Abstract
Recent observations with JWST have revealed that some galaxies have evolved already in the first billion years, exhibiting similar properties as have been observed at later times. We present the discovery and spectroscopic confirmation of a galaxy overdensity at z=7.88, consisting of 7 galaxies within a very close proximity. Some of those galaxies exhibit an enhancement of dust attenuation and more evolved stellar populations compared to the field counterparts at similar redshifts, indicating the evolution in that specific region may proceed even further. We introduce our ongoing JWST NIRCam pure-parallel imaging survey, BEACON, that envisions to sample many random sightlines of the universe and to construct a large galaxy sample of z~2 to z>10. Identification of luminous candidates, with spectroscopic followup, will enable statistical studies of early galaxy populations.
Introduction
Hierarchical structure formation is one of the fundamental features of our standard cosmological model. The first overdensities to collapse and form stars and galaxies play a particularly important role in the evolution of the universe and cosmic reionization (Tegmark et al. 1997). Identifying and studying the sources associated with these first overdensities thus provides critical insights into the evolution of galaxies, the intergalactic medium, and the underlying dark matter scaffolding (e.g., Mo & White 1996).
The clustering of sources around a luminous galaxy or quasar includes an expected excess of fainter companions, under a broad assumption that galaxy luminosity is correlated with the mass of the dark-matter host halo. Such early overdensities are thought to be the seeds of today’s galaxy clusters, and sites where galaxy formation and the evolution of the surrounding gas is progressing more rapidly compared to the mean of the universe. As such, the identification of galaxy over-densities at high redshift (z>6) has been of particular interest in the literature (e.g., Trenti et al. 2012, Castellano et al. 2016,2018,2022, Harikane et al. 2019, Tilvi et al. 2020, Hu et al. 2021, Endsley & Stark 2022, Larson et al. 2022). Furthermore, galaxy overdensities serve as ideal laboratories for studying the ionization of neutral hydrogen around galaxy systems; the presence of a large ionizing bubble may boost the fraction of escaping Lyman-alpha photons, which otherwise are scattered and absorbed by surrounding neutral hydrogen (Miralda-Escude 1998, Dijkstra 2014, Mason & Gronke 2020; see also Trapp et al. 2022).
JWST has opened up a path for us to directly observe those building blocks in the first billion years. With the unprecedented sensitivity in near-infrared wavelengths, it enables us to see the rest-frame optical features of early galaxies, i.e. those seen and studied in the SDSS for galaxies today. Early studies have been watershed with a number of discoveries, including luminous galaxies, record-breaking redshift, and evolved stellar populations. The equipped sensitive spectroscopy is capable of determining the detailed chemical composition and enrichment history in earlier times.
Here, we first present our discovery of a galaxy overdensity at z=7.88, merely ~650 million years after the big bang, consisting of 7 member galaxies in a small sky area. We then present our effort of investigating oxygen abundances in galaxies and their redshift evolution from our Cycle 1 GTO program. Lastly, we present our latest observing program, BEACON, that exploits the pure-parallel capability of JWST to identify early galaxies from many sightlines.
JWST’s confirmation of an early galaxy overdensity
As part of the JWST ERS program GLASS-ERS (PID1324), we observed the sightline of a cluster of galaxies, Abell 2744. Our observations were gained from the strong lensing magnification, amplified by this foreground cluster, making faint galaxy light detectable at high significance. In our spectroscopic observations, we assigned the spectroscopic slits on 7, previously known, galaxy candidates at z~8. These galaxies were previously identified in near-infrared observations by the Hubble Space Telescope but remained as “photometric” candidates i.e. sources that have no spectroscopic information.
Surprisingly, our spectroscopic observations not only confirmed the redshift of all 7 galaxies, but found that they are at the same redshift, z=7.88 (Morishita et al. 2023). The spatial distribution of the 7 galaxies is also tight, within an area of ~300 kpc in radius (Figure 1). Hereafter, we refer to this overdensity system as A2744-z7p9OD.
Their very tight proximity indicates that these galaxies formed and evolved in the excess area of matter density in the early universe. Such an overdensity is expected to grow to one of the most massive systems in the universe today. By looking into simulations, we identify a similar overdensity in the EAGLE simulation — the simulation predicts that an overdensity system like A2744-z7p9OD would have grown to a massive cluster of galaxies, with the total halo mass of ~10^15 Msun, comparable to the Coma cluster.
However, the prediction made from our comparison to simulations may suffer from a few uncertainties. One of such is the uncertainty in the halo mass estimate at the observed redshift. While an overdensity of 7 galaxies in such a small area could be a signature of a large-scale structure, it may well be part of a proto-group system; leading to a much smaller halo mass. To be definitive about the halo mass, and therefore the future evolution of the system, one solution is to characterize the system. This includes the search of other member galaxies to a larger extent (Morishita 2024, in prep.).
Besides the characterization of A2744-z7p9OD as part of a large-scale structure, the high-sensitivity imaging and spectra allow us to investigate the physical properties of the individual member galaxies. Remarkably, two pairs of the member galaxies (ZD3 & ZD6 and YD4 & YD7) show evidence of interaction in their morphologies, indicating potential enhancement of dust and ISM metallicity. Indeed, the spectra of these galaxies exhibit a red rest-frame UV slope, i.e. attenuation by dust (also see recent study by Hashimoto et al. 2023). While direct confirmation of such dust will require resolving sub-mm observations from, e.g., ALMA, this further characterizes this early overdensity as a place where galaxy evolution might be advanced, compared to field galaxies.
Toward better understanding of galaxy evolution in the early universe
We note that this finding is from a merely single pointing of JWST. Similarly, other remarkable findings of early luminous galaxies reported from the Cycle1 observations are from a very small portion of sky. The universe is characterized by fluctuation of matter density, which causes biases in pencil beam observations, known as cosmic variance. To mitigate such biases, we will have to sample the universe for a larger area and correct our statistics built upon a limited number of observations. To better understand the census of galaxies, we have initiated a large imaging program in Cycle2, BEACON. The BEACON project takes advantage of the pure-parallel opportunities available by the observatory, where a secondary instrument can be utilized while the primary instrument is observing their target. Since our goal is to construct an unbiased sample of early galaxies, the pure-parallel mode is ideal.
We note that there exist previous efforts of such using HST’s pure-parallel mode. However, it is worth highlighting that HST has wavelength coverage only up to 1.6um. Without sufficient red sensitivity, the contamination fraction in the selected high-z candidates was known to be large. JWST’s NIRCam pure-parallel imaging, on the other hand, affords much higher purity and robust characterizations of the candidates’ properties (see below).
Similar to existing extragalactic observations, our observations are configured with multi-band filters of the NIRCam instrument, with 4-8 filters depending on the availability. Those filters span a wide range of wavelength, 0.8-5.0 micron, allowing us not only to identify high-z galaxies but also lower redshift galaxies and enhance legacy values. With the allocated ~ 600 hrs, we will be able to explore ~0.4deg2 of sky, or ~200 pointings. Figure 2 shows the predicted numbers of high-z galaxy candidates from our program.
Summary
In this paper, we reported the discovery of an early galaxy overdensity, A2744-z7p9OD, the physical properties of the member galaxies enabled by JWST, and future prospects for improving our further understanding of the universe. The BEACON program is such an ideal path for us to robustly investigate the census of early luminous galaxies and their clustering properties, which could have a critical impact on a wide range of research.
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