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The era when the strong ultraviolet radiation from the first stars ionizes the surrounding hydrogen
atoms into protons and electrons is known as the epoch of reionization, which can be studied directly
using sensitive radio telescopes. These should determine when reionization occurred, and they would
inform the design of a proposed new telescope that would measure how the cavities of ionized hydrogen
created by the light from the first generations of stars, galaxies, and black holes expand into the
surrounding gas. In the long term, realization of the full potential of this approach would require in the
following decade a detailed mapping of the transition in the early universe from proto-galactic lumps of
gas and dark matter into the first objects, a goal of the proposed worldwide effort to construct the lowfrequency
Square Kilometer Array (SKA-Low) as discussed in the subsection “Radio-Millimeter-
Submillimeter” under “OIR and RMS on the Ground” in Chapter 3. Studies of the intergalactic medium,
which accounts for most of the baryons in the universe, at more recent times could be transformed by an
advanced UV-optical space telescope to succeed the Hubble Space Telescope (HST), equipped with a
high-resolution UV spectrograph.
Galaxies are composed not just of stars orbiting dense concentrations of dark matter. They also
contain gas and central, massive black holes. When the gas flows rapidly onto a central black hole, it
radiates powerfully and a quasar is formed. Meanwhile the black hole rapidly puts on weight. It is already
known from observations that these black holes can grow very soon after the galaxies form. However, the
manner in which this happens is still a mystery. These accreting black holes can be seen back to the
earliest times using the proposed space-based Wide Field Infrared Survey Telescope (WFIRST) and the
International X-ray Observatory (IXO), and the masses of the black holes can be measured using a
GSMT.
Simulations show that the first galaxies were likely relatively small and that the giant galaxies
observed today grew by successive mergers. Observations of mergers should be possible using JWST,
ALMA, WFIRST, and GSMT. As galaxies merge it is likely that their black holes merge as well. The
proposed Laser Interferometer Space Antenna (LISA) mission will search for the signatures of these
processes by scanning the skies for the bursts of gravitational waves produced during these early mergers
when the black holes are relatively small. (LISA will not be sensitive to the mergers of more massive
black holes.) An important part of the strategy is to search for associated flashes of electromagnetic
radiation that are expected as part of these events. The proposed Large Synoptic Survey Telescope
(LSST) will be ideally suited to this task and, working with a GSMT, should make it possible to pinpoint
and date the sites of black hole merger events.
In summary, this survey committee recommends improving understanding of the history of the
universe by observing how the first galaxies and black holes form and grow. To do so requires that
current capabilities be supplemented with the priority ground- and space-based activities identified in this
survey; see Box 7.1.
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