J. Anderson

Space Telescope Science Institute, Baltimore, Maryland, United States

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Publications (141)308.28 Total impact

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    ABSTRACT: We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar Planet Search field (3'x3'), together with three more Advanced Camera for Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and F814W filters, approximately every two weeks for two years, with the principal aim of detecting a hidden population of isolated black holes and neutron stars through astrometric microlensing. Proper motions were measured with an accuracy of ~0.1 mas/yr (~4 km/s) at F606W~25.5 mag, and better than ~0.5 mas/yr (20 km/s) at F606W~28 mag, in both axes. Proper-motion measurements allowed us to separate disk and bulge stars and obtain a clean bulge color-magnitude diagram. We then identified for the first time a white dwarf (WD) cooling sequence in the Galactic bulge, together with a dozen candidate extreme horizontal branch stars. The comparison between theory and observations shows that a substantial fraction of the WDs (30%) are systematically redder than the cooling tracks for CO-core H-rich and He-rich envelope WDs. This evidence would suggest the presence of a significant number of low-mass WDs and WD - main sequence binaries in the bulge. This hypothesis is further supported by the finding of two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.
    The Astrophysical Journal 06/2014; 790(2). · 6.73 Impact Factor
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    ABSTRACT: We use Chandra and XMM observations of the globular clusters $\omega$ Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favored by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.
    06/2014;
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    ABSTRACT: We present the results of a photometric search for variable stars in the core of the Galactic globular cluster M4. The input data are a large and unprecedented set of deep Hubble Space Telescope WFC3 images (large program GO-12911; 120 orbits allocated), primarily aimed at probing binaries with massive companions by detecting their astrometric wobbles. Though these data were not optimised to carry out a time-resolved photometric survey, their exquisite precision, spatial resolution and dynamic range enabled us to firmly detect 38 variable stars, of which 20 were previously unpublished. They include 19 cluster-member eclipsing binaries (confirming the large binary fraction of M4), RR Lyrae, and objects with known X-ray counterparts. We improved and revised the parameters of some among published variables.
    05/2014;
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    ABSTRACT: We present the results of a photometric search for variable stars in the core of the Galactic globular cluster M4. The input data are a large and unprecedented set of deep Hubble Space Telescope WFC3 images (large program GO-12911; 120 orbits allocated), primarily aimed at probing binaries with massive companions by detecting their astrometric wobbles. Though these data were not optimised to carry out a time-resolved photometric survey, their exquisite precision, spatial resolution and dynamic range enabled us to firmly detect 38 variable stars, of which 20 were previously unpublished. They include 19 cluster-member eclipsing binaries (confirming the large binary fraction of M4), RR Lyrae, and objects with known X-ray counterparts. We improved and revised the parameters of some among published variables.
    04/2014;
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    ABSTRACT: We combine new and archival Chandra observations of the globular cluster NGC 6752 to create a deeper X-ray source list, and study the faint radio millisecond pulsars (MSPs) of this cluster. We detect four of the five MSPs in NGC 6752, and present evidence for emission from the fifth. The X-rays from these MSPs are consistent with thermal emission from the neutron star surfaces, with significantly higher fitted blackbody temperatures than other globular cluster MSPs (though we cannot rule out contamination by nonthermal emission or other X-ray sources). NGC 6752 E is one of the lowest-L_X MSPs known, with L_X(0.3-8 keV)=1.0+0.9-0.5*10^30 ergs/s. We check for optical counterparts of the three isolated MSPs in the core using new HST ACS images, finding no plausible counterparts, which is consistent with their lack of binary companions. We compile measurements of L_X and spindown power for radio MSPs from the literature, including errors where feasible. We find no evidence that isolated MSPs have lower L_X than MSPs in binary systems, omitting binary MSPs showing emission from intrabinary wind shocks. We find weak evidence for an inverse correlation between the estimated temperature of the MSP X-rays and the known MSP spin period, consistent with the predicted shrinking of the MSP polar cap size with increasing spin period.
