arXiv subjects

Bruce E. Woodgate

Publications and source records attributed to Bruce E. Woodgate.

At least 19 recordsLinked to original sources

arXiv2014

Variability of Disk Emission in Pre-Main Sequence and Related Stars. III. Exploring Structural Changes in the Pre-transitional Disk in HD 169142

We present near-IR and far-UV observations of the pre-transitional (gapped) disk in HD 169142 using NASA's Infrared Telescope Facility and Hubble Space Telescope. The combination of our data along with existing data sets into the broadband spectral energy distribution reveals variability of up to 45% between ~1.5-10 μm over a maximum timescale of 10 years. All observations known to us separate into two distinct states corresponding to a high near-IR state in the pre-2000 epoch and a low state in the post-2000 epoch, indicating activity within the <1 AU region of the disk. Through analysis of the Pa β and Br γ lines in our data we derive a mass accretion rate in May 2013 of (1.5 - 2.7) x 10^-9 Msun/yr. We present a theoretical modeling analysis of the disk in HD 169142 using Monte-Carlo radiative transfer simulation software to explore the conditions and perhaps signs of planetary formation in our collection of 24 years of observations. We find that shifting the outer edge (r = 0.3 AU) of the inner disk by 0.05 AU toward the star (in simulation of accretion and/or sculpting by forming planets) successfully reproduces the shift in NIR flux. We establish that the ~40-70 AU dark ring imaged in the NIR by Quanz et al. (2013) and Momose et al. (2013) and at 7 mm by Osorio et al. (2014) may be reproduced with a 30% scaled density profile throughout the region, strengthening the link to this structure being dynamically cleared by one or more planetary mass bodies.

astro-ph.SR↗

arXiv2014

Probing for Exoplanets Hiding in Dusty Debris Disks: Disk Imaging, Characterization, and Exploration with HST/STIS Multi-Roll Coronagraphy

Spatially resolved scattered-light images of circumstellar (CS) debris in exoplanetary systems constrain the physical properties and orbits of the dust particles in these systems. They also inform on co-orbiting (but unseen) planets, systemic architectures, and forces perturbing starlight-scattering CS material. Using HST/STIS optical coronagraphy, we have completed the observational phase of a program to study the spatial distribution of dust in ten CS debris systems, and one "mature" protoplanetrary disk all with HST pedigree, using PSF-subtracted multi-roll coronagraphy. These observations probe stellocentric distances > 5 AU for the nearest stars, and simultaneously resolve disk substructures well beyond, corresponding to the giant planet and Kuiper belt regions in our Solar System. They also disclose diffuse very low-surface brightness dust at larger stellocentric distances. We present new results inclusive of fainter disks such as HD92945 confirming, and better revealing, the existence of a narrow inner debris ring within a larger diffuse dust disk. Other disks with ring-like sub-structures, significant asymmetries and complex morphologies include: HD181327 with a posited spray of ejecta from a recent massive collision in an exo-Kuiper belt; HD61005 suggested interacting with the local ISM; HD15115 & HD32297, discussed also in the context of environmental interactions. These disks, and HD15745, suggest debris system evolution cannot be treated in isolation. For AU Mic's edge-on disk, out-of-plane surface brightness asymmetries at > 5 AU may implicate one or more planetary perturbers. Time resolved images of the MP Mus proto-planetary disk provide spatially resolved temporal variability in the disk illumination. These and other new images from our program enable direct inter-comparison of the architectures of these exoplanetary debris systems in the context of our own Solar System.

