Johan Richard

We study the populations of stellar clumps in three high-redshift galaxies, at z = 4.92, 4.88, and 4.03, gravitationally lensed by the foreground galaxy clusters MS1358, RCS0224, and MACS0940, respectively. The lensed galaxies consist of multiple counter-images with large magnifications, mostly above $\mu > 5$ and in some cases reaching $\mu > 20$. We use rest-frame UV observations from the HST to extract and analyse their clump populations, counting 10, 3, and 11 unique sources, respectively. Most of the clumps have derived effective radii in the range $R_{\rm eff}=10\!-\!100$ pc, with the smallest one down to 6 pc, i.e. consistent with the sizes of individual stellar clusters. Their UV magnitudes correspond to $\rm SFR_{UV}$ mostly in the range $0.1\!-\!1\ {\rm M_\odot \, yr}^{-1}$; the most extreme ones, reaching ${\rm SFR_{UV}}=5\ {\rm M_\odot \, yr}^{-1}$ are among the UV-brightest compact ($R_{\rm eff} < 100$ pc) star-forming regions observed at any redshift. Clump masses span a broad range from 106 to $10^9\,{\rm M}_\odot$; stellar mass surface densities are comparable and in many cases larger than the ones of local stellar clusters, while being typically 10 times larger in size. By compiling published properties of clump populations at similar spatial resolution between redshifts 0 and 5, we find a tentative evolution of $\Sigma_{\rm SFR}$ and $\Sigma _{M_\star }$ with redshift, especially when very compact clumps ($R_{\rm eff}\leqslant 20$ pc) are considered. We suggest that these trends with redshift reflect the changes in the host galaxy environments where clumps form. Comparisons with the local universe clumps/star clusters shows that, although rare, conditions for elevated clump $\Sigma_{\rm SFR}$ and $\Sigma _{M_\star }$ can be found.