Zooming on the internal structure of $z\simeq6$ galaxies
  We present zoom-in, AMR, high-resolution ($\simeq 30$ pc) simulations of
high-redshift ($z \simeq 6$) galaxies with the aim of characterizing their
internal properties and interstellar medium. Among other features, we adopt a
star formation model based on a physically-sound molecular hydrogen
prescription, and introduce a novel scheme for supernova feedback, stellar
winds and dust-mediated radiation pressure. In the zoom-in simulation the
target halo hosts "Dahlia", a galaxy with a stellar mass $M_*=1.6\times
10^{10}$M$_\odot$, representative of a typical $z\sim 6$ Lyman Break Galaxy.
Dahlia has a total H2 mass of $10^{8.5}$M$_\odot$, that is mainly concentrated
in a disk-like structure of effective radius $\simeq 0.6$ kpc and scale height
$\simeq 200$ pc. Frequent mergers drive fresh gas towards the center of the
disk, sustaining a star formation rate per unit area of $\simeq 15 $M$_\odot$
yr$^{-1}$ kpc$^{-2}$. The disk is composed by dense ($n \gtrsim 25$ cm$^{-3}$),
metal-rich ($Z \simeq 0.5 $ Z$_\odot$) gas, that is pressure-supported by
radiation. We compute the $158\mu$m [CII] emission arising from {Dahlia}, and
find that $\simeq 95\%$ of the total [CII] luminosity
($L_{[CII]}\simeq10^{7.5}$ L$_\odot$) arises from the H2 disk. Although $30\%$
of the CII mass is transported out of the disk by outflows, such gas negligibly
contributes to [CII] emission, due to its low density ($n \lesssim 10$
cm$^{-3}$) and metallicity ($Z\lesssim 10^{-1}$Z$_\odot$). Dahlia is
under-luminous with respect to the local [CII]-SFR relation; however, its
luminosity is consistent with upper limits derived for most $z\sim6$ galaxies.
