Advanced Camera For Surveys Images Research Articles

Dynamical Evolution of Globular Cluster Systems Formed in Galaxy Mergers: DeepHubble Space TelescopeAdvanced Camera for Surveys Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the giant elliptical galaxy NGC 3610, a well-established remnant of a dissipative galaxy merger. These observations supersede previous, shallower observations that revealed the presence of a population of metal-rich globular clusters of intermediate age (~1.5-4 Gyr). We detect a total of 580 cluster candidates, 46% more than from the previous WFPC2 images. The new photometry strengthens the significance of the previously found bimodality of the color distribution of clusters. Peak colors in V - I are 0.93 ± 0.01 and 1.09 ± 0.01 for the blue and red subpopulations, respectively. The luminosity function of the inner 50% of the metal-rich (red) population of clusters differs markedly from that of the outer 50%. In particular, the luminosity function of the inner 50% of the red clusters shows a flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue clusters. This is consistent with predictions of recent models of cluster disruption for the age range mentioned above and for metallicities that are consistent with the peak color of the red clusters, as predicted by population synthesis models. The radial surface density profile of red clusters follows that of the underlying galaxy light more closely than in elliptical galaxies, which is consistent with the intermediate-age nature of the red clusters. We determine the specific frequency of clusters in NGC 3610 and find a present-day value of SN = 1.4 ± 0.6. Using published age estimates for the diffuse light of NGC 3610, as well as cluster disruption models, we estimate that this value will increase to SN = 3.8 ± 1.7 at an age of 10 Gyr, which is consistent with typical SN values for elliptical galaxies. Our findings constitute further evidence in support of the notion that metal-rich cluster populations formed during major mergers involving gas-rich galaxies can evolve dynamically (through disruption processes) into the red, metal-rich cluster populations that are ubiquitous in normal giant ellipticals.

Hidden activity in high-redshift spheroidal galaxies from mid-infrared and X-ray observations in the GOODS-North field

We exploit very deep mid-infrared (MIR) and X-ray observations by Spitzer and Chandra in the GOODS-North field to identify signs of hidden [either starburst or active galactic nucleus (AGN)] activity in spheroidal galaxies between z≃ 0.3 and 1. Our reference is a complete sample of 168 morphologically classified spheroidal (elliptical/lenticular) galaxies with zAB < 22.5 selected from GOODS Advanced Camera for Surveys imaging. Of these 19 have 24-μm detections in the GOODS catalogue, half of which have an X-ray counterpart in the 2-Ms Chandra catalogue (six detected in the 2–10 keV X-rays hard band), while about 25 per cent have 1.4-GHz fluxes larger than 40 μJy. Traces of hidden activity in the spheroidal population are also searched for in the deep X-ray images and 14 additional galaxies are detected in X-rays only. The nature of the observed MIR emissions is investigated by modelling their spectral energy distributions based on the available multiwavelength photometry, including X-ray, ultraviolet, optical, near-infrared, MIR and radio fluxes, and optical spectroscopy. The amount of dust derived from the infrared emission observed by Spitzer appears in excess of that expected by mass loss from evolved stars. When the available independent diagnostics are compared, in general, they provide consistent classifications about the nature of the activity in our spheroidal population. Given that, in principle, none of these diagnostics alone can be considered as conclusive, only trough a panchromatic comparison of them, we can reach an accurate comprehension of the underlying physical processes. In particular, our multiwavelength analysis of the X-ray and MIR properties leads us to conclude that at least eight of the 19 24-μm bright sources should hide an obscured AGN, while the X-ray undetected sources are more likely dominated by star formation. We conclude that ∼30 objects (∼20 per cent) of the original flux-limited sample of 168 spheroidal galaxies in the GOODS-North are detected during phases of prominent activity, of both stellar and quasar origin. Due to the short expected lifetimes of the infrared and X-ray emissions, this fraction might imply a significant level of activity in this class of galaxies during the relatively recent cosmic epochs –z∼ 0.3–1– under investigation.

A Population of Binaries in the Asymptotic Giant Branch of 47 Tucanae?

We have used a set of archived Hubble Space Telescope Advanced Camera for Surveys images to probe the evolved populations of the globular cluster 47 Tucanae. We find an excess of asymptotic giant branch (AGB) stars in the cluster core. We interpret this feature as the signature of an extrapopulation likely made by the progeny of massive stars originated by the evolution of binary systems. Indeed, the comparison with theoretical tracks suggests that the AGB population of 47 Tuc can be significantly contaminated by more massive stars currently experiencing the first ascending red giant branch.

