Reduction-Based Creative Telescoping for Algebraic Functions

Continuing a series of articles in the past few years on creative telescoping using reductions, we develop a new algorithm to construct minimal telescopers for algebraic functions. This algorithm is based on Trager's Hermite reduction and on polynomial reduction, which was originally designed for hyperexponential functions and extended to the algebraic case in this paper

On the Age and Metallicity Estimation of Spiral Galaxies Using Optical and Near-Infrared Photometry

In integrated-light, some color-color diagrams that use optical and near-infrared photometry show surprisingly orthogonal grids as age and metallicity are varied, and they are coming into common usage for estimating the average age and metallicity of spiral galaxies. In this paper we reconstruct these composite grids using simple stellar population models from several different groups convolved with some plausible functional forms of star formation histories at fixed metallicity. We find that the youngest populations present (t<2 Gyr) dominate the light, and because of their presence the age-metallicity degeneracy can be partially broken with broad-band colors, unlike older populations. The scatter among simple stellar population models by different authors is, however, large at ages t<2 Gyr. The dominant uncertainties in stellar population models arise from convective core overshoot assumptions and the treatment of the thermally pulsing asymptotic giant branch phase and helium abundance may play a significant role at higher metallicities. Real spiral galaxies are unlikely to have smooth, exponential star formation histories, and burstiness will cause a partial reversion to the single-burst case, which has even larger model-to-model scatter. Finally, it is emphasized that the current composite stellar population models need some implementation of chemical enrichment histories for the proper analysis of the observational data.Comment: 33 pages, 15 figures. Accepted to ApJ (Apr 2007). The major surgery was Fig. 1

A Search for Environmental Effects on Type Ia Supernovae

We use integrated colors and B and V absolute magnitudes of Type Ia supernova (SN) host galaxies in order to search for environmental effects on the SN optical properties. With the new sample of 44 SNe we confirm the conclusion by Hamuy et al. (1996a) that bright events occur preferentially in young stellar environments. We find also that the brightest SNe occur in the least luminous galaxies, a possible indication that metal-poorer neighbourhoods produce the more luminous events. The interpretation of these results is made difficult, however, due to the fact that galaxies with younger stellar populations are also lower in luminosity. In an attempt to remove this ambiguity we use models for the line strengths in the absorption spectrum of five early-type galaxies, in order to estimate metallicities and ages of the SN host galaxies. With the addition of abundance estimates from nebular analysis of the emission spectra of three spiral galaxies, we find possible further evidence that luminous SNe are produced in metal-poor neighborhoods. Further spectroscopic observations of the SN host galaxies will be necessary to test these results and assist in disentangling the age/metallicity effects on Type Ia SNe.Comment: 14 pages, 5 figures, to appear in the September 2000 issue of The Astronomical Journa

Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN

We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black–white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated

The stellar population histories of early-type galaxies. III. The Coma Cluster

We present stellar population parameters of twelve early-type galaxies (ETGs) in the Coma Cluster based on spectra obtained using the Low Resolution Imaging Spectrograph on the Keck II Telescope. Our data allow us to examine in detail the zero-point and scatter in their stellar population properties. Our ETGs have SSP-equivalent ages of on average 5-8 Gyr with the models used here, with the oldest galaxies having ages of ~10 Gyr old. This average age is identical to the mean age of field ETGs. Our ETGs span a large range in velocity dispersion but are consistent with being drawn from a population with a single age. Specifically, ten of the twelve ETGs are consistent within their formal errors of having the same age, 5.2+/-0.2 Gyr, over a factor of more than 750 in mass. We therefore find no evidence for downsizing of the stellar populations of ETGs in the core of the Coma Cluster. We suggest that Coma Cluster ETGs may have formed the majority of their mass at high redshifts but suffered small but detectable star formation events at z~0.1-0.3. Previous detections of 'downsizing' from stellar populations of local ETGs may not reflect the same downsizing seen in lookback studies of RSGs, as the young ages of the local ETGs represent only a small fraction of their total masses. (abridged)Comment: 49 pages, 20 figures (19 EPS, 1 JPEG). MNRAS, in press. For version with full resolution of Fig. 1 see http://www.astro.rug.nl/~sctrager/coma.pdf; for Table 2, see http://www.astro.rug.nl/~sctrager/coma_table2.pdf; for Table B3, see http://www.astro.rug.nl/~sctrager/coma_tableB3.pd

