Research

These facilities are used to map the both the continuum and spectral line emission from the gas and dust in these galaxies. Two areas where we have much activity is the study of the densest and coldest component of the ISM, molecular clouds, and the most rarefied and hottest component, the hot, ionized gas.

The molecular phase of the ISM is important as it is most directly connected to star formation in both the Milky Way and other galaxies. The UMass group is endeavoring to better understand the physical and dynamical properties of the molecular gas and put this phase in context wth the global cycling of gas in galaxies - which is essential to understand galactic evolution. It is well known that stars are formed in molecular cloud cores, but we now seek a better understanding of how this occurs. Observational programs to study the gravitational infall signature and outflow signature associated with the birth of stars are in progress. There is also an incredible chemical complexity and diversity in molecular clouds, and today we also seek to explain the nature of this chemistry and understand its implications. For decades we have made use of the Five College Radio Astronomy (FCRAO) 14-m telescope located on the Quabbin Reservoir in central Massachusetts for much of our observational studies, however soon the Large Millimeter Telescope (LMT) will be available and will provide greatly improved resolution and sensitivity for these studies.

High-energy activities in the ISM are manifested through the creation of a pervasive hot gas component. The presence of this rarefied interstellar component can profoundly affect the geometry and dynamics of the cooler phases of the ISM, the propagation of cosmic rays and UV/soft X-ray photons, the strength and topology of the magnetic field, the galactic disk-halo interaction, the distribution of metal abundances, and so forth. However, there is little agreement yet on such basic issues as how much hot gas there is, which thermal state the gas is typically in, how the gas is distributed relative to cooler phases, what fraction of supernova mechanical energy is deposited into the hot gas, and where the energy is transferred or dissipated. To address these issues, UMass astronomers have a multiwavelength programs in place to characterize the global hot gas component. We are using our Galactic center region as a unique laboratory to study detailed physical processes involved in heating, mass loading, and outflows of hot gas. We have obtained data or observing time from HST, FUSE, Chandra, and XMM-Newton to map hot gas and to study its interplay with other components in nearby galaxies. We will further examine the relationship of the global hot gas properties to the star formation rate, morphological type, and clustering environment of galaxies.

Mapping, for which the LMT will be the premier instrument in the world, is key to distinguishing among the chemical and physical processes that take place in the cometary environment, such as direct sublimation from the icy nucleus, chemical processing in the coma, and photodissociation of larger molecules. The first detection of new cometary molecules has also been accomplished by our group.

Cometary observations have shown that there is a striking similarity between the composition of comets and the composition of the icy mantles observed on interstellar grains. This has led to an appreciation that the physical processes at work in the interstellar medium are similar to those that were operating during the formative states of the solar system. Some investigators even speculate that this similarity arises because interstellar grain mantles survived accretion into the solar system and were incorporated into the comets. In either case, study of the chemical and physical processes at work in interstellar clouds will lead to an improved understanding of the origins of the molecules observed in comets and the physical conditions under which they were formed.

Stars are formed deep within giant molecular clouds in the galaxy, shrouding star formation in a fundamental yet unsolved mystery. It is a process that spans magnitudes in scale and is strongly coupled to the cloud's dynamics. The cloud is influenced by gravity, the interstellar magnetic field, supersonic turbulence, and mechanical and radiative feedback from the newborn stars themselves. The primary challenge to both theorists and observers is to determine the role each plays in the star formation process as these relate to the fraction of a cloud's mass converted into stars, the formation of massive stars and young stellar clusters, the distribution of angular momentum, and the quenching of star formation.

With its sensitivity to point sources and low surface brightness emission coupled with its imaging array instruments in the 1-3mm bands, the LMT can make significant contributions to this effort by measuring both the large scale low-density envelopes of giant molecular clouds and the high density cores from which stars and clusters condense.

Magneto-Turbulence in Molecular Clouds

Turbulent gas flows and the magnetic properties involved are key to regulating star formation and configuring the mass distribution of cores within them. By studying the molecular line emission of giant molecular clouds, measurements made by the LMT can assess the conditions in which the turbulent energy spectrum departs from the norm, which may signal zones of energy dissipation or injection, and may also help in determining the role of the magnetic fields.

Properties of Protostellar and Protocluster Cores

The protostellar and protocluster cores that emerge within the cloud are the precise sites of star formation. These cores strongly radiate in the 1mm band from cold dust within them. Imaging the thermal emission from dust grains over the extent of a molecular cloud using the LMT's millimeter-wavelength cameras provides a direct census of active or potential sites of star formation. The emission can be used to derive radial profiles of density for individual cores that can be compared to theoretical predictions and compile the core mass distribution function. Insight to the star formation process is further revealed by observations that probe the chemistry and kinematics of dense gas, as these trace the initial conditions prior to protostellar collapse.

Protostellar Disks

The gravitational collapse of dense rotating cores within a molecular cloud results in the creation of a central protostar surrounded by a flattened spinning disk of gaseous material. In this accretion disk, mass is transported inward toward the star and angular momentum is transported outward. Eventually, around the time newly formed planets inhibit further growth of the star, the disk moves into a phase known as a debris disk, where it resembles something not so different from our own asteroid belt, with lots of dust and planetismals.

