Space Telescope Science Institute

About: Space Telescope Science Institute is a facility organization based out in Baltimore, Maryland, United States. It is known for research contribution in the topics: Galaxy & Stars. The organization has 2448 authors who have published 14154 publications receiving 947296 citations. The organization is also known as: STScI.

Spectroscopic and photometric evidence of two stellar populations in the Galactic globular cluster NGC 6121 (M 4)

Abstract: Aims. We present abundance analysis based on high resolution spectra of 105 isolated red giant branch (RGB) stars in the Galactic Globular Cluster NGC 6121 (M 4). Our aim is to study its star population in the context of the multi-population phenomenon recently discovered to affect some Globular Clusters. Methods. The data have been collected with FLAMES+UVES, the multi-fiber high resolution facility at the ESO/VLT@UT2 telescope. Analysis was performed under LTE approximation for the following elements: O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Ba, and NLTE corrections were applied to those (Na, Mg) strongly affected by departure from LTE. Spectroscopic data were coupled with high-precision wide-field UBVIC photometry from WFI@2.2 m telescope and infrared JHK photometry from 2MASS. Results. We derived an average [Fe/H] = −1.07 ± 0.01 (internal error), and an α enhancement of [α/Fe] =+ 0.39 ± 0.05 dex (internal error). We confirm the presence of an extended Na-O anticorrelation, and find two distinct groups of stars with significantly different Na and O content. We find no evidence of a Mg-Al anticorrelation. By coupling our results with previous studies on the CN band strength, we find that the CN strong stars have higher Na and Al content and are more O depleted than the CN weak ones. The two groups of Na-rich, CN-strong and Na-poor, CN-weak stars populate two different regions along the RGB. The Na-rich group defines a narrow sequence on the red side of the RGB, while the Na-poor sample populate a bluer, more spread portion of the RGB. In the U vs. U − B color magnitude diagram the RGB spread is present from the base of the RGB to the RGB-tip. Apparently, both spectroscopic and photometric results imply the presence of two stellar populations in M 4. We briefly discuss the possible origin of these populations.

Massive Dark Matter Halos and Evolution of Early-Type Galaxies to z ~ 1

Abstract: The combination of gravitational lensing and stellar dynamics breaks the mass-anisotropy degeneracy and provides stringent constraints on the distribution of luminous and dark matter in early-type (E/S0) galaxies out to z ≈ 1. We present new observations and models of three lens systems (CFRS 03.1077, HST 14176+5226, HST 15433+5352) and the combined results from the five field E/S0 lens galaxies at z ≈ 0.5-1.0 analyzed as part of the Lenses Structure and Dynamics (LSD) Survey. Our main results are as follows: (1) Constant mass-to-light ratio models are ruled out at greater than 99% CL for all five E/S0 galaxies, consistent with the presence of massive and extended dark matter halos. The range of projected dark matter mass fractions inside the Einstein radius is fDM = 0.37-0.72, or 0.15-0.65 inside the effective radius Re for isotropic models. (2) The average effective power-law slope of the total (luminous plus dark; ρtot ∝ r) mass distribution is γ' = 1.75 ± 0.10 (1.57 ± 0.16) for Osipkov-Merritt models with anisotropy radius ri = ∞ (Re) with an rms scatter of 0.2 (0.35), i.e., marginally flatter than isothermal (γ' = 2). The ratio between the observed central stellar velocity dispersion and that from the best-fit singular isothermal ellipsoid (SIE) lens model is fSIE = σ/σSIE = 0.87 ± 0.04 with 0.08 rms, consistent with flatter-than-isothermal density profiles. Considering that γ' > 2 and fSIE > 1 have been reported for other systems (i.e., B1608+656 and PG 1115+080), we conclude that there is a significant intrinsic scatter in the slope of the mass-density profile of lens galaxies (rms ~ 15%), similar to what is found for local E/S0 galaxies. Hence, the isothermal approximation is not sufficiently accurate for applications that depend critically on the slope of the mass-density profile, such as the measurement of the Hubble constant from time delays. (3) The average inner power-law slope γ of the dark matter halo is constrained to be γ = 1.3 (68% CL), if the stellar velocity ellipsoid is isotropic (ri = ∞), or an upper limit of γ < 0.6, if the galaxies are radially anisotropic (ri = Re). The observed range of slopes of the inner dark matter distribution is consistent with the results from numerical simulations only for an isotropic velocity ellipsoid and if baryonic collapse and star formation do not steepen dark matter density profiles. (4) The average stellar mass-to-light ratio evolves as d log(M*/LB)/dz = -0.72 ± 0.10, obtained via a fundamental plane analysis. An independent analysis based on lensing and dynamics gives an average d log(M*/LB)/dz = -0.75 ± 0.17. Both values indicate that the mass-to-light ratio evolution for our sample of field E/S0 galaxies is slightly faster than those in clusters, consistent with the hypothesis that field E/S0 galaxies experience secondary bursts (~10% in mass) of star formation at z < 1. These findings are consistent with pure luminosity evolution of E/S0 galaxies in the past 8 Gyr and would be hard to reconcile with scenarios involving significant structural and dynamical evolution.

