[Show abstract][Hide abstract] ABSTRACT: We measure the spatial density of F turnoff stars in the Sagittarius dwarf
tidal stream, from Sloan Digital Sky Survey (SDSS) data, using statistical
photometric parallax. We find a set of continuous, consistent parameters that
describe the leading Sgr stream's position, direction, and width for 15 stripes
in the North Galactic Cap, and 3 stripes in the South Galactic Cap. We produce
a catalog of stars that has the density characteristics of the dominant leading
Sgr tidal stream that can be compared with simulations. We find that the width
of the leading (North) tidal tail is consistent with recent triaxial and
axisymmetric halo model simulations. The density along the stream is roughly
consistent common disruption models in the North, but possibly not in the
South. We explore the possibility that one or more of the dominant Sgr streams
has been mis-identified, and that one or more of the `bifurcated' pieces is the
real Sgr tidal tail, but we do not reach definite conclusions. If two dwarf
progenitors are assumed, fits to the planes of the dominant and `bifurcated'
tidal tails favor an association of the Sgr dwarf spheroidal galaxy with the
dominant Southern stream and the `bifurcated' stream in the North. In the North
Galactic Cap, the best fit Hernquist density profile for the smooth component
of the stellar halo is oblate, with a flattening parameter q = 0.53, and a
scale length of r_0 = 6.73. The Southern data for both the tidal debris and the
smooth component of the stellar halo do not match the model fits to the North,
although the stellar halo is still overwhelmingly oblate. Finally, we verify
that we can reproduce the parameter fits on the asynchronous Milkyway@home
volunteer computing platform.
The Astronomical Journal 04/2013; 145(6). DOI:10.1088/0004-6256/145/6/163 · 4.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In statistical photometric parallax, the knowledge of the distribution
of absolute magnitudes of a population of stars is used to statistically
determine the density distribution of that population, even though the
distance to each individual star cannot be determined. We use a maximum
likelihood technique and the 0.5 PetaFLOPS MilkyWay@home volunteer
computing platform to calculate the density of the leading tidal tail of
the Sagittarius dwarf Galaxy using Sloan Digital Sky Survey data from
Data Release 7. While the width and position of the leading tidal, as a
function of angular distance for the Sgr dwarf, are a good match to the
Law & Majewski (2010) triaxial halo model, there is some deviation
in the density along the leading tidal tail. We compare the orbital
planes of the dominant leading and trailing tidal tails of Sgr, as well
as the orbital planes of the bifurcated pieces, and conclude that it is
not obvious which or how many of these tidal streams is associated with
the Sgr dwarf galaxy. This research was supported by NSF grant AST
[Show abstract][Hide abstract] ABSTRACT: F turnoff stars are important tools for studying Galactic halo substructure
because they are plentiful, luminous, and can be easily selected by their
photometric colors from large surveys such as the Sloan Digital Sky Survey
(SDSS). We describe the absolute magnitude distribution of color-selected F
turnoff stars, as measured from SDSS data, for eleven globular clusters in the
Milky Way halo. We find that the M_g distribution of turnoff stars is
intrinsically the same for all clusters studied, and is well fit by two half
Gaussian functions, centered at mu=4.18, with a bright-side sigma=0.36, and
with a faint-side sigma=0.76. However, the color errors and detection
efficiencies cause the observed sigma of the faint-side Gaussian to change with
magnitude due to contamination from redder main sequence stars (40% at 21st
magnitude). We present a function that will correct for this
magnitude-dependent change in selected stellar populations, when calculating
stellar density from color-selected turnoff stars. We also present a consistent
set of distances, ages and metallicities for eleven clusters in the SDSS Data
Release 7. We calculate a linear correction function to Padova isochrones so
that they are consistent with SDSS globular cluster data from previous papers.
We show that our cluster population falls along the Milky Way Age-Metallicity
Relationship (AMR), and further find that isochrones for stellar populations on
the AMR have very similar turnoffs; increasing metallicity and decreasing age
conspire to produce similar turnoff magnitudes and colors for all old clusters
that lie on the AMR.
The Astrophysical Journal 11/2011; 743(2). DOI:10.1088/0004-637X/743/2/187 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent studies of the Milky Way halo have shown that there are many tidal debris streams and other substructures that can be detected from the spatial distributions of halo stars. We are attempting to describe the spatial structure through maximum likelihood fitting of a smoothly varying component plus a set of additional components that describe the tidal debris. The Sagittarius tidal debris stream in particular is modeled by a set of piecewise linear cylinders with a density that falls of as a Gaussian from the central axis of the cylinder. We show the highest likelihood fit to the density of SDSS F turnoff stars along the Sagittarius stream, and the results of a test of the sensitivity of the likelihood fits to the angle between the stream direction and the angle at which the data is sliced into pieces. This research was funded by NSF REU supplements to AST 06-07618, SEI(AST) 06-12213, and the NASA/NY Space Grant.
