Evolution in the Cluster Early-type Galaxy Size-Surface Brightness Relation at z =~ 1

We investigate the evolution in the distribution of surface brightness, as a function of size, for elliptical and S0 galaxies in the two clusters RDCS J1252.9-2927, z=1.237 and RX J0152.7-1357, z=0.837. We use multi-color imaging with the Advanced Camera for Surveys on the Hubble Space Telescope to determine these sizes and surface brightnesses. Using three different estimates of the surface brightnesses, we find that we reliably estimate the surface brightness for the galaxies in our sample with a scatter of < 0.2 mag and with systematic shifts of \lesssim 0.05 mag. We construct samples of galaxies with early-type morphologies in both clusters. For each cluster, we use a magnitude limit in a band which closely corresponds to the rest-frame B, to magnitude limit of M_B = -18.8 at z=0, and select only those galaxies within the color-magnitude sequence of the cluster or by using our spectroscopic redshifts. We measure evolution in the rest-frame B surface brightness, and find -1.41 \+/- 0.14 mag from the Coma cluster of galaxies for RDCS J1252.9-2927 and -0.90 \+/- 0.12 mag of evolution for RX J0152.7-1357, or an average evolution of (-1.13 \+/- 0.15) z mag. Our statistical errors are dominated by the observed scatter in the size-surface brightness relation, sigma = 0.42 \+/- 0.05 mag for RX J0152.7-1357 and sigma = 0.76 \+/- 0.10 mag for RDCS J1252.9-2927. We find no statistically significant evolution in this scatter, though an increase in the scatter could be expected. Overall, the pace of luminosity evolution we measure agrees with that of the Fundamental Plane of early-type galaxies, implying that the majority of massive early-type galaxies observed at z =~ 1 formed at high redshifts.Comment: Accepted in ApJ, 16 pages in emulateapj format with 15 eps figures, 6 in colo

The Discovery of Two New Satellites of Pluto

Pluto's first known moon, Charon, was discovered in 1978 (Christy 1978) and has a diameter about half that of Pluto (Buie 1992,Young 1994, Sicardy 2005), which makes it larger relative to its primary than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful (Stern 1991, Stern 1994, Stern 2003), but they were not sensitive to objects <=150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites (Stern 1994). Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), which makes Pluto the first Kuiper belt object (KBO) known to have multiple satellites. These new satellites are much smaller than Charon (diameter~1200 km), with P1 ranging in diameter from 60-165 km depending on the surface reflectivity, and P2 about 20% smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ~38 days (P1) and ~25 days (P2). The implications of the discovery of P1 and P2 for the origin and evolution of the Pluto system, and for the satellite formation process in the Kuiper belt, are discussed in a companion paper (Stern 2006).Comment: Preprint of a paper accepted for publication in the journal Natur

Advanced camera for surveys photometry of the cluster RDCS 1252.9-2927: The color-magnitude relation at z=1.24

We investigate the color-magnitude (CM) relation of galaxies in the distant X-ray-selected cluster RDCS 1252.9 - 2927 z = 1.24 at using images obtained with the Advanced Camera for Surveys 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 similar to30 elliptical galaxies. Simulations using the latest stellar population models indicate an age scatter for the elliptical galaxies of about 34%, with a mean age tau(L) greater than or similar to 2.6 Gyr (corresponding to z(L) greater than or similar to 2.7), and the last star formation occurring at z(end) greater than or similar to 1.5 Transforming to rest-frame (U-B), we conclude that the slope end and scatter in the CM relation for morphologically selected early-type galaxies show little or no evidence of evolution out to z approximate to 1.2. Thus, elliptical galaxies were already well established in X-ray - luminous clusters when the universe was a third of its present age

Coronagraphic imaging of 3C 273 with the advanced camera for surveys

The nearby and luminous QSO 3C 273 was imaged in 2002 July with the High Resolution Channel (HRC) of the Advanced Camera for Surveys (ACS) in coronagraphic mode in F475W (g), F606W (V), and F814W (I) as part of the Early Release Observations (ERO) program. After subtraction of the remaining PSF of the QSO, these images offer the most detailed view yet of the morphology and colors of the host galaxy of this QSO. We find that the central light distribution is elongated along the jet axis and its outer edge is delineated by an arc, centered on the jet at a radius of similar to2."6 from the QSO and bluer than the surrounding galaxy. This system is embedded in an extended galactic halo. Compared with early-type galaxies of similar redshifts and luminosities, the light distribution of 3C 273 is flatter in the core, likely from suppression by dust, but similar in the outer halo. The QSO is displaced from the isophotal center of the galaxy by similar to1."4. Previously known emission-line extensions are confirmed and new morphological features are identified, including a dramatic spiral-shaped plume, two faint. laments, a dust lane, and a knot along the jet axis. Part of the inner jet is unambiguously detected in all three bandpasses, and its morphology matches that of a MERLIN radio map. Different mechanisms that could explain the morphology of 3C 273 are considered, such as scattered QSO radiation, a face-on disk, and a merger event

