Early results from GLASS-JWST. V: the first rest-frame optical size-luminosity relation of galaxies at $z>7$

Abstract

We present the first rest-frame optical size-luminosity relation of galaxies at $z>7$, using the NIRCam imaging data obtained by the GLASS James Webb Space Telescope Early Release Science (GLASS-JWST-ERS) program, providing the deepest extragalactic data of the ERS campaign. Our sample consist of 19 photometrically selected bright galaxies with $m_{\text{F444W}}\le 27.8$ at $7<z<9$ and $m_{\text{F444W}}<28.2$ at $z\sim 9-15$. We measure the size of the galaxies in 5 bands, from the rest-frame optical ($\sim 4800\tilde{\mathrm{A}}$) to the ultra-violet (UV; $\sim 1600\tilde{\mathrm{A}}$) based on the Sérsic model, and analyze the size-luminosity relation as a function of wavelength. Remarkably, the data quality of NIRCam imaging is sufficient to probe the half-light radius $r_e$ down to $\sim 100$ pc at $z>7$. Given the limited sample size and magnitude range, we first fix the slope to that observed for larger samples in rest-frame UV using HST samples. The median size $r_0$ at the reference luminosity $M=-21$ decreases slightly from rest-frame optical ($600\pm 80$ pc) to UV ($450\pm 130$ pc). We then re-fit the size-luminosity relation allowing the slope to vary. The slope is consistent with ${\hat{I}}^2\sim 0.2$ for all bands except F150W, where we find a marginally steeper slope of ${\hat{I}}^2=0.53\pm 0.15$. The steep UV slope is mainly driven by the smallest and faintest galaxies. If confirmed by larger samples, it implies that the UV size-luminosity relation breaks toward the faint end as suggested by lensing studies.

Accepted by ApJL, 9 pages, 3 figures, 2 tables