Dr. Kulkarni's Research Group

The lightest chemical elements formed within minutes after the Big Bang, but virtually all elements heavier than Helium formed in stellar cores and supernova explosions. How did these other elements evolve to their current abundance levels? The evolution of chemical elements over the last >13 billion years is a crucial constraint on studies of the evolution of stars and interstellar gas within and surrounding galaxies. Moreover, understanding the “cosmic saga" of the chemical elements is fundamental to understanding the formation of planets and emergence of life!

Dr. Kulkarni and her team use a number of telescopes around the world such as the Keck and Subaru telescopes in Hawaii, the Magellan telescopes and the Very large Telescope in Chile, and the Hubble Space Telescope to spectroscopically measure the element abundances of galaxies at various redshifts. This allows us to trace the history of chemical element production over more than 90% of the age of the Universe.

Interstellar dust grains are microscopic solid particles making up a tiny fraction of the diffuse material in between stars. Despite its small fraction, interstellar dust has profound effects on the physical and chemical properties of interstellar matter. At the same time, absorption and scattering by interstellar dust can greatly alter the apparent properties of distant galaxies. Correcting for the effect of dust is crucially important while interpreting the observations of the distant universe. Yet, we know very little about the dust in the distant universe. Often, the corrections are made by simply assuming that the dust at high redshifts is similar to the dust in the Milky Way or its nearby satellite galaxies the Large and Small Magellanic Clouds.  

The team is investigating the chemical composition of interstellar dust in distant galaxies by directly looking for its spectral signatures. In particular, the team is studying the silicate content of this dust by searching for the silicate absorption features in the spectra of background quasars. The team has been using observations made with NASA’s Spitzer Space Telescope for this purpose. This work has led to the first detections of silicate dust in distant galaxies and the possible existence of crystalline silicates. 

The group is now investigating the connections between the dust and gas contents of the interstellar matter in galaxies, combining archival Infrared data from the James Webb Space Telescope and the Spitzer and Herschel space observatories and Optical/UV data from the Hubble Space Telescope as well as ground based observatories.

Observations of interstellar molecules in distant galaxies are essential for investigating the chemical evolution of galaxies. Absorption line systems in quasar spectra, especially the damped Lyman alpha (DLA) absorbers, provide excellent venues for directly studying the interstellar matter (ISM) in distant galaxies, selected independently of the galaxy luminosities. A few cold, dusty quasar absorption have been discovered using radio surveys and the Sloan Digital Sky Survey. These absorbers, far richer in dust/molecules than the general DLA population, give us rare opportunities to probe molecular clouds at high redshift. We have been searching for absorption signatures of molecules such as CO, CN, and H2O in high-redshift galaxies using spectrographs on the ESA/NASA Herschel Space Observatory. 

Detecting the light emitted by stars in galaxies gets rapidly more difficult with increasing distance. An alternative, more sensitive way to detect distant galaxies is by means of the absorption signature they imprint on the spectra of even more distant background sources such as quasars. However, understanding how these “absorber” galaxies relate to present-day galaxies still requires detection of the light emitted by the underlying galaxies. The difficulty in detecting these galaxies stems from their faintness and their small angular separation from the considerably brighter background quasars. The group has been using high spatial resolution imaging with adaptive optics systems on Keck and Gemini telescopes to detect the faint absorber galaxies, in collaboration with Drs. M. Chun, (Institute for Astronomy, University of Hawaii) and M. Makamiya (University of Hawaii, Hilo). The high resolution allows us to quantify the shapes and sizes of the galaxies, and their impact parameters from the background quasars. Combining this information with the spectroscopic information, we hope to understand the connections between the stellar properties and the interstellar gas within these galaxies.

The technique of integral field spectroscopy allows a powerful tool to obtain 3-D maps of gas kinematics in galaxies. In a project in collaboration with Drs. C. Peroux (Laboratoire d’Astrophysique de Marseille), N. Bouche (Laboratoire d’Astrophysique de Toulouse-Tarbes), and others, the group is using VLT/SINFONI and Gemini-NIFS to obtain 3-D maps of the redshifted Hydrogen Balmer-alpha line emission and absorption-selected galaxies. This allows us to not just confirm the redshift of the galaxy but measure its star formation rate and map the velocity dispersion of its interstellar gas.

Polar Ring Galaxies (PRGs) are visually spectacular objects, consisting of a robustly star forming ring of gas, dust and stars orbiting a plane perpendicular to the major axis of a central S0. Since the ring material experiences the gravitational potential in the polar plane, PRGs offer unique probes of the shapes of the dark matter halo. Furthermore, polar rings represent star forming environments distinct from the disks of spiral galaxies and collisional ring galaxies, where density waves act to collect the ISM and trigger star formation. We have been using Spitzer 3.6 and 4.5 micron images and Gemini optical images for a comprehensive sample of PRGs to obtain insights into the dynamics and star formation history of PRGs. 

Gaps in the existing atomic database are currently the greatest weakness in deriving element abundances from astrophysical spectra, e.g., spectra of distant galaxies. Both oscillator strengths and recombination coefficients are uncertain or unknown for many elements beyond Magnesium. These elements are crucial for the study of high-redshift galaxies as their ground-accessible, unsaturated lines give accurate abundances. In a collaborative research project with Drs. G. Ferland (University of Kentucky), and R. Kisielius and P. Bogdanovich (Vilnius University), the team is working on improving the atomic data needed for high-redshift galaxies and applying the resultant spectral simulations to observational datasets.