Supermassive black holes in BCGs

AbstractWe observed a sample of three Brightest Cluster Galaxies (BCGs), Abell 1836-BCG, Abell 2052-BCG, and Abell 3565-BCG, with the Advanced Camera for Surveys (ACS) and the Imaging Spectrograph (STIS) on board the Space Telescope. For each target galaxy we obtained high-resolution spectroscopy of the Hα and [N II] λ6583 emission lines at three slit positions, to measure the central ionized-gas kinematics. ACS images in three different filters (F435W, F625W, and FR656N) have been used to determine the optical depth of the dust, stellar mass distribution near the nucleus, and intensity map. We present supermassive black hole (SBH) mass estimates for two galaxies which show regular rotation curves and strong central velocity gradients, and an upper limit on the SBH mass of the third one. For the SBHs of Abell 1836-BCG and Abell 3565-BCG, we derived M• = 4.8+0.8−0.7 × 109 M⊙ and M• = 1.3+0.3−0.4 × 109 M⊙ at 1 σ confidence level, respectively. For the SBH of Abell 2052-BCG, we found M• ≤ 7.3 × 109 M⊙.

Evolution of the Color‐Magnitude Relation in High‐Redshift Clusters: Early‐Type Galaxies in the Lynx Supercluster atz∼ 1.26

Color-magnitude relations have been derived in the clusters RX J0849+4452 and RX J0848+4453 at z~1.26. The color-magnitude relation was determined from Advanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight color-magnitude relation very similar in color, although the two clusters present different X-ray luminosities and shapes, with RX J0849+4452 being three times more X-ray luminous and more compact, and having a temperature two times higher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452 and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/- 0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot, gives a mean luminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453. Even at a lookback time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place - apparently formed some 2.5~Gyr earlier at z~3

Identification of the Red Supergiant Progenitor of Supernova 2005cs: Do the Progenitors of Type II‐P Supernovae Have Low Mass?

The stars that end their lives as supernovae (SNe) have been directly observed in only a handful of cases, mainly because of the extreme difficulty of identifying them in images obtained prior to the SN explosions. Here we report the identification of the progenitor for the recent Type II-plateau (core collapse) SN 2005cs in pre-explosion archival images of the Whirlpool Galaxy (M51) obtained with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). From high-quality ground-based images of the SN obtained with the Canada-France-Hawaii Telescope, we precisely determine the position of the SN and are able to isolate the SN progenitor to within 004 in the HST ACS optical images. We further pinpoint the SN location to within 0005 from HST ACS ultraviolet images of the SN, confirming our progenitor identification. From photometry of the SN progenitor obtained with the pre-SN ACS images, and also from limits to its brightness in pre-SN HST NICMOS images, we infer that the progenitor is a red supergiant star of spectral type K3-M4 with initial mass 10 ± 3 M☉. We also discuss the implications of the SN 2005cs progenitor identification and its mass estimate. There is an emerging trend that the most common Type II-plateau SNe originate from low-mass supergiants (8-20 M☉).

Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field

We present Hubble Space Telescope Advanced Camera for Surveys images and color-magnitude diagrams for 25 nearby galaxies with radial velocities VLG < 500 km s-1. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, are found to be the nearest known representatives of blue compact dwarf objects. Using high-quality data on distances and radial velocities of 110 nearby field galaxies, we derive their mean Hubble ratio to be 68 km s-1 Mpc-1 with a standard deviation of 15 km s-1 Mpc-1. Peculiar velocities of most of the galaxies, Vpec = VLG - 68D, follow a Gaussian distribution with σv = 63 km s-1 but with a tail toward high negative values. Our data display the known correlation between peculiar velocity and galaxy elevation above the Local Supercluster plane. The small observed fraction of galaxies with high peculiar velocities, Vpec < -500 km s-1, may be understood as objects associated with nearby groups (Coma I, Eridanus) outside the local volume.

