(Robert) Shane Bussmann

email:	rsbussmann.spam@as.spam.arizona.edu (remove .spam for functional form)
telephone:	520-621-2026
address:	Department of Astronomy/Steward Observatory 933 N. Cherry Ave. Tucson, AZ 85721

About me

I am a fifth year grad student in the Astronomy department at the University of Arizona. I will be applying for post-doc positions in fall 2009 and I plan to graduate the following spring. My thesis advisor is Arjun Dey, an astronomer at the National Optical Astronomy Observatory (NOAO). In my free time I like to cook, take the dog for a walk, and play ultimate frisbee.

Research: Thesis work

Recent evidence has accumulated which suggests that in the distant universe, star-formation was more intense and occurred primarily in dusty galaxies with very red optical-infrared colors. My particular interest is in using astronomical observations at optical, infrared, sub-mm, and radio wavelengths to understand how these distant, dusty galaxies form and evolve.

Dey et al. (2008) combined data from the NOAO Deep Wide-Field Survey (NDWFS) and from Spitzer Space Telescope coverage of the same field to identify a population of galaxies that are extremely faint in the optical, yet bright in the infrared (IR). These galaxies are called dust obscured galaxies, or DOGs. Spectroscopic follow-up with the Keck telescopes and the Infrared Spectrograph aboard Spitzer has demonstrated that they lie at a redshift of 2 (with a dispersion of 0.5). The number densities, 24-micron flux densities, and redshifts imply the DOG population is very luminous, and may contribute up to 25% of the IR luminosity density at z~2 (Dey et al. 2008). DOG clustering properties indicate they reside in massive dark matter haloes and may evolve into some of the most massive galaxies observed today (Brodwin et al. 2008). My thesis involves studying these important objects in three complementary ways.

DOG morphologies: I am working with high spatial resolution data from the Hubble Space Telescope (HST) in the optical (ACS and WFPC2) and the IR (NICMOS) to study the morphological properties of DOGs. Although the DOGs selected for the first part of this project show mid-IR spectral features consistent with the presence of an obscured active galactic nucleus (AGN), nearly all are resolved in the NICMOS images and have half-light radii of 1-5 kpc (Bussmann et al. 2009). Very few objects show signs of on-going merger activity. This indicates that DOGs either represent the later stages of a merging system or they are not produced by major mergers. For the second half of this project, I am analyzing another set of HST data on a sample of DOGs selected to have a peak in their continuum emission at rest-frame 1-2 microns, typically attributed to stellar populations. Preliminary results indicate diffuse, irregular morphologies are more common in the new sample and a higher fraction of sources show features of an on-going major merger.

IR luminosities and dust properties of DOGs: Estimates of the IR luminosities of DOGs suggest they may contribute up to 25% of the total IR luminosity density at z~2. However, these estimates are based on extrapolations of their spectral energy distributions (SEDs) from 24 microns out to 1mm, and can be uncertain by up to an order of magnitude. Measurements of DOG fluxes in the sub-mm are crucial to confirming the current estimates of their IR luminosities. Part of my thesis involves analyzing 350 micron imaging of a small sample of DOGs obtained with the SHARC-II bolometer array at the Caltech Sub-mm Observatory and 1mm imaging of a subset of these sources with the CARMA mm-wave interferometer. These DOGs have mid-IR spectral features typical of obscured AGN, and the sub-mm data indicate that warm dust SEDs (similar to Mrk 231, a nearby galaxy dominated by an obscured AGN) are appropriate representations of the full DOG SED (Bussmann et al., submitted).

Comparing DOGs and SMGs: A complementary method of identifying high-redshift IR luminous galaxies is to survey for sub-mm bright galaxies (SMGs). These objects have number densities, IR luminosities, and clustering properties similar to DOGs, suggesting there may be a link between the two populations. One speculative explanation proposed by Dey at al. (2008) is that the two populations are related in an evolutionary sense. A catastrophic event in the life of a massive galaxy (one example would be a major merger) leads to an accumulation of gas in deep potentials. This triggers intense star formation, quickly resulting in the formation of large quantities of cold dust. Such a system will be luminous at sub-millimeter wavelengths (as an SMG). At some point, as the star-formations proceeds, accretion on to the central super-massive black hole triggers an AGN, which heats the dust to warmer temperatures. It is at this point that the system would have a warmer characteristic dust temperature and be selected as a DOG. Eventually, the AGN may be powerful enough to destroy or expel most of the dust (or star-formation proceeds to the point where the dust and gas are sufficiently consumed), and the system evolves into a massive elliptical galaxy. We can test this hypothesis by obtaining carefully selected samples of both SMGs and DOGs and comparing tracers of the evolutionary state of these systems. Examples include quantities such as the gas mass, stellar mass, and star formation rate. Undertaking this comparison will form the final part of my thesis work.

Research: Star Formation in Nearby Galaxies

HCN(3-2) Survey:I am interested in understanding how star formation rates (SFRs) of different types of galaxies in the local universe (within a few hundred Mpc) correlate with the amount of dense gas they have within them. My part entails leading a team of observers to conduct a >200 hour program at the Sub-Millimeter Telescope (SMT) on Mt. Graham to observe HCN(3-2) emission and publish our results. Our primary result is that the SFR does not increase linearly with the amount of dense gas as traced by HCN(3-2). This feature is consistent with expectations of theoretical models which predict that the molecular line luminosity depends upon the average gas density of the source relative to the critical density of the emission line (Bussmann et al. 2008).

Research: Star Formation in Our Galaxy

During my first two and a half years, I worked with Chris Walker on studies of molecular outflows from Young Stellar Objects (YSOs) in rho-Ophiuchus (L1688). rho-Oph is a young (< 1 Myr), nearby (~120 pc) molecular cloud that harbors protostars which drive outflows of material that stir up the surrounding environment. My primary project was to map the structure and energetics of a molecular outflow from a protostar known as Elias 29 by observing CO(3-2) emission using the SMT (Bussmann et al. 2007, ApJ, 657L, 33).

Research: Sub-mm Instrumentation