On the Metallicity-Color Relations and Bimodal Color Distributions in Extragalactic Globular Cluster Systems

We perform a series of numerical experiments to study how the nonlinear metallicity--color relations predicted by different stellar population models affect the color distributions observed in extragalactic globular cluster systems. % We present simulations in the $UBVRIJHK$ bandpasses based on five different sets of simple stellar population (SSP) models. The presence of photometric scatter in the colors is included as well. % We find that unimodal metallicity distributions frequently ``project'' into bimodal color distributions. The likelihood of this effect depends on both the mean and dispersion of the metallicity distribution, as well as of course on the SSP model used for the transformation. % Adopting the Teramo-SPoT SSP models for reference, we find that optical--to--near-IR colors should be favored with respect to other colors to avoid the bias effect in globular cluster color distributions discussed by \citet{yoon06}. In particular, colors such as \vh\ or \vk are more robust against nonlinearity of the metallicity--color relation, and an observed bimodal distribution in such colors is more likely to indicate a true underlying bimodal metallicity distribution. Similar conclusions come from the simulations based on different SSP models, although we also identify exceptions to this result.Comment: ApJ accepte

The MASSIVE Survey - I. A Volume-Limited Integral-Field Spectroscopic Study of the Most Massive Early-Type Galaxies within 108 Mpc

Massive early-type galaxies represent the modern-day remnants of the earliest major star formation episodes in the history of the universe. These galaxies are central to our understanding of the evolution of cosmic structure, stellar populations, and supermassive black holes, but the details of their complex formation histories remain uncertain. To address this situation, we have initiated the MASSIVE Survey, a volume-limited, multi-wavelength, integral-field spectroscopic (IFS) and photometric survey of the structure and dynamics of the ~100 most massive early-type galaxies within a distance of 108 Mpc. This survey probes a stellar mass range M* > 10^{11.5} Msun and diverse galaxy environments that have not been systematically studied to date. Our wide-field IFS data cover about two effective radii of individual galaxies, and for a subset of them, we are acquiring additional IFS observations on sub-arcsecond scales with adaptive optics. We are also acquiring deep K-band imaging to trace the extended halos of the galaxies and measure accurate total magnitudes. Dynamical orbit modeling of the combined data will allow us to simultaneously determine the stellar, black hole, and dark matter halo masses. The primary goals of the project are to constrain the black hole scaling relations at high masses, investigate systematically the stellar initial mass function and dark matter distribution in massive galaxies, and probe the late-time assembly of ellipticals through stellar population and kinematical gradients. In this paper, we describe the MASSIVE sample selection, discuss the distinct demographics and structural and environmental properties of the selected galaxies, and provide an overview of our basic observational program, science goals and early survey results.Comment: 19 pages, 14 figures. ApJ (2014) vol. 795, in pres

The MASSIVE Survey II: Stellar Population Trends Out to Large Radius in Massive Early Type Galaxies

We examine stellar population gradients in ~100 massive early type galaxies spanning 180 < sigma* < 370 km/s and M_K of -22.5 to -26.5 mag, observed as part of the MASSIVE survey (Ma et al. 2014). Using integral-field spectroscopy from the Mitchell Spectrograph on the 2.7m telescope at McDonald Observatory, we create stacked spectra as a function of radius for galaxies binned by their stellar velocity dispersion, stellar mass, and group richness. With excellent sampling at the highest stellar mass, we examine radial trends in stellar population properties extending to beyond twice the effective radius (~2.5 R_e). Specifically, we examine trends in age, metallicity, and abundance ratios of Mg, C, N, and Ca, and discuss the implications for star formation histories and elemental yields. At a fixed physical radius of 3-6 kpc (the likely size of the galaxy cores formed at high redshift) stellar age and [alpha/Fe] increase with increasing sigma* and depend only weakly on stellar mass, as we might expect if denser galaxies form their central cores earlier and faster. If we instead focus on 1-1.5 R_e, the trends in abundance and abundance ratio are washed out, as might be expected if the stars at large radius were accreted by smaller galaxies. Finally, we show that when controlling for \sigmastar, there are only very subtle differences in stellar population properties or gradients as a function of group richness; even at large radius internal properties matter more than environment in determining star formation history.Comment: 17 pages, 9 figures, accepted by ApJ; resubmitted with updated reference

Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study

We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The sample includes 14 galaxies covering a wide range of physical properties: morphology, total absolute magnitude, integrated color. We take advantage of the latter characteristic of the sample to check existing empirical calibrations of absolute SBF magnitudes both in the I- and V-passbands. Additionally, by comparing our SBF and color data with the Teramo-SPoT simple stellar population models, and other recent sets of population synthesis models, we discuss the feasibility of stellar population studies based on fluctuation magnitudes analysis. The main result of this study is that multiband optical SBF data and integrated colors can be used to significantly constrain the chemical composition of the dominant stellar system in the galaxy, but not the age in the case of systems older than 3 Gyr. SBF color gradients are also detected and analyzed. These SBF gradient data, together with other available data, point to the existence of mass dependent metallicity gradients in galaxies, with the more massive objects showing a non--negligible SBF versus color gradient. The comparison with models suggests that such gradients imply more metal rich stellar populations in the galaxies' inner regions with respect to the outer ones.Comment: ApJ Accepte

