Local universe

Studies of the nearby universe encompass a region of approximately 1 billion light years in radius, over which the effects of cosmic evolution are small.  Within this volume galaxies and associated objects are essentially frozen in their present day configurations.

As this is the part of the universe that can be observed in most detail, most of our knowledge about the structures and internal evolutionary processes of galaxies and systems of galaxies comes from studies within the local universe. In addition near field cosmology focuses on studies of the current states of galaxies to ascertain how they formed and evolved.

Any successful model of galaxies must not only reproduce youthful galaxies seen at high redshifts, but also the present day products of this evolution.  The basic approach in this area involves combining multi-wavelength data with theoretical models to define key physical processes that control the nature of galaxies.

Run by Matthew Bershady, Jay Gallagher, Sebastian Heinz, Snezana Stanimirovic, and Eric Wilcots.

Star Forming Galaxies

The conversion of gas into stars is a fundamental characteristic of galaxies. Building an understanding of star formation processes involves several astrophysical areas. Radio measurements supply information on star formation reservoirs in the form of atomic and molecular gas, respectively revealed by 21-cm and cm-to-submillimeter radio observations. To this end Wisconsin astronomers make use of a range of radio facilities from single-dish telescopes to interferometers, including the Very Large Array and in the near future, the Atacama Large Millimeter array (ALMA).

On the other side of the equation several of our projects are exploring the nature of galactic stellar populations.  These assessments include fundamental measurements of stellar masses through the analysis of disk structures to model fitting of spectral energy distributions to determine the mixes of stellar ages.  This work is largely based on UV-optical-near infrared photometric and spectroscopic data taken with a variety of ground-based telescopes, but especially WIYN and SALT, as well as with other unique facilities, such as the Hubble Space Telescope.

The question of star formation feedback and regulation also are of particular interest at UW-Madison. Observations show that once galaxies settle down from an initial phase of exuberant star formation, most systems maintain roughly constant stellar production rates until they become dead red stellar fossils.  This behavior implies that processes exist which stabilize star formation rates. For example, the energy from supernova blasts may dynamically heat the ISM, thereby reducing star formation over the longer term, while also locally stimulating star formation around a young stellar system.  Research on this topic by UW-Madison astronomers includes measurements of the physical state and kinematics of stars in gas. Investigations of starburst galaxies, cases where galaxies have star formation rates that are too high to sustain over cosmic time scales, are especially useful in revealing how feedback works, as well as offering insights into other processes, such as the generation of galactic winds.

Run by Jay Gallagher.

AGN Feedback

Black holes have a significant impact on their environment. Over the past 10 years, a consesus has emerged that galaxy clusters are heated by the action of relativistic jets, blasting into the intergalactic medium from the supermassive black hole of the central massive elliptical galaxy of the cluster. This heating might be enough to counter-balance the radiative losses the cluster gas is experiencing constantly, as it emits X-rays that can be observed from space based X-ray telescopes.

How this heating occurs and whether the heat can, in fact, be distributed to just the right locations in the cluster is still an issue of contentious debate. We are running state-of-the-art hydro simulations of galaxy clusters under the influence of energy injection from a central jet. We use fully cosmologically evolved clusters and inject powerful, collimated jets into them from the cluster center. A movie of this process can be accessed from Fig. 2. The key difference to earlier studies is that we include, for the first time, the dynamic state of the cluster, rather than assuming that it is a spherically symmetric, hydro-static atmosphere. This has important consequences for the dynamical evolution of radio sources (jets and the large scale bubbles of hot gas they blow into the intergalactic medium).

One of the important early results is that internal cluster dynamics, excited both by the action of the jets themselves and simply due to residual motions from previous dynamical encounters, as well as turbulence induced by cluster substructure (e.g., moving galaxies) can redistribute the gas in the cluster center in such a way that it erases any channels that were previously carved by the jet. This means that subsequent powerful jet outbursts can efficiently couple with the dense gas of the inner cluster, making it possible for the jet to heat the gas that needs it most (the coldest gas near the center of the cluster).

Run by Sebastian Heinz.

Galaxy Kinematics

Galaxy kinematics – the measurement of the motions of luminous gas and stars — provides fundamental tools for answering basic questions about when and how disk galaxies formed. For example, these motions allow us to infer the distribution of dynamical mass within galaxies, and hence the shape of their gravitational potentials. Kinematics also enable us to probe the dynamical state of the luminous baryons trapped within theses potentials by providing measures of angular momentum and dynamical temperature (the ratio of kinetic energy in random motion vs. ordered rotation). The combination of of kinematic measurements of gas (sticky) and stars (collisionless) allows us to separate the energetics due to mass accretion and dynamical instabilities from stellar and AGN feed-back. This in turn allows us to disentangle if and when gas is lost from the disk to the intergalactic medium, or recycled back into the disk.

Our department has a strong observational and theoretical focus on galaxy kinematics and their dynamical interpretation, as well as on stellar and AGN feedback, underpinned by the development of unique instrumentation for our telescope facilities. We have expertise in the study of galaxy kinematics over a wide range of galaxy types from large, normal spiral galaxies to irregular, late-type systems, to star-bursts at low and high redshift. Our observational strengths include optical integral-field spectroscopy (IFS) of ionized gas and stars, and neutral-hydrogen studies using single-dish and
aperture-synthesis arrays. We have lead the development of IFS systems on the WIYN 3.5m telescope and the Wisconsin H-alpha Mapper (WHAM); we are developing new IFS systems for our 11m telescope, SALT; and we are members of the US SKA consortium.

Questions for which we actively pursue answers include:

How are galaxy disks assembled and heated?  An unambiguous prediction of cold-dark-matter structure-formation scenarios is that disks form late, at relatively recent times. The dynamically cold nature of disks limits the accretion rate at any given time, but provides little further constraint on the history of matter accretion onto disks, or their heating via dynamical or feedback processes. Observationally, the matter accretion rate has not been determined at any epoch. Several programs here are filling this gap in our knowledge by studying a variety of galaxy types at different look-back times and

How much mass is actually in galaxy disks — is their rotation maximally supported at small radii by the mass of the disk? The answer to this question has profound implications for the shape of dark-matter halos, and hence on galaxy formation scenarios. Equally important, the answer to this questions provides the mass-to-light ratio of disk stellar populations, and thereby places limits on the faint-end of the initial mass function in galaxies outside of the
Milky Way, and other forms of dark-matter in the disk (e.g., stellar remnants, molecular gas, or sticky dark-matter). Some of the unique instruments developed here are being used to measure the mass in galaxy disks today.

A common theme throughout our collaborative research is spectroscopy — a tool which allows us to tie our kinematic measurements into studies of abundances and stellar populations essential for a complete picture of the life-cycle of baryons in the universe.

Run by Matthew Bershady.