    03/2014; 441(1).
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    ABSTRACT: The M4 Core Project with HST is designed to exploit the Hubble Space Telescope to investigate the central regions of M4, the Globular Cluster closest to the Sun. In this paper we combine optical and near-infrared photometry to study multiple stellar populations in M4. We detected two sequences of M-dwarfs containing ~38% (MS_I) and ~62% (MS_II) of MS stars below the main-sequence (MS) knee. We compare our observations with those of NGC2808, which is the only other GCs where multiple MSs of very low-mass stars have been studied to date. We calculate synthetic spectra for M-dwarfs, assuming the chemical composition mixture inferred from spectroscopic studies of stellar populations along the red giant branch, and different Helium abundances, and we compare predicted and observed colors. Observations are consistent with two populations, one with primordial abundance and another with enhanced nitrogen and depleted oxygen.
    01/2014; 439(2).
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    ABSTRACT: The Tarantula Nebula (a.k.a. 30 Doradus) in the Large Magellanic Cloud is one of the most famous objects in astronomy, with first astronomical references being more than 150 years old. Today the Tarantula Nebula and its ionizing cluster R136 are considered one of the few known starburst regions in the Local Group and an ideal test bed to investigate the temporal and spatial evolution of a prototypical starburst on a sub-cluster scale. The Hubble Tarantula Treasury Project (HTTP) is a panchromatic imaging survey of the stellar populations and ionized gas in the Tarantula Nebula that reaches into the sub-solar mass regime (<0.5 M⊙). HTTP utilizes the capability of the Hubble Space Telescope to operate the Advanced Camera for Surveys and the Wide Field Camera 3 in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow-band Hα images. The program was awarded 60 orbits of HST time and is built on the existing 30 orbits monochromatic proper motion program GO-12499 (PI Lennon). The combination of all these bands provides a unique view of the region: the resulting maps of the Tarantula’s stellar content provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early universe. At the same time access to detailed properties of individual stars allows us to begin to reconstruct the temporal and spatial evolution of the Tarantula Nebula over space and time on a sub-parsec scale. We will deliver high-level data products (i.e. star and cluster catalogs, co-registered stacked images). HTTP will become the definitive catalog of the field, and have lasting value for future. HTTP also has an educational and public outreach component aimed to stimulate interest in STEM disciplines among people with visual impairments. “Reach for the Stars: Touch, Look, Listen, Learn” is a free eBook that explains how stars form and evolve using images from HTTP. The eBook utilizes emerging technology that works in conjunction with the built-in accessibility features in the Apple iPad to allow totally blind users to interactively explore complex astronomical images.
    01/2014;
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    ABSTRACT: We present the first data release of the Hubble Space Telescope Frontier Fields program, a new Director's Discretionary program to carry out ultra-deep observations of six lensing clusters and parallel deep blank fields, probing the most distant galaxies currently observable. During the three-year program, each cluster is being observed for 140 orbits over two epochs, probing to 29th magnitude. We present here the first epoch of the cluster Abell 2744, observed to 70 orbits on the main cluster with WFC3/IR (in F105W, F125W, F140W and F160W) and on the parallel field with ACS (in F435W, F606W, F814W). We present the design of the pipeline for the data processing and calibration, including a new approach to ACS self-calibration. We discuss the various data products that we are distributing as high-level science products through the Mikulski Archive for Space Telescopes (MAST) at STScI, including distortion-corrected "drizzled" mosaics in all the filters, released throughout the course of the observations, as well as the final full-depth mosaics and related products. We deliver these high-level science products to the community on a rapid timescale to enable the widest scientific use of these data, as well as ensuring a public legacy dataset of the highest possible quality that is of lasting value to the entire community.
