Properties of the brightest young stellar clumps in extremely lensed galaxies at redshifts 4 to 5

Abstract

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 abo v e μ> 5 and in some cases reaching μ> 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 hav e deriv ed effectiv e radii in the range R 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 SFR UV mostly in the range 0 . 1 -1 M⊙yr -1 ; the most extreme ones, reaching SFR UV = 5 M⊙yr -1 are among the UV-brightest compact ( R eff < 100 pc) star-forming regions observed at any redshift. Clump masses span a broad range from 10 6 to 10 9 M⊙; 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 tentati ve e volution of _ SFR and _ M _ with redshift, especially when very compact clumps ( R eff ≤ 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 Σ SFR and Σ M*can be found.