The Globular Cluster System of the Virgo Dwarf Elliptical Galaxy VCC 1087

We present an analysis of the globular cluster (GC) system of the nucleated dwarf elliptical galaxy VCC 1087 in the Virgo Cluster based on Keck LRIS spectroscopy and archival Hubble Space Telescope Advanced Camera for Surveys imaging. We estimate that VCC 1087 hosts a total population of 77 ± 19 GCs, which corresponds to a relatively high V-band specific frequency of 5.8 ± 1.4. The g475 - z850 color distribution of the GCs shows a blue (metal-poor) peak with a tail of redder (metal-rich) clusters similar in color to those seen in luminous elliptical galaxies. The luminosity function of the GCs is lognormal and peaks at M = -7.2 ± 0.3, M = -8.1 ± 0.2. These peak positions are consistent with those found for luminous Virgo elliptical galaxies, suggesting either the lack of or, surprisingly similarly, the dynamical destruction processes of GCs among dwarf and giant galaxies. Spectroscopy of a subsample of 12 GCs suggests that the GC system is old and coeval (10 Gyr), with a fairly broad metallicity distribution (-1.8 [M/H] -0.8). In contrast, an integrated spectrum of the underlying galaxy starlight reveals that its optical luminosity is dominated by metal-rich, intermediate-age stars. The radial velocities of the GCs suggest rotation close to the major axis of the galaxy, and this rotation is dynamically significant with (vrot/σlos)* > 1. A compilation of the kinematics of the GC systems of nine early-type galaxies shows surprising diversity in the (vrot/σlos) parameter for GC systems. In this context, the GC system of VCC 1087 exhibits the most significant rotation-to-velocity dispersion signature. Dynamical mass modeling of the velocity dispersion profile of the GCs and galaxy stars suggests fairly constant mass-to-light ratios of ~3 out to 6.5 kpc. The present observations can entertain both baryonic and nonbaryonic solutions, and GC velocities at larger radii would be most valuable with regard to this issue. Finally, we discuss the evolution of VCC 1087 in terms of the galaxy harassment scenario and conclude that this galaxy may well be the remains of a faded, tidally perturbed Sc spiral.

Hubble Space TelescopeACS Images of the GG Tauri Circumbinary Disk

Hubble Space Telescope Advanced Camera for Surveys images of the young binary GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002 and are the most detailed of this system to date. They confirm features previously seen in the disk including a gap apparently caused by shadowing from circumstellar material, an asymmetrical distribution of light about the line of sight on the near edge of the disk, enhanced brightness along the near edge of the disk due to forward scattering, and a compact reflection nebula near the secondary star. New features are seen in the ACS images: two short filaments along the disk, localized but strong variations in the disk intensity (gaplets), and a spur or filament extending from the reflection nebulosity near the secondary. The back side of the disk is detected in the V band for the first time. The disk appears redder than the combined light from the stars, which may be explained by a varied distribution of grain sizes. The brightness asymmetries along the disk suggest that it is asymmetrically illuminated by the stars due to extinction by nonuniform circumstellar material or that the illuminated surface of the disk is warped by tidal effects (or perhaps both). Localized, time-dependent brightness variations in the disk are also seen.

The Araucaria Project: The Effect of Blending on the Cepheid Distance to NGC 300 from Advanced Camera for Surveys Images

We have used the Advanced Camera for Surveys aboard the Hubble Space Telescope to obtain F435W, F555W and F814W single-epoch images of six fields in the spiral galaxy NGC 300. Taking advantage of the superb spatial resolution of these images, we have tested the effect that blending of the Cepheid variables studied from the ground with close stellar neighbors, unresolved on the ground-based images, has on the distance determination to NGC 300. Out of the 16 Cepheids included in this study, only three are significantly affected by nearby stellar objects. After correcting the ground-based magnitudes for the contribution by these projected companions to the observed flux, we find that the corresponding Period-Luminosity relations in V, I and the Wesenheit magnitude W_I are not significantly different from the relations obtained without corrections. We fix an upper limit of 0.04 magnitudes to the systematic effect of blending on the distance modulus to NGC 300. As part of our HST imaging program, we present improved photometry for 40 blue supergiants in NGC 300.

