Dust and gas in luminous proto-cluster galaxies at z = 4.05: the case for different cosmic dust evolution in normal and starburst galaxies
Résumé
We measure the dust and gas content of the three submillimeter 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-H2 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'CO/Mdust and Mdust/M⋆
ratios than z =
0 MS galaxies, but by z ~ 2 the difference appears to be blurred,
probably due to differential metallicity evolution. SB galaxies appear to slowly evolve in
their
L'CO/Mdust and Mdust/M⋆
ratios all the way to z> 6 (consistent with rapid
enrichment of SB events), while MS galaxies rapidly increase in Mdust/M⋆
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 Mdust/M⋆,
consistent with recent PdBI and ALMA upper limits. We conclude that the Mdust/M⋆
ratio could be a powerful tool for distinguishing starbursts from normal galaxies at
z>
4.
Origine : Publication financée par une institution
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