A massive quiescent galaxy at redshift 4.658

A. C. Carnall thanks the Leverhulme Trust for their support via a Leverhulme Early Career Fellowship. R. J. McLure, J. S. Dunlop, D. J. McLeod, V. Wild, R. Begley, C. T. Donnan and M. L. Hamadouche acknowledge the support of the Science and Technology Facilities Council. F. Cullen acknowledges support from a UKRI Frontier Research Guarantee Grant (grant reference EP/X021025/1). A. Cimatti acknowledges support from the grant PRIN MIUR 2017 - 20173ML3WW 001.The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics (1; 2; 3; 4; 5). The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years (6; 7; 8). Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang (9; 10), however, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of M∗ = 3.8 ± 0.2 × 1010 M⊙, which formed over a ≃ 200 Myr period before this galaxy quenched its star formation activity at z=6.5+0.2−0.5, when the Universe was ≃ 800 million years old. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies.PostprintPeer reviewe

The stellar metallicities of massive quiescent galaxies at 1.0 < z < 1.3 from KMOS+VANDELS

We present a rest-frame UV-optical stacked spectrum representative of massive quiescent galaxies at $1.010.8$. The stack is constructed using VANDELS survey data, combined with new KMOS observations. We apply two independent full-spectral-fitting approaches, measuring a total metallicity, [Z/H]=$-0.13\pm0.08$ with Bagpipes, and [Z/H]=$0.04\pm0.14$ with Alf, a fall of $\sim0.2-0.3$ dex compared with the local Universe. We also measure an iron abundance, [Fe/H] =$-0.18\pm0.08$, a fall of $\sim0.15$ dex compared with the the local Universe. We measure the alpha enhancement via the magnesium abundance, obtaining [Mg/Fe]=$0.23\pm$0.12, consistent with similar-mass galaxies in the local Universe, indicating no evolution in the average alpha enhancement of log$(M_*/\rm{M_\odot})=11$ quiescent galaxies over the last $\sim8$ Gyr. This suggests the very high alpha enhancements recently reported for several bright $z\sim1-2$ quiescent galaxies are due to their extreme masses, log$(M_*/\rm{M_\odot})\gtrsim11.5$, rather than being typical of the $z\gtrsim1$ population. The metallicity evolution we observe with redshift (falling [Z/H], [Fe/H], constant [Mg/Fe]) is consistent with recent studies. We recover a mean stellar age of $2.5^{+0.6}_{-0.4}$ Gyr, corresponding to a formation redshift, z_\rm{form}=2.4^{+0.6}_{-0.3}. Recent studies have obtained varying average formation redshifts for $z\gtrsim1$ massive quiescent galaxies, and, as these studies report consistent metallicities, we identify different star-formation-history models as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at $z\gtrsim1$. Combining these with precise JWST $z>2$ quiescent-galaxy stellar-mass functions will provide an independent test of formation redshifts derived from spectral fitting.Comment: 16 pages, 3 figures, accepted for publication in Ap

Constraining quenching mechanisms at high redshift: the sizes, masses and star-formation histories of massive galaxies

Observations of the local and high-redshift Universe have revealed a clear bi-modality in the galaxy population. This is apparent in galaxy colours, morphologies and star-formation rates, with galaxies being categorised into two distinct populations; star-forming and quiescent. The existence of this bimodality means that one or more mechanisms must be able to cease, or quench, star-formation in galaxies. One of the key questions in extragalactic astronomy is to identify which quenching mechanisms are most important and on what timescales they operate. In this thesis, I use a combination of photometry and spectroscopy to examine the evolution of the quiescent galaxy population at high redshift in an attempt to piece together a coherent picture of their evolution over the past ten billion years of cosmic history. Some of the foundational results that have shaped our understanding of galaxy evolution are the relationships between galaxy stellar mass, size and age. In addition to the bi-modality observed up to z ∼ 2, it has also been shown that galaxies which formed earlier in cosmic time are more massive than their later counterparts, a phenomenon often referred to as “downsizing”. In this thesis, ultra-deep spectroscopic data along with HST and JWST imaging from large-scale surveys is used to study the sizes, masses and star-formation histories of massive galaxies from z = 0.25 to z = 2.25. Firstly, I present a study exploring the most massive galaxies at 0.6 < z < 1.3 using ultra-deep spectroscopy and photometry from the VANDELS and LEGA-C ESO public spectroscopic surveys. I investigate the relationships between galaxy stellar mass, physical size and age (using a well-known proxy for age predominant in quiescent galaxy spectra; the Dn4000 index), and demonstrate for the first time that downsizing is clearly evident in both our quiescent samples. I present a toy model to explain the size evolution of massive quiescent galaxies from z = 1.3 to z = 0.6 based on minor mergers. Next, I present the results of full spectral fitting of the VANDELS quiescent sample at 1.0 < z < 1.3, examining in further detail the relationship between stellar mass and age, this time using galaxy formation and quenching times. Upon further investigation into these trends, it becomes apparent that there is a sample of quiescent galaxies which appear to have experienced very short periods of star-formation, followed by abrupt quenching. These galaxies indicate higher levels of α-enhancement compared to the rest of the quiescent galaxies in my sample. This implies that there is an important mechanism at work which can end star-formation on very short timescales. One galaxy in my sample formed less than a billion years after the Big Bang, experiencing a short episode of star-formation before quickly becoming quiescent, based on star-formation and quenching timescales. Building on these key results at z ∼ 1, I investigate whether these trends can be seen at higher redshift using data obtained from the JWST PRIMER and JADES surveys. Due to the improved sensitivity and spatial resolution of JWST imaging, it is possible to examine the sizes and morphologies of star-forming and quiescent galaxies to lower stellar masses than previously possible. Over the redshift range 0.25 < z < 2.25, I find that low-mass quiescent galaxies in my sample exhibit Sérsic indices and sizes qualitatively consistent with these galaxies quenching by infall into cluster environments. Contrastingly, the sizes and morphologies of more massive quiescent galaxies are consistent with having quenched by internal feedback mechanisms. In summary, this thesis aims to provide a quantitative investigation of the evolution of quiescent galaxies over cosmic time. The unprecedented size and quality of the datasets used throughout this work have led to many important findings, enabling a deeper understanding of the properties of quiescent galaxies, and the overall formation and evolution of galaxies across time

A massive quiescent galaxy at redshift 4.658

The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics (1; 2; 3; 4; 5). The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years (6; 7; 8). Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang (9; 10), however, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift, z = 4.658, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of M∗ = 3.8 ± 0.2 × 1010 M⊙, which formed over a ≃ 200 Myr period before this galaxy quenched its star formation activity at z=6.5+0.2−0.5, when the Universe was ≃ 800 million years old. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies