matched by an increase of residual star-formation, as judged from the weak detection of [O II]λ3727 emission, pointing to a flattening of the steep increase in gas fractions previously seen from z ∼ 0 to 1.8. However, radio 3GHz stacked emission implies either much stronger dust-obscured star formation or substantial further evolution in radio-mode AGN activity with respect to z ∼ 1.5.

HST grism spectroscopy of z ∼ 3 massive quiescent galaxies

“…Their global strength was quantified and compared to that of the 4000 Å break by means of the D B /D n 4000 ratio, which is systematically higher than 1. This spectroscopically confirms on a galaxy-by-galaxy basis the PSB nature of massive bright QGs, which was pointed out before by means of stacking (D'Eugenio et al 2020) and by individual highresolution spectra (Forrest et al 2020b). More observational efforts are required to explore to which extent strong Balmer absorption lines are spread among objects with a lower massto-light ratio.…”

An Ancient Massive Quiescent Galaxy Found in a Gas-rich z ∼ 3 Group

“…This form of rapid quenching has been extensively proposed to explain populations of QGs at z ∼ 3-4 (e.g., Schreiber et al 2018b;D'Eugenio et al 2020aD'Eugenio et al , 2020bForrest et al 2020aForrest et al , 2020bValentino et al 2020). However, these are usually found to be PSBs, with t 50 < 0.8 Gyr (Figure 5).…”

“…Following the analysis in similar studies (Gobat et al 2012;Schreiber et al 2018b;D'Eugenio et al 2020a;Valentino et al 2020), we define t 50 , which is the time elapsed since the epoch of "half-mass formation" up until the time of observation. We determine this quantity to be 1.6 ± 0.4 Gyr at 90% confidence, after marginalizing on all the unknown parameters (SFH, metallicity, dust attenuation law and A V , and redshift).…”

“…Hydrodynamical simulations and semianalytical models have found it difficult to reproduce massive (10 11 M * ) quiescent galaxies (QGs) beyond z  3 (Steinhardt et al 2016;Schreiber et al 2018b;Cecchi et al 2019), while observations have been pushing the redshift boundary by detecting such systems at z ∼ 3-4 (Gobat et al 2012;Glazebrook et al 2017;Schreiber et al 2018a;D'Eugenio et al 2020a;Forrest et al 2020aForrest et al , 2020bSaracco et al 2020;Valentino et al 2020). Most of the systems so far observed belong to a population of recently quenched "poststarburst" (PSB) galaxies with their last star formation episode occurring over the last <0.8 Gyr.…”

The evolution of the gas fraction of quiescent galaxies modeled as a consequence of their creation rate

“…In addition, we also included f gas estimates derived from the (median) stacked FIR spectral energy distributions of ETGs at z ∼ 1.8 (G18; 977 galaxies), z ∼ 1.2, z ∼ 0.8, and z ∼ 0.5 (1394, 1536, and 563 galaxies, respectively; Magdis et al, submitted, hereafter M20). Finally, at higher redshift (z ∼ 3), we converted star formation rates (SFR) estimated from spectroscopy (Schreiber et al 2018c;D'Eugenio et al 2020) into gas masses assuming the star formation efficiency found by G18. As a consequence of our z max = 3.5, we did not include higher-redshift quiescent galaxies (Glazebrook et al 2017;Schreiber et al 2018b;Tanaka et al 2019;Valentino et al 2020) in the analysis and considered z ∼ 3 galaxies as pSB.…”