Structural Characteristics and Stellar Composition of Low Surface Brightness Disk Galaxies

We present UBVI surface photometry of a sample of low surface brightness (LSB) disk galaxies. LSB disk galaxies are fairly well described as exponential disks with no preferred value for either scale length, central surface brightness, or rotational velocity. Indeed, the distribution of scale lengths is indistinguishable from that of high surface brightness spirals, indicating that dynamically similar galaxies (e.g., those with comparable Rv^2) exist over a large range in surface density. These LSB galaxies are strikingly blue. The complete lack of correlation between central surface brightness and color rules out any fading scenario. Similarly, the oxygen abundances inferred from HII region spectra are uncorrelated with color so the low metallicities are not the primary cause of the blue colors. While these are difficult to interpret in the absence of significant star formation, the most plausible scenario is a stellar population with a young mean age stemming from late formation and subsequent slow evolution. These properties suggest that LSB disks formed from low initial overdensities with correspondingly late collapse times.Comment: Astronomical Journal, in press 45 pages uuencoded postscript (368K) including 9 multipart figures also available by anonymous ftp @ ftp.ast.cam.ac.uk /pub/ssm/phot.uu CAP-30-210442962983742937

Chemical abundances in low surface brightness galaxies: Implications for their evolution

Low Surface Brightness (LSB) galaxies are an important but often neglected part of the galaxy content of the universe. Their importance stems both from the selection effects which cause them to be under-represented in galaxy catalogs, and from what they can tell us about the physical processes of galaxy evolution that has resulted in something other than the traditional Hubble sequence of spirals. An important constraint for any evolutionary model is the present day chemical abundances of LSB disks. Towards this end, spectra for a sample of 75 H 2 regions distributed in 20 LSB disks galaxies were obtained. Structurally, this sample is defined as having B(0) fainter than 23.0 mag arcsec(sup -2) and scale lengths that cluster either around 3 kpc or 10 kpc. In fact, structurally, these galaxies are very similar to the high surface brightness spirals which define the Hubble sequence. Thus, our sample galaxies are not dwarf galaxies but instead have masses comparable to or in excess of the Milky Way. The basic results from these observations are summarized

Multifunctional Nanoparticle/Lipid Self-Assemblies

Lipid bilayer membranes provide the physical barriers that define the structure, function, and stability of living cells. Specifically, they play a key role in governing molecular and phase toxicity, anesthetic action, the bioaccumulation of hydrophobic or amphiphatic pollutants, and cell-surface interactions. In addition to their ubiquity in nature, vesicles (or liposomes) self-assembled from synthetic lipid bilayers provide a biologically inspired route for the design of, for example, drug delivery systems, nanostructured materials and devices, and biosensors. We currently have two research thrusts in our laboratory related to lipid assemblies: (1) Utilizing synthetic lipid bilayers as model cell membranes to study nanomaterial and biomolecular interactions and (2) designing new multifunctional hybrid lipid assemblies for therapeutic applications. In this seminar, I will describe the techniques we use to design these systems, to evaluate their thermodynamic and transport properties (e.g. within lipid bilayers), which are governed by intermolecular and surface interactions, and to impart function. Specific projects that will be discussed include gauging nanoparticle toxicity, elucidating peptide-membrane interaction mechanisms for drug delivery, and creating inorganic nanoparticle/lipid assemblies

Dark Matter Annihilations in the Large Magellanic Cloud

The flat rotation curve obtained for the outer star clusters of the Large Magellanic Cloud is suggestive of an LMC dark matter halo. From the composite HI and star cluster rotation curve, I estimate the parameters of an isothermal dark matter halo added to a `maximum disk.' I then examine the possibility of detecting high energy gamma-rays from non-baryonic dark matter annihilations in the central region of the Large Magellanic Cloud.Comment: 2pp, plain TeX,

Age Dating Merger Events in Early Type Galaxies via the Detection of AGB Light

A thorough statistical analysis of the J-H vs. H-K color plane of all detected early type galaxies in the 2MASS catalog with velocities less than 5000 km/s has been performed. This all sky survey is not sensitive to one particular galactic environment and therefore a representative range of early type galaxy environments have been sampled. Virtually all N-body simulation so major mergers produces a central starburst due to rapid collection of gas. This central starburst is of sufficient amplitude to change the stellar population in the central regions of the galaxy. Intermediate age populations are given away by the presence of AGB stars which will drive the central colors redder in H-K relative to the J- H baseline. This color anomaly has a lifetime of 2-5 billion years depending on the amplitude of the initial starburst Employing this technique on the entire 2MASS sample (several hundred galaxies) reveals that the AGB signature occurs less than 1% of the time. This is a straightforward indication that virtually all nearby early type galaxies have not had a major merger occur within the last few billion years