We present the photometric calibration of the Advanced Camera for Surveys (ACS). The ACS was installed in the Hubble Space Telescope (HST) in 2002 March. It comprises three cameras: the Wide Field Channel (WFC), optimized for deep near-IR survey imaging programs; the High Resolution Channel (HRC), a high-resolution imager that fully samples the HST point-spread function (PSF) in the visible; and the Solar Blind Channel (SBC), a far-UV imager. A significant amount of data has been collected to characterize the on-orbit performance of the three channels. We give here an overview of the performance and calibration of the two CCD cameras (WFC and HRC) and a description of the best techniques for reducing ACS CCD data. The overall performance is as expected from prelaunch testing of the camera. Surprises were a better-thanpredicted sensitivity in the visible and near-IR for both the WFC and HRC and an unpredicted dip in the HRC UV response at ∼3200 A ˚. On-orbit observations of spectrophotometric standard stars have been used to revise the prelaunch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zero points for all filters in three magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of longwavelength photons, the width of the PSF increases significantly in the near-IR, and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: one based on the observation of two star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and "hot" pixel population.

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 investigate the color-magnitude (CM) relation of galaxies in the distant X-ray selected cluster RDCS 1252.9-2927 at z = 1.24 using images obtained with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope in the F775W and F850LP bandpasses. We select galaxies based on morphological classifications extending about 3.5 mag down the galaxy luminosity function, augmented by spectroscopic membership information. At the core of the cluster is an extensive early-type galaxy population surrounding a central pair of galaxies that show signs of dynamical interaction. The early-type population defines a tight sequence in the CM diagram, with an intrinsic scatter in observed (i 775 −z 850 ) of 0.029 ± 0.007 mag based on 52 galaxies, or 0.024 ± 0.008 mag for ∼ 30 ellipticals. Simulations using the latest stellar population models indicate an age scatter for the ellipticals of about 34%, with a mean age τ L ∼ > 2.6 Gyr (corresponding to z L ∼ > 2.7), and the last star formation occurring at z end ∼ > 1.5. Transforming to rest-frame (U−B), we conclude that the slope and scatter in the CM relation for morphologically selected early-type galaxies show little or no evidence for evolution out to z ≈ 1.2. Thus, elliptical galaxies were already well established in X-ray luminous clusters when the universe was a third of its present age.

We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color magnitude diagrams (CMDs) extend ∼3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5±0.5 Gyr (Lindsay 1), 6.5±0.5 Gyr (Kron 3), 6±0.5 Gyr (NGC 339), 6±0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turn-offs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud (LMC) and the Milky Way (MW). Interestingly, our intermediateage star clusters have a metallicity spread of ∼0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally concentrated blue straggler star candidates in NGC 416, while for the other clusters these are not present. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (∼50 kpc), and the farthest cluster, Lindsay 38 (∼67 kpc), have a relative distance of ∼17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, Kron 3) lie in the north-western part of the SMC, while the youngest (NGC 419) is located near the SMC main body.