UV-continuum $\beta$ slopes of individual $z \sim 2-6$ clumps and their evolution

TL;DR

This study analyzes the UV continuum slopes of 166 star-forming clumps in high-redshift galaxies to understand their physical properties and evolution, revealing that clumps are sites of intense, young star formation with lower dust extinction than their host galaxies.

Contribution

It provides the first detailed comparison of UV slopes between individual clumps and entire galaxies at high redshift, highlighting differences in star formation and dust properties.

Findings

Clumps have median β ≈ -2.4, indicating young, massive stars.

Clumps follow known β-redshift and β-UV magnitude relations.

Detection of extremely blue clumps suggests low-metallicity stars.

Abstract

We study the ultraviolet (UV) continuum $\beta$ slope of a sample of 166 clumps, individual star-forming regions observed in high redshift galaxies. They are hosted by 67 galaxies with redshift between 2 and 6.2, strongly lensed by the Hubble Frontier Fields cluster of galaxies MACS J0416.1-2403. The $\beta$ slope is sensitive to a variety of physical properties, such as the metallicity, the age of the stellar population, the dust attenuation throughout the galaxy, the stellar initial mass function (IMF), and the star-formation history (SFH). The aim of this study is to compare the $\beta$ values of individual clumps with those measured on the entire galaxy, to investigate possible physical differences between these regions and their hosts. We found a median value of $\beta \sim -2.4$, lower than that of integrated galaxies. This result confirms that clumps are sites of intense star formation, populated by young, massive stars, whose spectrum strongly emits in the UV. This is also consistent with the assumption that the dust extinction at the location of the clumps is lower than the average extinction of the galaxy, or that clumps have a different IMF or SFH. We made use of the correlations, discovered for high-redshift galaxies, of the $\beta$ value with those of redshift and UV magnitude, $M_{UV}$, finding that clumps follow the same relations, extended to much fainter magnitudes ($M_{UV}<-13$). We also find evidence of eight clumps with extremely blue ($\beta \lesssim -2.7$) slopes, which could be the signpost of low-metallicity stars and constrain the emissivity of ionizing photons at high redshift.