[show abstract][hide abstract] ABSTRACT: We combined optical Hubble Space Telescope ACS images with mid-infrared Spitzer data of the two young star clusters NGC346 and NGC602 in the Small Magellanic Cloud, to study how local and global conditions may affect the process of star formation. We found that, while general conditions such as metallicity, or the mass or morphological type of the parent galaxy do not strongly affect the process of star formation, local conditions such as the gas and stellar density can affect both how star formation occurs and propagates, and also the evolution of a star cluster from early times.
Astrophysics and Space Science 08/2013; · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the photometric catalogs for the star-forming cluster NGC 602 in the wing of the Small Magellanic Cloud covering a range of wavelengths from optical HST/ACS (F555W, F814W) and SMARTS/ANDICAM (V, I) to infrared (Spitzer/IRAC 3.6, 4.5, 5.8, and 8 micron and MIPS 24 micron). Combining this with IRSF (InfraRed Survey Facility) near-infrared photometry (J, H, Ks), we compare the young main sequence (MS) and pre-main sequence (PMS) populations prominent in the optical with the current young stellar object (YSO) populations revealed by the infrared (IR). We analyze the MS and PMS population with isochrones in color-magnitude diagrams to derive ages and masses. The optical data reveal ~565 PMS candidates, low mass Stage III YSOs. We characterize ~40 YSOs by fitting their spectral energy distributions (SEDs) to a grid of models (Robitaille et al. 2007) to derive luminosities, masses and evolutionary phase (Stage I-III). The higher resolution HST images reveal that ~70% of the YSO candidates are either multiples or protoclusters. For YSOs and PMS sources found in common, we find a consistency in the masses derived. We use the YSO mass function to derive a present-day star-formation rate of ~0.2-1.0 Msun/yr/kpc^2, similar to the rate derived from the optical star formation history suggesting a constant star formation rate for this region. We demonstrate a progression of star formation from the optical star cluster center to the edge of the star forming dust cloud. We derive lifetimes of a few 10^5 years for the YSO Stages I and II. Comment: 55 pages, 18 Figures; High resolution images available form author Accepted to ApJ
The Astrophysical Journal 12/2010; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The ACS/WFC detector consists of two CCDs, each of which is read out through two amplifiers. While reading each quadrant of the detector, the electronic crosstalk between the amplifiers induces faint, typically negative, mirror-symmetric ghost images on the other three quadrants. The effect is strongest for high-signal offending (source) pixels. Analysis of the pre-SM4 crosstalk showed that its impact on ACS/WFC science was insignificant and can be ignored in most science applications. In this report, we analyze crosstalk after SM4. Crosstalk due to low-signal offenders is much weaker than before SM4 and does not produce ghosts similar to those seen in pre-SM4 images. For high-signal offending pixels, we find substantial differences between the gain=1 e^-/DN and gain=2 e^-/DN cases. For the default gain setting of 2, the crosstalk is similar to what it was before the SM4, up to 5-8 e^-" per pixel on the same CCD. For gain=1, the crosstalk is 100 e^- per pixel for saturated offending pixels on the same CCD, which is more than an order of magnitude above the pre- SM4 level. The crosstalk from saturated pixels is 20-30 e^- per pixel on the other CCD, which is also much higher than it was before SM4. For gain=1.4, the crosstalk is very similar to that at gain=2.
[show abstract][hide abstract] ABSTRACT: We present analysis of the young stellar and proto-stellar populations
in star-forming clusters NGC 602 in the SMC and N206 (Henize 206) in the
LMC. Deep, photometric data over the wavelength range 0.36 to 24 μm
allow us to probe stellar populations and recent star formation using a
combination of Color-Magnitude Diagram analysis, model Spectral Energy
Distribution fitting, and image inspection. We combine both ground- and
space-based observations to develop a more complete picture of star
formation and its progression in these regions, from embedded, infrared
bright YSOs (Young Stellar Objects) to faint optical Pre-Main Sequence
sources to optically bright O and B type stars. Infrared data are drawn
from the Spitzer Legacy Projects Surveying the Agents of a Galaxy's
Evolution (SAGE-LMC and SAGE-SMC). Near infrared data are from InfraRed
Survey Facility (IRSF) Magellanic Clouds Point Source Catalog of Kato,
et al. (2007). N206 optical data are taken from the Magellanic Cloud
Photometric Survey (MCPS), and NGC 602 optical data are from Hubble/ACS
(Advanced Camera for Surveys) and ground-based SMARTS (Small and
Moderate Aperture Research Telescope System) observations.
