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Junqueira T.C.,Leibniz Institute for Astrophysics Potsdam | Chiappini C.,Leibniz Institute for Astrophysics Potsdam | Lepine J.R.D.,University of Sao Paulo | Minchev I.,Leibniz Institute for Astrophysics Potsdam | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

In the last few decades, many efforts have been made to understand the effect of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of the fundamental parameters of the spiral structure is its angular velocity, or pattern speed Ωp, which determines the location of resonances in the disc and the spirals' radial extent. The most direct method for estimating the pattern speed relies on backward integration techniques, trying to locate the stellar birthplace of open clusters. Here, we propose a new method based on the interaction between the spiral arms and the stars in the disc. Using a sample of around 500 open clusters from the New Catalogue of Optically Visible Open Clusters and Candidates, and a sample of 500 giant stars observed by Apache Point Observatory Galactic Evolution Experiment, we find Ωp = 23.0 ± 0.5 kms-1 kpc-1, for a local standard of rest rotation V0 = 220 km s-1 and solar radius R0 = 8.0 kpc. Exploring a range in V0 and R0 within the acceptable values, 200-240 km s-1 and 7.5-8.5 kpc, respectively, results only in a small change in our estimate of Ωp, that is within the error. Our result is in close agreement with a number of studies which suggest values in the range 20-25 km s-1 kpc-1. An advantage of our method is that we do not need knowledge of the stellar age, unlike in the case of the birthplace method, which allows us to use data from large Galactic surveys. The precision of our method will be improved once larger samples of disc stars with spectroscopic information will become available thanks to future surveys such as 4MOST. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kupper A.H.W.,Columbia University | Balbinot E.,University of Surrey | Bonaca A.,Yale University | Johnston K.V.,Columbia University | And 4 more authors.
Astrophysical Journal | Year: 2015

Using the example of the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), we demonstrate how observational data on tidal streams can be efficiently reduced in dimensionality and modeled in a Bayesian framework. Our approach combines detection of stream overdensities by a Difference-of-Gaussians process with fast streakline models of globular cluster streams and a continuous likelihood function built from these models. Inference is performed with Markov chain Monte Carlo. By generating ≈107 model streams, we show that the unique geometry of the Pal 5 debris yields powerful constraints on the solar position and motion, the Milky Way and Pal 5 itself. All 10 model parameters were allowed to vary over large ranges without additional prior information. Using only readily available SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is 8.30 ± 0.25 kpc, and the transverse velocity is 253 ± 16 km s-1. Both estimates are in excellent agreement with independent measurements of these two quantities. Assuming a standard disk and bulge model, we determine the Galactic mass within Pal 5s apogalactic radius of 19 kpc to be (2.1±0.4)× 1011M⊙. Moreover, we find the potential of the dark halo with a flattening of = - q 0.95+ z 0.12 0.16 to be essentially spherical-at least within the radial range that is effectively probed by Pal 5. We also determine Pal 5s mass, distance, and proper motion independently from other methods, which enables us to perform vital crosschecks. Our inferred heliocentric distance of Pal 5 is - 23.6+0.7 0.8 kpc, in perfect agreement with, and more precise than, estimates from isochrone fitting of deep Hubble Space Telescope (HST) imaging data. We conclude that finding and modeling more globular cluster streams is an efficient way to map out the structure of our Galaxy to high precision. With more observational data and by using additional prior information, the precision of this mapping can be significantly increased © 2015. The American Astronomical Society. All rights reserved.