    01/2014;
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    ABSTRACT: Using over 13 years of archival HST observations of the relativistic jet in the archetypal radio galaxy M87, we have produced astrometric speed measurements of the optically bright synchrotron emitting plasma components in the jet with unprecedented accuracy. Building on previous work showing the superluminal nature of the jet in the optical, we have found that the jet motion is incredibly complex, with both transverse motions and flux variations which can be seen very clearly by eye in the timeseries of deep exposures. These observations of M87 provide us with a unique dataset with which to refine theoretical models of the largescale jet structure, potentially addressing open questions such as the jet collimation mechanism, bulk acceleration and deceleration in the jet, and the presence of a helical structure. I will also present very recent results using data from the HST archive on the optical counterjet and nuclear regions of M87 and discuss the larger implications of these detailed studies of one of the most nearby AGN jets.
    01/2014;
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    ABSTRACT: We present a new measurement of the stellar Initial Mass Function (IMF) based on ultra-deep, high-resolution photometry of >5,000 stars in the outskirts of the Small Magellanic Cloud (SMC) galaxy. The stellar main sequence of the SMC is measured in the color-magnitude diagram (CMD) down to ~30th magnitude, and is cleanly separated from the foreground star cluster population using proper motions. We simulate the SMC population by extracting stellar masses (single and unresolved binaries) from specific IMFs, and converting those masses to luminosities in our bandpasses. The corresponding photometry for these simulated stars is drawn directly from a rich cloud of million artificial stars, thereby accounting for the real photometric scatter and completeness of the data. Over a continuous and well populated mass range of M = 0.37-0.93 Msun, we demonstrate that the IMF is well represented by a single power-law form with slope α = -1.90 ([+0.15] [-0.10]) (3 σ error) (i.e., dN/dM propto M^(α), Salpeter α = -2.35). Our results indicate that the IMF does not turn over to a more shallow power law form within this mass range.
    01/2014;
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    ABSTRACT: Globular clusters come in a variety of flavours; their different components and different evolution histories leave imprints on their internal dynamics, which makes dynamical modelling a valuable tool for understanding their properties. Velocity dispersion profiles and mass profiles are key when investigating energy equipartition and core collapse or when hunting for intermediate mass black holes. Proper motion data are particularly useful as they provide two velocity components, which allows us to determine anisotropy profiles as well. From analyses by Bellini et al. carried out in the context of the HST Proper Motion (HSTPROMO) collaboration we now have HST proper motion data for some two dozen Galactic globular clusters. Each dataset typically contains proper motion data with accuracies of order 1 km/s for tens of thousands of stars within the central arcminute. We have developed new modelling techniques specifically designed to work with large, high-quality, discrete datasets of this nature, which we have successfully tested and applied to omega Centauri. We will briefly discuss our modelling efforts and then present some preliminary results for the new HST datasets, including velocity dispersion profiles, mass profiles, and energy equipartition.
    01/2014;
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    ABSTRACT: We demonstrate the power and usability of the DrizzlePac image processing tools developed at the Space Telescope Science Institute. These tools are available to the astronomical community, to align, distortion-correct, and combine stacks of images such as the Frontier Fields mosaics. Using 'cosmic-ray cleaned' images, we test various techniques for producing source catalogs to refine the image alignment. We present methodology for aligning images across visits, across filters, across detectors, and finally to an absolute reference catalog. The alignment solutions, or 'headerlet' files, will be made available to community as 'High Level Science Products' which may be applied to archival data in order to reduce the amount of work needed to re-process the Frontier Fields dataset. We also describe methodology for optimizing the drizzling 'pixfrac' (or drop size) of the final image for any given plate scale in order to provide the best signal-to-noise trade-off between pixel sampling and background noise.
