The Sizes of Candidate $z\sim9-10$ Galaxies: confirmation of the bright CANDELS sample and relation with luminosity and mass

TL;DR

This study measures the sizes of candidate galaxies at redshift 9-10, confirming their small sizes and using this data to explore galaxy size evolution, luminosity, and mass relations at these early cosmic times.

Contribution

The paper provides the first size measurements of luminous $z\sim9-10$ galaxy candidates and extends the size evolution analysis to these redshifts, confirming the size-luminosity and size-mass relations.

Findings

Mean size of 0.13" at $z\sim9-10$ matches extrapolated lower-redshift sizes.

Size constraints help distinguish high-redshift galaxies from contaminants.

Size evolution follows a $(1+z)^{-1.0 ext{±}0.1}$ trend from $z=4$ to $z=10$.

Abstract

Recently, a small sample of six $z\sim9-10$ candidates was discovered in CANDELS that are $\sim10-20\times$ more luminous than any of the previous $z\sim9-10$ galaxies identified over the HUDF/XDF and CLASH fields. We measure the sizes of these candidates to map out the size evolution of galaxies from the earliest observable times. Their sizes are also used to provide a valuable constraint on whether these unusual galaxy candidates are at high redshift. Using galfit to derive sizes from the CANDELS F160W images of these candidates, we find a mean size of 0.13$\pm$0.02" (or 0.5$\pm$0.1 kpc at $z\sim9-10$). This handsomely matches the 0.6 kpc size expected extrapolating lower redshift measurements to $z\sim9-10$, while being much smaller than the 0.59" mean size for lower-redshift interlopers to $z\sim9-10$ photometric selections lacking the blue IRAC color criterion. This suggests that source size may be an effective constraint on contaminants from $z\sim9-10$ selections lacking IRAC data. Assuming on the basis of the strong photometric evidence that the Oesch et al. 2014 sample is entirely at $z\sim9-10$, we can use this sample to extend current constraints on the size-luminosity, size-mass relation, and size evolution of galaxies to $z\sim10$. We find that the $z\sim9-10$ candidate galaxies have broadly similar sizes and luminosities as $z\sim6$-8 counterparts with star-formation-rate surface densities in the range of $\rm \Sigma_{SFR}=1-20\, M_\odot~ yr^{-1}\, kpc^{-2}$. The stellar mass-size relation is uncertain, but shallower than those inferred for lower-redshift galaxies. In combination with previous size measurements at z=4-7, we find a size evolution of $(1+z)^{-m}$ with $m=1.0\pm0.1$ for $>0.3L^*_{z=3}$ galaxies, consistent with the evolution previously derived from $2 < z < 8$ galaxies.