Unveiling dust, molecular gas, and high star formation efficiency in extremely UV bright star-forming galaxies at $z\sim 2.1-3.6$

Abstract

We analysed ALMA FIR (1.3 mm) dust continuum and CO emission of 12 starburst galaxies at $z\sim 2.1-3.6$, selected for their extreme brightness in the rest-UV with $M_{\mathrm{UV}}=-23.4$ to $-24.7$. We also analysed VLT HAWK-I $H$- and $K_{\mathrm{s}}$-band images. The galaxies are characterised by negligible dust attenuations with blue UV spectral slopes ($-2.62$ to $-1.84$), very young stellar populations of $\sim 10$ Myr, and powerful starbursts with a high mean specific star formation rate of $112\mathrm{Gy}{\mathrm{r}}^{-1}$, placing them $\sim 1.5$~dex above the main sequence at similar redshifts and stellar masses ($M_{\mathrm{stars}}\sim (1.5-4.6)\times 10^9{M}_{\odot }$). The FIR dust continuum emission revealed in 9 galaxies yields IR luminosities of $(5.9-28.3)\times 10^{11}{L}_{\odot }$ and large dust masses barely produced by SNe within the 10~Myr timescale. The CO emission detected in 8 galaxies evidence large molecular gas masses with a mean molecular gas fraction of 82%. The corresponding star formation efficiencies reach $\gtrsim 40$%, with amazingly short molecular gas depletion timescales between <13 Myr and 71 Myr. These unique properties never reported in previously studied galaxies highlight that these galaxies are likely caught at the very beginning of their stellar mass build-up and undergo a very efficient and fast conversion of gas into stars that can only result from the gas collapse within very short free-fall times. We find that the feedback-free starburst model seems to be able to explain the formation of these galaxies. To reconcile the co-spatial FIR dust emission with the UV-bright unattenuated emission, we speculate about radiation-driven outflows that can temporarily remove dust at the location of the starburst and expel dust at large distances in line with the measured large FIR effective radii ($1.7\mathrm{kpc}-5\mathrm{kpc}$) in comparison to very compact stellar radii.

21 pages, 8 figures, 4 tables, 1 Appendix. Astronomy & Astrophysics in press