Multiply lensed star forming clumps in the A521-sys1 galaxy at redshift 1

Abstract

We study the population of star-forming clumps in A521-sys1, a z = 1.04 system gravitationally lensed by the foreground (z = 0.25) cluster Abell 0521. The galaxy presents one complete counter-image with a mean magnification of µ ∼ 4 and a wide arc containing two partial images of A521-sys1 with magnifications reaching µ > 20, allowing the investigations of clumps down to scales of R eff < 50 pc. We identify 18 unique clumps with a total of 45 multiple images. Intrinsic sizes and UV magnitudes reveal clumps with elevated surface brightnesses, comparable to similar systems at redshifts z 1.0. Such clumps account for ∼ 40% of the galaxy UV luminosity, implying that a significant fraction of the recent star-formation activity is taking place there. Clump masses range from 10 6 M to 10 9 M and sizes from tens to hundreds of parsec, resulting in mass surface densities from 10 to 10 3 M pc −2 , with a median of ∼ 10 2 M pc −2 . These properties suggest that we detect star formation taking place across a wide range of scale, from cluster aggregates to giant star-forming complexes. We find ages of less than 100 Myr, consistent with clumps being observed close to their natal region. The lack of galactocentric trends with mass, mass density, or age and the lack of old migrated clumps can be explained either by dissolution of clumps after few ∼ 100 Myr or by stellar evolution making them fall below the detectability limits of our data.

Star formation at the smallest scales; A JWST study of the clump populations in SMACS0723

“…Overall, we observe that as star formation proceeds for a longer time (from IB to 100Myr) the ages of the clumps get on average older and, therefore, resulting masses (central panels) become slightly larger for the 100Myr model. These trends have already been remarked in the literature (Adamo et al 2013;Messa et al 2022), who find similar behavior and dependencies.…”

“…Since stellar clumps will appear with different sizes and resolutions depending on the magnification and distortion, fixed aperture photometry will produce significant contamination and might introduce systematics. We, therefore, simultaneously fit for clump size and fluxes, using the method initially developed by Messa et al (2019) and later improved and adapted to take into account lensing models as presented in Messa et al (2022). A similar approach has also been adopted by Meštrić et al (2022).…”

“…Following the method by Messa et al (2022), we determine an analytical expression of the PSF shape in each band, by fitting the resulting PSF image with an analytical function described by the combination of a Moffat, Gaussian, and 4th degree power-law profiles. This fit provides a good description of the PSF up to a radius of ∼0.5 , which is significantly larger than the physical region used to fit the clump light distribution.…”

“…According to the testing performed by Messa et al (2019Messa et al ( , 2022, we assume 0.4 pixel as the minimum resolvable size in the F150W and F200W frames. Sizes are reported as upper limits in clumps where at least one of the axes (typically the minor axes) is ≤ 0.4 pixel.…”

“…From the observational perspective, clump ages, important to determine clump survival, have been difficult to pin down (e.g., Cava et al 2018). Initial HST studies of clumps in lensed galaxies at redshift 2, find age ranges between a few and tens of Myr up to a few hundreds of Myr (e.g., Adamo et al 2013;Johnson et al 2017;Messa et al 2022;Vanzella et al 2022b). The main limitation has been so far the lack of homogeneous coverage of the UV-optical rest-frame of galaxies at redshift > 2 (e.g., see Meštrić et al 2022, who included ground-based data, although limiting, to improve age and mass estimates of clumps), a key phase for star formation and galaxy evolution.…”