We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of ten of the nearest and brightest nuclear clusters residing in late-type spiral galaxies, in seven bands that span the near-ultraviolet to the near-infrared. Structural prop
erties of the clusters were measured by fitting two-dimensional surface brightness profiles to the images using GALFIT. The clusters exhibit a wide range of structural properties. For six of the ten clusters in our sample, we find changes in the effective radius with wavelength, suggesting radially varying stellar populations. In four of the objects, the effective radius increases with wavelength, indicating the presence of a younger population which is more concentrated than the bulk of the stars in the cluster. However, we find a general decrease in effective radius with wavelength in two of the objects in our sample, which may indicate extended, circumnuclear star formation. We also find a general trend of increasing roundness of the clusters at longer wavelengths, as well as a correlation between the axis ratios of the NCs and their host galaxies. These observations indicate that blue disks aligned with the host galaxy plane are a common feature of nuclear clusters in late-type galaxies, but are difficult to detect in galaxies that are close to face-on. In color-color diagrams spanning the near-UV through the near-IR, most of the clusters lie far from single-burst evolutionary tracks, showing evidence for multi-age populations. Most of the clusters have integrated colors consistent with a mix of an old population (> 1 Gyr) and a young population (~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity to populations with a mix of ages that would not be possible to achieve with imaging in optical bands only.
Hundreds of Type 2 quasars have been identified in Sloan Digital Sky Survey (SDSS) data, and there is substantial evidence that they are generally galaxies with highly obscured central engines, in accord with unified models for active galactic nuclei
(AGNs). A straightforward expectation of unified models is that highly obscured Type 2 AGNs should show little or no optical variability on timescales of days to years. As a test of this prediction, we have carried out a search for variability in Type 2 quasars in SDSS Stripe 82 using difference-imaging photometry. Starting with the Type 2 AGN catalogs of Zakamska et al. (2003) and Reyes et al. (2008), we find evidence of significant g-band variability in 17 out of 173 objects for which light curves could be measured from the Stripe 82 data. To determine the nature of this variability, we obtained new Keck spectropolarimetry observations for seven of these variable AGNs. The Keck data show that these objects have low continuum polarizations (p<~1% in most cases) and all seven have broad H-alpha and/or MgII emission lines in their total (unpolarized) spectra, indicating that they should actually be classified as Type 1 AGNs. We conclude that the primary reason variability is found in the SDSS-selected Type 2 AGN samples is that these samples contain a small fraction of Type 1 AGNs as contaminants, and it is not necessary to invoke more exotic possible explanations such as a population of naked or unobscured Type 2 quasars. Aside from misclassified Type 1 objects, the Type 2 quasars do not generally show detectable optical variability over the duration of the Stripe 82 survey.
We present the direct imaging discovery of an extrasolar planet, or possible low-mass brown dwarf, at a projected separation of 55 +/- 2 AU (1.058 +/- 0.007 arcsec) from the B9-type star Kappa And. The planet was detected with Subaru/HiCIAO during th
e SEEDS survey, and confirmed as a bound companion via common proper motion measurements. Observed near-infrared magnitudes of J = 16.3 +/- 0.3, H = 15.2 +/- 0.2, Ks = 14.6 +/- 0.4, and L = 13.12 +/- 0.09 indicate a temperature of ~1700 K. The galactic kinematics of the host star are consistent with membership in the Columba association, implying a corresponding age of 30 +20 -10 Myr. The system age, combined with the companion photometry, points to a model-dependent companion mass ~12.8 MJup. The host stars estimated mass of 2.4-2.5 Msun places it among the most massive stars ever known to harbor an extrasolar planet or low-mass brown dwarf. While the mass of the companion is close to the deuterium burning limit, its mass ratio, orbital separation, and likely planet-like formation scenario imply that it may be best defined as a `Super-Jupiter with properties similar to other recently discovered companions to massive stars.
GJ 758 B is a cold (~600K) companion to a Sun-like star at 29 AU projected separation, which was recently detected with high-contrast imaging. Here we present photometry of the companion in seven photometric bands from Subaru/HiCIAO, Gemini/NIRI and
Keck/NIRC2, providing a rich sampling of the spectral energy distribution in the 1-5 micron wavelength range. A clear detection at 1.58 micron combined with an upper limit at 1.69 micron shows methane absorption in the atmosphere of the companion. The mass of the companion remains uncertain, but an updated age estimate indicates that the most likely mass range is ~30-40 Mjup. In addition, we present an updated astrometric analysis that imposes tighter constraints on GJ 758 Bs orbit and identifies the proposed second candidate companion, GJ 758 C, as a background star.
We propose a new concept for a very low background multi-ton liquid xenon Dark Matter experiment. The detector consists of two concentric spheres and a charge readout device in the centre. Xenon between the two spheres forms a self-shield and veto de
vice. The inner surface of the central sphere is coated with CsI to form an internal photocathode with minimum of 2pi coverage for any event in the active volume. Photoelectrons from the CsI photocathode drift toward the charge readout micro-structure in the centre of the detector. Both scintillation and ionisation are measured simultaneously for background rejection and 3-D event mapping. In addition to external shielding, the low background is achieved by eliminating PMTs and by using low radioactivity pure materials throughout the detector. We present detailed calculations of the charge readout system and design details. The detector is expected to probe the full SUSY parameter space.
