A First Look at Spatially Resolved Star Formation at $4.8<z<6.5$ with JWST FRESCO NIRCam Slitless Spectroscopy

Abstract

We present the first results on the spatial distribution of star formation in 454 star-forming galaxies at $4.8<z<6.5$ using H-Alpha emission-line maps and F444W imaging tracing the stellar continuum from JWST FRESCO NIRCam Slitless Spectroscopy. Star-forming galaxies with stellar masses $6.8\le$log($M_{\ast }/{\mathrm{M}}_{\odot }$)$<11.1$ have positive H-Alpha equivalent width profiles, providing direct evidence for the inside-out growth of galaxies just after the epoch of reionisation. GALFIT is used to calculate half-light radii, $R_{\mathrm{eff}}$ and central surface densities within 1 kiloparsec, ${\Sigma }_{1\mathrm{kpc}}$. At a fixed stellar mass of log$(M_{\ast }/{\mathrm{M}}_{\odot })=9.5$, ${\Sigma }_{1\mathrm{kpc},\mathrm{H}\alpha }$ is $1.04\pm 0.05$ times higher than ${\Sigma }_{1\mathrm{kpc},\mathrm{C}}$, $R_{\mathrm{eff},\mathrm{H}\alpha }$ is $1.18\pm 0.03$ times larger than $R_{\mathrm{eff},\mathrm{C}}$ and both $R_{\mathrm{eff}}$ measurements are less than 1 kiloparsec. These measurements suggest the rapid build-up of compact bulges just after the epoch of reionisation. By comparing to work done at lower redshifts with HST WFC3 Slitless Spectroscopy as part of the 3D-HST ($z=1$) and CLEAR ($z=0.5$) surveys, we find that $R_{\mathrm{eff}}(z)$ evolves at the same pace for H$\alpha$ and the continuum, but ${\Sigma }_{1\mathrm{kpc}}(z)$ evolves faster for H$\alpha$. As a function of the Hubble parameter, $\frac{R_{\mathrm{eff},\mathrm{H}\alpha }}{R_{\mathrm{eff},\mathrm{C}}}=1.1h(z)$ and $\frac{{\Sigma }_{1\mathrm{kpc},\mathrm{H}\alpha }}{{\Sigma }_{1\mathrm{kpc},\mathrm{C}}}=h(z{)}^{1.3}$. These functions suggest that the inside-out growth of the disk dominates the inside-out growth of the bulge towards lower redshifts. This is supported by the redshift evolution in EW(H$\alpha$) profiles, where there is rapid increase in EW(H$\alpha$) with radius within the half-light radius at $z=5.3$ but only significantly increasing EW(H$\alpha$) with radius in the outer disk at $z=0.5$.

12 pages, 6 figures, accepted for publication by A&A

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