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.

GEMS : Galaxy Fitting Catalogs and Testing Parametric Galaxy Fitting Codes : GALFIT and GIM2D

Abstract: In the context of measuring the structures of intermediate-redshift galaxies with HST ACS surveys, we tune, test, and compare two widely used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic models to both simulated and real galaxy data. Our study focuses on the GEMS survey with the sensitivity of typical HST survey data, and we include our final catalog of fit results for all 41,495 objects detected in GEMS. Using simulations, we find that fitting accuracy depends sensitively on galaxy profile shape. Exponential disks are well fit and have small measurement errors, whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the large amount of light at large radii. Both codes provide reliable fits with little systematic error for galaxies with effective surface brightnesses brighter than that of the sky; the formal uncertainties returned by these codes significantly underestimate the true uncertainties (as estimated using the simulations). We find that GIM2D suffers significant systematic errors for spheroids with close companions owing to the difficulty of effectively masking out neighboring galaxy light; there appears to be no work-around to this important systematic in GIM2D's current implementation. While this crowding error affects only a small fraction of galaxies in GEMS, it must be accounted for in the analysis of deeper cosmological images or of more crowded fields with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors because it can simultaneously fit the profiles of multiple companions as well as the galaxy of interest. We find GALFIT's robustness to nearby companions and factor of 20 faster runtime speed are important advantages over GIM2D for analyzing large HST ACS data sets.

BeppoSAX Observations of Unprecedented Synchrotron Activity in the BL Lacertae Object Markarian 501

Abstract: The BL Lacertae object Markarian 501, one of only three extragalactic sources (with Mrk 421 and 1ES 2344+514) so far detected at TeV energies, was observed with the BeppoSAX satellite in 1997 April 7, 11, and 16 during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA, and CAT Cherenkov telescopes. Over the whole 0.1-200 keV range, the spectrum was exceptionally hard (??1, with F? ? ???), indicating that the X-ray power output peaked at (or above) ~100 keV. This represents a shift of at least 2 orders of magnitude with respect to previous observations of Mrk 501, a behavior never seen before in this or any other blazar. The overall X-ray spectrum hardens with increasing intensity, and at each epoch it is softer at larger energies. The correlated variability from soft X-rays to the TeV band points to models in which the same population of relativistic electrons produces the X-ray continuum via synchrotron radiation and the TeV emission by inverse Compton scattering of the synchrotron photons or other seed photons. For the first time in any blazar, the synchrotron power is observed to peak at hard X-ray energies. The large shift of the synchrotron peak frequency with respect to previous observations of Mrk 501 implies that intrinsic changes in the relativistic electron spectrum caused the increase in emitted power. Due to the very high electron energies, the inverse Compton process is limited by the Klein-Nishina regime. This implies a quasi-linear (as opposed to quadratic) relation of the variability amplitude in the TeV and hard X-ray ranges (for the synchrotron self-Compton model) and an increase of the inverse Compton peak frequency smaller than that of the synchrotron peak frequency.

The Ultraviolet Spectroscopic Properties of Local Starbursts: Implications at High Redshift

Abstract: We report the results of a systematic study of the vacuum ultraviolet (λ 1150-2000 A) spectra of a sample of 45 starburst and related galaxies observed with the IUE satellite. These span broad ranges in metallicity (from 0.02 to 3 times solar), bolometric luminosity (~107-4 × 1011 L☉), and galaxy properties (e.g., including low-mass dwarf galaxies, normal disk galaxies, and massive galactic mergers). The projected size of the IUE spectroscopic aperture is typically 1 to several kpc and therefore usually encompasses the entire starburst and is similar to the aperture sizes used for spectroscopy of high-redshift galaxies. Our principal conclusion is that local starbursts occupy a very small fractional volume in the multidimensional manifold defined by such fundamental parameters as the extinction, metallicity, and vacuum-UV line strengths (both stellar and interstellar) of the starburst and the rotation speed (mass) and absolute magnitude of the starburst's "host" galaxy. More metal-rich starbursts are redder and more heavily extinguished in the UV, are more luminous, have stronger vacuum-UV lines, and occur in more massive and optically brighter host galaxies. We advocate using these local starbursts as a "training set" to learn how to interpret the rest frame UV spectra of star-forming galaxies at high redshift better, and we stress that the degree of similarity between local starbursts and high-redshift galaxies in this multidimensional parameter space can already be tested empirically. The results on local starbursts suggest that the high- redshift "Lyman Dropout" galaxies are typically highly reddened and extinguished by dust (by an average factor of 5-10 in the UV), may have moderately high metallicities (0.1-1 times solar?), are probably building galaxies with stellar surface-mass densities similar to present-day ellipticals, and may be suffering substantial losses of metal-enriched gas that can "pollute" the intergalactic medium.

Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies

Abstract: We present the large-scale correlation function measured from a spectroscopic sample of 46,748 luminous red galaxies from the Sloan Digital Sky Survey. The survey region covers 0.72 h^{-3} Gpc^3 over 3816 square degrees and 0.16

Tidal Decay of Close Planetary Orbits

Abstract: The 4.2-day orbit of the newly discovered planet around 51~Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as $\sim10\,$hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun.