The Typical Massive Quiescent Galaxy at z$\sim$3 is a Post-starburst

Understand

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$\sim$0.6 mag) and a strong Balmer break, larger than the 4000 A break, corresponding to a fairly young age of $\sim$300 Myr.
• This formally classifies a substantial fraction of classically selected quiescent galaxies at z $\sim$ 3 as post-starbursts, marking their convergence to the quenching epoch.
• The rapid spectral evolution with respect to z $\sim$ 1.5 quiescent galaxies is not matched by an increase of residual star-formation, as judged from the weak detection of [O II]3727A emission, pointing to a flattening of the steep increase in gas fractions previously seen from z $\sim$ 0 to 1.8.