We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z > 1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these objects, and to 170 previously reported SNe Ia, have been determined using empirical relations between light-curve shape and luminosity. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the -2expansion history, the transition between the two epochs is constrained to be at z = 0.46 ± 0.13. The data are consistent with the cosmic concordance model of Ω M ≈ 0.3, Ω Λ ≈ 0.7 (χ 2 dof = 1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant, we measure Ω M = 0.29± 0.05 0.03 (equivalently, Ω Λ = 0.71). When combined with external flat-Universe constraints including the cosmic microwave background and large-scale structure, we find w = −1.02± 0.13 0.19 (and w < −0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc 2 . Joint constraints on both the recent equation of state of dark energy, w 0 , and its time evolution, dw/dz, are a factor of ∼ 8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w 0 = −1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.

The Sloan Digital Sky Survey (SDSS) is an imaging and spectroscopic survey that will eventually cover approximately one-quarter of the celestial sphere and collect spectra of %10 6 galaxies, 100,000 quasars, 30,000 stars, and 30,000 serendipity targets. In 2001 June, the SDSS released to the general astronomical community its early data release, roughly 462 deg 2 of imaging data including almost 14 million detected objects and 54,008 follow-up spectra. The imaging data were collected in drift-scan mode in five bandpasses (u, g, r, i, and z); our 95% completeness limits for stars are 22.0, 22.2, 22.2, 21.3, and 20.5, respectively. The photometric calibration is reproducible to 5%, 3%, 3%, 3%, and 5%, respectively. The spectra are flux-and wavelength-calibrated, with 4096 pixels from 3800 to 9200 Å at R % 1800. We present the means by which these data are distributed to the astronomical community, descriptions of the hardware used to obtain the data, the software used for processing the data, the measured quantities for each observed object, and an overview of the properties of this data set.

We have compiled L 0 (3.4-4.1 m) and M 0 (4.6-4.8 m) photometry of 63 single and binary M, L, and T dwarfs obtained at the United Kingdom Infrared Telescope using the Mauna Kea Observatory filter set. This compilation includes new L 0 measurements of eight L dwarfs and 13 T dwarfs and new M 0 measurements of seven L dwarfs, five T dwarfs, and the M1 dwarf Gl 229A. These new data increase by factors of 0.6 and 1.6, respectively, the numbers of ultracool dwarfs (T eff P 2400 K) for which L 0 and M 0 measurements have been reported. We compute L bol , BC K , and T eff for 42 dwarfs whose flux-calibrated JHK spectra, L 0 photometry, and trigonometric parallaxes are available, and we estimate these quantities for nine other dwarfs whose parallaxes and flux-calibrated spectra have been obtained. BC K is a well-behaved function of near-infrared spectral type with a dispersion of $0.1 mag for types M6-T5; it is significantly more scattered for types T5-T9. T eff declines steeply and monotonically for types M6-L7 and T4-T9, but it is nearly constant at $1450 K for types L7-T4 with assumed ages of $3 Gyr. This constant T eff is evidenced by nearly unchanging values of L 0 -M 0 between types L6 and T3. It also supports recent models that attribute the changing near-infrared luminosities and spectral features across the L-T transition to the rapid migration, disruption, and/or thinning of condensate clouds over a narrow range of T eff . The L 0 and M 0 luminosities of early-T dwarfs do not exhibit the pronounced humps or inflections previously noted in the I through K bands, but insufficient data exist for types L6-T5 to assert that M L 0 and M M 0 are strictly monotonic within this range of types. We compare the observed K, L 0 , and M 0 luminosities of L and T dwarfs in our sample with those predicted by precipitating-cloud and cloud-free models for varying surface gravities and sedimentation efficiencies. The models indicate that the L3-T4.5 dwarfs generally have higher gravities (log g = 5.0-5.5) than the T6-T9 dwarfs (log g = 4.5-5.0). The predicted M 0 luminosities of late-T dwarfs are 1.5-2.5 times larger than those derived empirically for the late-T dwarfs in our sample. This discrepancy is attributed to absorption at 4.5-4.9 m by CO, which is not expected under the condition of thermochemical equilibrium assumed in the models. Our photometry and bolometric calculations indicate that the L3 dwarf Kelu-1 and the T0 dwarf SDSS J042348.57À041403.5 are probable binary systems. We compute log (L bol /L ) = À5.73 AE 0.05 and T eff = 600-750 K for the T9 dwarf 2MASSI J0415195À093506, which supplants Gl 570D as the least luminous and coolest brown dwarf presently known.

