UV and NIR size of the low-mass field galaxies: the UV compact galaxies

TL;DR

This study uses high-resolution HST images to analyze the size and growth modes of low-mass and massive galaxies, revealing an 'outside-in' growth in some low-mass galaxies and confirming 'inside-out' growth in massive ones.

Contribution

It provides new observational evidence of 'outside-in' growth in low-mass galaxies using UV and NIR size measurements, expanding understanding of galaxy formation modes.

Findings

Massive galaxies follow 'inside-out' growth consistent with previous studies.

Some low-mass galaxies exhibit 'outside-in' growth with compact UV morphology.

Growth modes transition smoothly around stellar mass of 10^8-10^9 solar masses.

Abstract

Most of the massive star-forming galaxies are found to have `inside-out' stellar mass growth modes, which means the inner parts of the galaxies mainly consist of the older stellar population, while the star forming in the outskirt of the galaxy is still ongoing. The high-resolution HST images from Hubble Deep UV Legacy Survey (HDUV) and Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) projects with the unprecedented depth in both F275W and F160W bands are the perfect data sets to study the forming and formed stellar distribution directly. We selected the low redshift ($0.05 < z_{\rm spec} < 0.3$) galaxy sample from the GOODS-North field where the HST F275W and F160W images are available. Then we measured the half light radius in F275W and F160W bands, which are the indicators of the star formation and stellar mass. By comparing the F275W and F160W half light radius, we find the massive galaxies are mainly follow the `inside-out' growth which is consistent with the previous results. Moreover, the HST F275W and F160W images reveal that some of the low-mass galaxies ($<10^8M_\odot$) have the `outside-in' growth mode: their images show a compact UV morphology, implying an ongoing star formation in the galaxy centre, the stars in the outskirts of the galaxies are already formed. The two modes transit smoothly at stellar mass range about $10^{8-9}M_\odot$ with a large scatter. We also try to identify the possible neighbour massive galaxies from the SDSS data, which represent the massive galaxy sample. We find that all of the spec-z selected galaxies have no massive galaxy nearby. Thus the `outside-in' mode we find in the low-mass galaxies are not likely originated from the environment.