Abstract
Under hierarchical assembly, galaxy formation begins as haloes collapse in the very early Universe and gas subsequently cools sufficiently to form stars. How do galaxies grow into the diversity of colours and structures seen in the local Universe? The high sensitivity of JWST and its wavelength coverage offers a revolutionary new view of the high-redshift galaxy population and its evolution into the present-day population. Spectroscopy with the Near Infrared Spectrograph (NIRSpec) in particular has proven powerful, revealing the spectral energy distributions and emission line kinematics of up to 200 galaxies simultaneously. Spectral modelling provides crucial insight into the properties of the stars and interstellar medium, as well as signatures of black hole activity. Combined, these different measurements are beginning to paint the picture of the early assembly of today’s massive galaxies and galaxies like our own Milky Way. Early results with JWST suggest a rapid growth in galaxy stellar mass and their central black holes in the early Universe, with morphologies and kinematics that are already surprisingly ‘mature’ and galaxy and black hole masses that are surprisingly high. Ongoing large spectroscopic surveys will obtain spectroscopy for nearly over 15,000 sources at redshifts z>1 by the end of 2024. These large statistical samples will be critical to place the early findings into a cosmological context, and unravel the evolution of the broader high-redshift galaxy population.
Introduction
In the local Universe there is a rich diversity of galaxies. Some look similar to our own Milky Way, with prominent, blue spiral arms where star formation takes place. Other galaxies are much redder in colour and also differ strongly in shape: whereas the spiral galaxies have disky structures, the reddest galaxies tend to be spheroidal. In contrast, current widely-accepted theories indicate that the very early Universe was more homogeneous. In the standard cosmological model, cold dark matter haloes collapse first, after which gas could cool and collapse to form stars (White & Rees 1978). These first haloes are thought to form the seeds of the massive galaxies observed at the present day. Therefore, a major open question in extragalactic astronomy is how galaxies grew from initially gas-rich structures to the great variety of systems seen today.
The early galaxies continue to grow rapidly after the initial formation of stars. Broadly, two growth mechanisms can be identified. Cold gas streams can efficiently provide fuel for star formation despite a hot circumgalactic medium in the halo (e.g. Dekel et al. 2009). At later times mergers between galaxies are thought to play a crucial role, with massive galaxies being almost entirely formed through the accretion of nearby galaxies (e.g. Oser et al. 2010). These two modes are difficult to distinguish observationally, and instead, the combined stellar mass growth is usually traced statistically. Observations from ground-based near-infrared instruments as well as with the Hubble Space Telescope (HST) have shown that the most massive galaxies were already in place at redshift z~2 (only 3 Gyr after the Big Bang; Muzzin et al. 2013; McDermid et al. 2015), indicating that these systems formed or accreted their stellar mass on rapid timescales.
Our current understanding of the early Universe has been limited by the spectral coverage and sensitivity of the HST. At lower redshifts (z<2), HST has been tremendously successful in uncovering the structures, star formation activity and stellar masses of galaxies. However, at higher redshifts HST probes rest-frame ultraviolet (UV) emission and only has the sensitivity to map the very brightest high-redshift galaxies. A large population of objects has therefore been missed by HST due to the faint UV emission of high-redshift galaxies or obscuration of UV light by dust. The earliest phase of galaxy formation and growth thus still remains unclear. How did galaxies in the early Universe build up their stellar mass? When did galaxies start to resemble the colours and structures seen in the present-day Universe?
Spectroscopy with JWST
The James Webb Space Telescope (JWST) provides the observations needed to tackle these longstanding questions. With its extremely high sensitivity and wavelength coverage in the near- and mid-infrared, JWST has enabled the efficient and highly complete detection of early galaxies (z>2) by their emission at rest-frame optical wavelengths. Unique to JWST is its ability to perform spectroscopy in the near-infrared with the Near-Infrared Spectrograph (NIRSpec; Jakobsen et al. 2022; Ferruit et al. 2022). The multi-object spectroscopic mode of NIRSpec allows for the simultaneous observation of 200 high-redshift galaxies, thereby delivering an unprecedented number of spectra of early galaxies. These spectra are taken at different spectral resolutions: the low-resolution mode is ideal for obtaining the precise redshift of a galaxy and an overview of the galaxy properties, by revealing spectral breaks and a broad range of emission lines. The higher spectral resolutions de-blend individual emission lines, enabling a more detailed view of the chemical properties of the gas and stars within galaxies, and also provide insight into the internal dynamics of galaxies.
Rapid mass and structural assembly at high redshift
Within the first months of science observations, imaging surveys with the Near-Infrared Camera (NIRCam) onboard JWST detected several thousand objects that were previously missed by HST and ground-based telescopes (e.g. Finkelstein et al. 2023; Rieke et al. 2023; Weaver et al. 2024). In the context of the questions above, these observations yielded two major surprises.
