(Robert) Shane Bussmann
Email: rsbussmann AT as DOT arizona DOT edu
Telephone: 520-621-2026
Address: Department of Astronomy
Steward Observatory
933 N. Cherry Ave.
Tucson, AZ 85721
CV (pdf)

About me:

I am a sixth 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/summer. My thesis advisor is Arjun Dey, an astronomer at the National Optical Astronomy Observatory (NOAO). In my free time I like to play ultimate frisbee, cook, and take the dog for a walk.

My Thesis: Probing the nature of dust-obscured galaxies at z~2

In the distant universe, star-formation was far more intense than it is today and it 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.

By combining the deep, wide-field imaging capabilities of the Spitzer Space Telescope and ground-based optical telescopes, Dey at al. (2008) have shown that a simple color selection of R-[24]>14 (F_24/F_R > 1000) successfully identifies a significant population of IR-luminous, dust-obscured galaxies (DOGs) at z~2 that inhabit massive dark matter haloes (M_DM ~ 10^12-13 Msun, Brodwin et al. 2008). My thesis involves studying these important objects with the goal of understanding the physical mechanisms responsible for their behavior.

Comparing DOGs and SMGs (Bussmann et al., in prep.)

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 star-formation 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.

IR luminosities and dust properties of DOGs (Bussmann et al., 2009, ApJ, 705, 184)

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 rely on assumptions regarding the shape 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.

Rest-frame UV to far-IR SEDs of DOGs with 350 micron observations normalized by rest-frame 8 micron flux density. These DOGs show a clear power-law in the mid-IR. Warm dust template SEDs like Mrk 231 fit the far-IR data much better than cold dust SEDs such as Arp 220 (Bussmann et al., submitted).

SEDs of DOGs in GOODS-N (Pope et al., 2008)

The origin of the IR luminosity in DOGs is uncertain. A study of similar sources down to a 24 micron flux density limit of F_24 = 40 micro-Jy by Fiore et al. (2008) in the Chandra Deep Field-South used a stacking analysis of X-ray spectra and found that 80% are Compton-thick AGN. On the other hand, Dey et al. (2008) examined mid-IR SEDs of DOGs and found evidence that DOGs with fainter 24-micron flux densities showed more evidence for star-formation rather than AGN activity. Determining the relative importance of starburst and AGN contributions to DOGs requires deep multi-wavelength data of a large sample of DOGs.

In this study, we use data from the GOODS-N field (where deep imaging and/or spectroscopy is available from the X-ray through the radio; see Giavalisco et al. 2004) to identify a sample of 79 faint (F_24 > 100 micro-Jy) DOGs and constrain their IR luminosities, determine the relative role of AGN and star formation activity and compare with SMGs. Nearly all are Compton-thick AGN by the criteria of Fiore et al. (2008), but Spitzer spectroscopy and photometry suggests that 80% are likely dominated by star formation. This set of DOGs have an average L_IR of 10^12 L_sun, or about a factor of 8 times less luminous than bright (F_850 > 5 mJy) SMGs. The composite faint DOG SED is similar to SMGs in the far-IR (dust temperature of ~30 K), but has a higher mid-IR to far-IR luminosity ratio by a factor of about 3 compared to SMGs.

Rest-frame mid-IR to far-IR composite SED of star-forming DOGs (black dots represent average luminosity density). Best-fit CE01+Draine models (solid curve), normalized composite SMG SED (dotted curve), and scaled SMG composite with additional hot (T=350 K) dust (dashed curve) are shown for comparison. Green bars indicate 5-sigma depths of planned deep surveys at 100 and 450 microns with Herschel/PACS and SCUBA-2 and show that the majority of DOGs will be detected by these surveys (Pope et al., 2008).

DOG morphologies, part 1 (Bussmann et al. 2009)

I used 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 this particular study 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. Very few objects show signs of on-going merger activity. This indicates that these AGN-dominated DOGs either represent the later stages of a merging system or they are not produced by major mergers.

Representative cutouts of NICMOS images of DOGs. The morphologies are diverse, with one source being undetected. Although all sources were selected to have mid-IR features typical of obscured AGN, nearly all show extended emission (Bussmann et al. 2009).

