Starburst to quiescent from HST/ALMA: Stars and dust unveil minor
  mergers in submillimeter galaxies at $z \sim 4.5$
  Dust-enshrouded, starbursting, submillimeter galaxies (SMGs) at $z \geq 3$
have been proposed as progenitors of $z \geq 2$ compact quiescent galaxies
(cQGs). To test this connection, we present a detailed spatially resolved study
of the stars, dust and stellar mass in a sample of six submillimeter-bright
starburst galaxies at $z \sim 4.5$. The stellar UV emission probed by HST is
extended, irregular and shows evidence of multiple components. Informed by HST,
we deblend Spitzer/IRAC data at rest-frame optical finding that the systems are
undergoing minor mergers, with a typical stellar mass ratio of 1:6.5. The FIR
dust continuum emission traced by ALMA locates the bulk of star formation in
extremely compact regions (median $r_{\rm{e}} = 0.70 \pm 0.29$ kpc) and it is
in all cases associated with the most massive component of the mergers (median
$\log (M_{*}/M_{\odot}) = 10.49 \pm 0.32$). We compare spatially resolved UV
slope ($\beta$) maps with the FIR dust continuum to study the infrared excess
($\rm{IRX} = L_{\rm{IR}}/L_{\rm{UV}}$)-$\beta$ relation. The SMGs display
systematically higher $\rm{IRX}$ values than expected from the nominal trend,
demonstrating that the FIR and UV emissions are spatially disconnected.
Finally, we show that the SMGs fall on the mass-size plane at smaller stellar
masses and sizes than cQGs at $z = 2$. Taking into account the expected
evolution in stellar mass and size between $z = 4.5$ and $z = 2$ due to the
ongoing starburst and mergers with minor companions, this is in agreement with
a direct evolutionary connection between the two populations.