The SAGE Project is supported by NASA/Spitzer grant 1275598 and NASA
American Astronomical Society Meeting Abstracts #215; 01/2010
[show abstract][hide abstract] ABSTRACT: Within the framework of a large coordinated HST effort to study the star formation processes and history of several fields in the SMC, we present the detailed history of the young cluster NGC602. In particular, we explore the characteristics of the many candidate pre main sequence (PMS) stars. We combine a new set of PMS stellar tracks for the metallicity Z = 0.004 with a stellar population synthesis model that takes into account all evolutionary phases. The best star formation is searched by comparing observed and synthetic CMDs.
Memorie della Societa Astronomica Italiana Supplementi. 01/2010;
[show abstract][hide abstract] ABSTRACT: The Advanced Camera for Surveys (ACS) has been continuously exposed to the space radiation environment since being installed during Hubble Servicing Mission 3B in March 2002. We have been monitoring and calibrating the effects of radiation damage in the CCDs during this 7-year period, which includes several changes in operating temperature and procedures for annealing at +20C. Radiation damage on-orbit leads to an increase of the mean dark current, a continuous growth of permanent hot pixels (defined to be greater than 0.08 e-/sec), and loss of charge transfer efficiency (CTE, see related paper by Chiaberge). In July 2006, coincident with the switch to the redundant side 2 electronics, the operating temperature of the CCDs was lowered from -77 C to -81 C, which caused the dark current rate and the hot pixel contamination to be reduced by more than 50%. Following the ACS failure in January 2007, the camera experienced a relatively warm 28-month "pseudo-annealing" period at -10 C. We summarize the radiation damage history of the two Wide Field Channel (WFC) CCDs, and our corresponding bias and dark calibration strategy for ACS science data, with results following the on-orbit installation of the new controller electronics box (CEB-R) during Hubble Servicing Mission 4 in May 2009. In general, we have found the effects of ongoing radiation damage, and its impact on our bias and dark calibration, to be largely predictable given the inflight age of the CCDs -- as if our repaired camera had been working all along. The horizontal striping issue (see related paper by Grogin) is based in the new electronics -- it is not an effect of radiation damage -- but we are investigating how to mitigate its impact on our calibration reference files.
[show abstract][hide abstract] ABSTRACT: The newly replaced CCD electronics box (CEB-R) of the Hubble Space
Telescope (HST) Advanced Camera for Surveys (ACS) features a
programmable SIDECAR ASIC manufactured by Teledyne. The CEB-R not only
restores the functionality of the ACS Wide Field Camera (WFC), but it
allows optimization of the WFC's imaging performance via on-orbit
adjustment of CCD bias and clock voltages and serial-data transmission
timing. We describe the strategy, preparation, execution, and results of
the ACS Optimization Campaign, an unprecedented on-orbit extension of
ground-based integration and testing that was conducted during the HST
Servicing Mission Observatory Verification period.
[show abstract][hide abstract] ABSTRACT: The NIRSpec Demonstration Model (DM) has recently completed its cryogenic test campaign. The main objective was to verify the structural and optical integrity of the NIRSpec design, as well as to check the cryo-chamber and the entire suite of optical ground support equipment. The NIRSpec optical train was representative up to the plane of the Microshutter Array, and telescope-like illumination was provided via custom-designed light sources. In this poster, we summarize the set-up, goals, procedures, and initial results from the DM test campaign.