Faint galaxies in deep advanced camera for surveys observations

We present the analysis of the faint galaxy population in the Advanced Camera for Surveys (ACS) Early Release Observation fields VV 29 ( UGC 10214) and NGC 4676. These observations cover a total area of 26.3 arcmin(2) and have depths close to that of the Hubble Deep Fields in the deepest part of the VV 29 image, with 10 sigma detection limits for point sources of 27.8, 27.6, and 27.2 AB magnitudes in the g(F475W), V-F606W, and I-F814W bands, respectively. Measuring the faint galaxy number count distribution is a difficult task, with different groups arriving at widely varying results even on the same data set. Here we attempt to thoroughly consider all aspects relevant for faint galaxy counting and photometry, developing methods that are based on public software and that are easily reproducible by other astronomers. Using simulations we determine the best SExtractor parameters for the detection of faint galaxies in deep Hubble Space Telescope observations, paying special attention to the issue of deblending, which significantly affects the normalization and shape of the number count distribution. We confirm, as claimed by Bernstein, Freedman, & Madore, that Kron-like magnitudes, such as the ones generated by SExtractor, can miss more than half of the light of faint galaxies, what dramatically affects the slope of the number counts. We show how to correct for this effect, which depends sensitively not only on the characteristics of the observations, but also on the choice of SExtractor parameters. We present catalogs for the VV 29 and NGC 4676 fields with photometry in the F475W, F606W, and F814W bands. We also show that combining the Bayesian software BPZ with superb ACS data and new spectral templates enables us to estimate reliable photometric redshifts for a significant fraction of galaxies with as few as three filters. After correcting for selection effects, we measure slopes of 0.32 +/- 0.01 for 22 25.5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function (alpha = -1.5), with a strong merging rate following the prescription of Glazebrook et al., phi* proportional to / (1 + Qz), with Q = 4

A large population of `Lyman-break' galaxies in a protocluster at redshift z ~ 4.1

The most massive galaxies and the richest clusters are believed to have emerged from regions with the largest enhancements of mass density relative to the surrounding space. Distant radio galaxies may pinpoint the locations of the ancestors of rich clusters, because they are massive systems associated with ‘overdensities’ of galaxies that are bright in the Lyman-α line of hydrogen. A powerful technique for detecting high-redshift galaxies is to search for the characteristic ‘Lyman break’ feature in the galaxy colour, at wavelengths just shortwards of Lyα, which is due to absorption of radiation from the galaxy by the intervening intergalactic medium. Here we report multicolour imaging of the most distant candidate protocluster, TN J1338–1942 at a redshift z ≈ 4.1. We find a large number of objects with the characteristic colours of galaxies at that redshift, and we show that this excess is concentrated around the targeted dominant radio galaxy. Our data therefore indicate that TN J1338–1942 is indeed the most distant cluster progenitor of a rich local cluster, and that galaxy clusters began forming when the Universe was only ten per cent of its present age

Ultracompact dwarf galaxies in Abell 1689: A photometric study with the advanced camera for surveys

The properties of ultracompact dwarf (UCD) galaxy candidates in Abell 1689 (z = 0.183) are investigated, based on deep high-resolution images from the Hubble Space Telescope Advanced Camera for Surveys. A UCD candidate has to be unresolved, have i < 28 mag (M-V < -11.5 mag), and satisfy color limits derived from Bayesian photometric redshifts. We find 160 UCD candidates with 22 mag < i < 28 mag. We estimate that about 100 of these are cluster members, based on their spatial distribution and photometric redshifts. For i greater than or similar to 26.8 mag, the radial and luminosity distribution of the UCD candidates can be explained well by Abell 1689's globular cluster ( GC) system. For i less than or similar to 26.8 mag, there is an overpopulation of 15 +/- 5 UCD candidates with respect to the GC luminosity function. For i less than or similar to 26 mag, the radial distribution of UCD candidates is more consistent with the dwarf galaxy population than with the GC system of Abell 1689. The UCD candidates follow a color-magnitude trend with a slope similar to that of Abell 1689's genuine dwarf galaxy population, but shifted fainter by about 2-3 mag. Two of the three brightest UCD candidates (M-V similar or equal to -17 mag) are slightly resolved. At the distance of Abell 1689, these two objects would have King profile core radii of similar or equal to35 pc and r(eff) similar or equal to 300 pc, implying luminosities and sizes 2-3 times those of M32's bulge. Additional photometric redshifts obtained with late-type stellar and elliptical galaxy templates support the assignment of these two resolved sources to Abell 1689 but also allow for up to four foreground stars among the six brightest UCD candidates. Our findings imply that in Abell 1689 there are greater than or equal to10 UCDs with M-V < -12.7 mag, probably created by stripping "normal" dwarf or spiral galaxies. Compared with the UCDs in the Fornax Cluster - the location of their original discovery - they are brighter, larger, and have colors closer to normal dwarf galaxies. This suggests that they may be in an intermediate stage of the stripping process. Checking the photometric redshifts of the brightest UCD candidates with spectroscopy would be the next step to definitely confirm the existence of UCDs in Abell 1689