[Show abstract][Hide abstract] ABSTRACT: Modern surveys are producing enormous amounts of data that can only be navigated via the use of the ever increasing computational resources available. For example, the SDSS has taken a large amount of photometric data that can be used to discover and study substructure in the Galactic spheroid. A maximum likelihood method was developed and applied to color-selected F turnoff stars from two stripes of SDSS data, to determine the spatial characteristics of the Sagittarius dwarf tidal debris that exists within these stripes. The Sagittarius tidal debris in stripes 79 and 86 were detected at the positions (l,b,R) = (163.311 °, -48.400 °, 30.23 kpc) and (l,b,R) = (34.775 °, -72.342 °, 26.08 kpc) and were found to have a FWHM of 6.53±0.54 kpc and 5.71±0.26 kpc and also to contain ≈9,500 and ≈16,700 F turnoff stars, respectively. The debris pieces are axially aligned with the directions (^X,^Y,^Z) = (0.758 kpc, 0.254 kpc, -0.600 kpc) and (^X,^Y,^Z) = (0.982 kpc, 0.084 kpc, 0.167 kpc), respectively. The results of probabilistically separating the tidal debris from the stellar spheroid are also presented.
[Show abstract][Hide abstract] ABSTRACT: Volunteer computing grids offer significant computing power at relatively low cost to researchers, while at the same time generating public interest in different scientific projects. However, in order to be used effectively, their heterogeneity, volatility and restrictive computing models must be overcome. As these computing grids are open, incorrect or malicious results must also be handled. This paper examines extending the BOINC volunteer computing framework to allow for asynchronous global optimization as applied to scientific computing problems. The asynchronous optimization method used is resilient to faults and the heterogeneous nature of volunteer computing grids, while allowing scalability to tens of thousands of hosts. A work verification strategy that does not require the validation of every result is presented. This is shown to be able to effectively reduce the need for verification done to less than 30% of the reported results, without degrading the performance of the asynchronous search methods. An asynchronous version of particle swarm optimization (APSO) is presented and com- pared to previously used asynchronous genetic search (AGS) using the MilkyWay@Home BOINC computing project. Both search methods are shown to scale to MilkyWay@Home's current user base, over 75,000 heterogeneous and volatile hosts, something not possible for traditional optimization methods. APSO is shown to provide faster convergence to optimal results while being less sensitive to its search parameters. The verification strategy presented is shown to be effective for both AGS and APSO.
Fifth International Conference on e-Science, e-Science 2009, 9-11 December 2009, Oxford, UK; 01/2009
[Show abstract][Hide abstract] ABSTRACT: We present spatial properties of the Sagittarius dwarf tidal stream at a
number of points across the sky. Using a proven maximum likelihood
method for characterizing the properties of tidal debris, we calculate
the spatial best-fit parameters of tidal debris, as well as the Galactic
stellar spheroid, within a number of small (2.5 degree wide stripes)
volumes. Using stars from SDSS DR6 having the colors of blue F turnoff
stars with reddening corrected color values 0.1 < (g-r) < 0.3, we
are able to determine the position, orientation, size, and number of
stars within the tidal debris for each volume, and compare to the number
of similar stars in the Sgr core. Utilizing a probabilistic separation
technique, we are able to extract the Sgr tidal debris from the stellar
spheroid. This extraction allows for the separate study of both the
debris and remaining spheroid stars. The separation also facilitates the
study of the Galactic potential, and ultimately the dark matter halo, as
the Sgr stream provides a good means of constraining models used in
N-body simulations of tidal disruption. This research is supported by
the National Science Foundation under grant SEI(AST) 06-12213, with
additional support from NASA New York Space Grant and John A. Huberty.
[Show abstract][Hide abstract] ABSTRACT: We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u, g, r, i, z photometry to study Milky Way halo substructure in the area around the north Galactic cap. A simple color cut (g - r < 0.4) reveals the tidal stream of the Sagittarius dwarf spheroidal galaxy, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB composite image created from three magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.