[show abstract][hide abstract] ABSTRACT: The newly installed Wide Field Camera 3 on the Hubble Space Telescope will be extensively used by GOs during Cycle 17 for imaging astrophysical phenomena in the UV, optical, and IR. During SMOV4 and Cycle 17, the WFC3 Instrument Team will measure the photometric performance of both the UVIS and IR cameras. These observations will target a set of bright spectrophotometric standard stars and two nearby star clusters. We describe the observational strategy to measure the photometric zeropoints of all filters and the transformation equations to relate the fluxes of objects measured in these filters to other popular ground and space based systems. A detailed discussion of the rationale for the choice of targets, and the analysis plan, are also presented.
Space Telescope WFC Instrument Science Report. 06/2009;
[show abstract][hide abstract] ABSTRACT: (Abridged). We present HST photometry of the late-type dwarf galaxy NGC 1705 observed with the WFPC2 in the U and B bands, and with the ACS/HRC in the U, V, and I bands. We cross-correlate these data with previous ones acquired with the WFPC2 in V and I and derive multiband Color-Magnitude diagrams (CMDs) of the cross-identified individual stars and candidate star clusters. For the central regions of the galaxy, where HST-NICMOS J and H photometry is also available, we present U, B, V, I, J, H CMDs of the 256 objects with magnitudes measured in all bands. While our previous study based on V, I, J and H data allowed us to trace the star formation history of NGC 1705 back to a Hubble time, the new data provide a better insight on its recent evolution. With the method of the synthetic CMDs, we confirm the presence of two strong bursts of star formation (SF). The older of the two bursts (B1) occurred between ~ 10 and 15 Myr ago, while the younger burst (B2) started ~ 3 Myr ago, and it is still active. The stellar mass produced by B2 amounts to ~ 10^6 msun, and it is a factor of ~ 3 lower for B1. The interburst phase was likely characterized by a much lower level of SF rather than by its complete cessation. The two bursts show distinct spatial distributions: while B1 is centrally concentrated, B2 is more diffused, and presents ring and arc-like structures that remind of an expanding shell. This suggests a feedback mechanism, in which the expanding superbubble observed in NGC 1705, likely generated by the (10-15) Myr burst, triggered the current strong SF activity. From the HRC data, we identified 12 star clusters (plus the SSC) in the central region of NGC 1705, 10 of which have photometry in all the UBVIJH bands. Using the GALEV models, we derived ages from ~ 10 Myr to ~ 1 Gyr, and masses between ~ 10^4 and 10^5 msun for the clusters.
The Astronomical Journal 05/2009; · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Deep Hubble Space Telescope/Advanced Camera for Surveys photometry of the young cluster NGC 602, located in the remote low-density "wing" of the Small Magellanic Cloud (SMC), reveals numerous pre-main-sequence (PMS) stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history (SFH) into recent times and constraining several stellar population properties, such as the present-day mass function (PDMF), the initial mass function, and the binary fraction. To better characterize the PMS population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the SMC (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the SFH in the region by comparing observed and synthetic color-magnitude diagrams. The derived PDMF for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching (0.3-0.7) × 10–3 M ☉ yr–1 in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.
The Astronomical Journal 02/2009; 137(3):3668. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a study of progressive Star Formation in young Magellanic Cloud clusters by examining their stellar and protostellar populations across a range of wavelengths. Infrared data from the SAGE (Surveying the Agents of a Galaxy's Evolution) Spitzer Legacy programs SAGE-LMC and SAGE-SMC are complemented by HST optical imaging, Magellanic Clouds Photometric Survey optical photometry, and near-IR data from the IRSF (InfraRed Survey Facility) Magellanic Clouds Point Source Catalog. Optical stellar populations are dated via the examination of Color-Magnitude Diagrams and comparison to theoretical isochrones. Embedded, infrared bright Young Stellar Objects (YSOs) are explored through the use of T. Robitaille's Spectral Energy Distribution fitter. We couple this analysis with the consideration of star forming environments as revealed through imaging across the optical and infrared wavelength regimes. Most embedded YSOs are found in the peripheral regions of clusters in which young Main Sequence stars dominate the optical, suggesting a progression of star formation. A detailed study of the young cluster NGC 602 in the Wing of the SMC reveals a population of pre-Main Sequence, T-Tauri type sources and young Main Sequence stars at cluster center bordered by dusty arcs containing embedded YSOs. This indicates a probable progression of star formation from cluster center to edge over time scales of less than 5 Myrs. Our analysis also shows that several IR YSOs correspond to multiple optical sources, raising important questions about the ambiguity between sources observed in different wavelengths and resolutions. Preliminary results from LMC star formation regions, including N11, are also discussed. The SAGE Project is supported by NASA/Spitzer grant 1275598 and NASA NAG5-12595.