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arXiv2012

Hot Gas, Cold Gas and Sub-Halos in a Lyman-alpha Blob at Redshift 2.38

We present integral field spectroscopy of a Lyman-alpha blob at redshift 2.38, with a spectral resolution three times better than previous published work. As with previous observations, the blob has a chaotic velocity structure, much of which breaks up into multiple components. Our spectroscopy shows, however, that some of these multiple components are extremely narrow: they have velocity widths of less than 100 km/s. Combining these new data with previous observations, we argue that this Lyman-alpha blob resides in a dark-matter halo of around 10^13 solar masses. At the centre of this halo are two compact red massive galaxies. They are surrounded by hot gas, probably a super-wind from merger-induced nuclear starbursts. This hot gas has shut down star formation in the non-nuclear region of these galaxies, leading to their red-and-dead colours. A filament or lump of infalling cold gas is colliding with the hot gas phase and being shocked to high temperatures, while still around 30kpc from the red galaxies. The shock region is self-absorbed in Lyman-alpha but produces C IV emission. Further out still, the cold gas in a number of sub-halos is being lit up, most likely by a combination of tidally triggered star formation, bow-shocks as they plough through the hot halo medium, resonant scattering of Lyman-alpha from the filament collision, and tidal stripping of gas which enhances the Lyman-alpha escape fraction. The observed Lyman-alpha emission from the Blob is dominated by the sum of the emission from these sub-halos. On statistical grounds, we argue that Lyman-alpha blobs are not greatly elongated in shape, and that most are not powered by ionisation or scattering from a central active galactic nucleus or starburst.

astro-ph.CO↗

arXiv2010

ACCESS: Enabling an Improved Flux Scale for Astrophysics

Improvements in the precision of the astrophysical flux scale are needed to answer fundamental scientific questions ranging from cosmology to stellar physics. The unexpected discovery that the expansion of the universe is accelerating was based upon the measurement of astrophysical standard candles that appeared fainter than expected. To characterize the underlying physical mechanism of the "Dark Energy" responsible for this phenomenon requires an improvement in the visible-NIR flux calibration of astrophysical sources to 1% precision. These improvements will also enable large surveys of white dwarf stars, e.g. GAIA, to advance stellar astrophysics by testing and providing constraints for the mass-radius relationship of these stars. ACCESS (Absolute Color Calibration Experiment for Standard Stars) is a rocket-borne payload that will enable the transfer of absolute laboratory detector standards from NIST to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of R = 500 across the 0.35-1.7 micron bandpass. Among the strategies being employed to minimize calibration uncertainties are: (1) judicious selection of standard stars (previous calibration heritage, minimal spectral features, robust stellar atmosphere models), (2) execution of observations above the Earth's atmosphere (eliminates atmospheric contamination of the stellar spectrum), (3) a single optical path and detector (to minimize visible to NIR cross-calibration uncertainties), (4) establishment of an a priori error budget, (5) on-board monitoring of instrument performance, and (6) fitting stellar atmosphere models to the data to search for discrepancies and confirm performance.

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arXiv2006

Optical Emission Band Morphologies of the Red Rectangle

We present narrow-band images of the Red Rectangle (RR) nebula which reveal the distinct morphologies of this intriguing nebula in different optical emission bands. The morphology of the RR nebula in blue luminescence (BL) and extended red emission (ERE) are almost mutually exclusive. We also present the optical detection of the circum-binary disk of the RR in the light of the BL. The total intensities from the two optical band emissions (BL and ERE) when summed over the nebula are of comparable magnitude. Their spatial distributions with respect to the embedded illumination sources lead us to suggest that they may be attributed to different ionization stages of the same family of carriers.

astro-ph↗

arXiv2005

The Evolution of Damped Lyman-alpha Absorbers: Metallicities and Star Formation Rates

The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the solar value at low redshifts. These surprising results appear to contradict the near-solar mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.

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arXiv2005

The Herbig Ae star HD 163296 in X-rays

Chandra X-ray imaging spectroscopy of the nearby Herbig Ae star HD 163296 at 100 AU angular resolution is reported. A point-like, soft (kT~0.5 keV), emission-line source is detected at the location of the star with an X-ray luminosity of 4.0e29 erg/s. In addition, faint emission along the direction of a previously-detected Ly-alpha-emitting jet and Herbig-Haro outflow may be present. The relatively low luminosity, lack of a hard spectral component, and absence of strong X-ray variability in HD 163296 can be explained as originating from optically-thin shock-heated gas accreting onto the stellar surface along magnetic field lines. This would require a (dipole) magnetic field strength at the surface of HD 163296 of at least ~100 G and perhaps as high as several kG. HD 163296 joins the T Tauri star TW Hya in being the only examples known to date of pre-main-sequence stars whose quiescent X-ray emission appears to be completely dominated by accretion.

