Compact, bulge dominated structures of spectroscopically confirmed
  quiescent galaxies at z~3
  We study structural properties of spectroscopically confirmed massive
quiescent galaxies at $z\approx 3$ with one of the first sizeable samples of
such sources, made of ten $10.8<\log(M_{\star}/M_{\odot})<11.3$ galaxies at
$2.4 < z < 3.2$ in the COSMOS field whose redshifts and quiescence are
confirmed by HST grism spectroscopy. Although affected by a weak bias toward
younger stellar populations, this sample is deemed to be largely representative
of the majority of the most massive and thus intrinsically rarest quiescent
sources at this cosmic time. We rely on targeted HST/WFC3 observations and fit
S\'ersic profiles to the galaxy surface brightness distributions at $\approx
4000$ angstrom restframe. We find typically high S\'ersic indices and axis
ratios (medians $\approx 4.5$ and $0.73$, respectively) suggesting that, at
odds with some previous results, the first massive quiescent galaxies may
largely be already bulge-dominated systems. We measure compact galaxy sizes
with an average of $\approx 1.4$kpc at $\log(M_{\star}/M_{\odot})\approx 11.2$,
in good agreement with the extrapolation at the highest masses of previous
determinations of the stellar mass - size relation of quiescent galaxies, and
of its redshift evolution, from photometrically selected samples at lower and
similar redshifts. This work confirms the existence of a population of compact,
bulge dominated, massive, quiescent sources at $z\approx 3$, providing one of
the first statistical estimates of their structural properties, and further
constraining the early formation and evolution of the first quiescent galaxies.
