arXiv subjects

Glenn Schneider

Publications and source records attributed to Glenn Schneider.

83 records · Page 5Linked to original sources

arXiv2005

The NICMOS Ultra Deep Field: Observations, Data Reduction, and Galaxy Photometry

This paper describes the observations and data reduction techniques for the version 2.0 images and catalog of the NICMOS Ultra Deep Field Treasury program. All sources discussed in this paper are based on detections in the combined NICMOS F110W and F160W bands only. The NICMOS images are drizzled to 0.09 arc second pixels and aligned to the ACS UDF F850LP image which was rebinned to the same pixel scale. These form the NICMOS version 2.0 UDF images. The catalog sources are chosen with a conservative detection limit to avoid the inclusion of numerous spurious sources. The catalog contains 1293 objects in the 144 x 144 arc sececonds NICMOS subfield of the UDF. The 5 sigma signal to noise level is an average 0.6 arc second diameter aperture AB magnitude of ~27.7 at 1.1 and 1.6 microns. The catalog sources, listed in order of right ascension, satisfy a minimum signal to noise criterion of 1.4 sigma in at least 7 contiguous pixels of the combined F110W and F160W image

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arXiv2001

Keck adaptive optics observations of TW Hydrae Association members

Adaptive optics (AO) on 8-10 m telescopes is an enormously powerful tool for studying young nearby stars. It is especially useful for searching for companions. Using AO on the 10-m W.M. Keck II telescope we have measured the position of the brown dwarf companion to TWA5 and resolved the primary into an 0.055 arcsecond double. Over the next several years follow-up astrometry should permit an accurate determination of the masses of these young stars. We have also re-observed the candidate extrasolar planet TWA6B, but measurements of its motion relative to TWA6A are inconclusive. We are carrying out a search for new planetary or brown dwarf companions to TWA stars and, if current giant planet models are correct, are currently capable of detecting a 1 Jupiter-mass companion at ~1 arcsecond and a 5 Jupiter-mass companion at ~0.5 arcsecon around a typical TWA member.

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arXiv2000

A Multivariate Analysis of Galaxies in the Hubble Deep Field North

We use the ultraviolet and optical WFPC2 and near-infrared NICMOS images of the Hubble Deep Field North to measure and statistically compare an array of parameters for over 250 of the galaxies it contains. These parameters include redshift, rest-frame visible asymmetry and concentration, bolometric luminosity and extinction-corrected star formation rate. We find only one strong correlation, between bolometric luminosity and star formation rate, from which early-type galaxies noticeably deviate. When our asymmetry measurements are combined with those of a sample of nearby galaxies covering the full Hubble sequence, we find a weak correlation between redshift and rest-frame visible asymmetry, consistent with the qualitative evidence of galaxy morphological evolution from these and other deep Hubble Space Telescope images. The mean values of these asymmetry measurements show a monotonic increase with redshift interval over the range 0 < z < 2, increasing by a factor of approximately three. If this trend is real, it suggests that galaxy morphological evolution is a gradual process that is continuing through the present cosmological epoch. There is evidence that the dominant source of this evolution is the "minor" mergers of disk galaxies with smaller companions, which could also transform late-type spiral galaxies into early-type spiral galaxies. Interestingly, in contrast to local galaxies we find no correlations between star formation rate and either UV or visible asymmetry. This could arise if the star formation of high-redshift galaxies proceeds in episodes that are short (~ 100 Myr) relative to the time scales over which galaxy mergers produce strong asymmetries (~ 500 Myr), a result suggested by the high star formation rates of Lyman break galaxies.

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arXiv2000

A Search for the Damped Ly-alpha Absorber at z=1.86 toward QSO 1244+3443 with NICMOS