American Astronomical Society Meeting Abstracts #213; 01/2009
[show abstract][hide abstract] ABSTRACT: Using an i - z dropout criterion, we determine the space density of z ~ 6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin2, or ~0.5 ± 0.1 objects arcmin-2 down to zAB ~ 27.3 (6 σ), or a completeness-corrected ~0.5 ± 0.2 objects arcmin-2 down to zAB ~ 26.5 (including one probable z ~ 6 active galactic nucleus). Combining deep ISAAC data for our RDCS 1252-2927 field (JAB ~ 25.7 and Ks,AB ~ 25.0; 5 σ) and NICMOS data for the Hubble Deep Field-North (J110,AB and H160,AB ~ 27.3, 5 σ), we verify that these dropouts have relatively flat spectral slopes, as one would expect for star-forming objects at z ~ 6. Compared with the average-color (β = -1.3) U-dropout in the Steidel et al. z ~ 3 sample, i-dropouts in our sample range in luminosity from ~1.5L* (zAB ~ 25.6) to ~0.3L* (zAB ~ 27.3) with the exception of one very bright candidate at z850,AB ~ 24.2. The half-light radii vary from 009 to 021, or 0.5 kpc to 1.3 kpc. We derive the z ~ 6 rest-frame UV luminosity density (or star formation rate density) by using three different procedures. All three procedures use simulations based on a slightly lower redshift (z ~ 5) V606-dropout sample from Chandra Deep Field-South ACS images. First, we make a direct comparison of our findings with a no-evolution projection of this V-dropout sample, allowing us to automatically correct for the light lost at faint magnitudes or lower surface brightnesses. We find 23% ± 25% more i-dropouts than we predict, consistent with no strong evolution over this redshift range. Adopting previous results to z ~ 5, this works out to a mere 20% ± 29% drop in the luminosity density from z ~ 3 to z ~ 6. Second, we use the same V-dropout simulations to derive a detailed selection function for our i-dropout sample and compute the UV-luminosity density [(7.2 ± 2.5) × 1025 ergs s-1 Hz-1 Mpc-3 down to zAB ~ 27]. We find a 39% ± 21% drop over the same redshift range (z ~ 3-6), consistent with the first estimate. This is our preferred value and suggests a star formation rate of 0.0090 ± 0.0031 M☉ yr-1 Mpc-3 to zAB ~ 27, or ~0.036 ± 0.012 M☉ yr-1 Mpc-3 by extrapolating the luminosity function to the faint limit, assuming α = -1.6. Third, we follow a very similar procedure, except that we assume no incompleteness, and find a rest-frame continuum luminosity that is ~2-3 times lower than our other two determinations. This final estimate is to be taken as a lower limit and is important if there are modest changes in the colors or surface brightnesses from z ~ 5 to z ~ 6 (the other estimates assume no large changes in the intrinsic selectability of objects). We note that all three estimates are well within the canonical range of luminosity densities necessary for reionization of the universe at this epoch by star-forming galaxies.
The Astrophysical Journal 12/2008; 595(2):589. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of MV = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, it would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~4-5 Myr, its derived mass is less than 1.3 × 106 M☉. Thus, the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intracluster media in the distant and younger universe.