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arXiv2004

Finding Extrasolar Planets in the Stellar Graveyard with the Hubble Space Telescope

In order to directly image an extrasolar planet, the large contrast between a star and a companion planet must be overcome. White Dwarfs (WDs) are the remnants of stars > 1 M_sun and are orders of magnitude dimmer, making searches for planets and brown dwarfs (BDs) possible. We present the preliminary results of a survey of 7 hydrogen white dwarfs with photospheric metal lines (DAZs) for substellar and planetary objects, using NICMOS on the Hubble Space Telescope (HST). The extremely high contrast available with the NICMOS coronagraph allows 7-12 M_Jup planets to be detected to within 10-30 AU of the primary WD. This represents the first concerted search for planetary objects around stars that were more massive than the sun. Our results show that there are candidates around three of the seven stars, which are awaiting confirmation through common proper motion studies, some of which can only be confirmed through another observation with HST. Continuing studies of WDs will constrain planet formation scenarios for more massive stars and provide a sample of planets to study with future space missions such as JWST and SIM.

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arXiv2004

The Distribution of Ly-alpha-Emitting Galaxies at z=2.38: Paper 2, Spectroscopy

In Paper 1 of this series we identified an 80 co-moving Mpc filament of candidate Lyman-alpha emitting galaxies at redshift 2.38. In this paper we present spectroscopy of the 37 galaxy candidates. Our spectroscopy reached a surface brightness limit of 5.0E-17 erg/cm^2/s/arcsec^2. Of the 14 candidates down to this limit, 12 were confirmed to be Lyman-alpha emitting galaxies at the filament redshift. We also obtained spectral confirmation for six of the lower surface brightness candidates, all of which also lay at the filament redshift. In addition, we identify a foreground cluster of QSOs at z=1.65.

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arXiv2003

The Distribution of Lya-Emitting Galaxies at z=2.3

We present the detection of 34 Ly-alpha emission-line galaxy candidates in a 80x80x60 co-moving Mpc region surrounding the known z=2.38 galaxy cluster J2143-4423. The space density of Ly-alpha emitters is comparable to that found by Steidel et al. when targeting a cluster at redshift 3.09, but is a factor of 5.8 +/- 2.5 greater than that found by field samples at similar redshifts. The distribution of these galaxy candidates contains several 5-10 Mpc scale voids. We compare our observations with mock catalogs derived from the VIRGO consortium Lambda-CDM n-body simulations. Fewer than 1% of the mock catalogues contain voids as large as we observe. Our observations thus tentatively suggest that the galaxy distribution at redshift 2.38 contains larger voids than predicted by current models. Three of the candidate galaxies and one previously discovered galaxy have the large luminosities and extended morphologies of "Ly-alpha blobs".

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arXiv2002

Emission Line Galaxies in the STIS Parallel Survey I: Observations and Data Analysis

In the first three years of operation STIS obtained slitless spectra of approximately 2500 fields in parallel to prime HST observations as part of the STIS Parallel Survey (SPS). The archive contains almost 300 fields at high galactic latitude (|b|>30) with spectroscopic exposure times greater than 3000 seconds. This sample contains 220 fields (excluding special regions and requiring a consistent grating angle) observed between 6 June 1997 and 21 September 2000, with a total survey area of about 160 square arcminutes. At this depth, the SPS detects an average of one emission line galaxy per three fields. We present the analysis of these data, and the identification of 131 low to intermediate redshift galaxies detected by optical emission lines. The sample contains 78 objects with emission lines that we infer to be redshifted [OII]3727 emission at 0.43<z<1.7. The comoving number density of these objects is comparable to that of H-alpha emitting galaxies in the NICMOS parallel observations. One quasar and three probable Seyfert galaxies are detected. Many of the emission-line objects show morphologies suggestive of mergers or interactions. The reduced data are available upon request from the authors.

