Research

I am primarily interested in how star formation in galaxies is affected by environment, with a focus on compact group configurations. Determining when the most recent episode of star formation occurred and if there is current star formation will provide insight into how the galaxy's cold gas is being processed. I currently combine optical spectroscopy and photometry to uncover the past star formation and interaction history of Southern Compact Group galaxies. In the future, I hope to combine this data with UV archival data to support far-infrared and radio observations of the cold gas distribution in these galaxies. I am also interested in determining if our own Local Group is undergoing the same processes observed in compact groups.

Compact groups of galaxies are configurations with similar densities to clusters but lower velocity dispersions. These arrangements tend to house galaxies with a wide range of morphologies suggesting that their cold gas supply is not completely depleted. We would expect the star formation activity to be enhanced in these systems but numerous studies have shown that, besides a few systems, this is not the case. These findings present many questions. How do the star formation histories of galaxies in compact groups differ from those in other environments? Does the variation in morphology imply that these groups are in various stages of interaction? Is star formation activity increased for a very brief amount of time as group members begin their interactions, quickly using up the cold gas? Is the supply of cold gas cut off as the galaxies enter a more massive dark matter halo, causing star formation to slowly ramp down over time? In order to answer these and other questions, we need information on the star formation histories of galaxies in different stages of group interaction.

The star formation history of galaxies is written in the chemical signatures of their stars. Understanding the ages and metallicities of the stellar populations in compact groups of galaxies will provide a timeline for star formation episodes and the chemical composition of the gas that formed these stars. We expect that galaxies that undergo a rapid processing of gas via star formation would house stars with higher than average metallicities. While those that have their gas completely removed would show a more uniform distribution in the metallicity of their stellar populations. Galaxies that reside in compact groups may show metallicity properties somewhere in between these two scenarios based on their stage of group evolution.

I am currently measuring the metallicity and ages of the stellar populations of Southern Compact Group (SCGs) galaxies using the Goodman Spectrograph on the 4m SOAR Telescope and the spectral synthesis code STARLIGHT. The groups I am observing show a wide range of morphologies which may imply that they are in various stages of group interaction. Most of these galaxies do not have measured redshifts, so group membership is still uncertain. Any galaxies that are revealed to be outside the group from a redshift analysis will be used as field galaxies for comparison. My research will determine when the most recent episode of star formation occurred in each galaxy and whether there is any current star formation. This will provide insight into how the cold gas is being processed in these galaxies. If a galaxy is a member of a group and has young stars and higher metallicities, we can infer that rapid star formation was triggered recently in this galaxy. If a similar galaxy has low to medium metallicity values; this may indicate that star formation is proceeding as it does for field galaxies.

Along with spectroscopic data, I am overseeing a project involving a post-baccalaureate student and a current undergraduate who are taking deep images of these SCGs using the PROMPT array on Cerro Tololo. Multiple images of each group are taken and stacked to increase the signal to noise and enhance any signs of interactions such as tidal tails and bridges between group members. The students will also use GALFIT to obtain surface brightness profiles of the galaxies and look for skewed isophotes which are indicators of past interactions.

Full thesis can be found here.

TL;DR version here. 

Gerald Cecil

JoEllen McBride

Yang Cui

University of North Carolina at Chapel Hill, Physics and Astronomy Department, Chapel Hill, NC

We are in the process of constructing a deployable integral field unit [d-IFU] for the Goodman Spectrograph on the 4.1m SOAR telescope in Chile. The Goodman Spectrograph utilizes volume-phase holographic gratings with 0.15 arcsec/pixel scaling over a wavelength range of 320-850nm. The proposed instrument will consist of four three axis probes each housing a 61-100 fused fiber bundle. These bundles can be positioned anywhere within the 5x9.5 arcmin^2 field of view of the SOAR telescope and will obtain a 0.5" sampling of any object within their 4" diameters. Fibers are separated above the collimator by slotted v-groove assemblies, then fed back to the Goodman slit where the light is dispersed onto the CCD. Images below show progress from concept design to final product. The ultimate goal is a multitude of larger (100-120 fiber) bundles deployed over a much larger field of view (i.e. 6 degree field of view of the UK Schmidt Telescope). This instrument will be most useful for obtaining spatially detailed spectra of clustered targets (i.e. globular clusters and galaxies).

Recent Publications

Optics Express Journal, 2010

AAS Meeting #215 2010

JoEllen McBride

University of North Carolina at Chapel Hill, Physics and Astronomy Department, Chapel Hill, NC

Gerald Cecil

University of North Carolina at Chapel Hill, Physics and Astronomy Department, Chapel Hill, NC

Joss Bland-Hawthorn

University of Sydney, Physics Department, Sydney, Aus.

