We present Hubble Space Telescope NIC2 morphologies of a spectroscopic sample of massive galaxies at z \sim 2.3 , by extending our sample of 9 compact quiescent galaxies ( r _ { e } \sim 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 r _ { e } = 2.8 kpc .
The three galaxies with the highest star formation rates ( \gtrsim 100 M _ { \odot } yr ^ { -1 } ) have irregular and clumpy morphologies .
The AGN host galaxies are more similar to the compact quiescent galaxies in terms of their structures ( r _ { e } \sim 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 .