First, multiple groups reported the discovery of highly ‘mature’ galaxies, as they found that the morphologies of galaxies at redshift z~2-4 (i.e. when the Universe was 1.5-3.0 Gyr old) already look like the well-organised galaxy structures observed at the present day (e.g. Ferreira et al. 2022, 2023; Kartaltepe et al. 2023). This is surprising, as previous studies with HST had indicated that the very early galaxies appear more clumpy, and less disky than galaxies in the local Universe. As shown in Figure 1, this may be due to the increased sensitivity of JWST: some objects that looked clumpy in HST imaging, appear as smooth disks with spiral arms in the JWST imaging, even when comparing at the same observed wavelength. However, imaging alone cannot answer whether these systems are truly like the objects we see at the present day: galaxies are projected at a random inclination angle on the sky, and it is therefore impossible to distinguish a rotating disk from an elongated structure with turbulent kinematics. Spectroscopy can reveal the kinematic properties of galaxies and answer whether the high-redshift galaxies have dynamical properties that are consistent with those observed at z~0. In de Graaff et al. (2023) we used NIRSpec in its multi-object spectroscopic mode to, for the first time, study the kinematic properties for a sample of 6 high-redshift (z~6-7) galaxies with low stellar masses and compact sizes. These early galaxies were found to have diverse kinematic structures: some show clear evidence of rotation, consistent with disky galaxies, whereas others are highly turbulent in nature. On the other hand, Nelson et al. (2023b) discovered a very massive galaxy with kinematic properties similar to that of galaxies at much lower redshifts. These studies likely reflect an important dependence of the morphology and kinematics on the overall galaxy mass, and will require further investigation with larger samples.
Figure 1: (a) HST (left) and JWST NIRCam (right) images of galaxies at high redshift (z~2), adapted from Ferreira et al. (2023). (b) NIRSpec observations of early galaxies at z~6 (adapted from de Graaff et al. 2023) provide insight into their kinematic structures.
Second, a substantial fraction of the newly-discovered sources with JWST were found to appear extraordinarily bright for their estimated redshift or to have unusually red colors and SEDs (e.g. Finkelstein et al. 2023; Nelson et al. 2023a; Pérez-González et al. 2023). The physical properties of these sources are still poorly understood, but many of these new objects have been suggested to be very massive already at a very early epoch. Of particular interest is a subset of high-redshift objects that have even been shown to be in tension with current well-established models of cosmology and/or galaxy formation (Labbe et al. 2023), because the estimated masses of these systems exceed all model predictions.
However, the imaging so far cannot determine the true nature of these sources, due to the coarse wavelength sampling of the different image filters. Only spectroscopy with JWST/NIRSpec can confirm whether these objects are truly at the claimed high redshifts (z~7) and reveal the stellar population properties. Kocevski et al. (2023) presented spectroscopic observations obtained for one of the sources from Labbé et al. (2023) as part of the Cosmic Evolution Early Release Science (CEERS) program (Figure 2), and revealed a slightly lower redshift (z=5.6) than estimated based on imaging alone (z=8.3). More importantly, the spectroscopy showed clear evidence for a luminous active galactic nucleus. This therefore showed that the stellar mass inferred previously was overestimated, as the light was assumed to originate solely from the stellar population.
Figure 2: Left: false-color image from JWST/NIRCam of one of the candidate extremely massive galaxies identified by Labbé et al. (2023). Right: the NIRSpec spectrum reveals a broad emission line, indicative of an active galactic nucleus. Figure adapted from Kocevski et al. (2023).
Conclusions & Outlook
JWST observations thus far have demonstrated the great potential of the different instruments to transform our understanding of galaxy evolution at high redshifts. The NIRSpec instrument in particular has proven to be essential to provide insight into the growth of galaxies and their structural properties. Nevertheless, many studies have been based on early observations and hence on modest sample sizes. Future work based on spectroscopic samples covering a wider range in stellar mass and redshift will be crucial to establish the growth and evolution of the broader galaxy population.
Further observations are also critical to uncover the properties of newly-discovered sources. Although one candidate for an extremely massive early galaxy was shown to be at a lower redshift and of AGN nature, many massive galaxy candidates remain. It is still unclear if all objects will turn out to have actively accreting black holes, or whether some sources may indeed have masses that are incompatible with current theories. Follow-up observations with JWST/NIRSpec that systematically target these highly-sought-after sources are planned for Cycle 2 (program 4106, PI Nelson; program 4233, PI de Graaff) and will soon reveal their true nature.
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