Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution

We present a study of the spectroscopic and photometric properties of galaxies in six nearby clusters. We perform a partial correlation analysis on our dataset to investigate whether the correlation between star formation rates in galaxies and their environment is merely another aspect of correlations of morphology, stellar mass, or mean stellar age with environment, or whether star formation rates vary independently of these other correlations. We find a residual correlation of ongoing star formation with environment, indicating that even galaxies with similar morphologies, stellar masses, and mean stellar ages have lower star formation rates in denser environments. Thus, the current star formation gradient in clusters is not just another aspect of the morphology-density, stellar mass-density, or mean stellar age-density relations. Furthermore, the star formation gradient cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. Our results constitute a true ``smoking gun'' pointing to the effect of environment on the later evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap

Reproductive isolation and interspecific hybridization in the threatened species, Sidalcea nelsoniana

In addition to its longstanding recognition as an influential evolutionary process, interspecific hybridization is increasingly regarded as a potential threat to the genetic integrity and survival of rare plant species, manifested through gamete wasting, increased pest and disease pressures, outbreeding depression, competitive exclusion, and genetic assimilation. Alternatively, hybridization has also been interpreted as a theoretically beneficial process for rare species suffering from low adaptive genetic diversity and accumulated genetic load. As such, interspecific hybridization, and the underlying pre- and post-mating reproductive barriers that influence its progression, should be considered fundamental components of conservation planning for many rare species, particularly those predisposed to hybridization by various ecological, genetic, and anthropogenic risk factors. In this study I evaluate the nature and efficacy of preand post-mating hybridization barriers in the threatened species, Sidalcea nelsoniana, which is sympatric (or nearly so) with three other congeners in the scarce native grasslands of the Willamette Valley in western Oregon. These four perennial species share a high risk of hybridization due to their mutual proximity, common occupation of disturbed habitats, susceptibility to anthropogenic dispersal, predominantly outcrossing mating systems, their capability of longlived persistence and vegetative expansion, and demonstrated hybridization tendencies among other members of the family and genus. Results show S. nelsoniana is reproductively isolated from all three of its congeners by a complex interplay of pre- and post-mating barriers. Although S. nelsoniana overlaps with S. campestris in the ecological attributes of flowering time, fine-scale geographic distribution, and pollinators, interspecific hybridization is discouraged through the post-mating barrier of sexual incompatibility (expressed primarily as reduced seed set). Hybridization between S. nelsoniana and S. virgata is limited by the premating barrier of asynchronous flowering (temporal isolation) and the supplemental post-mating barrier of sexual incompatibility, though the completeness of the latter varies in relation to crossing direction and S. virgata phenotype. Lastly, although S. nelsoniana and S. cusickii exhibit full interspecific sexual compatibility and produce fully fertile hybrids, hybridization in this species pair is discouraged by the pre-mating barrier of geographic isolation (the two species are narrowly parapatric). Additional findings of this study show that 1) pollinators transferred significantly more S. nelsoniana pollen to the heterospecific flowers of S. virgata and S. cusickii than to conspecific flowers (or those of S. campestris) in a mixed species array, 2) anthropogenic disturbance is ubiquitous across extant S. nelsoniana study populations so cannot be conclusively linked with the breakdown of spatial reproductive barriers, 3) polyploidy is confirmed in the species group and appears to directly influence the observed patterns of interspecific sexual compatibility and the chromosome numbers of hybrids, and 4) Fl hybrids exhibit a mosaic of parental, intermediate, and transgressive phenotypic characters that render hybrid discrimination difficult (especially in light of pronounced parental phenotypic variability). Ultimately, despite the presence of pre- and post-mating reproductive barriers, hybridization between Sidalcea nelsoniana and its local congeners is still possible through human intervention. Preserving the species' current genetic integrity will require prevention of Sidalcea dispersal that could lead to the breakdown of spatial, temporal, and sexual barriers between species. Development of useful molecular markers will be needed to help recognize hybrids in the wild, as traditional morphological methods appear inadequate for this endeavor

