Observing Galaxy Assembly with the James Webb Space Telescope

Rogier A. Windhorst, Rogier.Windhorst@asu.edu

The advantages of Webb

Three factors will make Webb exquisitely suitable for detecting the faintest and furthest galaxies, and for charting the history of their assembly: its large aperture compared with Hubble, its superb infrared (IR) detectors, and the dark IR sky background at its operating location, the second Lagrange point, L2.

The history of galaxy assembly began when the first stars were born—at First Light, about 200 million years after the Big Bang. At that time, the universe was ~20× smaller than it is today. Starlight that originated then, and is observed now, has been shifted in wavelength towards the IR portion of the spectrum, where Webb’s performance is optimized. An additional advantage of the IR is its ability to penetrate dust, which can obscure the processes of galaxy assembly at shorter wavelengths. Webb is designed to study the epoch of First Light and the entire subsequent process of galaxy assembly.

The process of galaxy assembly

Galaxies are the basic building blocks of the universe, where the supply of atoms and molecules in dark-matter (DM) halos cycles continuously through the birth and death of stars. Analytical theories and numerical models have suggested—and Hubble observations have shown—that galaxies build up over cosmic time in a hierarchical manner. The first star clusters and dwarf galaxies, born from the essentially unpolluted hydrogen and helium left over from the Big Bang, formed around the earliest, most massive, so-called “Population III” stars.  Their first associations were low-mass, star-forming objects, which grew into larger structures—galaxies—by hierarchical clustering and merging under self-gravity of the DM halos.

In massive DM halos, the process of assembling galaxies accelerated, with more frequent mergers of progressively larger galaxies, and gas converted into stars at a faster rate. This acceleration resulted in bulge-dominated galaxies forming most of their stars early on, with a peak at redshifts z ≃ 3, when the universe was only ~2 billion years old. The result was the early-type or elliptical galaxies that we see in the local universe today. In these galaxies, fewer stars formed recently, as GALEX and Hubble’s Wide Field Camera 3 (WFC3) have shown in great detail (e.g., Rutkowski et al. 2012).

Lower-mass halo environments resulted in less violent galaxy assembly, with lower-mass galaxies forming with smaller bulges and more dramatic spiral-disk structures. This process, fed by a steady inflow of gas and the mergers of minor galaxies, peaked at redshifts z ≃ 1–2, when the universe was 2–3× smaller than today and only 3–6 billion years old. After this peak of the cosmic star-formation rate, galaxies evolved more passively, slowly running out of the gas needed to make significant generations of new stars. The outcome is today’s Hubble sequence of elliptical and spiral galaxies, which has been in place for roughly the last half of the Hubble time. In earlier times, the process of galaxy assembly was more chaotic, and the deep-field images (Figs. 1–2 [Readers: To appreciate the full detail of the images, please magnify both figures on your screen.]) simply do not show the familiar galaxy structures that we observe nearby.












Systematic factors affecting galaxy assembly

Three systematic factors in star formation provide context for the fundamentally new contributions that Webb will make in the field of galaxy formation. Star formation is rather inefficient, with most starbursts using up only a small fraction of the available gas. Drawn out over cosmic time, the process of galaxy assembly is therefore vulnerable to factors that can deplete the gas supply available for star formation. Webb’s unique observing powers—dust penetration, high spatial resolution, and exquisite photometric sensitivity—will enable us to study the roles and interplay of these factors.

1. Gas clearing by supermassive black holes. In a bulge-dominated galaxy, a supermassive black hole (SMBH), sinking to the galaxy’s gravitational center, consumes a small but consistent ~0.2% fraction of the baryonic mass (Ferrarese & Ford 2005; Haring & Rix 2004). The massive BHs left over from the initial Population III stars, which accreted gas and merged hierarchically over cosmic time, likely seeded SMBHs, and the exchange of gravitational radiation facilitated final-stage BH mergers. SMBHs probably grew in lockstep with the process of galaxy assembly itself, at least in the more massive, bulge-dominated early-type galaxies.

SMBHs are important for galaxy formation, because they have bad eating habits. They eat irregularly and gobble up steady flows of gas from their surrounding accretion disks. But they can also suddenly swallow huge chunks of food, including stars, and possibly even entire star clusters. When they eat, the inner accretion disk—which spins rapidly, generating strong magnetic fields—heats up to the point that a relativistic jet of particles forms, and a large amount of gas flows out from the region just outside the Schwarzschild radius. If the viewing angle is right, this extremely bright active nucleus is visible as a quasar or a Seyfert galaxy. Its associated outflow can deposit large amounts of mechanical energy into the intergalactic medium (IGM). For the more massive, bulge-dominated galaxies, this feedback mechanism can shut down the consumption of in-falling gas for a long time, significantly moderating the process of galaxy assembly.

Astronomers will use Webb’s unique spatial resolution and sensitivity to investigate SMBHs and correlate their growth with other characteristics of galaxy formation.

2. Dust generation and gas clearing by supernovae and stellar winds. Stars themselves provide feedback mechanisms in galaxy formation. The combined supernovae output from massive stars in a star-forming galaxy will deposit significant gas, mechanical energy, and dust particles into the IGM. The steadier mass loss from more numerous, lower-mass supergiants in the last stages of their lives also pollutes the IGM with gas and dust. The combination of these processes can be so intense that most of the subsequent starbursts and feeding of SMBHs occurs in regions shrouded by dust. Indeed, another bad eating habit of SMBHs is that they gobble their prodigious meals under a tablecloth of dust, making the process hard to observe with facilities other than Webb.

3. Starvation by cosmic expansion. Galaxies have—or will soon—run out of gas to form significant numbers of new stars. During the last half of the Hubble time, the more massive galaxies have already begun to run out, which has stabilized the Hubble sequence, and has caused elliptical galaxies to be almost, but not completely, “red and dead.” Eventually, dark energy (DE), which dominates the expansion of the universe, may permanently drive all galaxies away from one another. In the last 4.5 billion years, the expansion rate has accelerated, and it may eventually stop all galaxy assembly, leaving each galaxy to consume its remaining, surrounding gas—and its smaller neighbors—while it still can.

The process of galaxy assembly is thus complex and often rather messy. Dust hides it, and its nature has changed significantly over the last 13.5 billion years. Progress awaits Webb, which will probe more deeply to the dust than Hubble can, and will offer higher spatial resolution and sensitivity than Spitzer has.