Optical and X-Ray Identification of Faint Radio Sources in the GOODS CDF-S Advanced Camera for Surveys Field

We present optical and X-ray identifications for the 64 radio sources in the Great Observatories Origins Deep Survey Chandra Deep Field-South Advanced Camera for Surveys (ACS) field revealed in the Australia Telescope Compact Array 1.4 GHz survey of the Chandra Deep Field-South. Optical identifications are made using the ACS images and catalogs, while the X-ray view is provided by the Chandra X-Ray Observatory 1 Ms observations. Redshifts for the identified sources are drawn from publicly available catalogs of spectroscopic observations and multiband photometric-based estimates. Using this multiwavelength information we provide a first characterization of the faint radio source population in this region. The sample contains a mixture of star-forming galaxies and active galactic nuclei, as identified by their X-ray properties and optical spectroscopy. A large number of morphologically disturbed galaxies are found, possibly related to star formation. In spite of the very deep optical data available in this field, seven of the 64 radio sources have no optical identification to z850 ~ 28 mag. Only one of these is identified in the X-ray.

The discovery of two new galaxy-galaxy lenses from the Sloan Digital Sky Survey

The gravitational lens configuration where a background galaxy is closely aligned with a foreground galaxy can provide accurate measurement of the dark matter density profile in the foreground galaxy, free of dynamical assumptions. Currently, only three such galaxy-galaxy lenses are known where the lensed source has a confirmed redshift and is reasonably bright at optical wavelengths, and therefore suitable for observations with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). Two of these were discovered by noting an anomalous emission line (from the source) in the spectrum of a massive early-type galaxy (the lens). To find further galaxy-galaxy lenses suitable for ACS imaging, we have looked for anomalous emission lines in the luminous red galaxy (LRG) subsample of the Sloan Digital Sky Survey (SDSS) data release 3 (DR3) spectroscopic data base. Our search methodology has similarities to that applied by Bolton et al. (which has had recent success), but extends the upper redshift limit for lensed sources to z ≃ 1.4. Here, we report follow-up imaging and spectroscopic observations of two candidates, confirmed as gravitational lenses by the detection of multiple images in the line of [OII]λλ3726, 3729. In the first system, J145957.1-005522.8, the lens at z = 0.58 consists of two LRGs. The anomalous emission line is confirmed as [OII] by the detection of the corresponding Hy line, providing a source redshift of z = 0.94. In the second system, J230946.3-003912.9, the lens is a single LRG at z = 0.29, and the source redshift is z = 1.00, confirmed by partially resolving the [OII] doublet.

Feedback and Brightest Cluster Galaxy Formation: ACS Observations of the Radio Galaxy TN J1338−1942 atz= 4.1

We present deep optical imaging of the z = 4.1 radio galaxy TN J1338-1942, obtained using the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, as well as ground-based near-infrared imaging data from the European Southern Observatory (ESO) Very Large Telescope (VLT). The radio galaxy is known to reside within a large galaxy overdensity (both in physical extent and density contrast). There is good evidence that this region is the progenitor of a present-day rich galaxy cluster. TN J1338 is the dominant galaxy in the protocluster in terms of size and luminosity (in both the optical and near-infrared) and therefore seems destined to evolve into the brightest cluster galaxy. The high spatial resolution ACS images reveal several kiloparsec-scale features within and around the radio galaxy. The continuum light is aligned with the radio axis and is resolved into two clumps in the i775 and z850 bands. These components have luminosities ~109 L☉ and sizes of a few kpc. The estimated nebular continuum, scattered light, synchrotron- and inverse Compton-scattering contributions to the aligned continuum light are only a few percent of the observed total, indicating that the observed flux is likely dominated by forming stars. The estimated star formation rate for the whole radio galaxy is ~200 M☉ yr-1. A simple model in which the jet has triggered star formation in these continuum knots is consistent with the available data. A striking, but small, linear feature is evident in the z850 aligned light and may be indicative of a large-scale shock associated with the advance of the radio jet. The rest of the aligned light also seems morphologically consistent with star formation induced by shocks associated with the radio source, as seen in other high-z radio galaxies (e.g., 4C 41.17). An unusual feature is seen in Lyα emission. A wedge-shaped extension emanates from the radio galaxy perpendicularly to the radio axis. This naturally connects to the surrounding asymmetric, large-scale (~100 kpc) Lyα halo. We posit that the wedge is a starburst-driven superwind associated with the first major epoch of formation of the brightest cluster galaxy. The shock and wedge are examples of feedback processes due to both active galactic nucleus and star formation in the earliest stages of massive galaxy formation.

