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Feuillet D.K.,New Mexico State University | Bovy J.,University of Toronto | Holtzman J.,New Mexico State University | Girardi L.,National institute for astrophysics | And 4 more authors.
Astrophysical Journal | Year: 2016

We present a sample of 705 local giant stars observed using the New Mexico State University 1 m telescope with the Sloan Digital Sky Survey-III/Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectrograph, for which we estimate stellar ages and the local star formation history (SFH). The high-resolution (R ∼ 22,500), near infrared (1.51-1.7 μm) APOGEE spectra provide measurements of stellar atmospheric parameters (temperature, surface gravity, [M/H], and [α/M]). Due to the smaller uncertainties in surface gravity possible with high-resolution spectra and accurate Hipparcos distance measurements, we are able to calculate the stellar masses to within 30%. For giants, the relatively rapid evolution up the red giant branch allows the age to be constrained by the mass. We examine methods of estimating age using both the mass-age relation directly and a Bayesian isochrone matching of measured parameters, assuming a constant SFH. To improve the SFH prior, we use a hierarchical modeling approach to constrain the parameters of the model SFH using the age probability distribution functions of the data. The results of an α-dependent Gaussian SFH model show a clear age-[α/M] relation at all ages. Using this SFH model as the prior for an empirical Bayesian analysis, we determine ages for individual stars. The resulting age-metallicity relation is flat, with a slight decrease in [M/H] at the oldest ages and a ∼0.5 dex spread in metallicity across most ages. For stars with ages ≲1 Gyr we find a smaller spread, consistent with radial migration having a smaller effect on these young stars than on the older stars. © 2016. The American Astronomical Society. All rights reserved.

Gontcho S.G.A.,University of Barcelona | Miralda-Escude J.,University of Barcelona | Miralda-Escude J.,Catalan Institution for Research and Advanced Studies | Busca N.G.,University Paris Diderot | Busca N.G.,Laboratorio Interinstitucional Of E Astronomia Linea
Monthly Notices of the Royal Astronomical Society | Year: 2014

An analytical framework is presented to understand the effects of a fluctuating intensity of the cosmic ionizing background on the correlations of the Lyα forest transmission fraction measured in quasar spectra. In the absence of intensity fluctuations, the Lyα power spectrum should have the expected cold darkmatter power spectrumwith redshift distortions in the linear regime, with a bias factor bδ and a redshift distortion parameter β that depend on redshift but are independent of scale. The intensity fluctuations introduce a scale dependence in both bδ and β, but keeping their product bδβ fixed. Observations of the Lyα correlations and cross-correlations with radiation sources like those being done at present in the Baryon Oscillation Spectroscopic Survey of Sloan Digital Sky Survey third generation have the potential to measure this scale dependence, which reflects the biasing properties of the sources and absorbers of the ionizing background. We also compute a second term affecting the Lyα spectrum, due to shot noise in the sources of radiation. This term is very large if luminous quasars are assumed to produce the ionizing background and to emit isotropically with a constant luminosity, but should be reduced by a contribution from galaxies, and by the finite lifetime and anisotropic emission of quasars. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Lee Y.S.,Michigan State University | Beers T.C.,Michigan State University | An D.,Ewha Womans University | Ivezic Z.,University of Washington | And 10 more authors.
Astrophysical Journal | Year: 2011

We employ measurements of the [α/Fe] ratio derived from low-resolution (R 2000) spectra of 17,277 G-type dwarfs from the SEGUE survey to separate them into likely thin- and thick-disk subsamples. Both subsamples exhibit strong gradients of orbital rotational velocity with metallicity, of opposite signs, -20 to -30kms-1dex-1 for the thin-disk and +40 to +50kms-1dex-1 for the thick-disk population. The rotational velocity is uncorrelated with Galactocentric distance for the thin-disk subsample and exhibits a small trend for the thick-disk subsample. The rotational velocity decreases with distance from the plane for both disk components, with similar slopes (-9.0 1.0kms-1kpc-1). Thick-disk stars exhibit a strong trend of orbital eccentricity with metallicity (about -0.2 dex-1), while the eccentricity does not change with metallicity for the thin-disk subsample. The eccentricity is almost independent of Galactocentric radius for the thin-disk population, while a marginal gradient of the eccentricity with radius exists for the thick-disk population. Both subsamples possess similar positive gradients of eccentricity with distance from the Galactic plane. The shapes of the eccentricity distributions for the thin- and thick-disk populations are independent of distance from the plane, and include no significant numbers of stars with eccentricity above 0.6. Among several contemporary models of disk evolution that we consider, radial migration appears to have played an important role in the evolution of the thin-disk population, but possibly less so for the thick disk, relative to the gas-rich merger or disk heating scenarios. We emphasize that more physically realistic models and simulations need to be constructed in order to carry out the detailed quantitative comparisons that our new data enable. © 2011. The American Astronomical Society. All rights reserved.

Moraes B.,Brazilian Center for Research in Physics (CBPF) | Moraes B.,Laboratorio Interinstitucional Of E Astronomia Linea | Polarski D.,Montpellier University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We derive some basic equations related to the redshift drift, and we show how some dark energy (DE) properties can be retrieved from it. We consider, in particular, three kinds of DE models which exhibit a characteristic signature in their redshift drift while no such signature would be present in their luminosity-distances: a sudden change of the equation of state parameter w DE at low redshifts, oscillating DE, and finally an equation of state with spikes at low redshifts. Accurate redshift-drift measurements would provide interesting complementary probes for some of these models and for models with varying gravitational coupling. While the redshift drift would efficiently constrain models with a spike at z∼1, the signature of the redshift drift for models with large variations at very low redshifts z<0.1 would be unobservable, allowing a large arbitrariness in the present expansion of the Universe. © 2011 American Physical Society.

Schlesinger K.J.,Australian National University | Schlesinger K.J.,University of California at Santa Cruz | Schlesinger K.J.,Ohio State University | Johnson J.A.,Ohio State University | And 21 more authors.
Astrophysical Journal | Year: 2012

We present the metallicity distribution function (MDF) for 24,270 G and 16,847 K dwarfs at distances from 0.2 to 2.3 kpc from the Galactic plane, based on spectroscopy from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. This stellar sample is significantly larger in both number and volume than previous spectroscopic analyses, which were limited to the solar vicinity, making it ideal for comparison with local volume-limited samples and Galactic models. For the first time, we have corrected the MDF for the various observational biases introduced by the SEGUE target-selection strategy. SEGUE is particularly notable for its sample of K dwarfs, which are too faint to examine spectroscopically far from the solar neighborhood. The MDF of both spectral types becomes more metal-poor with increasing |Z|, which reflects the transition from a sample with small [α/Fe] values at small heights to one with enhanced [α/Fe] above 1 kpc. Comparison of our SEGUE distributions to those of two different Milky Way models reveals that both are more metal-rich than our observed distributions at all heights above the plane. Our unbiased observations of G and K dwarfs provide valuable constraints over the |Z|-height range of the Milky Way disk for chemical and dynamical Galaxy evolution models, previously only calibrated to the solar neighborhood, with particular utility for thin- and thick-disk formation models. © 2012. The American Astronomical Society. All rights reserved..

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