Research Experience: Kavan U. Ratnatunga

The Hubble Space Telescope ( HST ) Medium Deep Survey ( MDS ) has been the focus of most of my research since 1992. I have been a co-investigator of this survey project led by Prof. Richard Griffiths since its inception in 1985, and have been PI on two HST funded archival projects which expanded on the survey. This HST key project got well over a 1000 hours of pure parallel time using WFPC2 over the 4-years of the main survey and over 300 hours with WF/PC before refurbishment in December 1993. I designed and organized the MDS database and wrote all of the software required to extract a set of unbiased galaxy model parameters for every object detected on these fields. Using an automated 2-dimensional maximum likelihood analysis, the procedure can do bulge and disk decomposition of small galaxy images at faint magnitudes. The resulting catalog with over 200,000 galaxies has been put online for the larger astronomical community via an user searchable interactive web browser interface to the 12GB MDS database of reduced data on over 500 WFPC2 fields. The catalogs are currently being statistically analyzed to impose constraints on observational cosmology and the evolution of galaxies at high red-shift.

After a 1st-class honors B.Sc. degree ( 1976) from the Dept. of Physics, University of Ceylon ( phy.cmb ) in Colombo, Sri Lanka, my astronomy career started at the University of Pittsburgh where I completed a MS degree (1979) at the Dept. of Physics and Astronomy ( phyast.pitt ). I then moved to the Mount Stromlo and Siding Spring Observatories ( MSSSO ), of the Australian National University ( ANU ). For my PhD thesis (1983), I developed a procedure and surveyed three Schmidt Fields near the Kapteyn selected areas SA 127, SA 141, SA 189" to locate and study the kinematics of in-situ field K-giants in the outer regions our Galaxy, with Ken Freeman as my thesis advisor.

I completed three post-doctoral research appointments. First at the Institute of Advanced Study ( SNS-IAS ) in Princeton, during 1984/86, Dominion Astrophysical Observatory ( DAO ) in Canada during 1986/88 and NASA Goddard Space Flight Center ( GSFC-ADC ) during 1989/91. I joined Space Telescope Science Institute ( STScI ) in Spring 1992 to work on the HST-MDS key project and relocated to the Dept. of Physics and Astronomy, Johns Hopkins University ( pha.jhu ) in fall 1992 and then to the Dept. of Physics Carnegie Mellon University ( phys.cmu ) in fall 1996 with the same group. Although I relocated many times, the last 15 years has been scientifically very productive resulting in 50 papers in major refereed journals of which 15 were as first author. I have worked in many fields of astronomy. Starting from developing many skills for statistical analysis of Galactic structure and kinematics, I have moved to studying the morphology of faint galaxies at high red-shift, and the modeling newly discovered gravitational lens candidates in the MDS database. I have over the years established many long term collaborations.

At the Institute of Advanced Study in Princeton, working with John Bahcall, I formulated the kinematic Galaxy code ( IASG ) as a general purpose interactive software interface between theoretical models of the Galaxy and observables in astronomical data catalogs. This code allows direct comparison of kinematics in the observed frame of reference.

At the Dominion Astrophysical Observatory in Canada, I moved on to develop a numerical algorithm for analysis of stellar data catalogs to derive maximum likelihood estimates of parameters used in models of the Galaxy. The procedure can now simultaneously evaluate the parameters required to calibrate distances to the stars, and represent the kinematical distribution function, as well as check for residual systematic deviations of the published observations. It uses all available information: photo-electric photometry, trigonometric parallax, proper motion, and radial velocity, in a global maximization. The likelihood function is formulated using these `directly' observed quantities, individually integrating over distance the probability of the observables available for each star. The method can rigorously handle errors of observation and incompleteness of the catalogue over photometry, as well as detect any outliers.

This research was continued at NASA Goddard Space Flight Center where I was on a NRC research associateship with Jaylee Mead in the Astronomical Data Center. The procedure was applied to analyze a number of star catalogs. It was also tested extensively using simulated Monte Carlo catalogs to show that unbiased estimates of the kinematical parameters are derived with reliable error estimates. The procedure has resulted in a number of publications. In collaboration with Stefano Casertano we were awarded a NASA Grant to develop these algorithms for analysis of parallax and proper motion data from HIPPARCOS. A project which has been just completed.

A common aim of my research is to understand the structure of our own Galaxy and external galaxies. The goal has been to formulate theoretical computer models with galaxy parameters derived using maximum likelihood analysis of star and galaxy images which can be compared directly with observation. I have continued to create computer algorithms and use them for my astronomical research. The increasingly popular trend of using generic software packages does not necessarily get maximum information from the available observations, or produce unbiased estimates of model parameters from an astronomical catalog.