Dust and gas in luminous proto-cluster galaxies at z=4.05: the case for
  different cosmic dust evolution in normal and starburst galaxies
  We measure the dust and gas content of the three sub-millimeter galaxies
(SMGs) in the GN20 proto-cluster at $z=4.05$ using new IRAM Plateau de Bure
interferometer (PdBI) CO(4-3) and 1.2-3.3 mm continuum observations. All these
three SMGs are heavily dust obscured, with UV-based star formation rate (SFR)
estimates significantly smaller than the ones derived from the bolometric
infrared (IR), consistent with the spatial offsets revealed by HST and CO
imaging. Based also on evaluations of the specific SFR, CO-to-H$_2$ conversion
factor and gas depletion timescale, we classify all the three galaxies as
starbursts (SBs), although with a lower confidence for GN20.2b that might be a
later stage merging event. We place our measurements in the context of the
evolutionary properties of main sequence (MS) and SB galaxies. ULIRGs have 3-5
times larger $L'_{\rm CO}/M_{\rm dust}$ and $M_{\rm dust}/M_\star$ ratios than
$z=0$ MS galaxies, but by $z\sim2$ the difference appears to be blurred,
probably due to differential metallicity evolution. SB galaxies appear to
slowly evolve in their $L'_{\rm CO}/M_{\rm dust}$ and $M_{\rm dust}/M_\star$
ratios all the way to $z>6$ (consistent with rapid enrichment of SB events),
while MS galaxies rapidly increase in $M_{\rm dust}/M_\star$ from $z=0$ to 2
(due to gas fraction increase, compensated by a decrease of metallicities).
While no IR/submm continuum detection is available for indisputably normal
massive galaxies at $z>2.5$, we show that if metallicity indeed decrease
rapidly for these systems at $z>3$ as claimed in the literature, we should
expect a strong decrease of their $M_{\rm dust}/M_\star$, consistent with
recent PdBI and ALMA upper limits. We conclude that the $M_{\rm dust}/M_\star$
ratio could be a powerful tool for distinguishing starbursts from normal
galaxies at $z>4$.
