HST grism spectroscopy of z~3 massive quiescent galaxies: Approaching the metamorphosis

C. D'Eugenio; E. Daddi; R. Gobat; V. Strazzullo; P. Lustig; I. Delvecchio; S. Jin; A. Cimatti; M. Onodera

doi:10.1051/0004-6361/202040067

paper

HST grism spectroscopy of z~3 massive quiescent galaxies: Approaching the metamorphosis

arXiv:2012.02767 · doi:10.1051/0004-6361/202040067

Abstract

Tracing the emergence of the massive quiescent galaxy (QG) population requires the build-up of reliable quenched samples. We present Hubble Space Telescope WFC3/G141 grism spectra of 10 quiescent galaxy candidates selected at $2.5 < z < 3.5$ in the COSMOS field. Spectroscopic confirmation for the whole sample is obtained within 1-3 orbits based on the presence of strong spectral breaks and Balmer absorption lines. Combining their spectra with optical to near-IR photometry, star-forming solutions are formally rejected for the entire sample. Broad spectral indices are consistent with the presence of young A-type stars, which implies that the last major episode of star formation has taken place no earlier than $\sim$ 300-800 Myr prior to observation. Marginalising over three different slopes of the dust attenuation curve, we obtain short mass-weighted ages and an average peak star formation rate of SFR $\sim 1 0^{3}$ M $_{⊙}$ yr $^{- 1}$ at $z_{f or ma t i o n} \sim 3.5$ . Despite mid- and far-IR data are too shallow to determine the obscured SFR on a galaxy-by-galaxy basis, the mean stack emission from 3GHz data constrains the level of residual obscured SFR to be globally below 50 M $_{⊙}$ yr $^{- 1}$ , hence three times below the scatter of the coeval main sequence. Alternatively, the very same radio detection suggests a widespread radio-mode feedback by active galactic nuclei (AGN) four times stronger than in z $\sim$ 1.8 massive QGs. This is accompanied by a 30% fraction of X-ray luminous AGN with a black hole accretion rate per unit SFR enhanced by a factor of $\sim 30$ with respect to similarly massive QGs at lower redshift. The average compact, high Sérsic index morphologies of our galaxies, coupled with their young mass-weighted ages, suggest that the mechanisms responsible for the development of a spheroidal component might be concomitant with (or preceding) those causing their quenching.

arXiv commentAccepted for publication in A&A. 28 pages, 16 figures + additional plots in appendix A

HST grism spectroscopy of z~3 massive quiescent galaxies: Approaching the metamorphosis

Abstract

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