UV Continuum Spectroscopy of a 6 L * z = 5.5 Starburst Galaxy

We have obtained a high S/N (22.3 hr integration) UV continuum VLT FORS2 spectrum of an extremely bright (z850 = 24.3) z = 5.515 ± 0.003 star-forming galaxy (BD38) in the field of the z = 1.24 cluster RDCS 1252.9-2927. From HST Advanced Camera for Surveys imaging, this object was selected as a potential z ~ 6 Lyman break galaxy (LBG) based on its red i775 - z850 = 1.5 color. This object shows substantial continuum (0.41 ± 0.02 μJy at 1300 A) and low-ionization interstellar absorption features typical of LBGs at lower redshift (z ~ 3); this is the highest redshift LBG confirmed via metal-absorption spectral features. The equivalent widths of the absorption features are similar to z ~ 3 strong Lyα absorbers. No noticeable Lyα emission was detected (F ≤ 1.4 × 10-18 ergs cm-2 s-1, 3 σ). This object is at most amplified 0.3 mag from gravitational lensing by the foreground cluster. The delensed half-light radius of this object is 1.6 kpc (025), and the star formation rate derived from the rest-frame UV luminosity is SFRUV = 38 h M☉ yr-1 (142 h M☉ yr-1 corrected for dust extinction). In terms of recent determinations of the z ~ 6 UV luminosity function, this object appears to be 6L*. The Spitzer IRAC fluxes for this object are 23.3 and 23.2 AB mag (delensed) in the 3.6 and 4.5 μm channels, respectively, implying a mass of (1-6) × 1010 M☉ from population synthesis models. This galaxy is brighter than any confirmed z ~ 6 i-dropout to date in the z850 band, and in the 3.6 and 4.5 μm channels, and is the most massive starbursting galaxy known at z > 5.

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

Abstract We report on the discovery of a fifth lensed image in the large separation lensed quasar system SDSS J1004$+$4112. A faint point source located ${0\rlap {.}{}^{\mathrm {\prime \prime }}2}$ from the center of the brightest galaxy in the lensing cluster was detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth lensed image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves, and thereby the mass distribution of the lensing cluster.

Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch

Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I 29. Our photometry directly reveals the core helium burning stellar population (the red clump or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average [M/H] = -0.64 and dispersion 0.49 dex. The metallicity distribution function is virtually identical to that found in other halo fields observed previously with HST, but with an enhanced metal-rich population that was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity-sensitive colors of the RGB stars with the metallicity- and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be ~8 Gyr. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.

Hubble Space TelescopeACS Weak‐Lensing Analysis of the Galaxy Cluster RDCS 1252.9−2927 atz= 1.24

We present a weak-lensing analysis of one of the most distant massive galaxy clusters known, RDCS 1252.9-2927, at z = 1.24, using deep images from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). By taking advantage of the depth and angular resolution of the ACS images, we detect for the first time at z > 1 a clear weak-lensing signal in both the i (F775W) and z (F850LP) filters. We measure a 5 σ signal in the i band and a 3 σ signal in the shallower z-band image. The two radial mass profiles are found to be in very good agreement with each other, and they provide a measurement of the total mass of the cluster inside a 1 Mpc radius of M(< 1 Mpc) = (8.0 ± 1.3) × 1014 M☉ in the current cosmological concordance model, h = 0.70, Ωm = 0.3, and ΩΛ = 0.7, assuming a redshift distribution of background galaxies as inferred from the Hubble Deep Field surveys. A weak-lensing signal is detected out to the boundary of our field (3' radius, corresponding to 1.5 Mpc at the cluster redshift). We detect a small offset between the centroid of the weak-lensing mass map and the brightest cluster galaxy, and we discuss the possible origin of this discrepancy. The cumulative weak-lensing radial mass profile is found to be in good agreement with the X-ray mass estimate based on Chandra and XMM-Newton observations, at least out to R500 ≃ 0.5 Mpc.