Dumet-Montoya H.S.,Brazilian Center for Research in Physics (CBPF) | Dumet-Montoya H.S.,Laboratorio Interinstitucional Of E Astronomia Linea | Caminha G.B.,Brazilian Center for Research in Physics (CBPF) | Caminha G.B.,Laboratorio Interinstitucional Of E Astronomia Linea | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Owing to their computational simplicity, models with elliptical potentials (pseudo-elliptical) are often used in gravitational lensing applications, in particular for mass modeling using arcs and for arc statistics. However, these models generally lead to negative mass distributions in some regions and to dumbbell-shaped surface density contours for high ellipticities. Aims. We revisit the physical limitations of the pseudo-elliptical Navarro-Frenk-White (PNFW) model, focusing on the behavior of the mass distribution close to the tangential critical curve, where tangential arcs are expected to be formed. We investigate the shape of the mass distribution on this region and the presence of negative convergence. We obtain a mapping from the PNFW to the NFW model with elliptical mass distribution (ENFW). We compare the arc cross section for both models, aiming to determine a domain of validity for the PNFW model in terms of its mass distribution and for the cross section. Methods. We defined a figure of merit to i) measure the deviation of the iso-convergence contours of the PNFW model to an elliptical shape, ii) assigned an ellipticity ε Σ to these contours, iii) defined a corresponding iso-convergence contour for the ENFW model. We computed the arc cross section using the "infinitesimal circular source approximation". Results. We extend previous work by investigating the shape of the mass distribution of the PNFW model for a broad range of the potential ellipticity parameter ε and characteristic convergence K S ℓ. We show that the maximum value of ε to avoid dumbbell-shaped mass distributions is explicitly dependent on K S ℓ, with higher ellipticities (ε ≈ 0.5, i.e., ε Σ ≈ 0.65) allowed for small K S ℓ. We determine a relation between the ellipticity of the mass distribution ε Σ and ε valid for any ellipticity. We also derive the relation of characteristic convergences, obtaining a complete mapping from PNFW to ENFW models, and provide fitting formulae for connecting the parameters of both models. Using this mapping, the cross sections for both models are compared, setting additional constraints on the parameter space of the PNFW model such that it reproduces the ENFW results. We also find that the negative convergence regions occur far from the arc formation region and should therefore not be a problem for studies with gravitational arcs. Conclusions. We conclude that the PNFW model is well-suited to model an elliptical mass distribution on a larger ε-K S ℓ parameter space than previously expected. However, if we require the PNFW model to reproduce the arc cross section of the ENFW well, the ellipticity is more restricted, particularly for low K S ℓ. The determination of a domain of validity for the PNFW model and the mapping to ENFW models could have implications for the use of PNFW models for the inverse modeling of lenses and for fast arc simulations, for example. © 2012 ESO.

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.

Balbinot E.,Federal University of Rio Grande do Sul | Balbinot E.,Laboratorio Interinstitucional Of E Astronomia Linea | Santiago B.X.,Federal University of Rio Grande do Sul | Santiago B.X.,Laboratorio Interinstitucional Of E Astronomia Linea | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present an implementation of the matched-filter technique to detect tidal tails of globular clusters. The method was tested using the Sloan Digital Sky Survey (SDSS) data for the globular cluster Palomar 5 revealing its well-known tidal tails. We also ran a simulation of a globular cluster with a tidal tail where we successfully recover the tails for a cluster at the same position and with the same characteristics of NGC 2298. Based on the simulation we estimate that the matched-filter increases the contrast of the tail relative to the background of stars by a factor of 2.5 for the case of NGC 2298. We also present the photometry of the globular cluster NGC 2298 using the MOSAIC2 camera installed on the Cerro Tololo International Observatory (CTIO) 4-m telescope. The photometry covers ~3deg2 reaching V~ 23. A fit of a King profile to the radial density profile of NGC 2298 shows that this cluster has a tidal radius of which is twice as in the literature. The application of the matched-filter to NGC 2298 reveals several extra-tidal structures, including a leading and trailing tail. We also find that NGC 2298 has extra-tidal structures stretching towards and against the Galactic disc, suggesting strong tidal interaction. Finally, we assess how the matched-filter performs when applied to a globular cluster with and without mass segregation taken into account. We find that disregarding the effects of mass segregation may significantly reduce the detection limit of the matched-filter. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Balbinot E.,Federal University of Rio Grande do Sul | Balbinot E.,Laboratorio Interinstitucional Of E Astronomia Linea | Santiago B.X.,Federal University of Rio Grande do Sul | Santiago B.X.,Laboratorio Interinstitucional Of E Astronomia Linea | And 10 more authors.
Astrophysical Journal | Year: 2013

