MASSIVE AND NEWLY DEAD: DISCOVERY OF A SIGNIFICANT POPULATION OF GALAXIES WITH HIGH-VELOCITY DISPERSIONS AND STRONG BALMER LINES AT z ∼ 1.5 FROM DEEP KECK SPECTRA AND HST/WFC3 IMAGING

We present deep Keck/LRIS spectroscopy and Hubble Space Telescope/WFC3 imaging in the rest-frame optical for a sample of eight galaxies at z similar to 1.5 with high photometrically determined stellar masses. The data are combined with five Very Large Telescope/X-Shooter spectra. We find that these 13 galaxies have high velocity dispersions, with a median of sigma = 301 km s(-1). This high value is consistent with their relatively high stellar masses and compact sizes. We study their stellar populations using the strength of Balmer absorption lines, which are not sensitive to dust absorption. We find a large range in Balmer absorption strength, with many galaxies showing very strong lines indicating young ages. The median H delta(A) equivalent width, determined directly or inferred from the H10 line, is 5.4 angstrom, indicating a luminosity-weighted age of similar to 1 Gyr. Although this value may be biased toward higher values because of selection effects, high-dispersion galaxies with such young ages are extremely rare in the local universe. Interestingly, we do not find a simple correlation with rest-frame U - V color: some of the reddest galaxies have very strong Balmer absorption lines, which may indicate the importance of multiple bursts of star formation. These results demonstrate that many high-dispersion galaxies at z similar to 1.5 were recently quenched. This implies that there must be a population of star-forming progenitors at z similar to 2 with high velocity dispersions or linewidths, which are notoriously absent from CO/H alpha selected surveys.