Abstract
JWST is reshaping our understanding of galaxy evolution, from pushing the frontier of the earliest galaxies known to probing the inner lives of nearby galaxies in exquisite detail. JWST’s photometric and spectroscopic capabilities, reaching depths and spectral and spatial resolution never before achieved in the infrared, open up previously inaccessible diagnostics and parameter space that promise to broaden our understanding of the Universe. Here we present a non-comprehensive overview of how JWST is addressing the Big Questions in galaxy evolution − how do galaxies get their shapes? How do galactic ecosystems change over cosmic time? What is the role of black holes? and finally, when and why do galaxies stop forming new stars? − at what has facetiously become “low” redshift, z ≤ 8 − 9, in the community.
Introduction
Globular clusters, ancient and compact spherical collections of stars, have long been one of our most enigmatic neighbors. Our own Milky Way contains 150 of these structures, putting them practically in reach astronomically speaking. Globular clusters likely formed in the very early epochs, witness to early galaxy assembly, yet how they formed and what role they play in the galactic ecosystem remains a mystery after decades of active research (e.g. Harris & Racine, 1979). Part of the challenge is exactly that globular clusters are so very old and the process of aging erases clues as to their origin. And these relatively small − again, astronomically speaking − faint structures have defied observation earlier in the Universe at higher redshifts,[1] where we can use the travel time of their light to Earth as a time machine.
Enter the James Webb Space Telescope (JWST; Gardner et al., 2023). When the CANUS[2] team was examining the first JWST deep field in SMACS J0723, taken in the first month of JWST science operations, they discovered something miraculous. Around a galaxy (named the “Sparkler”) magnified by another foreground galaxy cluster are several compact sparkles (Figure 1), which they dated to be old stellar populations that probably formed only 500 million years after the Big Bang (Mowla et al., 2022) at z > 9! These stars, likely already ancient by the time their light began traveling to us some 9 billion years ago, present a unique new opportunity to push back the frontiers of our understanding of a previously inscrutable component of galaxies.
The study of galaxy evolution, established when the extended, fuzzy shapes of “spiral nebulae” were determined to be in fact be massive structures external to our own Milky Way, is a historical journey back to our origins. Across cosmic time, an individual galaxy will undergo dramatic transformations, accreting and consuming gas from the cosmic web which it processes into new stars, solar systems, and planets, growing in mass and undergoing structural changes. Understanding this process is one of the primary science drivers for JWST, which carries an instrument suite designed to peer back to the beginning of the Universe with unparalleled spatial and spectral detail. JWST’s three near-infrared instruments − NIRCam, NIRSpec, and NIRISS[3] − and one mid-infrared instrument, MIRI,[4] each provide a distinctive piece of the puzzle as we pursue the big questions in galaxy evolution: how do galaxies get their shapes? How does the galactic ecosystem within a galaxy change over cosmic time? What is the role of black holes? And finally, when and why do galaxies stop forming new stars?
From Hubble to High Redshift: The Shapes of Galaxies with JWST
A new era in astronomy was sparked with the Great Debate between Harlow Shapley and Heber Curtis, who disagreed on whether the un-star-like, extended shapes of “spiral nebulae” meant that massive structures existed outside of our Milky Way. The outcome was a literal redefining of our concept of the Universe and today we know that galaxies are not only independent structures, but that a galaxy’s shape encodes critical information as to its assembly and evolution, which was codified in the Hubble Sequence classification[5] (Hubble, 1926). Increasingly powerful telescopes culminating with the Hubble Space Telescope (HST) chased this sequence back through cosmic time for bright galaxies, finding that the ordered spirals and spheroids Edwin Hubble cataloged for his Sequence exist as early as ∼ 3 billion years after the Big Bang (z ∼ 3), but they are accompanied by an increase in the fraction of disordered peculiar and irregular galaxies, perhaps pointing to turbulent origins. However, the types of light we could see arriving from these early galaxies, ultraviolet (UV) and optical, can be obfuscated by showing only recently formed stars, or redirected by cosmic dust. Only observations in the near-infrared show the true shape of a galaxy outlined by the older, bulk stellar population.
