VLA and ALMA Imaging of Intense, Galaxy-Wide Star Formation in z ~ 2 Galaxies

TL;DR

This study uses high-resolution VLA and ALMA imaging to reveal that star formation in z~2 galaxies occurs galaxy-wide, often at stellar-mass concentrations, with implications for galaxy evolution and outflows.

Contribution

It provides the first direct, high-resolution evidence of galaxy-wide star formation in main-sequence galaxies at z~2, linking star formation distribution to galaxy morphology and AGN activity.

Findings

Star formation occurs over a median diameter of 4.2 kpc, comparable to stellar-mass distribution.

Median SFR surface density is 2.5 M$_{ ext{sun}}$ yr$^{-1}$ kpc$^{-2}$, capable of driving outflows.

Main-sequence SFGs are twice as large as bright submillimeter galaxies with higher SFRs.

Abstract

We present $\simeq$0$.\!\!^{\prime\prime}4$-resolution extinction-independent distributions of star formation and dust in 11 star-forming galaxies (SFGs) at $z = 1.3-3.0$. These galaxies are selected from sensitive, blank-field surveys of the $2' \times 2'$ Hubble Ultra-Deep Field at $\lambda = 5$ cm and 1.3 mm using the Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA). They have star-formation rates (SFRs), stellar masses, and dust properties representative of massive main-sequence SFGs at $z \sim 2$. Morphological classification performed on spatially-resolved stellar mass maps indicates a mixture of disk and morphologically disturbed systems; half of the sample harbor X-ray active galactic nuclei (AGN), thereby representing a diversity of $z \sim 2$ SFGs undergoing vigorous mass assembly. We find that their intense star formation most frequently occurs at the location of stellar-mass concentration and extends over an area comparable to their stellar-mass distribution, with a median diameter of $4.2 \pm 1.8$ kpc. This provides direct evidence for galaxy-wide star formation in distant, blank-field-selected main-sequence SFGs. The typical galactic-average SFR surface density is 2.5 M$_{\odot}$yr$^{-1}$kpc$^{-2}$, sufficiently high to drive outflows. In X-ray-selected AGN where radio emission is enhanced over the level associated with star formation, the radio excess pinpoints the AGN, which are found to be co-spatial with star formation. The median extinction-independent size of main-sequence SFGs is two times larger than those of bright submillimeter galaxies whose SFRs are $3-8$ times larger, providing a constraint on the characteristic SFR ($\sim300$ M$_{\odot}$yr$^{-1}$) above which a significant population of more compact star-forming galaxies appears to emerge.