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

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 $0.15-0.2{}^{\prime \prime }$ 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.

11 pages, 8 figures, 2 tables