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Origins
Understanding the dramatic evolution of galaxies over cosmic time through observations is a key
part of the committee’s recommended science program. Following the growth of cosmic structure and
learning empirically how the dark and luminous matter are connected is a major science goal for GSMT,
which, with its superb spectroscopic reach, would be able to measure redshifts and thus infer distances all
the way from our local neighborhood to the epoch of reionization and monitor the build-up of mass and
the rise and fall of star formation at visual wavelengths. Meanwhile CCAT would provide the
submillimeter perspective on the history of star formation over cosmic time. (See Figure 7.2 for an
illustration of the complementarity.) The “fossil record” of how our Milky Way galaxy was assembled
can be traced by studying resolved stellar populations with LSST and JWST, and by using the adaptive
optics capability on GSMT. GSMT would also be able to perform exquisite spectroscopy of the most
ancient, nearby stars. In the next decade, large-scale numerical simulations of the formation and
evolution of galaxies should achieve the spatial resolution and physical realism necessary to interpret
these observations successfully and to tell the story of how our galaxy was born.
Our understanding of star formation under a wide variety of physical conditions will benefit from
extensive surveys of the giant molecular clouds within which stars form. ALMA will and CCAT would
be major tools for this exploration. Complementary studies of the young stars spawned in these molecular
regions will require infrared surveys with high angular resolution both in our galaxy and in the
neighboring galaxies the Magellanic Clouds, using JWST in space and GSMT equipped with adaptive
optics on the ground.
Since solar flares create many cosmic rays that can cause mutations of genetic material,
understanding these flares is important for understanding the chances of a planet being habitable. Flares
on the more numerous low mass, cool stars may preclude some forms of life on orbiting planets—already
known and to be discovered. Studying flares from the Sun using optical techniques with ATST and at
radio frequencies—by using the proposed mid-scale innovations program candidate Frequency Agile
Solar Radiotelescope (FASR)—as well as stellar flares in far-off planetary systems using the proposed
IXO, could advance our understanding of planetary habitability.
Understanding the Cosmic Order
The critical constituents of galaxies—dark matter, stars, gas, dust, and super-massive black
holes—are strongly coupled to one another. The program recommended here will allow major progress
in our understanding of this cosmic order. Large multi-object spectroscopic surveys with new
instruments would measure the stellar populations and the internal motions of thousands of distant
galaxies in a single observation. High-angular-resolution optical and near-infrared integral-field-unit
spectrographs on intermediate-class and large-aperture ground-based telescopes would trace in detail the
internal velocity fields of galaxies. Meanwhile, while JWST will provide observations on the assembly of
galaxies over cosmic time, IXO would obtain X-ray observations of the warm and hot gas in the dark
matter halos that surround galaxies.
High-mass stars embedded in dense gas within galaxies can be inventoried with CCAT and
studied in detail with ALMA. These stars are thought to be the main agents for injecting mass and energy
into the interstellar medium and for driving galactic outflows. They do this through powerful stellar winds
and supernova explosions, both of which are also responsible for accelerating cosmic rays and amplifying
magnetic fields. The proposed ACTA facility will advance understanding of the mechanisms involved.
The cycling of gas from galaxies to the surrounding intergalactic medium and back again could also be
studied with a GSMT telescope, using high-resolution optical spectra to study gas absorption lines
highlighted by background quasars along many sight-lines, but a future UV space mission will be needed
for a complete inventory. This program of observations will move the subject of galaxy evolution from
one largely dominated by surveys to one of integrated measurements of the buildup of dark matter, gas,
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