We have carried out a high-resolution imaging search for the galaxy associated with the damped Lyman-alpha (DLA) absorber at z=1.859 toward the z_{em}=2.48 quasar QSO 1244+3443, using the HST and the NICMOS. Images were obtained in the broad filter F160W and the narrow filter F187N with camera 2 on NICMOS with the goal of detecting the rest-frame optical continuum and the H-alpha line emission from the DLA. After PSF subtraction, two weak features are seen at projected separations of 0.16-0.24" from the quasar. Parts of these features may be associated with the DLA absorber, although we cannot completely rule out that they could be artifacts of the point spread function (PSF). If associated with the DLA, the objects would be ~1-2 h_{70}^{-1} kpc in size with integrated flux densities of 2.5 and 3.3 mu Jy in the F160W filter, implying luminosities at lambda_{central}=5600 A in the DLA rest frame of 4.4-5.9 x 10^{9} h_{70}^{-2} L_{solar} at z=1.86, for q0=0.5. However, no significant H-alpha line emission is seen from these objects, suggesting low star formation rates (SFRs). Our 3 sigma upper limit on the SFR in the DLA is 1.3 h_{70}^{-2} M_{solar}/yr for q0 = 0.5 (2.4 h_{70}^{-2} M_{solar} yr^{-1} for q0 = 0.1). This together with our earlier result for LBQS 1210+1731 mark a significant improvement over previous constraints on the star formation rates of DLAs. A combination of low SFR and some dust extinction is likely to be responsible for the lack of H-alpha emission. Alternatively, the objects, may be associated with the quasar host galaxy. In any case, our observations suggest that the DLA is not a large bright proto-disk, but a compact object or a low-surface brightness galaxy. If the two features are PSF artifacts then the constraints on DLA properties are even more severe.

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arXiv2000

A Color Analysis of the NICMOS Parallel Image Archive

We present a photometric analysis of all high Galactic lattitude (|b| > 20d) broad-band parallel images taken by the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument during its initial lifetime in HST Cycle 7. These images, taken through the F110W and F160W filters, reach a mean 3-sigma limiting magnitude of approximately 22 in both bands, and cover a total area of approximately 92 square arcminutes. The reddest of the 358 galaxies detected have F110W-F160W colors and F160W magnitudes consistent with being a combination of both dusty star-forming and evolved early-type galaxies at 1 < z < 2. The surface density of these galaxies is comparable to that of the population of Extremely Red Objects (EROs) discovered in ground-based surveys (~ 100 deg-2), suggesting that EROs also represent a combination of both galaxy types in this redshift range. Roughly 10% of the detected galaxies appear to be blue compact dwarf galaxies at z < 1, a result consistent with studies of the HST Medium Deep Survey Fields. The surface density of these objects down to magnitude of 22 in F160W is ~ 300 deg-2. None of the 681 point sources detected have F110W-F160W colors matching those expected for QSOs whose continua have been significantly reddened by internal dust. Our data limit the surface density of such QSOs to be < 50 deg-2 down the mean limiting magnitudes of the sample images. Since the surface density of QSOs detected on the basis of ultraviolet excess in optical surveys to comparable depth is ~ 100 deg-2, this argues against the suggestion that dust-reddened QSOs comprise the undetected majority of the QSO population. The F110W-F160W color can also be used to identify unreddened QSOs at z > 8, but we find no such candidates. This is consistent with the evidence that QSO space density declines sharply at z > 5.

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arXiv2000

The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R=800) and existing spectral types and photometry and the models of D'Antona & Mazzitelli, we find that the distributions of ages of comparable samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of \~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass IMFs in these sites of clustered star formation are similar over a wide range of stellar densities and other environmental conditions. With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 Msun, and then rolls over into a power law that continues from about 1 Msun to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun and probably, after a modest completeness correction, even from 0.01-0.02 Msun. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against various properties of the observed IMF.

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arXiv2000

NICMOS Imaging of a Damped Lyman-alpha Absorber at z=1.89 toward LBQS 1210+1731 : Constraints on Size and Star Formation Rate

We report results of a high-resolution imaging search (in rest frame H-$α$ and optical continuum) for the galaxy associated with the damped Lyman-$α$ (DLA) absorber at $z=1.892$ toward the $z_{em}=2.543$ quasar LBQS 1210+1731, using HST/NICMOS. After PSF subtraction, a feature is seen in both the broad-band and narrow-band images, at a projected separation of 0.25$\arcsec$ from the quasar. If associated with the DLA, the object would be $\approx 2-3$ $h_{70}^{-1}$ kpc in size with a flux of $9.8 \pm 2.4$ $μ$Jy in the F160W filter, implying a luminosity at $λ_{central}=5500$ Å in the rest frame of $1.5 \times 10^{10}$ $h_{70}^{-2}$ L$_{\odot}$ at $z=1.89$, for $q_{0}=0.5$. However, no significant H-$α$ emission is seen, suggesting a low star formation rate (SFR) (3 $σ$ upper limit of 4.0 $h_{70}^{-2}$ M$_{\odot}$ yr$^{-1}$), or very high dust obscuration. Alternatively, the object may be associated with the host galaxy of the quasar. H-band images obtained with the NICMOS camera 2 coronagraph show a much fainter structure $\approx 4-5$ $h_{70}^{-1}$ kpc in size and containing four knots of continuum emission, located 0.7$\arcsec$ away from the quasar. We have probed regions far closer to the quasar sight-line than in most previous studies of high-redshift intervening DLAs. The two objects we report mark the closest detected high-redshift DLA candidates yet to any quasar sight line. If the features in our images are associated with the DLA, they suggest faint, compact, somewhat clumpy objects rather than large, well-formed proto-galactic disks or spheroids.