Hubble Space Telescope Advanced Camera for Surveys coronagraphic imaging of the AU microscopii debris disk

We present Hubble Space Telescope Advanced Camera for Surveys multicolor coronagraphic images of the recently discovered edge-on debris disk around the nearby (similar to10 pc) M dwarf AU Microscopii. The disk is seen between r = 0".75 and 15" (7.5-150 AU) from the star. It has a thin midplane with a projected FWHM thickness of 2.5-3.5 AU within r < 50 AU of the star that increases to 6.5-9 AU at r similar to 75 AU. The disk's radial brightness profile is generally flat for r < 15 AU, then decreases gradually (I proportional to r(1.8)) out to r approximate to 43 AU, beyond which it falls rapidly (I proportional to r(-4.7)). Within 50 AU the midplane is straight and aligned with the star, and beyond that it deviates by similar to3degrees, resulting in a bowed appearance that was also seen in ground-based images. Three-dimensional modeling of the disk shows that the inner region (r < 50 AU) is inclined to the line of sight by less than 1degrees and the outer disk by similar to3degrees. The inclination of the outer disk and moderate forward scattering (g approximate to 0.4) can explain the apparent bow. The intrinsic, deprojected FWHM thickness is 1.5-10 AU, increasing with radius. The models indicate that the disk is clear of dust within similar to12 AU of the star, in general agreement with the previous prediction of 17 AU based on the infrared spectral energy distribution. The disk is blue, being 60% brighter at B than I relative to the star. One possible explanation for this is that there is a surplus of very small grains compared with other imaged debris disks that have more neutral or red colors. This may be due to the low radiation pressure exerted by the late-type star. Observations at two epochs show that an extended source seen along the midplane is a background galaxy

Evolution of the color-magnitude relation in high-redshift clusters: Blue early-type galaxies and red pairs in RDCS J0910+5422

The color-magnitude relation has been determined for the RDCS J0910+5422 cluster of galaxies at redshift z = 1.106. Cluster members were selected from the Hubble Space Telescope Advanced Camera for Surveys (HST ACS) images, combined with ground-based near-IR imaging and optical spectroscopy. The observed early-type color-magnitude relation (CMR) in i(775) - z(850) versus z(850) shows an intrinsic scatter in color of 0.060 +/- 0.009 mag, within 10 from the cluster X-ray emission center. Both the elliptical and the S0 galaxies show small scatter about the CMR of 0.042 +/- 0.010 and 0.044 +/- 0.020 mag, respectively. From the scatter about the CMR, a mean luminosity weighted age t > 3: 3 Gyr (z(f) approximate to 3) is derived for the elliptical galaxies, assuming a simple stellar population modeling (single-burst solar metallicity). Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in i(775) - z(850), by 0.07 +/- 0.02 mag, than the ellipticals. The ellipticity distribution as a function of color indicates that the face-on S0s in this particular cluster have likely been classified as elliptical. Thus, if anything, the offset in color between the elliptical and S0 populations may be even more significant. The color offset between S0 and E galaxies corresponds to an age difference of approximate to 1 Gyr for a single-burst solar-metallicity model. A solar-metallicity model with an exponential decay in star formation will reproduce the offset for an age of 3.5 Gyr; i.e., the S0s have evolved gradually from star-forming progenitors. The early-type population in this cluster appears to be still forming. The blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct progenitors of the more evolved S0s that follow the same red sequence as elliptical galaxies in other clusters. Thirteen red galaxy pairs are observed, and the galaxies associated in pairs constitute similar to 40% of the CMR galaxies in this cluster