    01/2014;
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    ABSTRACT: We present an overview of the ongoing Hubble Space Telescope large program GO-12911. The program is focused on the core of M 4, the nearest Galactic globular cluster, and the observations are designed to constrain the number of binaries with massive companions (black holes, neutron stars, or white dwarfs) by measuring the “wobble” of the luminous (mainsequence) companion around the center of mass of the pair, with an astrometric precision of ∼50 µas. The high spatial resolution and stable medium-band PSFs of WFC3/UVIS will make these measurements possible. In this work we describe (i) the motivation behind this study, (ii) our observing strategy, (iii) the many other investigations enabled by this unique data set, and which of those our team is conducting, and (iv) a preliminary reduction of the first-epoch dataset collected on 2012 October 10. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Astronomische Nachrichten 12/2013; 334(10). · 1.40 Impact Factor
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    ABSTRACT: We collected F606W- and F814W-band time-series data of the Sagittarius low-reddening window in the Galactic bulge with the Advanced Camera far Surveys mounted on the Hubble Space Telescope. We sampled the region approximately every two weeks for one year, with the principal aim to detect a hidden population of isolated black holes and neutron stars in the Galactic disk through astrometric microlensing. We present preliminary results here based on a photometric catalog including ~3*10^5 stars down to F606W ~ 31 mag. Proper motions were also measured, with an accuracy of better than ~ 0.5 mas/yr at F606W ~ 28 mag in both coordinates. We were then able to separate disk and bulge stars and to obtain a clean bulge color-magnitude diagram. Together with a dozen candidate extreme horizontal branch stars we were able to identify for the first time a clearly defined white dwarf (WD) cooling sequence in the bulge. The comparison between theory and observations shows that a substantial fraction of the WDs (~40%) is systematically redder than the canonical cooling tracks for CO-core DA WDs. This evidence would suggest the presence of a significant number of He-core WDs in the bulge, formed in close binaries, as has been found in some Galactic globular and open clusters. The presence of close binaries in the bulge population is further supported by the finding of a candidate dwarf nova in outburst and a few candidate cataclysmic variables in quiescence in the same field.
    08/2013;
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    ABSTRACT: We derive the star formation history in four regions of the Small Magellanic Cloud (SMC) using the deepest VI color-magnitude diagrams (CMDs) ever obtained for this galaxy. The images were obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope and are located at projected distances of 0.5-2 degrees from the SMC center, probing the main body and the wing of the galaxy. We derived the star-formation histories (SFH) of the four fields using two independent procedures to fit synthetic CMDs to the data. We compare the SFHs derived here with our earlier results for the SMC bar to create a deep pencil-beam survey of the global history of the central SMC. We find in all the six fields observed with HST a slow star formation pace from 13 to 5-7 Gyr ago, followed by a ~ 2-3 times higher activity. This is remarkable because dynamical models do not predict a strong influence of either the LMC or the Milky Way (MW) at that time. The level of the intermediate-age SFR enhancement systematically increases towards the center, resulting in a gradient in the mean age of the population, with the bar fields being systematically younger than the outer ones. Star formation over the most recent 500 Myr is strongly concentrated in the bar, the only exception being the area of the SMC wing. The strong current activity of the latter is likely driven by interaction with the LMC. At a given age, there is no significant difference in metallicity between the inner and outer fields, implying that metals are well mixed throughout the SMC. The age-metallicity relations we infer from our best fitting models are monotonically increasing with time, with no evidence of dips. This may argue against the major merger scenario proposed by Tsujimoto and Bekki 2009, although a minor merger cannot be ruled out.
    The Astrophysical Journal 08/2013; 775(2). · 6.73 Impact Factor
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    ABSTRACT: Globular clusters trace the formation history of the spheroidal components of our Galaxy and other galaxies, which represent the bulk of star formation over the history of the Universe. The clusters exhibit a range of metallicities (abundances of elements heavier than helium), with metal-poor clusters dominating the stellar halo of the Galaxy, and higher-metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and were later engulfed along with their original host galaxies, and which were formed within it. Here we report an absolute age of 9.9 ± 0.7 billion years (at 95 per cent confidence) for the metal-rich globular cluster 47 Tucanae, determined by modelling the properties of the cluster's white-dwarf cooling sequence. This is about two billion years younger than has been inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than metal-poor halo clusters like NGC 6397.
