A dusty, normal galaxy in the epoch of reionization

Abstract: Candidates for the modest galaxies that formed most of the stars in the early universe, at redshifts $z > 7$, have been found in large numbers with extremely deep restframe-UV imaging. But it has proved difficult for existing spectrographs to characterise them in the UV. The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant UV-selected galaxy detected in dust emission is only at $z = 3.25$, and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at this early epoch. Here we report thermal dust emission from an archetypal early universe star-forming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be $z = 7.5\pm0.2$ from a spectroscopic detection of the Ly{\alpha} break. A1689-zD1 is representative of the star-forming population during reionisation, with a total star-formation rate of about 12M$_\odot$ yr$^{-1}$. The galaxy is highly evolved: it has a large stellar mass, and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at $z > 7$, in spite of the very short time since they first appeared.

• The paper demonstrates that A1689-zD1 exhibits rapid dust enrichment with a dust-to-gas ratio similar to the Milky Way, challenging early universe assumptions.
• It employs X-shooter spectroscopy and infrared measurements to determine star-formation rates between ~2.7 and 12 M☉ yr⁻¹, indicating advanced evolution at z ~7.5.
• The study implies that evolved, dusty, and metal-rich galaxies existed in the epoch of reionization, paving the way for deeper ALMA observations.

A Dusty, Normal Galaxy in the Epoch of Reionization

The paper discusses the discovery and detailed analysis of the galaxy A1689-zD1, situated in the early universe's epoch of reionization with a redshift of z = 7.5±0.2. This corresponds to a period when the universe was less than one billion years old. Using the X-shooter spectrograph on the Very Large Telescope, the researchers have presented compelling data on thermal dust emission from this archetypal early universe star-forming galaxy, revealing significant findings about its properties.

A1689-zD1 distinguishes itself as one of the oldest galaxies confirmed by spectroscopy through its stellar continuum, with a sharp Lyman-alpha break at 1035±24 nm defining its redshift. This galaxy is characterized by a substantial stellar mass, heavy dust enrichment, and a dust-to-gas ratio akin to the Milky Way's, indicating that dusty, evolved galaxies are present among fainter star-forming populations at z > 7, although metallicity and dust are typically rapidly enriched through supernovae.

One of the most notable aspects of A1689-zD1 is its total star-formation rate, reported as approximately 12 M☉ yr⁻¹. The researchers measure an uncorrected UV luminosity of ~1.8×10¹⁰ L☉, yielding a star-formation rate estimation of 2.7±0.3 M☉ yr⁻¹ based on UV emission. Meanwhile, the infrared analysis indicates even higher star-formation rates when considering the measured total infrared (TIR) luminosity of 6.2×10¹¹ L☉, approximating a rate of 9 M☉ yr⁻¹. The derived dust mass approximates 4×10⁷ M☉, considering a dust temperature of 35 K, which aligns with these findings of enriched dust content even in the significantly early universe epoch.

A1689-zD1's high dust-to-gas mass ratio, around 17×10⁻³, provides evidence of substantial metal enrichment, rendering it comparably enriched to modern local galaxies despite the brief time since the galaxy's inception. Calculations suggest a corresponding gas mass of 2×10¹⁰ M☉, further demonstrating evolved characteristics contrary to prior assumptions for galaxies in the epoch of reionization.

The implications of these findings are twofold: they challenge previous assumptions about the availability of dust and metals in the nascent universe and highlight the potential for detecting similar galaxies with facilities like ALMA. As these evolved systems appear amid younger galaxies, our understanding of cosmic and elemental evolution calls for a reevaluation, especially concerning star-forming processes and galaxy maturation in the earliest cosmic epochs. Future research could pivot towards understanding the formation mechanisms responsible for rapid dust enrichment and improved observational practices to capture such distant and intrinsic properties of early-universe galaxies comprehensively. These observations substantially contribute to the astrophysical discourse on the matter distribution and enrichment processes in formative neponic galaxies. Researchers will likely extend observational methodologies in the infrared spectrum to refine our comprehension of cosmological galaxy formation and evolution dynamics.