Resolved giant stars in the nearby Sculptor Group galaxies are being studied in deep ground-based images to trace disk truncation beyond the H I limit. We recently used the SOAR Optical Imager + Sloan r' filter on that 4.1-m telescope in worse than median Cerro Pachon seeing (0.8 arcsec FWHM). We summed 10,000 seconds of exposure of a 5.5' by 5.5' patch of the outer disk of NGC 7793 away from Sculptor Group dwarfs and bright field stars. Our stack does include background compact galaxies and clusters that must be excluded. Preliminary culling retains ~7000 stars and suggests that their photometry is complete to deeper than 27 mag at 3-sigma detection. We will report on the resulting final radial profile and its implications.

References

Bland-Hawthorn, J. et al, 2005, ApJ, 629, 239

Carignan, C., 1985, ApJ SS, 58, 107

Publications

AAS Meeting #215 2010 

The Sloan Digital Sky Survey (SDSS) is an ongoing survey of the Northern Galactic Sky (Stoughton et al, 2002). Spectroscopy has been completed on over 790,000 galaxies at the time of this thesis. Photometric measurements using the 120 megapixel camera have been completed on over one million galaxies. As of Data Release 6 (DR6), over 9500 square degrees of sky have been covered. The SDSS uses an automated deblender to identify objects as stars or galaxies. The deblender is used to separate overlapping objects by detecting individual peaks throughout the image. Since galaxies are comprised of diverse components (i.e. HII regions, supernovae, bulges, halos, etc.), it is often difficult for an automated deblender to decide what constitutes a galaxy. This can cause shredding of a particular galaxy, which occurs most commonly in large galaxies, especially those classified as late-type (Blanton et al, 2005). A galaxy dominated by two bright HII regions could be classified as one galaxy or two HII regions, turning this into a subjective problem.

This project aims to account for this shredding by re-fitting nearby galaxies with a Sersic profile. We chose early-type galaxies from the Nearby Galaxies Catalog (Tully, 1988) that were also fit by the SDSS. Our algorithm consisted of a two dimensional fit to the equation (Sersic, 1963)

Initial parameters were chosen interactively using an IDL visualization routine. From our fits, larger galaxies tended to yield brighter magnitudes than those measured by SDSS. This makes sense if the galaxy was initially shredded. Smaller galaxies tended to have fainter magnitudes than those measured by SDSS. This is also expected since SDSS uses only two types of profiles to fit their galaxies. An exponential profile to fit late-type galaxies and a de Vaucouleur profile (de Vaucouleur, 1948) to fit early-types. The de Vaucouleur profile tends to overestimate the flux of a galaxy. Our Sersic profile accounts for steeper radial light profiles better than the de Vaucouleur profile.

Full thesis can be found here. 

Full thesis can be found here. 

Full document can be found here. 

Fall 2017: 0.4m PROMPT array, Cerro Tololo, Chile, Used array for teaching my PHYS100 course at West Chester University

2014-2015: 4m SOAR Telescope, Cerro Pachon, Chile, MOS Goodman Spectrograph, Used the multi-slit capabilities of the Goodman Spectrograph to obtain spectra of Southern Compact Groups.

2013-2014: 4m SOAR Telescope, Cerro Pachon, Chile, Goodman Spectrograph, Commissioned and tested a robotic fiber bundle positioner.

Jul 2013: 21ft Green Bank Observatory, Green Bank, WV, Observed the planets using the 21 foot dish connected to the Skynet network of telescopes.

Jul 2012: 40ft Green Bank Observatory, Green Bank, WV, Observed the Moon for undergraduate projects.

2012-2016: 0.4m PROMPT array, Cerro Tololo, Chile, Used array for teaching introductory astronomy labs and mentoring undergraduates at UNC-Chapel Hill.

Mar 2010: 4m SOAR Telescope, Cerro Pachon, Chile, SOAR Optical Imager, Deep imaging of the outer optical disk of NGC 7793.

Feb 2008: 0.9m WIYN, Kitt Peak Observatory, Tucson, AZ, Looking for tidal tails in interacting galaxies using the MOSAIC imager.

2003-2004: 24in Britton Observatory, Dickinson College, Carlisle, PA, Observed variable stars for thesis projects.

Oct 2001: 0.9m NURO, Lowell Observatory, Flagstaff, AZ, Observed variable stars.