The Hubble Sequence beyond z=2 for Massive Galaxies: Contrasting Large
  Star-Forming and Compact Quiescent Galaxies
  We present Hubble Space Telescope NIC2 morphologies of a spectroscopic sample
of massive galaxies at z~2.3, by extending our sample of 9 compact quiescent
galaxies (r_e~0.9 kpc) with 10 massive emission-line galaxies. The
emission-line galaxies are classified by the nature of their ionized emission;
there are six star-forming galaxies and four galaxies hosting an active
galactic nucleus (AGN). The star-forming galaxies are the largest among the
emission-line galaxies, with a median size of re = 2.8 kpc. The three galaxies
with the highest star formation rates (> 100M \odot/yr) have irregular and
clumpy morphologies. The AGN host galaxies are more similar to the compact
quiescent galaxies in terms of their structures (re~1.1 kpc) and spectral
energy distributions. The total sample clearly separates into two classes in a
color-mass diagram: the large star-forming galaxies that form the blue cloud,
and the compact quiescent galaxies on the red sequence. However, it is unclear
how or even if the two classes are evolutionary related. Three out of six
massive star-forming galaxies have dense cores and thus may passively evolve
into compact galaxies due to fading of outer star-forming regions. For these
galaxies a reverse scenario, in which compact galaxies grow inside-out by star
formation is also plausible. We do caution though that the sample is small.
Nonetheless, it is evident that a Hubble sequence of massive galaxies with
strongly correlated galaxy properties is already in place at z > 2.