The Astrophysical Journal 12/2008; 642(2):L137. DOI:10.1086/504797 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a maximum likelihood method for determining the spatial properties of tidal debris and of the Galactic spheroid. With this method we characterize Sagittarius debris using stars with the colors of blue F turnoff stars in SDSS stripe 82. The debris is located at (α ,δ ,R) = (31.37°± 0.26°,0.0°,29.22 ± 0.20 kpc) , with a (spatial) direction given by the unit vector (− 0.991 ± 0.007 kpc ,0.042 ± 0.033 kpc ,0.127 ± 0.046 kpc) , in galactocentric Cartesian coordinates, and with FWHM = 6.74 ± 0.06 kpc. This 2.5° wide stripe contains 0.9% as many F turnoff stars as the current Sagittarius dwarf galaxy. Over small spatial extent, the debris is modeled as a cylinder with a density that falls off as a Gaussian with distance from the axis, while the smooth component of the spheroid is modeled with a Hernquist profile. We assume that the absolute magnitude of F turnoff stars is distributed as a Gaussian, which is an improvement over previous methods which fixed the absolute magnitude at . The effectiveness and correctness of the algorithm is demonstrated on a simulated set of F turnoff stars created to mimic SDSS stripe 82 data, which shows that we have a much greater accuracy than previous studies. Our algorithm can be applied to divide the stellar data into two catalogs: one which fits the stream density profile and one with the characteristics of the spheroid. This allows us to effectively separate tidal debris from the spheroid population, both facilitating the study of the tidal stream dynamics and providing a test of whether a smooth spheroidal population exists.
The Astrophysical Journal 12/2008; 683(2):750. DOI:10.1086/589681 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Large scale surveys are providing vast amounts of data that can help us understand and study tidal debris more easily and accurately. A maximum likelihood method for determining the spatial properties of this tidal debris and the stellar Galactic spheroid has been developed to take advantage of these huge datasets. We present the results of studying the Sagittarius dwarf tidal stream in two SDSS stripes taken in the southern Galactic Cap using this method. This study was done using stars with the colors of blue F turnoff stars in SDSS. We detected Sagittarius debris at the positions (l,b,R) = (163.311°,-48.400°,30.23 kpc) and (l,b,R) = (34.775°,-72.342°,26.08 kpc). These debris pieces were found to have a FWHM of 6.53+/-0.54 kpc and 5.71+/-0.26 kpc and also to contain ~9,500 and ~16,700 F turnoff stars, respectively. The debris pieces were also found to have (spatial) directions of (Xˆ,Yˆ,Zˆ) = (0.758,0.254,-0.600) and (Xˆ,Yˆ,Zˆ) = (0.982,0.945,0.167), respectively. Using the results of the algorithm, we have also probabilistically separated the tidal debris from the stellar spheroid and present those results as well.
[Show abstract][Hide abstract] ABSTRACT: This paper presents GMLE 1, a generic and distributed framework for maximum likelihood evaluation. GMLE is currently being applied to astroinformatics for determining the shape of star streams in the Milky Way galaxy, and to particle physics in a search for theory-predicted but yet unobserved sub-atomic particles. GMLE is designed to enable parallel and distributed executions on platforms ranging from supercomputers and high-performance homogeneous computing clusters to more heterogeneous Grid and Internet computing environments. GMLE's modular implementation seperates concerns of developers into the distributed evaluation frameworks, scientific models, and search methods, which interact through a simple API. This allows us to compare the benefits and drawbacks of different scientific models using different search methods on different computing environments. We describe and compare the performance of two implementations of the GMLE framework: an MPI version that more effectively uses homogeneous environments such as IBM's BlueGene, and a SALSA version that more easily accommodates heterogeneous environments such as the Rensselaer Grid. We have shown GMLE to scale well in terms of computation as well as communication over a wide range of environments. We expect that scientific computing frameworks, such as GMLE, will help bridge the gap between scientists needing to analyze ever larger amounts of data and ever more complex distributed computing environments. 1This work has been partially supported by the following grants: NSF AST No. 0607618, NSF IIS No. 0612213, NSF MRI No. 0420703 and NSF CAREER CNS Award No. 0448407. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
[Show abstract][Hide abstract] ABSTRACT: F-turnoff stars have been used to discover structure in the Milky Way spheroid, including the Sgr dwarf tidal stream, and are being used to trace the distribution of stars in the smooth component of the spheroid. Some studies have assumed a single average absolute magnitude for all spheroid stars with SDSS colors of 0.1 < (g-r) < 0.3 and others have assumed a Gaussian distribution of absolute magnitudes. This is particularly important as we use more sophisticated methods, including maximum likelihood methods to measure model parameters more precisely. The absolute magnitudes of spheroid stars from the Besançon Galactic model were studied to produce an absolute magnitude histogram reflecting the brightness distribution for F-turnoff stars in that model. Comparison of the histogram with the assumed distribution shows that, at least for the model stars, absolute magnitude is not sufficiently described by a simple Gaussian curve. Rather, the model shows a linear increase from 3.4 in absolute magnitude to the modal value, 5.7, and falls off quickly to 6.2. To test the accuracy of this result, data from the Sloan Digital Sky Survey were used to create histograms for ten globular clusters belonging to the spheroid. Qualitative comparisons show that the globular cluster absolute magnitude distributions resemble the currently assumed Gaussian distribution more closely than that obtained from the Besançon model. Furthermore, preliminary data suggest that the globular cluster histograms can be adequately described by a skewed Gaussian distribution peaking at 4.6 with corresponding left- and right-side standard deviations 0.6 and 1.0. This project was funded by the National Science Foundation under grant SEI(AST)- 0612213.
