A massive, quiescent galaxy at redshift of z=3.717 (1702.01751v3)

Abstract: In the early Universe finding massive galaxies that have stopped forming stars present an observational challenge as their rest-frame ultraviolet emission is negligible and they can only be reliably identified by extremely deep near-infrared surveys. These have revealed the presence of massive, quiescent early-type galaxies appearing in the universe as early as z$\sim$2, an epoch 3 Gyr after the Big Bang. Their age and formation processes have now been explained by an improved generation of galaxy formation models where they form rapidly at z$\sim$3-4, consistent with the typical masses and ages derived from their observations. Deeper surveys have now reported evidence for populations of massive, quiescent galaxies at even higher redshifts and earlier times, however the evidence for their existence, and redshift, has relied entirely on coarsely sampled photometry. These early massive, quiescent galaxies are not predicted by the latest generation of theoretical models. Here, we report the spectroscopic confirmation of one of these galaxies at redshift z=3.717 with a stellar mass of 1.7$\times$10$^{11}$ M$_\odot$ whose absorption line spectrum shows no current star-formation and which has a derived age of nearly half the age of the Universe at this redshift. The observations demonstrates that the galaxy must have quickly formed the majority of its stars within the first billion years of cosmic history in an extreme and short starburst. This ancestral event is similar to those starting to be found by sub-mm wavelength surveys pointing to a possible connection between these two populations. Early formation of such massive systems is likely to require significant revisions to our picture of early galaxy assembly.

Spectroscopic Confirmation of a Massive Quiescent Galaxy at Redshift z=3.717

The discovery of a massive, quiescent galaxy at a redshift z=3.717, documented by the authors, represents a significant advance in our understanding of the formation and evolution of galaxies in the early universe. This paper explores the intriguing challenge of identifying massive galaxies that have ceased forming stars in the early universe, where their rest-frame ultraviolet emissions are negligible. Such galaxies can be reliably identified by extremely deep near-infrared surveys, which have traditionally revealed the presence of massive, quiescent early-type galaxies appearing as early as z~2, significantly complicating our understanding of galaxy evolution.

The paper provides spectroscopic confirmation of a massive, quiescent galaxy with a redshift z=3.717, a significant earlier epoch than previously confirmed massive quiescent specimens. The galaxy in question, ZF-COSMOS-20115, exhibits a stellar mass of 1.7 × 10¹¹ M⊙, effectively making it an outlier compared to galaxy formation theories which don't predict such massive, quiescent systems at this redshift. The absorption line spectrum of the galaxy indicates no ongoing star-formation, and an age estimate places it at nearly half the age of the universe at this redshift. These observations imply that the galaxy underwent rapid star formation within the first billion years of cosmic history, in a remarkably brief but intense starburst event.

The observational evidence of this quiescent galaxy at z=3.717 raises questions about the formation and assembly processes in early galaxies. Typically, theoretical models, including those incorporating hydrodynamical simulations, predict that massive galaxies at such high redshifts would still be actively forming stars due to continual cosmic accretion processes. However, the detection of ZF-COSMOS-20115 challenges these models, revealing the need for significant theoretical revisions in our understanding of early galaxy assembly.

The spectroscopic analysis revealed strong Hydrogen Balmer absorption lines characteristic of post-starburst galaxies. This age-sensitive feature underscores the reliability of the observed galaxy's age estimates and rules out possible alternative models of dusty, highly star-forming regions that could mimic the quiescent spectral energy distribution (SED).

The paper further speculates about the ancestors of such massive galaxies, positing potential links to dust-obscured star-forming galaxies identified in sub-millimeter wavelength surveys. These potential progenitors exhibit high star formation rates exceeding 1000 M⊙ per year, potentially forming most of their stellar mass in single, significant starburst events. However, existing sub-millimeter surveys fail to detect many galaxies with the necessary extreme star formation rates.

The findings suggest two major implications. Firstly, they call for a reevaluation of current galaxy formation models, particularly regarding the efficiency and timescales of star formation in the early universe. Secondly, they hint at a mechanism of rapid baryonic mass assembly and subsequent quenching, which could have pivotal implications for the evolution of massive galaxies in general. The theoretical understanding of dark matter halo growth, conversion of baryons to stellar mass, and the corresponding quiescence of already formed massive systems must be adjusted to accommodate observations like ZF-COSMOS-20115.

Future work should direct effort towards larger, more comprehensive spectroscopic surveys and utilize forthcoming observational facilities to better characterize these early quiescent populations. The potential for significant advancements in this field is underscored by the capabilities of upcoming telescopes, which promise to shed further light on these complex and dynamic processes shaping the early universe.