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Based on the data from the Next Generation Virgo cluster Survey (NGVS), we statistically study the photometric properties of globular clusters (GCs), ultra-compact dwarfs (UCDs) and dwarf nuclei in the Virgo core (M87) region. We found an obvious negative color (g - z) gradient in GC system associate with M87, i.e. GCs in the outer regions are bluer. However, such color gradient does not exist in UCD system, neither in dwarf nuclei system around M87. In addition, we found that many UCDs are surrounded by extended, low surface brightness envelopes. The dwarf nuclei and UCDs show different spatial distributions from GCs, with dwarf nuclei and UCDs (especially for the UCDs with visible envelopes) lying at larger distances to the Virgo center. These results support the view that UCDs (at least for a fraction of UCDs) are more tied to dwarf nuclei than to GCs.
We discuss the stellar halos of massive elliptical galaxies, as revealed by our ambitious integral-field spectroscopic survey MASSIVE. We show that metallicity drops smoothly as a function of radius out to ~ 2.5 Re, while the [α/Fe] abundance ratios stay flat. The stars in the outskirts likely formed rapidly (to explain the high ratio of alpha to Fe) but in a relatively shallow potential (to explain the low metallicities). This is consistent with expectations for a two-phase growth of massive galaxies, in which the second phase involves accretion of small satellites. We also show some preliminary study of the gas content of these most MASSIVE galaxies.
Discovery of ultra-compact dwarfs (UCDs) in the past 15 years blurs the once thought clear division between classic globular clusters (GCs) and early-type galaxies. The intermediate nature of UCDs, which are larger and more massive than typical GCs but more compact than typical dwarf galaxies, has triggered hot debate on whether UCDs should be considered galactic in origin or merely the most extreme GCs. Previous studies of various scaling relations, stellar populations and internal dynamics did not give an unambiguous answer to the primary origin of UCDs. In this contribution, we present the first ever detailed study of global dynamics of 97 UCDs (rh ≳ 10 pc) associated with the central cD galaxy of the Virgo cluster, M87. We found that UCDs follow a different radial number density profile and different rotational properties from GCs. The orbital anisotropies of UCDs are tangentially-biased within ~ 40 kpc of M87 and become radially-biased with radius further out. In contrast, the blue GCs, which have similar median colors to our sample of UCDs, become more tangentially-biased at larger radii beyond ~ 40 kpc. Our analysis suggests that most UCDs in M87 are not consistent with being merely the most luminous and extended examples of otherwise normal GCs. The radially-biased orbital structure of UCDs at large radii is in general agreement with the scenario that most UCDs originated from the tidally threshed dwarf galaxies.
High spatial-resolution measurements of surface brightness fluctuations (SBFs) with the Hubble Space Telescope (HST) provide the most precise distances available to early-type galaxies beyond the Local Group. The observable SBF magnitude in a given bandpass is a basic property of any stellar system, corresponding to a ratio of the first and second moments of the stellar luminosity function. Calibration of the method has presented challenges, but we now have an excellent empirical determination of how the SBF observable varies with galaxy color in broad bandpasses at the red end of the optical spectrum, and we are working towards a similar calibration for HST's Wide-Field Camera 3 in the near-infrared wavelength range, where the SBF magnitudes are considerably brighter. From HST Advanced Camera for Surveys data, we have determined the relative distances of the Virgo and Fornax clusters to within a precision of 2%, and resolved their internal structures. More recent measurements allow us to tie the Coma cluster, the standard of comparison for distant cluster studies, to the same precise distance scale. The SBF method can be calibrated in an absolute sense either empirically using Cepheids or theoretically based on stellar population models. The agreement between model and empirical zero points provides an independent confirmation of the Cepheid distance scale.
