CLEAR: The Evolution of Spatially Resolved Star Formation in Galaxies between $0.5\lesssim z \lesssim1.7$ using H$\alpha$ Emission Line Maps

TL;DR

This study uses spatially resolved H-alpha maps from HST to investigate galaxy growth and star formation gradients between redshifts 0.5 and 1.7, revealing inside-out growth and evolving central star formation activity.

Contribution

It provides new measurements of galaxy size and star formation surface density evolution over redshift, using high-resolution H-alpha maps and comparing with cosmological simulations.

Findings

Galaxy effective radius is 1.2 times larger than stellar continuum at z~0.5.

No significant evolution in galaxy size from z~1.7 to z~0.5.

Decrease in central star formation surface density with decreasing redshift.

Abstract

Using spatially resolved H-alpha emission line maps of star-forming galaxies, we study the evolution of gradients in galaxy assembly over a wide range in redshift ($0.5<z<1.7$). Our $z\sim0.5$ measurements come from deep Hubble Space Telescope WFC3 G102 grism spectroscopy obtained as part of the CANDELS Lyman-alpha Emission at Reionization (CLEAR) Experiment. For star-forming galaxies with Log$(M_{*}/\mathrm{M}_{\odot})\geqslant8.96$, the mean H-alpha effective radius is $1.2\pm0.1$ times larger than that of the stellar continuum, implying inside-out growth via star formation. This measurement agrees within $1\sigma$ with those measured at $z\sim1$ and $z\sim1.7$ from the 3D-HST and KMOS-3D surveys respectively, implying no redshift evolution. However, we observe redshift evolution in the stellar mass surface density within 1 kiloparsec ($\Sigma_\mathrm{1kpc}$). Star-forming galaxies at $z\sim0.5$ with a stellar mass of Log$(M_{*}/\mathrm{M}_{\odot})=9.5$ have a ratio of $\Sigma_\mathrm{1kpc}$ in H-alpha relative to their stellar continuum that is lower by $(19\pm2)\%$ compared to $z\sim1$ galaxies. $\Sigma_{1\mathrm{kpc, H}\alpha}$/$\Sigma_{1\mathrm{kpc,Cont}}$ decreases towards higher stellar masses. The majority of the redshift evolution in $\Sigma_{1\mathrm{kpc,H}\alpha}$/$\Sigma_{1\mathrm{kpc,Cont}}$ versus stellar mass stems from the fact that Log($\Sigma_{1\mathrm{kpc, H}\alpha}$) declines twice as much as Log($\Sigma_{1\mathrm{kpc, Cont}}$) from $z\sim 1$ to 0.5 (at a fixed stellar mass of Log$(M_{*}/\mathrm{M}_{\odot})=9.5$). By comparing our results to the TNG50 cosmological magneto-hydrodynamical simulation, we rule out dust as the driver of this evolution. Our results are consistent with inside-out quenching following in the wake of inside-out growth, the former of which drives the significant drop in $\Sigma_{1\mathrm{kpc, H}\alpha}$ from $z\sim1$ to $z\sim0.5$.