Globular Clusters | Star-Forming Regions | Optical Spectroscopy
Population Studies of Extratidal Stars Around Globular Clusters
My current work focuses on identifying globular cluster stars which have been removed from the parent cluster and now live beyond its tidal influence. Since the globular clusters of the Galactic bulge are heavily obscured by attenuating dust and occupy densely crowded fields, special care must be taken to de-redden the photometry and decontaminate the sample. Using de-reddened NUV-optical six-band photometry from the Dark Energy Camera on the Blanco 4-m telescope at CTIO, it was possible to construct well populated color magnitude diagrams of both the field and the inner cluster. I used the optimal contrast color-magnitude filtering technique, as well as Gaia eDR3 proper motions, to identify candidate extratidal cluster stars.
Preliminary results suggest that the candidate extratidal stars are found preferentially in overdensities, either along the line connecting the cluster to the Galactic bulge, or in opposing sectors around the cluster. Population fractions and radial gradients among the candidate extratidal stars are being studied.
Multiple Populations in Globular Clusters
One of my current interests is in the phenomenon of multiple stellar populations in the globular clusters of the Milky Way bulge. For the better part of a century, globular clusters were thought of as the archetypical example of a single simple stellar population, but that picture has been completely turned on its head as overwhelming evidence points to multiple populations in almost all GCs where the stellar populations have been studied in detail.
In one recent project, I used resolved, de-reddened, NUV and optical stellar photometry to study the phenomenon of multiple stellar populations in the globular clusters of the Milky Way bulge, which due to high and variable extinction as well as extreme field crowding are comparatively understudied. One of the major tasks of the project was to overcome these observational challenges.
We find strong evidence for multiple populations among the red giant stars in all of the clusters studied.
Extinction Maps Toward Southern Galactic Bulge Globular Clusters
The globular clusters located in or around the Galactic bulge are notoriously difficult to study due to the interstellar dust clouds and filaments between the Sun and the Galactic center. Dust has the effect of attenuating light and distorting colors especially at optical wavelengths and shorter. Correcting photometry for the presence of interstellar dust is an essential prerequisite for extracting physical information from color-magnitude diagrams.
In this project I used NUV-optical-NIR photometry to create wide-field, high resolution maps of differential extinction and reddening around 14 globular clusters toward the Galactic bulge. The maps can be used to correct measured stellar colors and magnitudes, effectively lifting the veil of dust between the observer and the target globular clusters and enabling meaningful inferences about abundances in the atmospheres of stars in the distant clusters.
Gas Kinematics in Dwarf Galaxies
In another one of my current projects, I seek to understand the chemical evolution of dwarf irregular galaxies. These galaxies are a fraction of the size of the Milky Way, do not have recognizable spiral-like or ellipsoidal shapes, and are usually the sites of significant past and ongoing star formation. Despite all the star formation (and synthesis of heavy elements which accompanies star formation), these galaxies are almost always measured to have depleted heavy element abundances. The reasons for this are somewhat contentious.
I am using Indiana University's guaranteed institutional time on the WIYN 3.5-m telescope on Kitt Peak, AZ. to collect the spectroscopic data necessary to study the motions of the ionized gas in nearby dwarf irregular galaxies. The motion of the gas in these galaxies can tell us something about the extent to which energetic phenomena (such as supernovae and stellar winds) are causing the removal of enriched material as it becomes entrained in galactic-scale outflows.
The results indicate that massive stars are injecting significant kinetic energy into the ionized ISM, causing local turbulence as well as large scale directed flows.
Stellar Kinematics in Giant Elliptical Galaxies
The formation and evolution of giant elliptical galaxies has traditionally been linked to the collision of disk galaxies. While spiral disk galaxies feature ordered structure and non-random motions of the stars, e.g., the stars sweep out elliptical orbits along the disk, giant elliptical galaxies appear to have very little structure, and random stellar orbits. Therefore a natural hypothesis would be that an elliptical galaxy is the remnant of the collision between two or more spiral disk galaxies.
Using spectra from the multi-object spectrograph DEIMOS on the W.M. Keck II Telescope I was able to create seamless line of sight radial velocity maps for a large sample of elliptical and lenticular galaxies. These maps, and rotation curves also derived from the spectroscopic data, show that almost all galaxies have strong disk-like rotation in the inner parts that transitions to a slow co-rotating spheroid at around 2 effective radii. This is taken as strong evidence for the two-phase evolution scenario for early type galaxies, where an initial thick turbulent disk is slowly eroded as a spheroid of accreted material builds up around it over evolutionary timescales.
Nebular Abundances and Ionizations in Dwarf Galaxy NGC 6822
Using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope, observations were made of mid-infrared forbidden emission lines of hydrogen, neon, and sulfur in the H II regions of the nearby dwarf irregular galaxy NGC 6822, "Barnard's Galaxy". The observations cover the [S IV], [Ne II], [Ne III], and [S III] lines which arise from the S^3+, Ne^+, Ne^++, and S^++ ions. Since these are the dominant ionization states of neon and sulfur in these H II regions, a determination of the Ne/S abundance ratio was possible. This abundance ratio has implications for the chemical evolution of neon and sulfur. The median Ne/S ratio among the H II regions was found to be 11.6, similar to the value found in the higher metallicity galaxies M83 and M33. This value is at odds with the solar value of Ne/S = 6.5.
Since the fractional ionic abundances, e.g., <Ne++>/<S++>, are highly sensitive to the ionizing SED of the embedded stars, it was possible to test how well the observed H II regions were fit by different nebular models. The spherical ionization-bounded nebular models had fixed density but varying input stellar SEDs. It was found that the observations most closely track the supergiant O-star atmospheres with T_eff = 40-45 kK and solar abundances.