Carbon stars in the X-shooter Spectral Library

We provide a new collection of spectra of 35 carbon stars obtained with the ESO/VLT X-shooter instrument as part of the X-shooter Spectral Library project. The spectra extend from 0.3$\mu$m to 2.4$\mu$m with a resolving power above $\sim$ 8000. The sample contains stars with a broad range of (J-K) color and pulsation properties located in the Milky Way and the Magellanic Clouds. We show that the distribution of spectral properties of carbon stars at a given (J-K) color becomes bimodal (in our sample) when (J-K) is larger than about 1.5. We describe the two families of spectra that emerge, characterized by the presence or absence of the absorption feature at 1.53$\mu$m, generally associated with HCN and C$_2$H$_2$. This feature appears essentially only in large-amplitude variables, though not in all observations. Associated spectral signatures that we interpret as the result of veiling by circumstellar matter, indicate that the 1.53$\mu$m feature might point to episodes of dust production in carbon-rich Miras.Comment: 29 pages, 21 figures, 9 tables, Accepted for publication in A&

The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations

We analyze single-stellar-population (SSP) equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. These galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log $\sigma$, and [E/Fe]. SSP age and velocity dispersion can be taken as the two independent parameters that specify a galaxy's location in this ``hyperplane.'' The hyperplane can be decomposed into two sub-relations: (1) a ``Z-plane,'' in which [Z/H] is a linear function of log $\sigma$ and log t; and (2) a relation between [E/Fe] and $\sigma$ in which [E/Fe] is larger in high-$\sigma$ galaxies. Cluster and field ellipticals follow the same hyperplane, but their ($\sigma$,t) distributions within it differ. Nearly all cluster galaxies are old; the field ellipticals span a large range in SSP age. The tight Mg--$\sigma$ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on; the tightness of these relations does not necessarily imply a narrow range of ages at fixed $\sigma$. The relation between [E/Fe] and $\sigma$ is consistent with a higher effective yield of Type II SNe elements at higher $\sigma$. The Z-plane is harder to explain and may be a powerful clue to star formation in elliptical galaxies if it proves to be general. Present data favor a ``frosting'' model in which low apparent SSP ages are produced by adding a small frosting of younger stars to an older base population. If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant $\sigma$ in the Z-plane, as required. This favors star formation from well-mixed pre-enriched gas rather than unmixed low-metallicity gas from an accreted object. (Abridged)Comment: To be published in the June 2000 issue of the Astronomical Journal. 28 pages, 13 figures, uses emulateap

Radially extended kinematics and stellar populations of the massive ellipticals NGC1600, NGC4125 and NGC7619. Constraints on the outer dark halo density profile

We present high quality long slit spectra along the major and minor axes out to 1.5-2 Re (14-22 kpc) of three bright elliptical galaxies (NGC1600, NGC4125, NGC7619) obtained at the Hobby-Eberly Telescope (HET). We derive stellar kinematic profiles and Lick/IDS indices (Hbeta, Mgb, Fe5015, Fe5270, Fe5335, Fe5406). Moreover, for NGC4125 we derive gas kinematics and emission line strengths. We model the absorption line strengths using Simple Stellar Populations models that take into account the variation of [\alpha/Fe] and derive ages, total metallicity and element abundances. Overall, we find that the three galaxies have old and [\alpha/Fe] overabundant stellar populations with no significant gradients. The metallicity is supersolar at the center with a strong negative radial gradient. For NGC4125, several pieces of evidence point to a recent dissipational merger event. We calculate the broad band color profiles with the help of SSP models. All of the colors show sharp peaks at the center of the galaxies, mainly caused by the metallicity gradients, and agree well with the measured colors. Using the Schwarzschild's axisymmetric orbit superposition technique, we model the stellar kinematics to constrain the dark halos of the galaxies. We use the tight correlation between the Mgb strength and local escape velocity to set limits on the extent of the halos by testing different halo sizes. Logarithmic halos - cut at 60 kpc -minimize the overall scatter of the Mgb-Vesc relation. Larger cutoff radii are found if the dark matter density profile is decreasing more steeply at large radii.Comment: Accepted for publication in Astronomy and Astrophysic