The Big Bang was the defining moment in the history of the Universe. The hot, ionized matter in the Universe recombined to form neutral hydrogen and helium only a few 100,000 yr after this event. However, the intergalactic medium that we see today is highly ionized, and remains that way back to high redshifts. This means that the intergalactic medium was reionized at some time early in the history of the Universe, by a process that we are yet to identify. Observations of distant quasars reveal that the epoch of reionisation ended near z ~ 6. The detection of polarization in the cosmic microwave background suggests that reionisation began at a very high redshift of z ~ 15, although the uncertainty is large.

Most of the stars in the Universe formed over the redshift interval from z ~ 3 to z ~ 1. Despite significant uncertainties at redshifts higher than this, all of the available information suggests that conventionl star formation, which formed the bulk of the stellar mass in the Universe, could not on its own have powered the reionisation of the inter-galactic medium (IGM).

Similarly, the first truly dark Gunn-Peterson troughs have been detected in z ~ 6 quasars identified in the Sloan Digital Sky Survey. However, early indications from the quasar luminosity function are that insufficient numbers of quasars exist at higher redshifts to provide the necessary ionizing photon flux. Surveys for high-redshift quasars confirm the suspected turnover in the quasar luminosity function and the rapid decline in source density between z ~ 6 and z ~ 8.

GMTIFS will advance studies of cosmic reionisation in two fundamental ways:

• It will enable studies that identify the first-light objects at high redshifts, and
• It will enable studies of the structure of the IGM during the epoch of reionisation.

First-light objects

Reionisation is the signature of a major energetic event in cosmic history. The source of the ionizing phtons is uncertain, but the leading candidate is the unique first-generation of zero-metallicity Population III stars. Numerical simulations suggest that the first exceedingly low- or zero-metallicity stars to form were super-massive (M > 100 MSun). These short-lived massive single stars quickly destroyed themselves via stellar winds and supernovae, and in so doing imparted a significant amount of energy to the IGM.

This first generation of massive stars is probably too faint for detection by the next generation of Extremely Large Telescopes. However, their death throes, as hypernovae or gamma-ray bursts, are likely to have already been detected in transient surveys as sources just beyond the reach of current instrumentation capabilities for rapid follow-up.

The formation of the first condensed objects, be they galaxies or massive black holes, is likely to be accompanied by a second generation of more conventional star formation. These young galaxies may quickly be enshrouded in dust of their own making, but some of them will likely have clear escape paths for ultraviolet photons. The observed-frame far-red and near-infrared are the appropriate windows for observations of these young galaxies. Lyα emission is the strongest and most direct spectral signature, but Lyα may escape in only a small fraction of z > 5 objects. He II λ1640 emission may be a better tracer of primordial populations. Strong He II λ1640 emission is an indicator of high-mass Population III stars. The Lyman continuum break provides a more universal, but difficult to detect, signature of very high redshifts.

Numerous z > 6 galaxies have been detected in the Hubble Ultra-Deep Field with magnitudes of z' ~ 25-27. The GMTIFS IFS will permit spectroscopy of z > 6 objects in practical exposure times. The exceedingly compact nature of these sources means that there are significant sensitivity gains to be realized through the use of adaptive-optics-corrected spectroscopy.

The synergy between the GMT and JWST provides a potent set of tools for studies of the early Universe and first-light objects. JWST, with its unmatched sensitivity to broad-band light, will excel at identifying z > 6 galaxies from their broad-band colors. Early confirmation spectroscopy will be routine with the wide field of view of the JWST-NIRSpec MOS survey spectrograph, but the low spectral resolution of NIRSpec will provide only limited velocity resolution. GMTIFS will be the perfect platform to undertake follow-up spectroscopy with which to determine the internal dynamics and physical extent of this embryonic phase of galaxy evolution.

Figure 1 - Spatially-resolved Lyα emission-line properties for a simulated GMTIFS observation of UDFy-38135539 (Lehnert et al. 2010, Nature, 467, 940) with an emission-line flux of 6.1x10-18 erg cm-2 s-1 and a total integration time of 14 hr. Shown are the emission-line central intensity (10-15 erg cm-2 s-1 Å-1 spaxel-1; top-left), central velocity (km s-1; top-right), velocity dispersion (km s-1; bottom-left), and continuum intensity density (10-15 erg cm-2 s-1 Å; bottom-right).

Structure of the Inter-Galactic Medium

We have very little knowledge of the source populations in the redshift interval from z ~ 6 to z ~ 20 that provide the hard ultraviolet photons required to reionize the IGM, nor do we know much about the strucure of the resulting IGM at these times. What is needed is an unbiased, yet direct, probe of the period of reionisation on which is printed the signature of reionisation mechanics. Historically, quasars have provided such beacons in the distant Universe. However, quasars are poor tracers of the reionisation epoch because they have intrinsically complex spectral shapes, they strongly influence the properties of their local environment, and they are exceedingly rare at the early times that we wish to probe.

Gamma-ray bursts (GRBs) provide an alternative probe of the IGM at high redshifts. GRBs are some of the most distant objects detected in the Universe. When identified early enough, they can be extremely bright. So while an understanding of the physical nature of GRB events remains elusive, they can play a key role in determining the structure of the IGM during the reionisation of the Universe.

These high-redshift GRBs will be used to perform detailed analysis of the shape of the damping wing of Lyα absorption across the epoch of reionisation. Such measurements are possible only because of the otherwise unstructured power-law spectral shape of GRBs. This provides a unique probe of the structure of the IGM as a function of redshift along with access to IGM enrichment physics via classical studies of rest-frame ultraviolet metal-absorption-line systems (e.g., C IV λ1550 and Mg II λ2798). The local environment within the host galaxy will also be probed by such measurements, a proposition not possible in studies of emission-line galaxies at similar redshifts.

GMACS on GMT will perform a full tomographic reconstruction of the IGM by measuring the Lyman-α forest towards z = 3-5 Lyman Break galaxies, but these probes cannot access the epoch of reionisation. The GMTIFS IFS will probe to higher redshifts, but a full tomographic reconstruction will not be practical due to the low density of sight lines provided by GRBs. With sufficient spectral resolution observations of sufficiently bright GRBs, the structural state of the early Universe will be revealed with clarity.

The need for a rapid-response capability for target-of-opportunity observations is acutely apparent in the era of JWST because current indications are that this space-based facility will be difficult, if not impossible, to re-task for quick response. This restricts the role of JWST for such events to that of the important late-time high-sensitivity observations of the transient host environment once it has been unambiguously identified in prompt-response studies using GMTIFS.