INTERSTELLAR MEDIUM
The Department of Astronomy has an active program in observational and
theoretical cosmology to study the fundamental nature of the Universe
on the largest scales, the beginning and ultimate fate of the
Universe, the origin of structures such as galaxies and clusters of
galaxies, and the determination of what composes the major
constituents of the Universe. Specific topics of investigation
include quasar absorption line clouds, the growth of large-scale
structure, the nature of star-forming high-redshift galaxies,
microwave background observations, and galaxy clusters.
Distant galaxies hold the keys to many fundamental questions about the
history, fate, age, and composition of the Universe. They are our
fundamental unit of measure in tracing mass and light, since they
contain virtually all of the Universe's luminous matter. By using
the light travel time from those objects to peer back to the early
Universe, we can probe the conditions that led to the formation of
the galaxies and stars with which we are familiar today. The last
five years have witnessed a revolution in research on faint,
cosmologically distant galaxies, including the identification of a
large population of actively star-forming galaxies at redshifts of 3
or 4 and even greater. They are arguably the precursors of normal
galaxies today. UMass researchers in this area make use of the
Hubble Space Telescope and the 10-m Keck telescopes in Hawaii to
study these distant galaxies in detail.
It is generally accepted that most of the mass in the Universe is in the
form of dark matter that interacts with ordinary baryonic matter only
gravitationally. However, observations are almost always made on the
baryonic part of the Universe, such as galaxies, galaxy clusters and
quasars. We use computer simulations to bring these theories of the
underlying nature of the Universe closer to the observations. The
galaxies and clusters grew by gravitational instability from
small-amplitude density fluctuations in the early universe.
Gravitational clustering of the dark matter drives the evolution of
the mass distribution on large scales. Unlike previous simulations
that only modeled the dark matter, hydrodynamic simulations, which
incorporate the non-gravitational physics that influences the
evolution of the cosmic baryon distribution, dramatically enhance the
explanatory and predictive power of cosmological theories. We use
such simulations to model the cosmic history of the baryon component
of the universe, from the earliest times to the present day, with
particular attention to the distributions of galaxies and
intergalactic gas.
Simulations of high-redshift structure, supported by quasar absorption data,
indicate that most of the high-redshift baryons reside in the diffuse
medium that produces the Lyman-alpha forest, with smaller fractions
in galaxies and in hot gas surrounding galaxies. Continuation of high
resolution simulations to the present epoch will predict the division
of baryons in the local universe among stars, galactic gas, shock
heated intracluster gas, and the diffuse intergalactic medium. The
simulations aid the physical interpretation of emission and
absorption measurements at a variety of wavelengths and will allow
direct testing of cosmological theories.
Other UMass researchers are studying the primary and secondary anisotropies
in the cosmic microwave background. These measurements are made both
from the ground using the Caltech Submilimeter Observatory on Mauna
Kea and from balloon- borne telescopes launched from the US and
Antarctica. Precise measurements of the angular power spectrum of
the primary CMB anisotropies will discriminate between competing
cosmological models and, if the inflationary scenario is correct,
will accurately determine many of the physical parameters of the
universe. Observations of the Sunyaev-Zel'dovich (S-Z) effect in the
CMB spectrum provide a means to measure the Hubble constant out to
large redshifts.
According to recent simulations of the formation of structure in the universe,
much of the baryonic matter may be in a hot phase of the
intergalactic medium. This hot medium may contribute significantly to
the X-ray background in the 0.5-1 keV range. The characterization of
the contribution can be a key diagnosis of the thermal and chemical
condition of the medium. One approach being employed at UMass is to
observe X-ray shadows of extragalactic X-ray-absorbing clouds
against the cosmic X-ray background. X-ray observations, together
with follow-up optical photometry and spectroscopic measurements of
associated galaxies, provide a new and powerful way to explore galaxy
clusters and their environs. UMass astronomers are conducting a
systematic search for large-scale X-ray-emitting structures to
investigate imprints of initial density perturbations and to
constrain the mass composition of the Universe.
When completed, the Large Millimeter Telescope will play an important role
in observational cosmology. The LMT will have the sensitivity to
detect both the dust continuum and molecular spectral line emission
from protogalaxies almost at the edge of the observable universe,
directly probing the epoch of galaxy formation. The LMT will also be
the world's leading facility for studying the S-Z effect in distant
galaxy clusters and the primary anisotropies on small angular scales.