astro-ph↗

arXiv2000

Wide Field Imaging of the Hubble Deep Field South Region II: The Evolution of Galaxy Clustering at z<1

We present the galaxy-galaxy angular correlations as a function of photometric redshift in a deep-wide galaxy survey centered on the Hubble Deep Field South. Images were obtained with the Big Throughput Camera on the Blanco 4m telescope at CTIO, of 1/2 square degree in broad-band uBVRI, reaching ~24th mag. Approximately 40,000 galaxies are detected in the survey. We determine photometric redshifts using galaxy template fitting to the photometry. Monte Carlo simulations show that redshifts from these data should be reliable out to z~1, where the 4000 Angstrom break shifts into the I-band. The inferred redshift distribution, n(z), shows good agreement with the distribution of galaxies measured in the HDF North and the Canada-France Redshift Survey. After assigning galaxies to redshift bins with width Delta_z=0.33, we determine the two point angular correlation function in each bin. We find that the amplitude of the correlation, A_w, drops across the three bins to redshift z~1. Simple epsilon models of clustering evolution fit this result, with the best agreement for epsilon=0. Hierarchical cold-dark-matter models best fit in a low density, Lambda-dominated universe.

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arXiv2000

On the Gas Surrounding High Redshift Galaxy Clusters

Francis & Hewett (1993) identified two 10-Mpc scale regions of the high redshift universe that were seemingly very overdense in neutral hydrogen. Subsequent observations showed that at least one of these gas-rich regions enveloped a cluster of galaxies at redshift 2.38. We present improved observations of the three background QSOs with sightlines passing within a few Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen absorption at the cluster redshift, suggesting that this cluster (and perhaps all high redshift clusters) may be surrounded by a ~5 Mpc scale region containing ~ 10^12 solar masses of neutral gas. If most high redshift clusters are surrounded by such regions, we show that the gas must be in the form of many small (< 1 kpc), dense (> 0.03 cm^-3) clouds, each of mass < 10^6 solar masses. These clouds are themselves probably gathered into > 20 kpc sized clumps, which may be galaxy halos or protogalaxies. If this gas exists, it will be partially photoionised by the UV background. We predict the diffuse Ly-alpha flux from this photoionisation, and place observational limits on its intensity.

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arXiv2000

Redshift estimation from low-resolution prism SEDs with an NGST MOS

We discuss the utility of a low resolution prism as a component of a Multi-Object Spectrometer for NASA's proposed Next Generation Space Telescope (NGST). Low resolution prism spectroscopy permits simultaneous observation of the 0.6-5micron wavelength regime at R~50. To such data we can apply the modern techniques in spectral energy distribution (SED) fitting to determine source redshifts, sometimes called ``photometric redshifts''. Low resolution prism observations of galaxy SED's provide a significant advantage over multi-filter observations for any realistic observing strategy, and we have performed extensive simulations to quantify it. The advantage of the prism is largest in the regime of faint (K_AB>30) objects at high redshift (z>4), the primary discovery space of NGST. Many important objects will be too faint for follow up at higher spectral resolution, so prism observations are the optimal technique to study them. Prism observations also reduce the contamination of high redshift samples by lower redshift interlopers.

astro-ph↗

arXiv2000

Wide Field Imaging of the Hubble Deep Field-South Region I: Quasar Candidates

We present candidate quasars from a multi-color (uBVRI + narrow-band) imaging survey of 1/2 square degree around the Hubble Deep Field - South. We identify 154 candidate quasars with B < 23 using color selection, consistent with previously measured QSO number counts if we assume a 60% selection efficiency. The narrow-band filter (NB) was centered at 3958 A to detect Ly-alpha at the redshift of J2233-6033, the HDF-S QSO. We confirm the presence of Ly-alpha nebulosity extending ~12'' around the HDF-S QSO, reported by Bergeron et al. (1999). We detect 10 point-like objects in emission through the NB filter. Of these, 7 satisfy our QSO color selection criteria. One of emission-line objects is a B~20 radio-quiet quasar at z=1.56, 6.7' from the line of sight to the HDF-S QSO and ~12'' from the western edge of the WFPC2 deep field.

