An exquisitely deep view of quenching galaxies through the gravitational
  lens: Stellar population, morphology, and ionized gas
  This work presents an in-depth analysis of four gravitationally lensed red
galaxies at z = 1.6-3.2. The sources are magnified by factors of 2.7-30 by
foreground clusters, enabling spectral and morphological measurements that are
otherwise challenging. Our sample extends below the characteristic mass of the
stellar mass function and is thus more representative of the quiescent galaxy
population at z > 1 than previous spectroscopic studies. We analyze deep
VLT/X-SHOOTER spectra and multi-band Hubble Space Telescope photometry that
cover the rest-frame UV-to-optical regime. The entire sample resembles stellar
disks as inferred from lensing-reconstructed images. Through stellar population
synthesis analysis we infer that the targets are young (median age = 0.1-1.2
Gyr) and formed 80% of their stellar masses within 0.07-0.47 Gyr. Mg II
$\lambda\lambda 2796,2803$ absorption is detected across the sample.
Blue-shifted absorption and/or redshifted emission of Mg II is found in the two
youngest sources, indicative of a galactic-scale outflow of warm ($T\sim10^{4}$
K) gas. The [O III] $\lambda5007$ luminosity is higher for the two young
sources (median age less than 0.4 Gyr) than the two older ones, perhaps
suggesting a decline in nuclear activity as quenching proceeds. Despite
high-velocity ($v\approx1500$ km s$^{-1}$) galactic-scale outflows seen in the
most recently quenched galaxies, warm gas is still present to some extent long
after quenching. Altogether our results indicate that star formation quenching
at high redshift must have been a rapid process (< 1 Gyr) that does not
synchronize with bulge formation or complete gas removal. Substantial bulge
growth is required if they are to evolve into the metal-rich cores of
present-day slow-rotators.