JWST’s Hubble Sequence
JWST’s sensitivity and HST-like spatial resolution in the near-infrared wavelengths is already putting our previous view of galaxy shapes to the test. As demonstrated in Figure 2 (a), JWST can identify spiral arms and central bulge structures within disks that are not obvious or even entirely invisible with HST. The results have been surprising: rather than the chaotic, irregular shapes we expected from HST to be abundant, JWST has quickly revealed there are far more ordered, familiar shapes even earlier in the Universe than we ever imagined (e.g. Jacobs et al., 2023; Kartaltepe et al., 2023; Ferreira et al., 2023). In fact, the Hubble Sequence seems firmly in place only 1 billion years (z ~6) after the Big Bang! This new challenge to the current paradigm is further enhanced by access to a previously unseen population: faint galaxies with low stellar masses can now be identified and studied with JWST at early cosmic times. These faint galaxies likely live very different lives from their massive counterparts; their smaller gravitational wells make them more susceptible to disruption, say through interactions with neighbors. They may also play an outsized role in shaping their more massive counterparts: JWST is also revealing that elliptical galaxies − massive spheroids that have stopped forming stars − have a retinue of previously-undetected companions 900x less massive (and nearly a 100x too faint for HST!) than themselves (Figure 2 (b); Suess et al., 2023), long suspected culprits in creating the spheroidal shape of elliptical galaxies.
The Tumultuous Inner Lives of Galaxies
The birth, lifecycle, and death of stars are fundamental to who we are as this process is responsible for “polluting” the Universe with heavier elements − essential for planets and people! – than the hydrogen, helium, and trace amounts of lithium produced in the Big Bang. The recipe for forming stars within a galactic ecosystem is still poorly understood, however, particularly in the early Universe. We know that at a macro level, the efficiency of forming new stars peaked some 10 billion years ago (the so-called cosmic noon era at z ~ 1 − 3) and then declined to present day (Madau & Dickinson, 2014), likely due to less availability of the hydrogen gas required to form stars. We also know from studies of nearby galaxies that the process of star formation is incredibly complex, sensitive to conditions and energy transfer on scales much smaller than the galaxy itself.
The galactic ecosystem: gas, metals, and dust
The revolution in our understanding of galactic ecosystems with JWST will come largely because of two factors: unprecedented sensitive spectroscopy and spatial resolution in the infrared.
Spectroscopy is the gold standard of astronomical observations: it finely bins the light from galaxies, revealing discrete features that contain a wealth of information about the energetic processes in a galaxy. Though not as enchanting as the gorgeous images coming out of JWST, a galaxy’s spectrum is nevertheless the source of much excitement among astronomers, as was demonstrated when the very first press release of JWST data presented NIRSpec spectroscopy of a galaxy viewed just 600 million years after the Big Bang (Figure 3). Those in the know immediately spotted an innocuous little line of ionized oxygen that just so happens to be a gold standard for measuring the amount of heavy elements in a galaxy (Arellano-Córdova et al., 2022; Schaerer et al., 2022; Curti et al., 2023; Trump et al., 2023; Rhoads et al., 2023; Brinchmann, 2023; Laseter et al., 2024). This line was previously rarely used to track heavy elements over cosmic time because it was too faint for detection except in nearby galaxies and now, on its figurative first day, JWST was not only detecting this little line, but detecting it in a galaxy that emitted its light 13.1 billion years ago!
The unique capability of JWST to do sensitive spectroscopy in the near- and mid-infrared will give us new insight into several vital aspects of the galactic ecosystem. Using optical emission lines, JWST is measuring how many heavy elements are present in galaxies over cosmic time (e.g. D’Eugenio et al., 2023; Sanders et al., 2024; He et al., 2024), which tracks how stars are born and how pristine gas is accreted from the cosmic web. As we push earlier and earlier in the Universe, we expect these heavy elements to be increasingly rare, with huge implications for the first galaxies; and indeed, JWST is finding that these early ecosystems are harsh places of rapid star formation and extreme radiation fields (Cameron et al., 2023; Topping et al., 2024). JWST’s spectroscopy is so detailed that not only can we measure the strength of spectral features (like the emission lines in Figure 3) but also their shapes. The shapes of spectral features have a lot to tell us about how stars and gas are moving through a galaxy (called kinematics); for example, whether gas is flowing into the center of a galaxy, compressing to form a burst of star formation, or flowing out of the galaxy, expelling heavy elements into the cosmic web and removing fuel for new stars (de Graaff et al., 2023). And this is only the tip of the iceberg, JWST’s spectroscopy is going to be changing our view of the Universe for decades to come.
