Science Nugget—Charting Cosmic Reionization with TMT
  Richard Ellis, Caltech

The reionization of the intergalactic medium about 13 billion years ago was a landmark event in cosmic history. It brought the so-called "Dark Ages" to a close and rendered space transparent to ultraviolet photons. Essentially, it marks the beginning of the Universe that appears familiar to us today.

When did this remarkable event happen? What role did the first star-forming galaxies play and how did those early systems develop to become the galaxies we see at later times? These questions represent the final frontier in understanding the origin of stellar systems where current facilities are finding it very hard to make further progress.

TMT is just one of several future astronomy facilities that promise to enable great discoveries at early cosmic times. The successor to Hubble, the James Webb Space Telescope (JWST), is designed to image to unprecedented flux limits, and a variety of radio facilities will chart the spatial distribution of neutral hydrogen using the redshifted 21-centimeter line. How will TMT interface with such facilities and what will be its unique role?

Some mature galaxies are now being found at redshift 6, and polarization signals from the microwave background suggest scattering by free electrons residing in structures at redshifts between 10 and 20; accordingly, the intervening period is the important region to explore.

Theory suggests early sources of ionizing photons will carve out "bubbles" of ionized hydrogen (Figure 1). As these bubbles grow and overlap, reionization is completed. However, even the source of the ionizing photons is in dispute. In addition to young star-forming sources, contributions to the ionizing radiation may come from material spiraling into black holes or even from decaying subatomic particles.

TMT will search for observational signatures associated with young stars, including ultraviolet continuum radiation and Lyman-alpha emission from nearby gas clouds. For the earliest metal-free systems, spectacularly massive and short-lived stars with diagnostic Helium emission lines may also be present. Near-infrared spectroscopy and associated imaging are thus the tools needed to make progress. The signals may be diluted by dust extinction and scattering by neutral gas, so accurate predictions are hard to make.

As with all exploration, the most sublime excitement lies in not knowing what we might find! Reionization could happen gradually through the collective output of myriads of slowly-growing small systems shining sporadically over several hundred million years. Alternatively, it could be a spectacular event, narrowly focused in time.

And how large, physically, will these early sources be? Flexibility in instrumental parameters and effective use of adaptive optics over moderate fields of view will be essential to make progress in all potential situations.

Over the next decade or so, radio surveys promise to map the topology of the intergalactic medium. However, they will not locate any of the sources of reionizing photons. JWST will conduct wide field surveys to locate the most luminous star-forming sources, but its spectroscopic capabilities are limited. TMT’s forté and complementarity lies in its unique combination of exquisite angular resolution and sensitivity, for both imaging and spectroscopy in fields known beforehand to be of interest.

The TMT project has conducted conceptual design studies of two closely related infrared spectrographs relevant for studying cosmic reionization: IRIS, a single integral field unit (IFU) behind the facility adaptive optics unit capable of working at the diffraction limit; and IRMOS, a multiplexed version that can survey a 2-5 arcminute field with intermediate correction by adaptive optics.

Figure 2 illustrates how a set of deployable IFUs can be arranged in various ways to survey for redshifted emission within a bubble associated with a luminous ultraviolet source—perhaps one located with JWST—or in magnified field of a strong gravitational lens such as a foreground cluster of galaxies. By tracing the extent and strength of Lyman-alpha emission in bubbles at various redshifts, the fraction of escaping photons can be witnessed and modeled in detail. For sufficiently luminous sources, the emission line strengths of hydrogen and helium can be used to gauge the nature of the ionizing radiation and search for possible signatures of metal-free stars formed at the cosmic dawn.

Figure 1:
Cosmic reionization viewed at a redshift of 9.3. The box shows the distribution of neutral (maroon) and ionized (light blue) hydrogen around the most energetic star-forming sources (yellow dots). To understand the reionization process, we must connect the sources of ionizing photons to the growth rate of such bubbles. In conjunction with radio surveys, TMT’s infrared spectrograph IRMOS will offer unique capabilities for charting the distribution of star-forming sources across such bubbles. (Credit: Nick Gnedin/Fermilab)

Figure 2:
Two deployment modes for a multiplexed set of integral field units as conceived in the TMT instrument IRMOS. A region is chosen centered on the distribution of luminous ultraviolet continuum sources detected by JWST. The instrument can undertake detailed spectroscopic measurements in a distributed (targeted) mode or probe for faint Lyman alpha emission in smaller fields in a "blind" contiguous mapping mode. (Credit: Anna Moore/Caltech)

Animation:
The first stars in the Universe emitted not only visible light, but also energetic ultraviolet light and X-rays, which are able to ionize a hydrogen atom. These first stars, grouped in proto-galaxies, created small transparent HII regions around them. The HII regions increased in size until the neighboring regions merged together, clearing up the "fog" of neutral hydrogen, and making the Universe transparent to star light, as it is today.

Astronomers refer to this as "cosmic reionization," because the hydrogen gas in the Universe became ionized again, as it was in the very first stages of cosmic history.

This animation [25 MB mpg] shows a simulation of cosmological reionization between a redshift of 20 and 5. Initially, the computational cube is filled with neutral hydrogen gas (arbitrarily shown as the color maroon). The ionized gas glows blue, becoming totally transparent if it becomes very highly ionized. Yellow dots are star-forming galaxies.

As time goes on, the first galaxies ionize and destroy the neutral hydrogen around them, creating holes in the cube. Later, these holes merge and the Universe becomes ionized (transparent) again.

Credit: Nick Gnedin (Fermilab)