The discovery of rest-frame UV colour gradients and a diversity of dust morphologies in bright z ~ 7 Lyman-break galaxies

Abstract

We present deep ALMA dust continuum observations for a sample of luminous ($M_{\mathrm{UV}}<-22$) star-forming galaxies at $z\simeq 7$. We detect five of the six sources in the far-infrared (FIR), providing key constraints on the obscured star-formation rate (SFR) and the infrared-excess-$\beta$ (IRX-$\beta$) relation without the need for stacking. Despite the galaxies showing blue rest-frame UV slopes ($\beta \simeq -2$) we find that 35-75 percent of the total SFR is obscured. We find the IRX-$\beta$ relation derived for these $z\simeq 7$ sources is consistent with that found for local star-burst galaxies. Using our relatively high-resolution (FWHM $\simeq 0.7\mathrm{arcsec}$) observations we identify a diversity of dust morphologies in the sample. We find both compact emission that appears offset relative to the unobscured components and extended dust emission that is co-spatial with the rest-frame UV light. In the majority of the sources we detect strong rest-frame UV colour gradients (with up to $\Delta \beta \simeq 0.7$-$1.4$) as probed by the multi-band UltraVISTA ground-based data. The observed redder colours are spatially correlated with the location of the FIR detection. Our results show that even in bright Lyman-break galaxies at $z\simeq 7$ the peak of the star-formation is typically hosted by the fainter, redder, regions in the rest-frame UV, which have an obscured fraction of $f_{\mathrm{obs}}\ge 0.8$. As well as demonstrating the importance of dust obscured star-formation within the Epoch of Reionization, these observations provide an exciting taster of the rich spatially resolved datasets that will be obtained from JWST and high-resolution ALMA follow-up at these redshifts.

14 pages, 6 figures, 3 tables. Updated to MNRAS accepted version

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