Distant Red Galaxies in the Hubble Ultra Deep Field

We take advantage of the Hubble Ultra Deep Field data to study the rest-frame optical and UV morphologies of the novel population of distant red galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks = 21.5, five of which have photometric redshifts zphot ≳ 2, corresponding to a surface density of 0.9 arcmin-2. The surface brightness distributions of the zphot ≳ 2 galaxies are better represented by exponential disks than R1/4 laws. Two of the zphot ≳ 2 galaxies are extended, while three have compact morphologies. The rest-frame optical morphology of the zphot ≳ 2 galaxies is quite different from the rest-frame UV morphology: All the galaxies have red central components that dominate in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) H160-band images, as well as distinct off-center blue features that show up in (and often dominate) the Advanced Camera for Surveys (ACS) images. The mean measured effective radius of the zphot ≳ 2 galaxies is ⟨re⟩ = 1.9 ± 1.4 kpc, similar (within the errors) to the mean size of Lyman break galaxies at similar redshifts. All the DRGs are resolved in the ACS images, while four are resolved in the NICMOS images. Two of the zphot ≳ 2 galaxies are bright X-ray sources and hence host active galactic nuclei (AGNs). One of these galaxies is resolved in the ACS and NICMOS images, which means that the AGN does not dominate its rest-frame UV/optical spectral energy distribution (SED), while the other is unresolved in the NICMOS images and hence could have an AGN-dominated SED. The diverse rest-frame optical and UV morphological properties of DRGs derived here suggest that they have complex stellar populations consisting of both evolved populations, which dominate the mass and the rest-frame optical light, and younger populations that show up as patches of star formation in the rest-frame UV light, in many ways resembling the properties of normal local galaxies. This interpretation is supported by fits to the broadband SEDs, which for all five zphot ≳ 2 galaxies are best represented by models with extended star formation histories and substantial amounts of dust.

TheHubble Deep FieldNorth SCUBA Super-map - III. Optical and near-infrared properties of submillimetre galaxies

We present a new submillimetre (submm) super-map in the Hubble Deep Field North (HDF-N) region (Great Observatories Origins Deep Survey North, GOODS-N, field), containing 40 statistically robust sources at 850 μm. This map contains additional data, and several new sources, including one of the brightest blank-sky extragalactic submm sources ever detected. We have used the Advanced Camera for Surveys (ACS) Hubble Space Telescope (HST) images and ground-based near-infrared (IR) observations from the Great Observatories Origins Deep Survey (GOODS), along with deep radio observations, to develop a systematic approach for counterpart identification. With the depth achieved by this survey, optical counterparts have been found for all the radio-detected (RD) submm sources. We have used the colours, morphologies and photometric redshifts of these secure identifications to help identify counterparts to the radio-undetected (RU) sources, finding that certain combinations of optical properties can be used to successfully identify the counterpart to a submm source. 72 per cent of our sources with optical coverage have a unique optical counterpart using our new techniques for counterpart identification and an additional 18 per cent have more than one possibility that meet our criteria in the ACS images. Thus, only ∼10 per cent of our sources lack a plausible optical/near-IR counterpart, meaning that we have the first sample of Submillimetre Common User Bolometer Array (SCUBA) sources that is nearly completely identified in the optical. We have found a much higher extremely red object (ERO) rate than other submm surveys, as a result of the increased depth in the optical images. The median photometric redshift (and quartile range), from optical and near-IR data, is 1.7 (1.3-2.5) for the RD submm sources and rises to 2.3 (1.3-2.7) for the RU subsample. We find interesting correlations between the 850-μm flux and both the ι 775 magnitude and the photometric redshift, from which there appears to be an absence of high-redshift faint counterparts to the lower flux density SCUBA sources. While the quantitative morphologies span a range of values, in general the submm galaxies show larger sizes and a higher degree of asymmetry than other galaxy populations at the same redshifts. In the Appendix, we discuss several improvements in our data analysis procedure, including methods of testing for source reliability.

Evidence of Primordial Clustering around the QSO SDSS J1030+0524 at z = 6.28

We present tentative evidence of primordial clustering, manifested as an excess of color-selected objects in the field of the QSO SDSS J1030+0524 at redshift z = 6.28. We have selected objects red in i775 - z850 on the basis of Hubble Space Telescope Advanced Camera for Surveys imaging of a field centered on the QSO. Compared to data at comparable depth obtained by the Great Observatories Origins Deep Survey, we find an excess of objects with i775 - z850 ? 1.5 in the QSO field. The significance of the detection is estimated to be ~97% on the basis of the counts alone and increases to 99.4% if one takes into account the color distribution. If confirmed, this would represent the highest redshift example of galaxy clustering and would have implications on models for the growth of structure. Bias-driven clustering of first luminous objects forming in the highest peaks of the primordial density field is expected in most models of early structure formation. The redshift of one of the candidates has been found to be z = 5.970 by our spectroscopy with the Keck I Low Resolution Imaging Spectrometer, confirming the validity of our color selection.