    Nature 08/2013; 500(7460):51-3. · 38.60 Impact Factor
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    ABSTRACT: Globular clusters trace the formation history of the spheroidal components of both our Galaxy and others, which represent the bulk of star formation over the history of the universe. They also exhibit a range of metallicities, with metal-poor clusters dominating the stellar halo of the Galaxy, and higher metallicity clusters found within the inner Galaxy, associated with the stellar bulge, or the thick disk. Age differences between these clusters can indicate the sequence in which the components of the Galaxy formed, and in particular which clusters were formed outside the Galaxy and later swallowed along with their original host galaxies, and which were formed in situ. Here we present an age determination of the metal-rich globular cluster 47 Tucanae by fitting the properties of the cluster white dwarf population, which implies an absolute age of 9.9 (0.7) Gyr at 95% confidence. This is about 2.0 Gyr younger than inferred for the metal-poor cluster NGC 6397 from the same models, and provides quantitative evidence that metal-rich clusters like 47 Tucanae formed later than the metal-poor halo clusters like NGC 6397.
    07/2013;
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    ABSTRACT: The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar mass regime (< 0.5 Mo). HTTP utilizes the capability of HST to operate the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3) in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow band H$\alpha$ images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early Universe. Access to detailed properties of individual stars allows us to begin to reconstruct the evolution of the stellar skeleton of the Tarantula Nebula over space and time with parcsec-scale resolution. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.
    The Astronomical Journal 07/2013; 146:53. · 4.97 Impact Factor
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    ABSTRACT: The Sagittarius Stream is the brightest and most prominent stellar stream around the Milky Way (MW), and the only one with a widely agreed upon origin and parent galaxy. It therefore provides a unique tracer to constrain the properties of the MW's dark halo. The position, width, distance, mean line-of-sight velocity, and velocity dispersion of the Stream have been mapped in various parts on the sky. Detailed N-body modeling has shown that these dynamical/positional properties of the Stream are strongly dependent on the shape and extent of the MW dark halo. However, proper motions (PMs) are measured only for limited parts of the Stream with insufficient quality to constrain the dark halo properties with high confidence. To resolve this issue, we measured PMs of stars in four fields along the Sgr Stream using HST ACS/WFC. I will present the results of this study and discuss the implications.
    07/2013;
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    ABSTRACT: We present new UV observations for NGC288, taken with the WFC3 detector on board the Hubble Space Telescope, and combine them with existing optical data from the archive to explore the multiple-population phenomenon in this globular cluster (GC). The WFC3's UV filters have demonstrated an uncanny ability to distinguish multiple populations along all photometric sequences in GCs, thanks to their exquisite sensitivity to the atmospheric changes that are tell-tale signs of second-generation enrichment. Optical filters, on the other hand, are more sensitive to stellar-structure changes related to helium enhancement. By combining both UV and optical data we can measure helium variation. We quantify this enhancement for NGC288 and find that its variation is typical of what we have come to expect in other clusters.
    The Astrophysical Journal 06/2013; 775(1). · 6.73 Impact Factor

Publication Stats

1k Citations
308.28 Total Impact Points

Institutions

  • 2007–2014
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
    • Wichita State University
      Wichita, Kansas, United States
  • 2013
    • National Institute of Astrophysics
      Roma, Latium, Italy
    • Max Planck Institute for Astrophysics
      Arching, Bavaria, Germany
    • Universidad de La Laguna
      • Department of Astrophysics
      San Cristóbal de La Laguna, Canary Islands, Spain
    • University of California, Los Angeles
      • Division of Astronomy & Astrophysics
      Los Angeles, CA, United States
  • 2012
    • Instituto de Astrofísica de Canarias
      San Cristóbal de La Laguna, Canary Islands, Spain
    • The Astronomical Observatory of Brera
      Merate, Lombardy, Italy
  • 2011
    • Universität Heidelberg
      • Centre for Astronomy (ZAH)
      Heidelburg, Baden-Württemberg, Germany
  • 2007–2008
    • Rice University
      • Department of Physics and Astronomy
      Houston, TX, United States