The accretion phase for low mass protostars that will become sun-like stars is intriguing, as it is always accompanied by the simultaneous presence of a high velocity ejection of material into bipolar jets that emerge perpendicular to the plane of the disk. Although we know that accretion disks and jets of expelled material are always seen together, exactly how this pairing happens is a mystery. It is possible to solve this mystery by further exploring both the intersection of the disk and the star, where the jets are probably formed, and the innermost part of the disk, which spinning rapidly and has a magnetic field.

Many observational facilities are being used for these studies, including both space-based telescopes (Hubble, Spitzer, Chandra, XMM-Newton, GALEX, and the upcoming Herschel Space Telescope) and ground-based telescopes (optical telescopes, VLA, and the upcoming LMT).

The LMT will provide unprecedented sensitivity and speed to map the cold, dense gas and dust components of nearby galaxies. These new data will provide leverage for investigating the relationship between the newly formed stars and the gas from which those stars were born based on galaxy morphology, luminosity, and environment. The low density regions on the outskirts of galaxies provide an excellent testing ground for the thresholds of star formation and for investigating the nature (primordial or produced by gas ejection) of the gas envelopes surrounding galaxies.

Current projects using the Hubble, Chandra, and Spitzer Telescopes are investigating the resolved stellar populations and the interfaces between photo-ionized and shock-ionized regions in nearby galaxies, the output from energetic processes, the distribution and content of dust, and the extinction this dust produces in galaxies.

Galactic Structure and Dynamics

Theoretical study of galactic structure at UMass ranges from analytic perturbation theory to kinetic theory of N-body simulations to statistical inference from theoretical models and observational data. For example, new adiabatic theory and the "dispersion relation" approach can be applied to a variety of secular evolution problems in barred galaxy evolution, tidal excitation, Magellanic cloud heating and Milky Way kinematics. All of this work is numerically intensive. The perturbation theory, although formally analytic, demands numerical solutions.

To meet this need, three "suites" of parallel numerical codes for production work have been developed by Martin Weinberg:

1. a perturbation theory package called Orbit;
2. an N-body code using the expansion approach which is well suited to studying slow evolution over very long time scales; and
3. a Bayesian statistical package called the "Bayesian Inference Engine" (BIE).

This latter project grew out of the need to exploit 2MASS and other catalog data to constrain Milky Way and Local Group theory, but it is a stand-alone general Bayesian platform.

The Origin and Evolution of Galaxies and Large Scale Structures

The past decade has witnessed spectacular progress in our empirical exploration of the Universe. WMAP observations and the study of distant supernovae have ushered in a new era of precision cosmology, including discoveries into the geometry of the universe, the kinematics of the Hubble expansion, and the cosmic mass-energy content. However, structure formation continues to be elusive due to inability of measuring key properties of galaxies without large systematic errors and inaccuracies in predicting details about star formation. Cosmology surveys have pushed the most powerful ground- and space-based facilities to their limits to partially reveal the evolution of some types of galaxies; however, a number of fundamental limitations are becoming increasingly clear. due to biases in optical and near-IR based galaxy selection methods and the inability to measure results in spectroscopic redshift for galaxies at redshift z > 6.5. Consequently, a different, complementary approach is needed to obtain the complete picture of cosmic star formation history and galaxy evolution.

Galaxy Clusters

Clusters of galaxies, having nearly reached dynamical equilibrium, offer an impressive laboratory to test models of large scale structure formation and the dependence on environment of galaxy formation and evolution. Historically, mm-wavelength observations of clusters have focused on the redshift-independent brightness of the Sunyaev-Zel'dovich Effect (SZE), but with its high resolution and exquisite surface brightness sensitivity, the LMT offers a fundamentally new observational window into the study of galaxy clusters, groups, and other mass-biased environments. LMT users will map the distribution of the intracluster medium (ICM) with 6-10 times higher angular resolution than previous studies. This will in turn enable us to probe the formation process of clusters. Using the Redshift Search Receiver, LMT users will study starburst galaxies in order to better understand the rates of star formation within galaxy clusters.

The LMT also offers an opportunity to study cooling flows in clusters. It has long been understood that the high density and short cooling time in cluster centers should lead to a cooling flow, unless the cooling flow is shut off by an additional source of energy. LMT users will be able to investigate the nature of cooling flows and possible re-heating mechanisms through detailed mapping of the gas and dust distribution in nearby cooling clusters.

Dark Matter and the Structure of Galaxies

According to current theory of structure formation, the matter content of the universe is dominated by cold dark matter (CDM). Because of gravitational instability, perturbations in the CDM density distribution grow with time and form quasi-static clumps called dark matter halos. Luminous objects, such as galaxies and galaxy clusters, are assumed to form in the gravitational potential wells of CDM halos. Thus, a first step in understanding galaxy distribution in the universe is to understand how CDM halos are distributed in space and how galaxies interact with them.

The properties of the dark halo population can be studied in great detail through numerical simulaions and analytical modeling. One method of exploring CDM halo reaction with galaxies is based the conditional luminosity function model, which links galaxies and dark matter halos by matching the number density and clustering properties of galaxies with those of dark matter halos in the current CDM model. Another method uses galaxy systems identified from large redshift surveys of galaxies.