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.

The Second-Generation Guide Star Catalog: Description and Properties

Abstract: The Guide Star Catalog II (GSC-II) is an all-sky database of objects derived from the uncompressed Digitized Sky Surveys that the Space Telescope Science Institute has created from the Palomar and UK Schmidt survey plates and made available to the community. Like its predecessor (GSC-I), the GSC-II was primarily created to provide guide star information and observation planning support for Hubble Space Telescope. This version, however, is already employed at some of the ground-based new-technology telescopes such as GEMINI, VLT, and TNG, and will also be used to provide support for the James Webb Space Telescope (JWST) and GAIA space missions as well as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, one of the major ongoing scientific projects in China. Two catalogs have already been extracted from the GSC-II database and released to the astronomical community. A magnitude-limited (RF = 18.0) version, GSC2.2, was distributed soon after its production in 2001, while the GSC2.3 release has been available for general access since 2007. The GSC2.3 catalog described in this paper contains astrometry, photometry, and classification for 945,592,683 objects down to the magnitude limit of the plates. Positions are tied to the International Celestial Reference System; for stellar sources, the all-sky average absolute error per coordinate ranges from 02 to 028 depending on magnitude. When dealing with extended objects, astrometric errors are 20% worse in the case of galaxies and approximately a factor of 2 worse for blended images. Stellar photometry is determined to 0.13-0.22 mag as a function of magnitude and photographic passbands (RF , BJ , IN ). Outside of the galactic plane, stellar classification is reliable to at least 90% confidence for magnitudes brighter than RF = 19.5, and the catalog is complete to RF = 20.

An optical supernova associated with the X-ray flash XRF 060218.

Abstract: Long-duration gamma-ray bursts (GRBs) are associated with type Ic supernovae(1) that are more luminous than average(2-5) and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB - supernovae were thought to be rare events(6). Whether X-ray flashes - analogues of GRBs, but with lower luminosities and fewer gamma-rays - can also be associated with supernovae, and whether they are intrinsically 'weak' events or typical GRBs viewed off the axis of the burst(7), is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB - supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output(8) and the supernova radio flux(9). Our data, combined with radio and X-ray observations(8-10), suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB - supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB - supernovae.

The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-redshift Circumgalactic Medium

Abstract: We analyze the physical conditions of the cool, photoionized (T ∼ 10 4 K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of L ∼ L ∗ galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (nHii/nH 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, Rvir, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile nH = (10 −4.2±0.25 )(R/Rvir) −0.8±0.3 . Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the Hi column densities, we estimate a lower limit to the cool gas mass M cool CGM > 6.5 × 10

The Evolution of Early-Type Galaxies in Distant Clusters*

Abstract: We present results from an optical-infrared photometric study of early-type (E+S0) galaxies in 19 galaxy clusters out to z = 0.9. The galaxy sample is selected on the basis of morphologies determined from Hubble Space Telescope (HST) WFPC2 images and is photometrically defined in the K band in order to minimize redshift-dependent selection biases. Using new ground-based photometry in five optical and infrared bands for each cluster, we examine the evolution of the color-magnitude relation for early-type cluster galaxies, considering its slope, intercept, and color scatter around the mean relation. New multiwavelength photometry of galaxies in the Coma Cluster is used to provide a baseline sample at z ≈ 0 with which to compare the distant clusters. The optical-IR colors of the early-type cluster galaxies become bluer with increasing redshift in a manner consistent with the passive evolution of an old stellar population formed at an early cosmic epoch. The degree of color evolution is similar for clusters at similar redshift and does not depend strongly on the optical richness or X-ray luminosity of the cluster, which suggests that the history of early-type galaxies is relatively insensitive to environment, at least above a certain density threshold. The slope of the color-magnitude relationship shows no significant change out to z = 0.9, which provides evidence that it arises from a correlation between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in the optical-IR colors of the galaxies is small and nearly constant with redshift, which indicates that the majority of giant, early-type galaxies in clusters share a common star formation history, with little perturbation due to uncorrelated episodes of later star formation. Taken together, our results are consistent with models in which most early-type galaxies in rich clusters are old, formed the majority of their stars at high redshift in a well-synchronized fashion, and evolved quiescently thereafter. We consider several possible effects that may be introduced by the choice of morphologically recognizable elliptical and S0 galaxies in dense environments as a subject for study. In particular, the inclusion of S0 galaxies, which might be undergoing morphological transformation in clusters as part of the Butcher-Oemler effect, may influence the results of our investigation.

Hubble space telescope observations of young star clusters in NGC-4038/4039, 'the antennae' galaxies

Abstract: New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented NGC 4038/4039, nicknamed The Antennae, is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger Down to the limiting magnitude of V approximately 23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks The mean absolute magnitude of these objects is M(sub V) = -11 mag, with the brightest objects reaching M(sub V) approximately -15 Their mean apparent color indices ar U - V = -07 mag and V - 1 = 08 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (u - v)(sub 0) = -10 mag and (V - I(sub 0) = 05 Their mean effective radius, determined from slightly resolved images, is 18 pc (for H(sub 0) = 50 km/s /Mpc) Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galazy mergers Based on their U - V and V - I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations The cluster luminosity function (LF) is approximately a power law, phi(L)dL proportional to L(exp -178+/-005)dL, with no hint of a turnover at fainter magnitudes This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a Full Width at Half Maximum (FWHM) of approximately 3 mag Besides the blue clusters, we also find about a dozen extremely red objects with V - I greater than 30 The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent We propose that these objects may be very young star clusters still embedded in their placental dust cocoons