We report on the discovery of a new Milky Way (MW) companion stellar system located at (αJ 2000, θJ 2000) = (22h10m43. s15, 14°56′58″8). The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using CanadaFranceHawaii-Telescope/ MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of 31.9+1.0?1.6 kpc, with a half-light radius of rh = 7.24+1.94?1.29 pc and a concentration parameter of c = log10 (rt/rc) = 1.55. A systematic isochrone fit to its colormagnitude diagram resulted in log (age yr?1) = 10.07+0.05 ?0.03 and [Fe/H] = ?1.58+0.08 ?0.13. These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 95±6 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of MV = ?1.21±0.66. The resulting surface brightness is μV = 25.90 mag arcsec?2. Its position in the MV versus rh diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star clusterone of the faintest and lowest mass systems yet identified. © 2013. The American Astronomical Society. All rights reserved.

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.

de Simoni F.,Federal University of Fluminense | Sobreira F.,Laboratorio Interinstitucional Of E Astronomia Linea | Carnero A.,Laboratorio Interinstitucional Of E Astronomia Linea | Ross A.J.,University of Portsmouth | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We analyse the large-scale angular correlation function (ACF) of the CMASS luminous galaxies (LGs), a photometric-redshift catalogue based on the Data Release 8 (DR8) of the Sloan Digital Sky Survey-III. This catalogue contains over 600 000 LGs in the range 0.45 ≤ z ≤ 0.65, which was split into four redshift shells of constant width. First, we estimate the constraints on the redshift-space distortion (RSD) parameters bσ8 and fσ8, where b is the galaxy bias, f the growth rate and σ8 is the normalization of the perturbations, finding that they vary appreciably among different redshift shells, in agreement with previous results using DR7 data. When assuming constant RSD parameters over the survey redshift range, we obtain fσ8 = 0.69 ± 0.21, which agrees at the 1.5σ level with Baryon Oscillation Spectroscopic Survey DR9 spectroscopic results. Next, we performed two cosmological analyses, where relevant parameters not fitted were kept fixed at their fiducial values. In the first analysis, we extracted the baryon acoustic oscillation peak position for the four redshift shells, and combined with the sound horizon scale from 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) to produce the constraints ωm = 0.249 ± 0.031 and w = -0.885 ± 0.145. In the second analysis, we used the ACF full shape information to constrain cosmology using real data for the first time, findingωm = 0.280 ± 0.022 and fb = ωb/ωm = 0.211 ± 0.026. These results are in good agreement with WMAP7 findings, showing that the ACF can be efficiently applied to constrain cosmology in future photometric galaxy surveys. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Girardi L.,National institute for astrophysics | Girardi L.,Laboratorio Interinstitucional Of E Astronomia Linea
Annual Review of Astronomy and Astrophysics | Year: 2016

Low-mass stars in their core-helium-burning stage define the sharpest feature present in the color-magnitude diagrams of nearby galaxy systems: the red clump (RC). This feature has given rise to a series of methods aimed at measuring the distributions of stellar distances and extinctions, especially in the Magellanic Clouds and Milky Way Bulge. Because the RC is easily recognizable within the data of large spectroscopic and asteroseismic surveys, it is a useful probe of stellar densities, kinematics, and chemical abundances across the Milky Way disk; it can be applied up to larger distances than that allowed by dwarfs; and it has better accuracy than is possible with other kinds of giants. Here, we discuss the reasons for the RC narrowness in several sets of observational data, its fine structure, and the presence of systematic changes in the RC properties as regards age, metallicity, and the observed passband. These factors set the limits on the validity and accuracy of several RC methods defined in the literature. © 2016 by Annual Reviews.

Girardi L.,National institute for astrophysics | Girardi L.,Laboratorio Interinstitucional Of E Astronomia Linea
Astronomische Nachrichten | Year: 2016

We briefly describe the simulation of stellar populations in the Milky Way by means of the TRIdimensional modeL of thE GALaxy (TRILEGAL) code. Among the many possible uses of this kind of code, we emphasize their role for improving stellar evolution models. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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