Galaxies and Cosmology Seminar

Connor Painter, The University of Texas at Austin

Sparks in early galaxies: the first star clusters in BonFIRE

Star clusters have been revealed as ubiquitous features of high-redshift galaxies in deep JWST observations, products of vigorous star formation events in dense gas-rich environments. Their properties reflect the environmental conditions at their formation sites and serve as probes of the underlying physics in their host galaxies. Cosmological samples of star clusters from observations can be compared with analogous samples from simulations to constrain models of galaxy formation and potentially illuminate the origin of enigmatic present-day globular clusters. However, it is only in the past few years that the resolution required to resolve star clusters in cosmological volumes has been computationally feasible. In my work, I use BonFIRE, a suite of large-volume, premier-resolution cosmological simulations with state-of-the-art baryonic physics, to analyze thousands of galaxies and identify tens of thousands of star clusters from redshift 20 to 6. Massive galaxies show clumpy morphologies in agreement with observations, with 2% to 50% of their stellar mass contained in bound clusters. Lower-mass galaxies frequently show nearly 100% of their stellar mass contained in a single, central cluster. These cluster-like dwarfs form a new, distinct subpopulation from "normal" clusters in massive galaxies, leading to bimodality in global distributions of some cluster properties, such as metallicity. This sample is among the largest and most detailed extracted from a large-volume cosmological simulation and makes predictions testable by forthcoming high-redshift observations.

Tiger Hsiao, The University of Texas at Austin

An Incandescent Galaxy through a Cosmic Microscope at Cosmic Dawn

The first stars and galaxies are difficult to detect individually, but studying small-scale structures with JWST and gravitational lensing enhances the chances of discovering the first star clusters. JWST has already revealed such clusters in lensed fields, including the Sunrise Arc (z~6.0), the Cosmic Gems (z~9.6), and MACS0647-JD (z = 10.17), with spatial resolution down to ∼1 pc. In particular, MACS0647-JD, the brightest galaxy known at z > 10, has been resolved into two distinctive clumps with sizes of 70 and 20 pc in NIRCam imaging, with a possible companion located 3 kpc away. Using NIRSpec, we detected seven emission features, including the auroral line [O III] 4363 and the resolved [O II] doublet, which yield an electron temperature of 17,000 K and an electron density of log(nₑ) = 2.9, respectively, together with Hα and [O III] 5007 detections with MIRI. We present, for the first time at z > 10, a “direct” metallicity of 13% Zsun, suggesting that the universe had been rapidly enriched just ~460 Myr after the Big Bang. We also found an enhanced carbon-to-oxygen abundance ratio of log(C/O) = –0.44, which may reflect the chemical heritage of Pop III stars. These results highlight our progress toward JWST’s goal of understanding the first stars, star clusters, and galaxies.