Resolved Kennicutt-Schmidt law in two strongly lensed star-forming galaxies at redshift 1

TL;DR

This study investigates the Kennicutt-Schmidt law at various spatial scales in two strongly lensed galaxies at redshift 1, revealing that the law generally holds but with increased scatter at smaller scales, and that spatial correlation varies between galaxies.

Contribution

First direct resolution of the Kennicutt-Schmidt law at sub-100 parsec scales in high-redshift galaxies using gravitational lensing and ALMA/HST data.

Findings

Kennicutt-Schmidt law holds at galactic scales for z~1 galaxies.

Scatter in the law increases at smaller spatial scales.

Star-forming regions and molecular clouds become de-correlated below 1 kpc in one galaxy.

Abstract

We study the star formation rate (SFR) versus molecular gas mass ($M_\mathrm{mol}$) scaling relation from hundreds to thousands parsec scales in two strongly lensed galaxies at redshift $z\sim 1$, the Cosmic Snake and A521. We trace SFR using extinction-corrected rest-frame UV observations with the Hubble Space Telescope (HST), and $M_\mathrm{mol}$ using detections of the CO(4--3) line with the Atacama Large Millimetre/submillimetre Array (ALMA). The similar angular resolutions of our HST and ALMA observations of $0.15-0.2\,''$ combined with magnifications reaching $\mu>20$ enable us to resolve structures in the galaxies of sizes lower than $100\,\mathrm{pc}$. These resolutions are close to those of nearby galaxies studies. This allows us to investigate for the first time the Kennicutt-Schmidt (KS) law (SFR-$M_\mathrm{mol}$ surface densities) at different spatial scales, from galactic scales to $\sim 100\,\mathrm{pc}$ scales, in galaxies at $z\sim 1$. At integrated scales we find that both galaxies satisfy the KS law defined by galaxies at redshifts between 1 and 2.5. We test the resolved KS (rKS) law in cells of sizes down to $200\,\mathrm{pc}$ in the two galaxies. We observe that this relationship generally holds in these $z\sim 1$ galaxies although its scatter increases significantly with decreasing spatial scales. We check the scale dependence of the spatial correlation between the surface densities of SFR and $M_\mathrm{mol}$ by focussing on apertures centred on individual star-forming regions and molecular clouds. We conclude that star-forming regions and molecular clouds become spatially de-correlated at $\lesssim 1\,\mathrm{kpc}$ in the Cosmic Snake, whereas they appear de-correlated at all spatial scales (from $400\,\mathrm{pc}$ to $6\,\mathrm{kpc}$) in A521.