Examining both optical and optical-infrared color distributions of the globular cluster (GC) systems in large elliptical galaxies is the key to study how non-linearities in the color-metallicity relations of their GC systems are linked to bimodal optical color distributions. In order to do this for the core of the Coma cluster of galaxies (Abell 1656), centered on the giant elliptical galaxy NGC 4874, we have combined F160W (H160) near-infrared (NIR) imaging data acquired with the Wide Field Camera 3 IR Channel (WFC3/IR), installed on Hubble Space Telescope (HST) in 2009, with F475W (g475) and F814W (I814) optical imaging data from the HST Advanced Camera for Surveys (ACS). Since optical-NIR color distributions of extragalactic GC systems reflect the underlying features of the metallicity distributions, we have probed not only optical g475–I814 and optical-NIR I814–H160 color distributions but also the color-color relation for this GC system. The features of these color distributions have been quantitatively analyzed using the Gaussian Mixture Modeling code. We find that brighter GCs have a much redder mean color than fainter ones. The optical color distribution of the GC system in the Coma cluster core shows the typical bimodality, while the evidence for bimodality is significantly weaker in the optical-NIR color distribution.
The surface brightness fluctuation (SBF) method at near-infrared (NIR) wavelengths is a powerful tool for estimating distances to unresolved stellar systems with high precision. The IR channel of the Wide Field Camera 3 (WFC3), installed on board the Hubble Space Telescope (HST) in 2009, has a greater sensitivity and a wider field of view than the previous generation of HST IR instruments, making it much more efficient for measuring distances to early-type galaxies in the Local Volume. To take full advantage of its capabilities, we need to empirically calibrate the SBF distance method for WFC3's NIR passbands. We present the SBF measurements for the WFC3/IR F160W bandpass filter using observations of 16 early-type galaxies in the Fornax and Virgo Clusters. These have been combined with existing (g475–z850) color measurements from the Advanced Camera for Surveys Virgo and Fornax Cluster Surveys to derive a space-based H160-band SBF relation as a function of color. We have also compared the absolute SBF magnitudes to those predicted by evolutionary population synthesis models in order to study stellar population properties in the target galaxies.
We present a brief update on the ACS Virgo and Fornax Cluster Surveys — Hubble Space Telescope programs to obtain ACS imaging for 143 early-type galaxies in the two galaxy clusters nearest to the Milky Way. We summarize a selection of science highlights from the surveys as including new results on the central structure of early-type galaxies, the apparent continuity of photometric and structural parameters between dwarf and giant galaxies, and the properties of globular clusters, diffuse star clusters and ultra-compact dwarf galaxies.
We examine the photometric and structural properties of early-type galaxies derived from the ACS Virgo and Fornax Cluster Surveys, supplementing these data with previously published or newly reanalysed data for additional early-type galaxies in Virgo, Fornax and the Local Group. As we have noted elsewhere, Sérsic models are found to provide accurate representations of the observed brightness profiles on scales greater than a few percent of the effective radius, Re. On smaller scales, the brightness profiles of bright (MB ≳ −20) galaxies show central deficits with respect to the inward extrapolation of the Sérsic models; fainter galaxies usually show central excesses. Fainter than MB ≈ −20, we find a continuity in the photometric and structural scaling relations: i.e., between absolute magnitude, central surface brightness, Sérsic index, effective radius, and the surface brightness measured at, and averaged within, the effective radius. There is no evidence for a “dwarf/giant dichotomy” — a conclusion consistent with a number of recent studies but contrary to some earlier claims.
We present the effects due to alpha-element enhancement on surface brightness fluctuation magnitudes and broad-band colors in order to investigate the calibration zero-point. We study these effects at ages covering 0.1 to 13 Gyr and metallicities of Z=0.0003 to 0.04 using the Teramo group's isochrones, BaSTI. Our preliminary results indicate that alpha-enhanced models are needed to match massive red galaxies while solar-scaled models are fine for dwarf blue galaxies to reproduce their independently estimated ages and metallicities.
The superior resolution and large format of the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) make it a powerful new tool in the study of extra-galactic globular cluster (GC) systems. We summarize some early results on GC populations from the ACS GTO program, concentrating on the extreme cases of the isolated dwarf NGC 2915 and the core of the massive lensing cluster Abell 1689.
Many problems exist in current attempts to develop polycrystalline GaAs as a basis for thin-film solar cells. Some of these problems arise from the direct interaction of carriers, both dark and photo-generated, with grain boundaries. Others are more indirect; e.g., shunting currents due to the grain boundary-enhanced diffusion of contaminating impurities. This paper describes several of these effects, including the influence of system chemistry on grain properties, the correlation of device parameters with grain size, and grain boundary passivation experiments. A review of various approaches to solving the problems confronting the field is given, and an attempt is made to interpret reported observations in terms of existing theoretical models.
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