The Astrophysical Journal 12/2008; 585(2):750. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 i775 - z850 versus z850 shows an intrinsic scatter in color of 0.060 ± 0.009 mag, within 1' 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 > 3.3 Gyr (zf ≈ 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 i775 - z850, 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 ≈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 ~40% of the CMR galaxies in this cluster.
The Astrophysical Journal 12/2008; 639(1):81. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 "protocluster" 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 "wedge" 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.
The Astrophysical Journal 12/2008; 630(1):68. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z = 0.83 cluster MS 1054-0321. Our analyses are based on imaging performed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) in the F606W, F775W, and F850LP passbands and extensive spectroscopic data obtained with the Keck Low-Resolution Imaging Spectrograph. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z = 0.83. These results provide additional support for the hypothesis that the formation of galaxies in MS 1054-0321 began at redshifts considerably greater than unity. We also find a 2 σ deficit of intrinsically faint, red galaxies (i775 - z850 ≥ 0.5, Mi > -19) in this cluster. Although the significance is marginal, this trend may suggest that faint, red galaxies (which are common in z < 0.1 rich clusters) have not yet been created in this cluster at z = 0.83. The giant-to-dwarf ratio in MS 1054-0321 starts to increase inwards of the virial radius or when Σ > 30 Mpc-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. A physical process that begins to become effective at around the virial radius or Σ ~ 30 Mpc-2 may thus be responsible for the evolution of color and luminosity of cluster galaxies.
The Astrophysical Journal 12/2008; 621(1):188. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 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 (i775-z850) of 0.029 ± 0.007 mag based on 52 galaxies or 0.024 ± 0.008 mag for ~30 elliptical galaxies. Simulations using the latest stellar population models indicate an age scatter for the elliptical galaxies of about 34%, with a mean age τL 2.6 Gyr (corresponding to zL 2.7), and the last star formation occurring at zend 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 of 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.
The Astrophysical Journal 12/2008; 596(2):L143. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We combine imaging data from the Advanced Camera for Surveys (ACS) with VLT/FORS optical spectroscopy to study the properties of star-forming galaxies in the z = 0.837 cluster Cl 0152-1357. We have morphological information for 24 star-forming cluster galaxies, which range in morphology from late-type and irregular to compact early-type galaxies. We find that while most star-forming galaxies have r625 - i775 colors bluer than 1.0, eight are in the red cluster sequence. Among the star-forming cluster population, we find five compact early-type galaxies that have properties consistent with their identification as progenitors of dwarf elliptical galaxies. The spatial distribution of the star-forming cluster members is nonuniform. We find none within R ~ 500 Mpc of the cluster center, which is highly suggestive of an intracluster medium interaction. We derive star formation rates from [O II] λ3727 line fluxes and use these to compare the global star formation rate of Cl 0152-1357 to other clusters at low and intermediate redshifts. We find a tentative correlation between integrated star formation rates and TX, in the sense that hotter clusters have lower integrated star formation rates. Additional data from clusters with low X-ray temperatures are needed to confirm this trend. We do not find a significant correlation with redshift, suggesting that evolution is either weak or absent between z = 0.2 and 0.8.
The Astrophysical Journal 12/2008; 621(2):651. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. The disk has two conspicuous spiral rings and two large-scale spiral arms. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. The disk's color, scattering function, and infrared emission suggest that submicron-sized grains dominate the dust population observed in scattered light. CO observations indicate the presence of up to 60 M⊕ of gas. The dust grains are subject to the competing effects of expulsive radiation pressure (β > 1, where β is the ratio of the radiation and gravitational forces) and retentive gas drag. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a flyby encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk with initial radius 400 AU, a gas mass of 50 M⊕, and β = 4 and suggests that the system is being observed within 4000 yr of the flyby periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5MJ planet in an eccentric orbit on the planetesimal distribution of HD 141569A.
The Astrophysical Journal 12/2008; 627(2):986. · 6.73 Impact Factor