One-Bead Microrheology with Rotating Particles

We lay the theoretical basis for one-bead microrheology with rotating particles, i.e, a method where colloids are used to probe the mechanical properties of viscoelastic media. Based on a two-fluid model, we calculate the compliance and discuss it for two cases. We first assume that the elastic and fluid component exhibit both stick boundary conditions at the particle surface. Then, the compliance fulfills a generalized Stokes law with a complex shear modulus whose validity is only limited by inertial effects, in contrast to translational motion. Secondly, we find that the validity of the Stokes regime is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let

Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754

To better understand the mechanism or mechanisms that lead to AGN activity today, we measure the X-ray AGN fraction in a new sample of nearby clusters and examine how it varies with galaxy properties, projected cluster-centric radius, and cluster velocity dispersion. We present new wide-field Chandra X-ray Observatory observations of Abell 85, Abell 754 and the background cluster Abell 89B out to their virial radii. Out of seventeen X-ray sources associated with galaxies in these clusters, we classify seven as X-ray AGN with L_{X,B} > 10^{41} erg/s. Only two of these would be classified as AGN based on their optical spectra. We combine these observations with archival data to create a sample of X-ray AGN from six z < 0.08 clusters and find that 3.4+1.1/-0.8% of M_R 10^{41} erg/s. We find that more X-ray AGN are detected in more luminous galaxies and attribute this to larger spheriods in more luminous galaxies and increased sensitivity to lower Eddington-rate accretion from black holes in those spheroids. At a given X-ray luminosity limit, more massive black holes can be accreting less efficiently, yet still be detected. If interactions between galaxies are the principal drivers of AGN activity, then the AGN fraction should be higher in lower velocity dispersion clusters and the outskirts of clusters. However, the tendency of the most massive and early-type galaxies to lie in the centers of the richest clusters could dilute such trends. While we find no variation in the AGN fraction with projected cluster-centric radius, we do find that the AGN fraction increases significantly from 2.6+1.0/-0.8% in rich clusters to 10.0+6.2/-4.3% in those with lower velocity dispersions.Comment: Accepted by Astrophysical Journal, 17 pages using emulateapj.cls, 10 B & W Figures (degraded): Full resolution paper available at http://www.astronomy.ohio-state.edu/~sivakoff/AGN/XAGN_A85_A754.pd

Structure of Polyelectrolytes in Poor Solvent

We present simulations on charged polymers in poor solvent. First we investigate in detail the dilute concentration range with and without imposed extension constraints. The resulting necklace polymer conformations are analyzed in detail. We find strong fluctuations in the number of pearls and their sizes leading only to small signatures in the form factor and the force-extension relation. The scaling of the peak in the structure factor with the monomer density shows a pertinent different behavior from good solvent chains.Comment: 7 pages, 5 figures. submitted to EP

Spatial variations of the Sr i 4607 {\AA} scattering polarization peak

Context. The scattering polarization signal observed in the photospheric Sr i 4607 {\AA} line is expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect), but also to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular scales, but observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required. Aims. We aim to provide observational evidence of the polarization peak spatial variations, and to analyze the correlation they might have with granulation. Methods. Observations conjugating high spatial resolution and high spectropolarimetric precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the adaptive optics system and the newly installed image derotator. Results. Spatial variations of the scattering polarization in the Sr i 4607 {\AA} line are clearly observed. The spatial scale of these variations is comparable with the granular size. Small correlations between the polarization signal amplitude and the continuum intensity indicate that the polarization is higher at the center of granules than in the intergranular lanes.Comment: 5 pages, 4 figure

ROTSE All Sky Surveys for Variable Stars I: Test Fields

The ROTSE-I experiment has generated CCD photometry for the entire Northern sky in two epochs nightly since March 1998. These sky patrol data are a powerful resource for studies of astrophysical transients. As a demonstration project, we present first results of a search for periodic variable stars derived from ROTSE-I observations. Variable identification, period determination, and type classification are conducted via automatic algorithms. In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0 and m_v=15.5. About 90% of these objects are newly identified as variable. Examples of many familiar types are presented. All classifications for this study have been manually confirmed. The selection criteria for this analysis have been conservatively defined, and are known to be biased against some variable classes. This preliminary study includes only 5.6% of the total ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11 postscript figures and 1 gif