back to home

Main Areas of Research

My research addresses several interrelated problems in galaxy formation by looking at both the global evolution of the galaxy population and the detailed formation mechanisms of individual galaxy components. I am most interested in the puzzles and paradoxes that arise in trying to reconcile the current hierarchical galaxy formation paradigm with what we know about real galaxies, especially disk galaxies (since we live in one!). The observed universe is richer in disk galaxies than hierarchical models predict. Bulges of disk galaxies are often themselves "disky" and young, seemingly defying the theoretical picture in which bulges represent an earlier generation of merger-formed spheroids (elliptical E and lenticular S0 galaxies) around which disks have regrown. The abundance of disk galaxies and starburst activity increases abruptly below a threshold galaxy mass of a few times 10^10 Msun, while as a function of redshift, this threshold mass "downsizes" so that starbursts and disks were common up to higher mass scales at earlier times. By studying the components, structure, and dynamics of galaxies as a function of their mass and evolutionary history, my collaborators and I have taken steps toward reconciling disk galaxies with hierarchical models (but I never rule out that these models may be wrong!).

Notes in this color below indicate a sampling of possible projects for new group members. Some current student projects are here. Here are some computing tips.

Gas, Stellar, and Dark Matter Content of Galaxies
Morphological Evolution of Galaxies
Galaxy Dynamics and Scaling Relations
Instrumentation
The Nearby Field Galaxy Survey (NFGS)

  1. Gas, Stellar, and Dark Matter Content of Galaxies

    2. Student & Postdoc Opportunities: (1) direct calibration of galaxy stellar mass estimation methods at high redshift (DEEP2 and/or new Goodman Spectrograph data); (2) analysis of molecular-to-atomic gas ratios and participation in CARMA & Herschel surveys, with the long-term goal of using ALMA to measure evolution in galaxy gas content since z~1; (3) adaptation of photometric gas fraction techniques to higher redshift and/or spatially resolved data

    Research Highlights:

    • Existence of a tight correlation between U-K color and atomic-gas-to-stellar mass ratio in two independent surveys (HyperLeda/SDSS/2MASS: Kannappan (2004); NFGS: Wei, Kannappan, et al. 2007, in prep.), enabling determination of photometric gas fractions .

    • Existence of a threshold mass for gas richness, seen in photometric gas fractions in Kannappan (2004) and in both ionized and atomic gas detections in the NFGS (Wei, Kannappan, et al. 2007, in prep.). Below the threshold mass , gas fractions rise abruptly on both the red and blue sequences . This gas richness shift may be related to the starburst threshold mass at a few times 10^10 Msun.

    • Investigation of galaxy mass-to-light ratios and dark matter fractions using dynamical scaling relations in near-infrared and optical passbands, showing that K-band light is not a good tracer of dynamical mass, whereas B does better than expected. (Drory & Kannappan, joint talk given at UT Austin October 2003)

    • Comparison of stellar mass estimation techniques against dynamical masses, demonstrating systematic uncertainties in stellar mass scale between different galaxy classes -- e.g., late types, classical red-sequence E/S0s, and so-called "blue-sequence E/S0s" (Kannappan & Gawiser 2007)

  2. Morphological Evolution of Galaxies

    3. Student & Postdoc Opportunities: (1) search for newly forming disks in an E/S0 galaxy survey with deep optical and GALEX NUV/FUV imaging; (2) analysis of "downsizing" in disk building using MUSYC data to identify blue-sequence E/S0s at high redshift; (3) census of resonance structures (bars and rings) and their drivers; (4) direct measurement of whether blue-centered galaxies are forming "disky bulges"; (5) test for coordinated bulge and disk growth using Spitzer Space Telescope data tracing current and past star formation as a function of radius

    Research Highlights:

    • Statistical analysis of galaxies whose centers are bluer than their outer disks (likely reflecting disky bulge growth via gas inflow), arguing that this phenomenon (1) correlates with evidence of minor mergers and interactions, (2) represents an important mode of late-epoch bulge growth for most disk galaxies, leading to significant changes in bulge-to-disk ratio without destroying disks, and (3) is a natural consequence of hierarchical galaxy formation, consistent with high-z data on bulge rejuvenation. (Kannappan, Jansen, & Barton 2004; Talk given at the Zurich Meeting 8/2003)

    • Identification of morphologically classified E/S0 galaxies on the blue sequence in color-stellar mass space, with properties transitional between spiral/irregular types and classical red E/S0s. Analyzing blue-sequence E/S0s, we find (Kannappan, Guie, & Baker 2006, submitted): 1) a rare high-mass population produced by recent interactions & mergers, and 2) an abundant intermediate-to-low-mass population emerging below the gas-richness threshold mass found in Kannappan (2004; see section 1 above). The latter group may be rebuilding disks as predicted by hierarchical models of galaxy formation.