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arXiv1999

NICMOS Observations of the Pre-Main-Sequence Planetary Debris System HD 98800

Spectral energy distributions (SEDs) from 0.4 to 4.7 microns are presented for the two principal stellar components of HD~98800, A and B. The third major component, an extensive planetary debris system (PDS), emits > 20% of the luminosity of star B in a blackbody SED at 164 +/- 5K extending from mid-IR to millimeter-wavelengths. At 0.95 microns a preliminary upper limit of < 0.06 is obtained for the ratio of reflected light to the total from star B. This result limits the albedo of the PDS to < 0.3. Values are presented for the temperature, luminosity, and radius of each major systemic component. Remarkable similarities are found between the PDS and the interplanetary debris system around the Sun as it could have appeared a few million years after its formation.

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arXiv1999

NICMOS Imaging of the HR 4796A Circumstellar Disk

We report the first near infrared (NIR) imaging of a circumstellar annular disk around the young (~8 Myr), Vega-like star, HR 4796A. NICMOS coronagraph observations at 1.1 and 1.6 microns reveal a ring-like symmetrical structure peaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from the central A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2 deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement with recent 12.5 and 20.8 micron observations of a truncated disk (Koerner, et al. 1998). The ring is resolved with a characteristic width of less than 0.26 arcsec (17 AU) and appears abruptly truncated at both the inner and outer edges. The region of the disk-plane inward of ~60 AU appears to be relatively free of scattering material. The integrated flux density of the part of the disk that is visible (greater than 0.65 arcsec from the star) is found to be 7.5 +/- 0.5 mJy and 7.4 +/- 1.2 mJy at 1.1 and 1.6 microns, respectively. Correcting for the unseen area of the ring yields total flux densities of 12.8 +/- 1.0 mJy and 12.5 +/- 2.0 mJy, respectively (Vega magnitudes = 12.92 /+- 0.08 and 12.35 +/-0.18). The NIR luminosity ratio is evaluated from these results and ground-based photometry of the star. At these wavelengths Ldisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonable agreement between the stellar flux scattered in the NIR and that which is absorbed in the visible and re-radiated in the thermal infrared. The somewhat red reflectance of the disk at these wavelengths implies mean particle sizes in excess of several microns, larger than typical interstellar grains. The confinement of material to a relatively narrow annular zone implies dynamical constraints on the disk particles by one or more as yet unseen bodies.

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arXiv1998

NICMOS Observations of the Hubble Deep Field: Observations, Data Reduction, and Galaxy Photometry

This paper contains a catlog of sources observed in a portion of the Hubble Deep Field by the NICMOS instrument. Methods of observation and data analysis are discussed. The catalog has two parts. The first is a listing of all sources with signal to noise ratios greater than or equal to 2.5. The second part contains sources with signal to noise ratios less than 2.5. The second part is intended for researchers who wish to know the limits to which sources may appear at a location.

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arXiv1997

High-resolution near-infrared imaging of the Orion 114-426 silhouette disk

We present the first high-resolution near-infrared images of the edge-on silhouette circumstellar disk, Orion 114-426, made using NICMOS on the Hubble Space Telescope. Images taken against the bright nebular background of the ionized hydrogen Pa$α$ line at 1.87 micron show the major axis of the disk to be approximately 20% smaller than at 0.6 micron, from which we deduce the structure of the edge of the disk. Continuum images of diffuse polar lobes above and below the plane of the disk show a morphology and evolution with wavelength consistent with predictions for reflection nebulae in a diffuse envelope with large polar cavities, surrounding a thin, massless, Keplerian disk, centered on an otherwise hidden central star. We make use of our observations and reasonable assumptions about the underlying disk structure to show that the disk mass is at least 10 earth masses and plausibly $\geq 5\times 10^{-4}$ solar masses.

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