A new multifield determination of the galaxy luminosity function at z = 7-9 incorporating the 2012 Hubble Ultra-Deep Field imaging

Abstract: We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z ≃ 7 and 8, and a first estimate at z ≃ 9. An accurate determination of the form and evolution of the galaxy LF during this era is of key importance for improving our knowledge of the earliest phases of galaxy evolution and the process of cosmic reionization. Our analysis exploits to the full the new, deepest Wide Field Camera 3/infrared imaging from our Hubble Space Telescope (HST) Ultra-Deep Field 2012 (UDF12) campaign, with dynamic range provided by including a new and consistent analysis of all appropriate, shallower/wider area HST survey data. Our new measurement of the evolving LF at z ≃ 7 to 8 is based on a final catalogue of ≃600 galaxies, and involves a step-wise maximum-likelihood determination based on the photometric redshift probability distribution for each object; this approach makes full use of the 11-band imaging now available in the Hubble Ultra-Deep Field (HUDF), including the new UDF12 F140W data, and the latest Spitzer IRAC imaging. The final result is a determination of the z ≃ 7 LF extending down to UV absolute magnitudes M_1500 = −16.75 (AB mag) and the z ≃ 8 LF down to M_1500 = −17.00. Fitting a Schechter function, we find M*_1500 = −19.90^(+0.23)_(−0.28), log ϕ* = −2.96^(+0.18)_(−0.23) and a faint-end slope α = −1.90^(+0.14)_(−0.15) at z ≃ 7, and M*_1500 = −20.12^(+0.37)_(−0.48), log ϕ* = −3.35^(+0.28)_(−0.47) and α = −2.02^(+0.22)_(-0.23) at z ≃ 8. These results strengthen previous suggestions that the evolution at z > 7 appears more akin to ‘density evolution’ than the apparent ‘luminosity evolution’ seen at z ≃ 5 − 7. We also provide the first meaningful information on the LF at z ≃ 9, explore alternative extrapolations to higher redshifts, and consider the implications for the early evolution of UV luminosity density. Finally, we provide catalogues (including derived z_phot, M_1500 and photometry) for the most robust z ∼ 6.5-11.9 galaxies used in this analysis. We briefly discuss our results in the context of earlier work and the results derived from an independent analysis of the UDF12 data based on colour–colour selection.