This wonderous spectroscopy will team up with JWST’s ability to take images with high spatial resolution, revealing the inner workings even in smaller, distant galaxies. As just one example, a small but integral component of the galactic ecosystem is cosmic dust, small to large carbon and silicate grains that are pervasive in massive galaxies at later times. Cosmic dust is known to be highly efficient at absorbing the high-energy photons emitted by young stars and other energetic processes and re-emitting lower-energy photons with infrared wavelengths. Because of this, much activity happening in galaxies has been “hidden”, invisible to our UV and optical telescopes and even to HST’s infrared capabilities at high redshift, including the birth clouds of new stars. JWST can peer through cosmic dust in the near-infrared and directly measure the photons from small dust grains in the mid-infrared. Such became exquisitely apparent with early observations of nearby galaxies from the JWST PHANGS team;[6] Figure 4 shows MIRI images of two very different regions where new stars are being born (Williams et al., 2022). These observations identify new stars in their dusty cradles, catching star formation in its earliest stages. Combining MIRI observations with those at even longer wavelengths in the far-infrared from the ground-based interferometer ALMA reveals that only one of these regions is rich in molecular hydrogen, the fuel for star formation. That both are forming stars speaks to the complexity of the galactic ecosystem and the power of JWST in unraveling this complexity.
The mystery of Little Red Dots
JWST’s look at the inner lives of galaxies is not only testing our long-held models but also discovering types of galaxies that we never expected. Considerable excitement was sparked in the community with the discovery of Little Red Dots in the first billion years of the Universe using NIRSpec and NIRCam. These compact galaxies have peculiar ‘v-shaped’ spectra, emitting abundant photons at the short and long wavelength extremes (e.g. Matthee et al., 2023; Labbe et al., 2023). This shape defies our current models, as in normal galaxies, an excess of long wavelength photons seen by JWST would usually be caused by the aforementioned process of cosmic dust absorbing the high energy UV and reemitting in the near-infrared. Follow-up has only deepened the mystery: spectroscopy with NIRSpec has uncovered broad spectral features that signal active black holes among some of these Little Red Dots (as well as some brown dwarf stars just pretending to be galaxies; Kocevski et al., 2023; Harikane et al., 2023; Greene et al., 2023; Matthee et al., 2023; Maiolino et al., 2023a; Furtak et al., 2023; Kokorev et al., 2024). These moderately massive black holes are new territory; pre-JWST we could only see supermassive black holes in the form of luminous quasars this early in the Universe. That’s not the last word, however: examination of Little Red Dots with MIRI reveals that many are lacking in even the longer-wavelength photons seen by MIRI, uncharacteristic of active black holes and more characteristic of old stars (Williams et al., 2023; Pérez-González et al., 2024). The nature of these previously unknown and surprisingly abundant galaxies − whether they represent the early population of galaxies with moderately massive black holes − has important implications for the origin and growth of black holes and their contribution to galaxy evolution.
The Origin and Influence of Black Holes
Actively accreting black holes have long been an integral part of our models of galaxy evolution, invoked to create the conditions that can influence the gaseous contents of galaxies and ultimately shut down star formation. JWST now stands poised to revolutionize our understanding: from revealing the host galaxies of the most luminous quasars in the early Universe, to spotting moderately massive black holes for the first time beyond our immediate neighborhood, to finding long “missing” black holes, hidden by cosmic dust.
Which came first: the chicken or the egg?
A foundational relation in astronomy is the Magorrian relation (Kormendy & Ho, 2013), which tightly links the mass of a galaxy’s black hole to its mass in stars. This relation is well established up to 11 billion years ago, implying that galaxies and their black holes evolve in lockstep and revealing a classic chicken and the egg dilemma: did galaxies or black holes form first?
While ground-based optical telescopes and HST have discovered about one thousand luminous quasars up to z ~ 7 − 8 (e.g. Mortlock et al., 2011; Bañados et al., 2018; Matsuoka et al., 2018, 2023), the brightness of the quasar precluded making measurements of their host galaxies, if they were detected at all. Discovery of black holes down to intermediate masses (such as the Little Red Dots in the previous section) and the robust detection of their host galaxies is only possible with JWST at high redshift.
Using NIRSpec, JWST quickly shattered our record of the earliest black hole known, discovering two within ~ 200−300 Myr after the Big Bang (e.g. Goulding et al., 2023; Maiolino et al., 2023b,a), and several more within the first billion years (e.g. Harikane et al., 2023; Larson et al., 2023). Such early, massive black holes challenge the idea that their “seeds” form from the collapse of the first massive stars (Fan et al., 2023). Do these surprisingly large black holes have correspondingly massive host galaxies? Careful subtraction of the quasar’s light in NIRCam imaging has revealed no, their host galaxies are in fact rather wimpy (Figure 5; Ding et al., 2023; Stone et al., 2023a,b). The emerging picture is that the lockstep evolution of galaxies and their black holes breaks down in the early Universe; in other words, black holes may grow first and rapidly, leaving galaxies to play catch-up.
