HST/WFC3 NEAR-INFRARED SPECTROSCOPY OF QUENCHED GALAXIES AT z similar to 1.5 FROM THE WISP SURVEY: STELLAR POPULATION PROPERTIES
We combine Hubble Space Telescope (HST) G102 and G141 near-IR (NIR) grism spectroscopy with HST/WFC3UVIS, HST/WFC3-IR, and Spitzer/IRAC [3.6 mu m] photometry to assemble a sample ofmassive (log(M-star/M-circle dot) similar to 11.0) and quenched (specific star formation rate < 0.01 Gyr-1) galaxies at z similar to 1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy for quenched sources at these redshifts. In contrast to the local universe, z similar to 1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties. We find that their spectral energy distributions (SEDs) are well fitted with exponentially decreasing star formation histories and short star formation timescales (tau <= 100 Myr). Quenched galaxies also show a wide distribution in ages, between 1 and 4 Gyr. In the (u - r)(0)-versus-mass space quenched galaxies have a large spread in rest-frame color at a given mass. Most quenched galaxies populate the z similar to 1.5 red sequence (RS), but an important fraction of them (32%) have substantially bluer colors. Although with a large spread, we find that the quenched galaxies on the RS have older median ages (3.1 Gyr) than the quenched galaxies off the RS (1.5 Gyr). We also show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies off the RS. We derive the upper limit on the fraction of massive galaxies on the RS at z similar to 1.5 to be < 43%. We speculate that the young quenched galaxies off the RS are in a transition phase between vigorous star formation at z > 2 and the z similar to 1.5 RS. According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z similar to 1.5 RS is of the order of similar to 1Gyr.