Dynamical Mass and Heating of Galaxy Disks

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A longstanding problem in astrophysics is to understand how galaxies form and develop throughout their lifetimes. Such understanding is necessary to uncover how our Universe evolved and to gain insight into the origin of our own Milky Way Galaxy. One important aspect of understanding galaxy formation and evolution is the study of the motion and structure of a galaxy's disk system as observed through the action of the stars and gas of the galaxy. This project uncovers the archaeological record of mass-assembly and dynamical evolution of today's galaxy disks, revealed by relations between stellar motions and stellar populations. This basic research project will be conducted at a public University, where advances and discovery are combined with teaching and training to a large and diverse audience. The proposed project broadly disseminates knowledge and achieves scientific and technical literacy in these ways: (1) Research and teaching infrastructure will be enhanced by launching science projects on a new WIYN Observatory instrument. (2) STEM course development on galaxy dynamics and spectrograph instrumentation will intertwine with basic research as part of the PI's on-going faculty duties. (3) Graduate and undergraduate training of the future STEM workforce will include astronomical observations, data analysis, instrument commissioning, data mining, and collaborative research. (4) Educational tools on fiber-optics and their use in society will be developed, implemented, and assessed as part of a public project in our Space Place outreach facility, targeted at school-age students and adults. (5) A four-year lecture series on the Milky Way and external galaxies will be connected to this educational project, presenting expert knowledge on fiber-optic astronomical surveys to the public.The projects's primary objective is to couple the relative amplitude of rotation and velocity dispersion of gas and stars with stellar ages and abundances, used as chronometers, to probe the dynamical heating of disks over the past 4--6 Gyrs. This is analogous to what is being measured with resolved stars in ground-breaking studies of the Milky Way, here albeit more coarsely resolved yet broadly applied to a representative galaxy sample over a wide range in halo mass. This study is leveraged by a new integral-field unit (IFU) on the WIYN 3.5-m telescope and a large IFU survey being undertaken in the fourth generation of the Sloan Digital Sky Survey.A primary outcome of this work will be to answer the question of whether apparent variations in stellar mass-to-light ratios are due to changes in the stellar initial mass function, uncertainties in the late phases of stellar-evolution, or subtleties in the assumed distribution of luminous and dark matter. The approach enables accurate measurements of disk mass by explicitly tying the vertical gradients in disk stellar populations into a self-consistent dynamical model. In so doing, the study determines how recent disk heating-rates depend on galaxy properties and environment, thereby constraining the astrophysical source(s) of this phenomenon. The project calibrates a method for measuring stellar velocity dispersions via the asymmetric drift between gas and stars, a related dynamical phenomenon discovered decades ago in the Milky Way and now measurable in external galaxies. The method will be applied to the current generation of integral-field spectroscopic survey data on nearby galaxies and has application at high redshift. The broader implications of this project include the following: (1) Determining the vertical heating rate of spiral disks using spectrophotometric chronometers is novel and establishes a concrete picture of the stellar density profile in the largest dissipational structures in the universe. (2) Coupling differences between stellar and gas kinematics with stellar population ages provides a new means for estimating dynamical mass and calibrating the mass-to-light ratios of stellar populations. (3) Calibrating mass-to-light ratios places stringent constraints on late phases of stellar evolution, the efficiency with which star-formation locks baryons into stars, and the inner density profiles of dark halos.