The missing population of AGN hidden by cosmic dust
A key thorn in the side of efforts to link active black holes to evolution on the scale of entire galaxies has been our inability to catalog all black hole activity. Like young stars, black holes can exist in an extended cocoon of cosmic dust which absorbs the high energy photons escaping the accretion disk − superheated material falling onto the black hole − and re-emits them in the infrared. JWST MIRI’s spiritual predecessor, the Spitzer Space Telescope, was able to identify these hidden AGN if their infrared emission was very luminous, outshining the galaxy’s stars. JWST now has the sensitivity to take a much more complete census, identifying small excesses in infrared light with MIRI and specific spectral signatures with NIRSpec and MIRI from less luminous and very dusty active black holes (e.g. Alberts et al., 2020; Lyu et al., 2023) even at high redshift (e.g. Scholtz et al., 2023). And particularly, MIRI can take this census at z ~ 1 − 3, when both the efficiency of forming new stars and the rate of accretion of material onto black holes peaked. Early results show that MIRI increases the number of black holes identified in massive galaxies by more than 2x over X-ray surveys and can discover black holes in low-mass and high redshifts galaxies, newly accessible populations with JWST (Lyu et al., 2023). The hidden black holes can hide no longer, giving us a more complete view of the galaxy-black hole connection (Bonaventura et al., 2024).
The Emergence of Quenched Galaxies
The endgame for galaxies is the cessation of their star formation, a process called quenching. How and why galaxies quench (and stay quenched; Dome et al., 2024) has been a driving mystery in the field of galaxy evolution, with an array of proposed quenching mechanisms (Man & Belli, 2018). An amazing effort using ground- and space-based facilities has been expended to push back the frontier closer and closer to the emergence of the first quenched galaxies; however, the faint and red nature of these sources has limited these efforts to the most massive quiescent galaxies at z ≲ 4, ~ 1.5 billion years after the Big Bang.
The elusive first quiescent galaxies not only provide a boundary condition on the rapid assembly of early galaxy populations, they are not yet old enough that the signatures of their quenching mechanisms have been erased by aging and merger activity. Very quickly post-launch, JWST’s high sensitivity in the near-infrared uncovered remarkable evidence of rapid early growth, including some candidates for early, ultra-massive galaxies which, if confirmed by spectroscopy, would be difficult to explain with our current cosmological models (e.g. Labbe et al., 2023). Given that we know galaxies cannot grow indefinitely to arbitrary masses in the later Universe, this implied rapid, permanent quenching as early as z ~ 6.
Quenched galaxies are relatively rare and thus need sensitive imaging over large areas of the sky. Even given this, searches for quenched galaxies in our first, limited area deep surveys have found tantalizing evidence that galaxies no longer forming stars are surprisingly abundant in the first 2 billion years of the Universe (Carnall et al., 2023b; Alberts et al., 2023). Among these, NIRSpec spectroscopy of a quenched galaxy at z = 4.6582 finds that this galaxy likely formed all of its stars very rapidly, in just 200 million years, before shutting down, likely for good (Figure 6; Carnall et al., 2023a). And continuing in our theme, JWST is not just changing our view of massive galaxies, but opening up a whole new view of low-mass galaxies up to high redshifts. Combining NIRCam imaging with some of the deepest MIRI imaging so far observed (46 hours of exposure time in one filter!) has uncovered a surprise: not only are there quenched low-mass galaxies at z > 3 but the proximity of other galaxies may play an important role in their quenching (Sandles et al., 2023; Alberts et al., 2023). This tests a long-standing theory that low-mass galaxies will happily go on forming stars for longer than the age of the Universe without outside influence, unlike their massive counterparts. A gorgeous example of this is the discovery of a so-called Jekyll and Hyde pair − a dusty, vigorously star-forming galaxy with a massive quenched galaxy companion − that has several low-mass quenched galaxy companions (Figure 7; Alberts et al., 2023); together these galaxies make up the Cosmic Rose, just one of many examples of JWST images that fire the imagination and leave us in awe of our Universe.
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[1] “Redshift” is a non-linear scale that corresponds to the amount of time that has passed since a photon left its galaxy and arrived at Earth. It is often represented by the letter “z”.
[2] The CAnadian NIRISS Unbiased Cluster Survey (CANUCS).
[3] Near-Infrared Camera (NIRCam; Rieke et al., 2023), Near-Infrared Spectrograph (NIRSpec; Jakobsen et al., 2022), Near Infrared Imager and Slitless Spectrograph (Doyon et al., 2023, NIRISS).
[4] Mid-Infrared Instrument (MIRI; Wright et al., 2023).
[5] At base, the Hubble Sequence groups galaxies into spheroidal, disk-y or spiral, and irregular morphologies.
[6] Physics at High Angular resolution in Nearby GalaxieS (PHANGS; Lee et al., 2023).