astro-ph↗

arXiv1998

Galaxy Clusters and Large Scale Structure at High Redshifts

We present a detailed study of a rich galaxy cluster at z=2.38. We demonstrate that this cluster contains large overdensities of damped Ly-alpha absorption lines, of Ly-alpha emitting galaxies and of extremely red objects. The overdensity of extremely red objects in this field demonstrates that many are high z galaxies. The huge overdensities we measure for these three classes of object are much larger than the mass overdensities of typical clusters at this redshift, as predicted by CDM and related models. We suggest therefore that the distribution of damped Ly-alpha absorption line systems, of Ly-alpha emitting galaxies and of extremely red objects are all very strongly biassed, and that somehow a small overdensity of mass has increased the fraction of baryons in collapsed objects, in the volume occupied by the cluster, to close to unity (a factor of ~10 increase). We speculate that some unknown physical process, acting on the volume occupied by our cluster, caused the normally diffuse ionised inter-galactic medium to coalesce into small (< 10^8 Solar masses) blobs of neutral hydrogen, which produce the Ly-alpha absorption-lines. Star formation occurred within these blobs at z>5, enriching them with metals and producing stars, which after several mergers and ~ 0.5 Gyr of passive evolution form the extremely red objects. The Ly-alpha emitting galaxies are probably AGN, triggered perhaps by mergers of the small blobs.

astro-ph↗

arXiv1998

Imaging and spectroscopy of arcs around the most luminous X-ray cluster RX J1347.5-1145

The cluster RX J1347.5-1145, the most luminous cluster in the X-ray wavelengths, was imaged with the newly installed Space Telescope Imaging Spectrograph (STIS) on-board HST. Its relatively high redshift (0.451) and luminosity indicate that this is one of the most massive of all known clusters. The STIS images unambiguously show several arcs in the cluster. The largest two arcs (> 5 arcsec in length) are symmetrically situated on opposite sides of the cluster, at a distance of ~ 35 arcsec from the central galaxy. The STIS images also show approximately 100 faint galaxies within the radius of the arcs whose combined luminosity is ~ 4 x 10^11 Lsun. We also present ground-based spectroscopic observations of the northern arc which show one clear emission line at 6730 A, which is consistent with an identification as [OII] 3727 A, implying a redshift of 0.81 for this arc. The southern arc shows a faint continuum but no emission features. The surface mass within the radius of the arcs (240 kpc), as derived from the gravitational lensing, is 6.3 x 10^14 Msun. The resultant mass-to-light ratio of ~1200 is higher than what is seen in many clusters but smaller than the value recently derived for some `dark' X-ray clusters (Hattori et al. 1997). The total surface mass derived from the X-ray flux within the radius of the arcs is ~2.1 - 6.8 x 10^14 Msun, which implies that the ratio of the gravitational to the X-ray mass is ~1 to 3. The surface GAS mass within this radius is ~3.5 x 10^13 Msun, which implies that at least 6% of the total mass within this region is baryonic.

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arXiv1997

STIS Near Ultraviolet Time-Tagged Spectra of the Crab Pulsar

We present the spectrum and pulse profile of the Crab Pulsar in the near ultraviolet (1600-3200 Angstroms) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional Near-Ultraviolet Multi-Anode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2 arcsec by 2 arcsec aperture and the low dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse profile is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 A dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55+- 0.05 leads to a good fit to a power law with slope of -0.3+-0.2. Spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The pulse profile is stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption line equivalent widths of Mg I, Mg II and FeII are lower than expected based upon the implied HI column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.

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