An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-Redshift AGB Analogs

TL;DR

This study uses HST infrared observations to identify dust-producing AGB stars in metal-poor dwarf galaxies, revealing that such stars can produce dust early in galaxy evolution, impacting high-redshift galaxy dust content.

Contribution

It presents new HST data distinguishing C-type and M-type AGB stars in metal-poor galaxies, doubling known dusty AGB stars and providing evidence for dust production in low-metallicity M-type stars.

Findings

Most dusty AGB stars are carbon-rich.

Dust is present around M-type stars even at very low metallicity.

AGB stars can produce dust within 30 million years of galaxy formation.

Abstract

The survey for DUST in Nearby Galaxies with Spitzer (DUSTiNGS) identified several candidate Asymptotic Giant Branch (AGB) stars in nearby dwarf galaxies and showed that dust can form even in very metal-poor systems (Z ~ 0.008 $Z_\odot$). Here, we present a follow-up survey with WFC3/IR on the Hubble Space Telescope (HST), using filters that are capable of distinguishing carbon-rich (C-type) stars from oxygen-rich (M-type) stars: F127M, F139M, and F153M. We include six star-forming DUSTiNGS galaxies (NGC 147, IC 10, Pegasus dIrr, Sextans B, Sextans A, and Sag DIG), all more metal-poor than the Magellanic Clouds and spanning 1 dex in metallicity. We double the number of dusty AGB stars known in these galaxies and find that most are carbon rich. We also find 26 dusty M-type stars, mostly in IC 10. Given the large dust excess and tight spatial distribution of these M-type stars, they are most likely on the upper end of the AGB mass range (stars undergoing Hot Bottom Burning). Theoretical models do not predict significant dust production in metal-poor M-type stars, but we see evidence for dust excess around M-type stars even in the most metal-poor galaxies in our sample (12+log(O/H) = 7.26-7.50). The low metallicities and inferred high stellar masses (up to ~10 $M_\odot$) suggest that AGB stars can produce dust very early in the evolution of galaxies (~30 Myr after they form), and may contribute significantly to the dust reservoirs seen in high-redshift galaxies.