|The framework of large scale structure formation relies heavily on the idea of cold dark matter (CDM). The predictions of CDM on large scales seem to be consistent with observations, but it is not readily apparent that the theory's success extends to small (galaxy) scales. The velocity fields, rotation curve shapes and dark matter halo central densities of low surface brightness (LSB) galaxies are typically more consistent with round, cored dark matter halos than the triaxial, cuspy CDM halos predicted by numerical simulations. Discrepancies between the expected properties of CDM halos and galaxy observations are not exclusive to dark matter-dominated LSB galaxies; all normal spiral galaxies are found to reside in underdense halos. Reconciling the predictions of numerical simulations with observations of dark matter-dominated and spiral galaxies is thus currently one of the greatest challenges for CDM.
The addition of baryonic physics to simulations of galaxy formation has been instrumental in producing realistic massive and Milky Way-analog disk galaxies that are bulgeless or only moderately bulged. Additionally, in recent simulations of dwarf galaxies, processes that remove or redistribute baryons during galaxy formation have been shown to alter the inner shape (triaxial to more spherical) and density structure (cuspy to more core-like) of dark matter halos. It therefore seems that baryons may be integral to easing the tension between observations and theoretical expectations of CDM halo structure.
I am interested in using observational constraints on the unique star formation histories, structure, and kinematics of LSB galaxies to evaluate the plausibility that baryons can modify the dark matter halos of these galaxies. Are the observed properties of LSB galaxies such as gas-richness, low gas surface densities, low past and current star formation rates, blue colors, and low metal abundances consistent with galaxies which have undergone periods of star formation and baryonic mass loss via outflows?
To answer these questions, we have begun the MUSCEL (MUltiwavelength observations of the Structure, Chemistry, and Evolution of LSB galaxies) program. For details, please see our program webpage: click here for MUSCEL details
RINGS: The RSS Imaging spectroscopy Nearby Galaxy Survey
With the recently re-commissioned RSS spectrograph on SALT, we are anticipating high-resolution Fabry-Perot Halpha velocity fields of 19 nearby, normal spiral galaxies. We will combine these optical kinematic data with HI kinematic data and optical and near-infrared photometry to constrain the concentration, size and ellipticity of the dark halo. We are also interested in determining how non-circular flows and processes like star formation and related feedback influence the observed structure of galaxies (bars, rings, etc).
DiskFit: A Code for Modeling Asymmetries in Disk Galaxies
DiskFit is a code for modelling asymmetries in either photometry (fitting images) or kinematics (fitting velocity fields) of disk galaxies. For more details, see the DiskFit webpage.
As an NSF Astronomy & Astrophysics Postdoctoral Fellow at the Center for Cosmology at the University of California, Irvine, my collaborators and I worked to constrain the properties of dark matter halos formed in numerical simulations. We integrated my LSB galaxy velocity field observations into numerical simulations to probe the shape of the dark matter potential (spherical vs. triaxial), the density profile of the halo (cuspy vs. cored), and to determine the effects of baryonic processes (supernova feedback). We also explored models of thermal and non-thermal warm dark matter and self-interacting dark matter that naturally produce cored halos.
Boxy/peanut-shaped bulges and figure-of-eight position-velocity diagrams (PVDs) have been connected to the presence of bars in edge-on galaxies. We have observed the edge-on spiral NGC 2683 with the RC Spectrograph on the Kitt Peak 4m and the SparsePak IFU on the WIYN telescope. Both the long-slit PVD and the Halpha velocity field show complex kinematics consistent with the presence of a bar.