DOG morphologies, part 2 (Bussmann et al., in prep.)

For the second half of this project, I am analyzing another set of HST data on a sample of DOGs (bump 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.

These objects may represent a precursor stage to the AGN-dominated DOGs (if major mergers are the dominant production mechanism of DOGs), or they may represent a fully distinct branch of evolution for z~2 ULIRGs.

Representative cutouts of NICMOS images of bump DOGs. The morphologies are again diverse, although irregular and merger-like morphologies appear to be more common in the bump DOGs compared to the AGN-dominated DOGs (Bussmann et al., in prep.).

Star Formation Laws in Nearby Galaxies

Measurements of the relationship between molecular line luminosity and IR luminosity provide the critical information that reveals how vigorous star-formation is for a given amount of molecular gas mass. These star formation laws can then be used to predict star formation rates (SFRs) in models where the gas mass is known (e.g., numerical simulations of major mergers, semi-analytic models of galaxy evolution, etc.). The seminal paper on the star formation law by Kennicutt (1998) showed that the molecular gas mass -- as traced by CO(1-0) emission -- and the star formation rate are related by a Schmidt power-law with an index of N=1.4+/-0.15. Two pioneering studies extended this by measuring the dense molecular gas mass -- traced by HCN(1-0) -- both in nearby galaxies (Gao & Solomon, 2004) and in star-forming cores the Milky Way (Wu et al. 2005). These authors both found a slope consistent with unity, suggesting that the star formation efficiency (SFR/M_dense) is constant over many orders of magnitude in SFR. I have been involved in two projects that continue to explore the detailed nature of these star formation laws.

HCN(3-2) Survey with the SMT (Bussmann et al. 2008)

To test the hypothesis that dense gas mass tracers increase linearly with star formation rate, I led a team of observers in conducting a >100 hour program at the Sub-Millimeter Telescope (SMT) on Mt. Graham to measure HCN(3-2) luminosities in a sample of nearby galaxies spanning a broad range of IR luminosities. We found that the SFR increases more slowly than would be expected based on a linear extrapolation from the amount of dense gas as traced by HCN(3-2). This provides observational evidence that the star formation law depends on the critical density for excitation of the molecular tracer relative to the average gas density of the galaxy in question, a feature that is consistent with predictions from theoretical models (Narayanan et al. 2008, Krumholz & Thompson, 2008).

IR luminosity as a function of HCN(3-2) luminosity for a sample of nearby galaxies (data from Bussmann et al., 2008 and Gracia-Carpio et al. 2008). Best fit slope is shown with solid line and is less than unity at the 99% confidence level (inset).

A Comprehensive Study of SFR - molecular line luminosities (Juneau et al., ApJ accepted)

I am involved in an effort to analyze molecular line ratios in a sample of 34 nearby luminous and ultra-luminous infrared galaxies. We identify galaxies hosting active galactic nuclei (AGN) using a uniform classification based on integrated optical spectroscopy. We use different molecular emission line ratios of high-density to low-density tracers in conjunction with our AGN classification scheme to determine whether chemistry or other effects associated with the hard radiation field of the AGN could be driving the observed star formation laws.

Star Formation in Our Galaxy

In my first two years of grad school, I worked with Chris Walker and helped develop sub-mm heterodyne array receivers (the 7-pixel DesertSTAR receiver and the 64-pixel SuperCAM receiver). I also used these instruments to look for molecular outflows around protostars.

Proto-stellar Outflows (Bussmann et al. 2007)

My second year project was a study 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 the protostar known as Elias 29 by observing CO(3-2) emission using the SMT. Image at right shows a map of the molecular outflow around Elias 29, with blue-shifted emission in solid lines and red-shifted emission in dashed lines. There is some evidence of precession of the outflow axis, consistent with H_2 imaging (lower inset). A second outflow possibly associated with LFAM 26 is also present, with an axis consistent with the axis of H_2 emission (upper inset).

Map of molecular outflow -- traced by CO(3-2) emission -- around protostar Elias 29 (Bussmann et al., 2007).

Last Edited: 09.05.09