[Show abstract][Hide abstract] ABSTRACT: We present spatial positions of stars in the leading and trailing tidal tails of the Sagittarius dwarf galaxy debris. We show that the Sgr tidal tails do not explain the overdensity of stars in the direction of what has been named the Virgo Stellar Stream. The Virgo Stellar Stream is probably gravitationally unbound debris from another dwarf galaxy disruption. It covers 300 sq. deg. of sky, with a diameter of 10 kpc and a stellar mass of roughly 5 x 105 MSun. We demonstrate a new technique of using the color of a stellar population's turnoff to distinguish between structures which may overlap in space. This technique allowed us to clearly separate a new tidal debris stream that is slightly closer, has a bluer turnoff, and is much smaller than the Monoceros stream in the Galactic plane. Funding from the National Science Foundation AST 03-07571, AST 06-07618, and the NASA Space Grant.
[Show abstract][Hide abstract] ABSTRACT: We use Sloan Digital Sky Survey Data Release 5 photometry and spectroscopy to study a tidal stream that extends over 50 degrees in the North Galactic Cap. From the analysis of the path of the stream and the colors and magnitudes of its stars, the stream is about 20 kpc away at its nearest detection (the celestial equator). We detect a distance gradient -- the stream is farther away from us at higher declination. The contents of the stream are made up from a predominantly old and metal-poor population that is similar to the globular clusters M13 and M92. The integrated absolute magnitude of the stream stars is estimated to be M_r = -7.5. There istentative evidence for a velocity signature, with the stream moving at -40 km/s at low declinations and +100 km/s at high declinations. The stream lies on the same great circle as Complex A, a roughly linear association of HI high velocity clouds stretching over 30 degrees on the sky, and as Ursa Major II, a recently discovered dwarf spheroidal galaxy. Lying close to the same great circle are a number of anomalous, young and metal-poor globular clusters, including Palomar 1 and Ruprecht 106.
The Astrophysical Journal 06/2006; 658(1). DOI:10.1086/511302 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy. Comment: ApJ (Letters), in press
The Astrophysical Journal 04/2006; 647(2). DOI:10.1086/507324 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a maximum likelihood method for determining the spatial properties of tidal debris and those of the Galactic spheroid. This method is then applied to SDSS DR6 data to characterize debris from the Sagitarrius Dwarf tidal stream. Over small spatial extent, a 2.5-degree wide wedge, the debris is modeled as a cylinder with a density that falls off as a Gaussian with distance from the axis. The Galactic spheroid component is modeled using a Hernquist profile. We utilize stars with the colors of blue F turnoff stars having the reddening corrected colors: 0.1 < g - r < 0.3. We also assume that the absolute magnitude of these F turnoff stars are distributed as a Gaussian about a mean, M(g) = 4.2. The effectiveness and correctness of our algorithm is tested using a data set simulated to mimic that seen in the SDSS data. We then apply the algorithm to data extracted from the SDSS DR6 database on a wedge-by-wedge basis, by which we create a piece-wise view of the Sagittarius debris over the entire sky. This also allows for the comparison of the spheroid component at different positions on the sky. We then use our algorithm to separate the stellar data into two distinct catalogs: one with the spatial properties of the debris and one with the characteristics of the spheroid. This provides an effective means to examine the separate populations and to facilitate the study of stream kinematics and inspect the smooth nature of the spheroid.