    • Interpreting counterrotating gas and stars in galaxies. An inventory of gas-stellar counterrotation in the NFGS shows that this phenomenon occurs primarily in intermediate-to-low luminosity E/S0 galaxies (Kannappan & Fabricant 2001a,b) that are on or near the blue sequence (Kannappan, Guie, & Baker 2006, submitted), although one late-type dwarf shows possible counterrotation and many show kinematically decoupled gas and stars (Guie, Kannappan, Balog, & Berlind 2005). The E/S0 morphologies of most gas-stellar counterrotators may reflect formation via minor and/or gas-rich mergers of dwarf galaxies. Alternatively, counterrotating gas may represent fresh external gas accretion, as is likely for NGC7360 , whose extensive secondary gas and stellar disks are hard to explain without new growth.

  3. Galaxy Dynamics and Scaling Relations

    4. Student & Postdoc Opportunities: (1) mass modeling of dynamically "hot" and "cold" S0 galaxies aimed at integrating these transitional systems consistently into L-V (or Mass-V) studies; (2) application of artificial redshifting to low-z galaxy velocity fields from WIYN/SparsePak, with the goal of unbiased comparison to high-z data; (3) participation in one of the high- or low-z galaxy surveys planned for the Goodman Spectrograph on SOAR (commissioning now, fall 2007!)

    Research Highlights:

    • Analysis of the physical drivers of scatter in the relation between galaxy luminosity L and internal velocity V (V=rotation velocity for spirals and V=velocity dispersion for bulges and E/S0s), showing that (1) color and emission-line equivalent width, both tracers of recent star formation, are strong drivers of L-V residuals for non-dwarf spirals and galaxy bulges, and (2) kinematic asymmetries and other signs of disturbance are the main drivers of L-V scatter for dwarf galaxies. (Kannappan, Fabricant, & Franx 2002; Kannappan, Jansen, & Barton 2004; if you're interested in kinematic asymmetries check out Kannappan & Fabricant 2001c and Kannappan 2001 as well)

    • Quantitative evaluation of issues in interpreting evolution in the disk-galaxy L-V relation (Tully-Fisher relation) from high to low redshift: selection biases, differences in high- and low-z measurement techniques, and effects of kinematic anomalies. Accounting for such issues (1) resolves prior discrepancies between high-z Tully-Fisher studies (Kannappan et al. 2003; Kannappan et al. 2001) and (2) suggests that apparent slope evolution in the Tully-Fisher relation may be driven by a subpopulation of galaxies with kinematic anomalies and possibly interaction-triggered starbursts (Kannappan & Barton 2004).

    • Joint DEEP2 + NFGS study of evolution in disk-galaxy luminosity-velocity-radius scaling relations from z ~ 1 to 0. (Metevier, Kannappan, & DEEP2 team, Letter to be submitted fall 2007)

  4. Instrumentation

    5. Student & Postdoc Opportunities: I am currently supporting Chris Clemens' efforts to commission the Goodman Spectrograph on the SOAR telescope and enhance its capabilities (e.g., multi-slit, custom VPH gratings). Collaborative instrumentation work with Chris may be an option for a new group member with an interest in spectroscopy.

    IntroSpec is a fiber-fed CCD spectrograph for the 16-inch Knowles Telescope, used for astronomy classes at Harvard University. I built IntroSpec with Dan Fabricant and Charlie Hughes (Kannappan, Fabricant, & Hughes 2002; Kannappan & Fabricant 2000).

  5. The Nearby Field Galaxy Survey (NFGS)

    6. The Nearby Field Galaxy Survey was designed to represent the natural diversity of the local galaxy population over a wide range of luminosities, morphologies, and environments. The gas and stellar kinematic database (Kannappan 2001) adds to the photometric and spectrophotometric survey begun by Rolf Jansen, Dan Fabricant, Marijn Franx, and Nelson Caldwell.