This dissertation investigates the behavior of cold dark matter (CDM) on galaxy scales. We present well-resolved Ha velocity fields of the central regions of 17 dark matter-dominated low surface brightness (LSB) and dwarf galaxies observed with the DensePak Integrated Field Unit. We derive rotation curves from the two-dimensional data and compare them to published long-slit and HI rotation curves. We find broad consistency between the independent data sets. Under several assumptions about the velocity contribution from the baryons, we fit the dark matter component with cuspy NFW and cored pseudoisothermal halos. We find the data to be better described by cored dark matter halos. For the majority of galaxies, NFW halo fits either cannot be made or the implied concentrations are too low for LCDM. The shapes of the NFW rotation curves are also inconsistent with the galaxy rotation curves. We find that CDM predicts a substantial cusp mass excess near the centers of the galaxies and that the ratio of predicted to observed dark matter increases as baryons become more important. We investigate claims that systematic effects including beam smearing, slit misplacement and noncircular motions are responsible for slowly rising long-slit and HI rotation curves. We find the DensePak rotation curves to also be slowly rising, supporting the idea that this is an intrinsic feature of LSB rotation curves. We also model the two-dimensional NFW halo and test several modifications to the potential in an attempt to simultaneously reconcile both the NFW velocity field and rotation curve with observed galaxy data. We present mock DensePak velocity fields and rotation curves of axisymmetric and non-axisymmetric potentials. We find that a non-axisymmetric NFW potential with a constant axis ratio can reduce the cusp mass excess in the observed galaxy data, but the observer's line-of-sight must be along the minor axis of the potential, and the NFW pinch is not erased from the velocity field. We find that a non-axisymmetric NFW potential with a radially varying axis ratio tends to wash out the NFW pinch but introduces a twist to the velocity field.
Kuzio de Naray, R., McGaugh, S.S., de Blok, W.J.G., & Bosma A. 2006, ApJS, 165, 461
Kuzio de Naray, R., McGaugh, S.S., & de Blok, W.J.G. 2008, ApJ, 676, 920
In this project, we used optical long-slit spectra to measure the oxygen
abundances of the HII regions in a sample of low surface brightness (LSB)
galaxies. We used three techniques to measure the abundances: the O[III]
direct abundance measure, the R23 strong line method, and the Equivalent
Width method. We also used three approaches to investigate the galactic
chemical evolution of the LSB galaxies: the luminosity-metallicity (L-Z)
relation, the mass-metallicity relation, and the oxygen abundance versus
gas mass fraction. We found our sample of LSB galaxies to be metal-poor
with high gas mass fractions. This implies that the galaxies are at an
early stage in their evolution. We also found the L-Z relation of the
LSB galaxies to not be significantly different from the L-Z relation of
other galaxy types.
Selected for their high ionizations, low metallicity extragalactic HII regions
and diffuse, ionized gas in blue compact galaxies have traditionally been the
objects studied when measuring the primordial helium abundance. To test
whether or not different types of objects give consistent determinations of
the primordial helium abundance, we studied a sample of low metallicity HII
regions in LSB galaxies. Our HII regions were chosen because they have lower
mean ionizations. Though the error on our measure of the helium abundance
was too large to make any significant cosmological statements, we did find
that a sample of objects with low mean ionizations do not give significantly
different results than objects with high ionization.
For my senior honors thesis at Penn State, I wrote a population synthesis
program designed to probe the chemical and star formation history of a galaxy
via its planetary nebulae (PN). The code creates fully-described PN at
random times in their evolutionary history. Initial mass-final mass relations
determine the final mass of the PN central stars and post-asymptotic giant
branch stellar evolutionary tracks determine the temperature and luminosity of
the PN central star and the radius and density of the nebula. The CLOUDY
photoionization code is integrated into the program and is called to generate
the emission line strengths for the PN. The program output can be used to
study the Planetary Nebula Luminosity Function (PNLF) and serve as a
comparison to the populations of PN observed in actual galaxies.
Ciardullo, R., Kuzio, R.E., Simone, A. 2001 Bull. AAS, 33, 1510
Image Credit: Robin Ciardullo
I have used optical [OIII]5007 images to determine the planetary nebula
luminosity function (PNLF) distances to M33, NGC 2403 and NGC 3627. The
planetary nebulae (PN) are identified by blinking between the on-band
[OIII]5007 image and an off-band image. If it appears in the on-band image
and disappears in the off-band image, it is a PN. The apparent magnitudes of
the PN are used to construct the PNLF for each galaxy and to derive a
distance estimate. The PNLF distances were then compared to the Cepheid
Ciardullo, R., Feldmeier, J.J., Jacoby, G.H., Kuzio de Naray, R., Laychak, M.B., Durrell, P.R. 2002, ApJ,
Kuzio, R.E., Ciardullo, R., Feldmeier, J.J., Jacoby, G.H. 1999 Bull. AAS, 31, 1391