Star Formation at z~6: The UDF-Parallel ACS Fields

We report on the i-dropouts detected in two exceptionally deep ACS fields (B_{435}, V_{606}, i_{775}, and z_{850} with 10 sigma limits of 28.8, 29.0, 28.5, and 27.8, respectively) taken in parallel with the UDF NICMOS observations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 down to z_AB=28.1, or 1.4 i-dropouts arcmin^{-2}, with significant field-to-field variation (as expected from cosmic variance). This extends i-dropout searches some ~0.9^m further down the luminosity function than was possible in the GOODS field, netting a ~7x increase in surface density. An estimate of the size evolution for UV bright objects is obtained by comparing the composite radial flux profile of the bright i-dropouts (z<27.2) with scaled versions of the HDF-N + HDF-S U-dropouts. The best-fit is found with a (1+z)^{-1.57_{-0.53} ^{+0.50}} scaling in size (for fixed luminosity), extending lower redshift (1<z<5) trends to z~6. Adopting this scaling and the brighter i-dropouts from both GOODS fields, we make incompleteness estimates and construct a z~6 LF in the rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding a shape consistent with that found at lower redshift. To evaluate the evolution in the LF from z~3.8, we make comparisons against different scalings of a lower redshift B-dropout sample. Though a strong degeneracy is found between luminosity and density evolution, our best-fit model scales as (1+z)^{-2.8} in number and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UV luminosity density at z~6 which is just 0.38_{-0.07} ^{+0.09}x that at z~3.8. Our inclusion of size evolution makes the present estimate lower than previous z~6 estimates.Comment: 5 pages, 5 figures, accepted for publication in the Astrophysical Journal Letters, labelling to the left-hand axis of Figure 4 correcte

Initial Results from the Lightning Imaging Sensor (LIS) on the International Space Station (ISS) and Comparison with Observations from the Tropical Rainfall Measuring Mission (TRMM) LIS

No abstract availabl

The MASSIVE Survey - X. Misalignment between Kinematic and Photometric Axes and Intrinsic Shapes of Massive Early-Type Galaxies

We use spatially resolved two-dimensional stellar velocity maps over a $107"\times 107"$ field of view to investigate the kinematic features of 90 early-type galaxies above stellar mass $10^{11.5}M_\odot$ in the MASSIVE survey. We measure the misalignment angle $\Psi$ between the kinematic and photometric axes and identify local features such as velocity twists and kinematically distinct components. We find 46% of the sample to be well aligned ($\Psi < 15^{\circ}$), 33% misaligned, and 21% without detectable rotation (non-rotators). Only 24% of the sample are fast rotators, the majority of which (91%) are aligned, whereas 57% of the slow rotators are misaligned with a nearly flat distribution of $\Psi$ from $15^{\circ}$ to $90^{\circ}$. 11 galaxies have $\Psi \gtrsim 60^{\circ}$ and thus exhibit minor-axis ("prolate") rotation in which the rotation is preferentially around the photometric major axis. Kinematic misalignments occur more frequently for lower galaxy spin or denser galaxy environments. Using the observed misalignment and ellipticity distributions, we infer the intrinsic shape distribution of our sample and find that MASSIVE slow rotators are consistent with being mildly triaxial, with mean axis ratios of $b/a=0.88$ and $c/a=0.65$. In terms of local kinematic features, 51% of the sample exhibit kinematic twists of larger than $20^{\circ}$, and 2 galaxies have kinematically distinct components. The frequency of misalignment and the broad distribution of $\Psi$ reported here suggest that the most massive early-type galaxies are mildly triaxial, and that formation processes resulting in kinematically misaligned slow rotators such as gas-poor mergers occur frequently in this mass range.Comment: Accepted to MNRA

The Development of the Puerto Rico Lightning Detection Network for Meteorological Research

A land-based Puerto Rico Lightning Detection Network (PR-LDN) dedicated to the academic research of meteorological phenomena has being developed. Five Boltek StormTracker PCI-Receivers with LTS-2 Timestamp Cards with GPS and lightning detectors were integrated to Pentium III PC-workstations running the CentOS linux operating system. The Boltek detector linux driver was compiled under CentOS, modified, and thoroughly tested. These PC-workstations with integrated lightning detectors were installed at five of the University of Puerto Rico (UPR) campuses distributed around the island of PR. The PC-workstations are left on permanently in order to monitor lightning activity at all times. Each is networked to their campus network-backbone permitting quasi-instantaneous data transfer to a central server at the UPR-Bayam n campus. Information generated by each lightning detector is managed by a C-program developed by us called the LDN-client. The LDN-client maintains an open connection to the central server operating the LDN-server program where data is sent real-time for analysis and archival. The LDN-client also manages the storing of data on the PC-workstation hard disk. The LDN-server software (also an in-house effort) analyses the data from each client and performs event triangulations. Time-of-arrival (TOA) and related hybrid algorithms, lightning-type and event discriminating routines are also implemented in the LDN-server software. We also have developed software to visually monitor lightning events in real-time from all clients and the triangulated events. We are currently monitoring and studying the spatial, temporal, and type distribution of lightning strikes associated with electrical storms and tropical cyclones in the vicinity of Puerto Rico