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices (from Geballe et al. 2002, G02) that measure the strengths of the near infrared water and methane bands. We identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954−0935066 as T9, the latest and coolest dwarf found to date.We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system -2and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the G02 spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions and thicknesses of the clouds. We have identified a small group of late L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H 2 O and K I absorption, possibly due to an unusually small amount of condensates.The scatter seen in the H − K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H 2 opacity. The correlation of H − K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages > 10 8 yrs and the gravities implied by the H − K colors indicate that the T dwarfs in our sample have masses in the range 15 -75 M Jupiter . One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ∼ 4.2 -4.5 and mass ∼ 10 -15 M Jupiter .

We present 0.6È2.5 km, spectra of 27 cool, low-luminosity stars and substellar objects. Based R Z 400 on these and previously published spectra, we develop a preliminary spectral classiÐcation system for L and T dwarfs. For late L and T types the classiÐcation system is based entirely on four spectral indices in the 1È2.5 km interval. Two of these indices are derived from water absorption bands at 1.15 and 1.4 km, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K-band index also applicable to mid-to-late L dwarfs. Continuum indices shortward of 1 km used by previous authors to classify L dwarfs are found to be useful only through mid-L subclasses. We employ the 1.5 km water index and the 2.2 km methane index to complete the L classiÐcation through L9.5 and to link the new system with a modiÐed version of the 2MASS "" color-d ÏÏ index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from T0 to T8, based on all four indices, that is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H-band methane absorption.

We measure the morphology-density relation ( MDR) and morphology-radius relation (MRR) for galaxies in seven z $ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z 850 ¼ 24, corresponding to L /L Ã ¼ 0:21 and 0.30 at z ¼ 0:83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0:8 < z < 1:2 and that observed at z $ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, f EþS0 , with increasing density proceeds less rapidly at z $ 1 than it does at z $ 0. At z $ 1 and AE ! 500 galaxies Mpc À2 , we find h f EþS0 i ¼ 0:72 AE 0:10. At z $ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities AE k 40 galaxies Mpc À2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The f E -density relation exhibits no significant evolution between z ¼ 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

We analyze deep multicolor Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail, and many small counterimages are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counterimages. The model is refined as new counterimages are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1:0 < z < 5:5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies, and the light is more concentrated than the mass within r < 50 kpc h À1 . The projected mass profile flattens steadily toward the center with a shallow mean slope of dlog AE=dlog r ' À0:55 AE 0:1, over the observed range r < 250 kpc h À1 , matching well an NFW profile, but with a relatively high concentration, C vir ¼ 8:2 þ2:1 À1:8 . A softened isothermal profile (r core ¼ 20 AE 2 00 ) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend angles with redshift. The dependence on the cosmological parameters is weak owing to the proximity of A1689, z ¼ 0:18, constraining the locus, M þ Ã 1:2. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best-fitting lens model.

We present high-resolution R-band images of the central regions of 67 early-type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space T elescope (HST ). This homogeneously selected sample roughly doubles the number of early-type galaxies that have now been imaged at HST resolution and complements similar data on the central regions of radio galaxies and the bulges of spiral galaxies. Our sample strikingly conÐrms the complex morphologies of the central regions of early-type galaxies which have become apparent from previous studies with HST . In particular, we detect dust, either in the form of nuclear disks or with a Ðlamentary distribution, in 43% of all galaxies, in good agreement with previous estimates. In addition, we Ðnd evidence for embedded stellar disks in a remarkably large fraction of 51%. In 14 of those galaxies the disklike structures are misaligned with the main galaxy, suggesting that they correspond to stellar bars in S0 galaxies. We analyze the luminosity proÐles of the galaxies in our sample and classify galaxies according to their central cusp slope. To a large extent we conÐrm the results from previous HST surveys in that early-type galaxies reveal a clear dichotomy : the bright ellipticals are generally boxy and have luminosity proÐles that (M B [ [20.5) break from steep outer power laws to shallow inner cusps (referred to as "" core ÏÏ galaxies). The fainter ellipticals, on the other hand, typically have disky isophotes and luminosity proÐles that lack a clear break and have a steep central cusp (referred to as "" power-law ÏÏ galaxies). The advantages and shortcomings of classiÐcation schemes utilizing the extrapolated central cusp slope c are discussed, and it is shown that c might be an inadequate representation for galaxies whose luminosity proÐle slope changes smoothly with radius rather than resembling a broken power law. Thus, we introduce a new, alternative parameter and show how this a †ects the classiÐcation. In fact, we Ðnd evidence for an "" intermediate ÏÏ class of galaxies that cannot unambiguously be classiÐed as either core or power-law galaxies and that have central cusp slopes and absolute magnitudes intermediate between those of core and power-law galaxies. It is unclear at present, however, whether these galaxies make up a physically distinct class or whether distance and/or resolution e †ects cause them to lose their distinct core or power-law characteristics.