ATLAS: A High-cadence All-sky Survey System

Abstract: Technology has advanced to the point that it is possible to image the entire sky every night and process the data in real time. The sky is hardly static: many interesting phenomena occur, including variable stationary objects such as stars or QSOs, transient stationary objects such as supernovae or M dwarf flares, and moving objects such as asteroids and the stars themselves. Funded by NASA, we have designed and built a sky survey system for the purpose of finding dangerous near-Earth asteroids (NEAs). This system, the "Asteroid Terrestrial-impact Last Alert System" (ATLAS), has been optimized to produce the best survey capability per unit cost, and therefore is an efficient and competitive system for finding potentially hazardous asteroids (PHAs) but also for tracking variables and finding transients. While carrying out its NASA mission, ATLAS now discovers more bright ($m < 19$) supernovae candidates than any ground based survey, frequently detecting very young explosions due to its 2 day cadence. ATLAS discovered the afterglow of a gamma-ray burst independent of the high energy trigger and has released a variable star catalogue of 5$\times10^{6}$ sources. This, the first of a series of articles describing ATLAS, is devoted to the design and performance of the ATLAS system. Subsequent articles will describe in more detail the software, the survey strategy, ATLAS-derived NEA population statistics, transient detections, and the first data release of variable stars and transient lightcurves.

Low-resolution Spectroscopy of Gamma-ray Burst Optical Afterglows: Biases in the Swift Sample and Characterization of the Absorbers

Abstract: We present a sample of 77 optical afterglows (OAs) of Swift detected gamma-ray bursts (GRBs) for which spectroscopic follow-up observations have been secured. Our first objective is to measure the redshifts of the bursts. For the majority (90%) of the afterglows, the redshifts have been determined from the spectra. We provide line lists and equivalent widths (EWs) for all detected lines redward of Lyα covered by the spectra. In addition to the GRB absorption systems, these lists include line strengths for a total of 33 intervening absorption systems. We discuss to what extent the current sample of Swift bursts with OA spectroscopy is a biased subsample of all Swift detected GRBs. For that purpose we define an X-ray-selected statistical sample of Swift bursts with optimal conditions for ground-based follow-up from the period 2005 March to 2008 September; 146 bursts fulfill our sample criteria. We derive the redshift distribution for the statistical (X-ray selected) sample and conclude that less than 18% of Swift bursts can be at z > 7. We compare the high-energy properties (e.g., γ-ray (15-350 keV) fluence and duration, X-ray flux, and excess absorption) for three subsamples of bursts in the statistical sample: (1) bursts with redshifts measured from OA spectroscopy; (2) bursts with detected optical and/or near-IR afterglow, but no afterglow-based redshift; and (3) bursts with no detection of the OA. The bursts in group (1) have slightly higher γ-ray fluences and higher X-ray fluxes and significantly less excess X-ray absorption than bursts in the other two groups. In addition, the fractions of dark bursts, defined as bursts with an optical to X-ray slope βOX 39% in group (3). For the full sample, the dark burst fraction is constrained to be in the range 25%-42%. From this we conclude that the sample of GRBs with OA spectroscopy is not representative for all Swift bursts, most likely due to a bias against the most dusty sight lines. This should be taken into account when determining, e.g., the redshift or metallicity distribution of GRBs and when using GRBs as a probe of star formation. Finally, we characterize GRB absorption systems as a class and compare them to QSO absorption systems, in particular the damped Lyα absorbers (DLAs). On average GRB absorbers are characterized by significantly stronger EWs for H I as well as for both low and high ionization metal lines than what is seen in intervening QSO absorbers. However, the distribution of line strengths is very broad and several GRB absorbers have lines with EWs well within the range spanned by QSO-DLAs. Based on the 33 z > 2 bursts in the sample, we place a 95% confidence upper limit of 7.5% on the mean escape fraction of ionizing photons from star-forming galaxies. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programs 275.D-5022 (PI: Chincarini), 075.D-0270 (PI: Fynbo), 077.D-0661 (PI: Vreeswijk), 077.D-0805 (PI: Tagliaferri), 177.A-0591 (PI: Hjorth), 078.D-0416 (PI: Vreeswijk), 079.D-0429 (PI: Vreeswijk), 080.D-0526 (PI: Vreeswijk), 081.A-0135 (PI: Greiner), 281.D-5002 (PI: Della Valle), and 081.A-0856 (PI: Vreeswijk). Also based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. Some of the data obtained herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck foundation.