Showing papers by "Mike Irwin published in 2000"

The Hubble Deep Field South: Formulation of the observing campaign

Abstract: Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10-3 to 2 × 105 μm We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 224 quasar in the STIS field of view The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 A, and deep imaging of an adjacent field with NICMOS camera 3 at 11, 16, and 22 μm All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers

Discovery of High Proper-Motion Ancient White Dwarfs: Nearby Massive Compact Halo Objects?

Abstract: We present the discovery and spectroscopic identification of two very high proper-motion ancient white dwarf stars, found in a systematic proper-motion survey. Their kinematics and apparent magnitude clearly indicate that they are halo members, while their optical spectra are almost identical to the recently identified cool halo white dwarf WD 0346+246. Canonical stellar halo models predict a white dwarf volume density that is 2 orders of magnitude less than the ρ ~ 7 × 10-4 M☉ pc-3 inferred from this survey. With the caveat that the sample size is very small, it appears that a significant fraction, ~10%, of the local dark matter halo is in the form of very old, cool, white dwarfs.

The APM Survey for cool carbon stars in the Galactic halo – II. The search for dwarf carbon stars

Abstract: We present proper-motion measurements for carbon stars found during the APM Survey for Cool Carbon Stars in the Galactic halo as reported in an earlier paper by Totten & Irwin. Measurements are obtained using a combination of POSSI, POSSII and UKST survey plates supplemented where necessary by CCD frames taken at the Isaac Newton Telescope. We find no significant proper motion for any of the new APM colour-selected carbon stars and so conclude that there are no dwarf carbon stars present within this sample. We also present proper-motion measurements for three previously known dwarf carbon stars and demonstrate that these measurements agree favourably with those previously quoted in the literature, verifying our method of determining proper motions. Results from a complimentary program of JHK photometry obtained at the South African Astronomical Observatory are also presented. Dwarf carbon stars are believed to have anomalous near-infrared colours, and this feature is used for further investigation of the nature of the APM carbon stars. Our results support the use of JHK photometry as a dwarf/giant discriminator and also reinforce the conclusion that none of the new APM-selected carbon stars is a dwarf. Finally, proper-motion measurements combined with extant JHK photometry are presented for a sample of previously known halo carbon stars, suggesting that one of these stars, CLS29, is likely to be a previously unrecognized dwarf carbon star.

The INT Wide Field Imaging Survey(WFS)

Abstract: The 2.5m Isaac Newton Telescope(INT) is currently being used to carry out a major multi-colour, multi-epoch, CCD based wide field survey over an area of 100 square degrees. The survey parameters have been chosen to maximise scientific return over a wide range of scientific areas and to complement other surveys being carried out elsewhere. Unique aspects of the survey is that it concentrates on regions of sky that are easily accessible from telescopes in both Northern and Southern terrestrial hemispheres and that it the first public survey to use filters similar to that being used by the Sloan Digital Sky Survey. A major aim of the the INT Wide Field Survey program is to bridge the gap between the all-sky photographic 2 and 3 band surveys such as the Palomar and UK Schmidt sky surveys and the ultra-deep keyhole surveys such as the Hubble Deep Field.

The Montreal-Cambridge-Tololo Survey of Southern Subluminous Blue Stars: The South Galactic Cap

Abstract: Results for the south Galactic cap region of the Montreal-Cambridge-Tololo survey of blue subluminous stars are presented. This region overlaps the 840 deg2 region studied almost three decades ago by Slettebak & Brundage. We present a list of equatorial coordinates, photographic photometry, and spectroscopic identifications, as well as finding charts, for 188 blue objects [(U-B)pg ≤ -0.6] brighter than Bpg = 16.5 in this area. Completeness of the survey and comparisons with other similar efforts are discussed.

Magellanic Cloud Periphery Carbon Stars. IV. The SMC

Abstract: The kinematics of 150 carbon stars observed at moderate dispersion on the periphery of the Small Magellanic Cloud are compared with the motions of neutral hydrogen and early-type stars in the intercloud region. The distribution of radial velocities implies a configuration of these stars as a sheet inclined at 73° ± 4° to the plane of the sky. The near side, to the south, is dominated by a stellar component; to the north, the far side contains fewer carbon stars and is dominated by the neutral gas. The upper velocity envelope of the stars is closely the same as that of the gas. This configuration is shown to be consistent with the known extension of the SMC along the line of sight and is attributed to a tidally induced disruption of the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago. The dearth of gas on the near side of the sheet is attributed to ablation processes akin to those inferred in 1996 by Weiner & Williams to collisional excitation of the leading edges of Magellanic Stream clouds. Comparison with the 1989 kinematic data of Hardy, Suntzeff, & Azzopardi and Maurice, Martin, & Bouchet and the 1986 and 1988 data of Mathewson et al. leaves little doubt that forces other than gravity play a role in the dynamics of the H I.

Magellanic Cloud Periphery Carbon Stars IV: The SMC

Abstract: The kinematics of 150 carbon stars observed at moderate dispersion on the periphery of the Small Magellanic Cloud are compared with the motions of neutral hydrogen and early type stars in the Inter-Cloud region. The distribution of radial velocities implies a configuration of these stars as a sheet inclined at 73+/-4 degrees to the plane of the sky. The near side, to the South, is dominated by a stellar component; to the North, the far side contains fewer carbon stars, and is dominated by the neutral gas. The upper velocity envelope of the stars is closely the same as that of the gas. This configuration is shown to be consistent with the known extension of the SMC along the line of sight, and is attributed to a tidally induced disruption of the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr ago. The dearth of gas on the near side of the sheet is attributed to ablation processes akin to those inferred by Weiner & Williams (1996) to collisional excitation of the leading edges of Magellanic Stream clouds. Comparison with pre LMC/SMC encounter kinematic data of Hardy, Suntzeff, & Azzopardi (1989) of carbon stars, with data of stars formed after the encounter, of Maurice et al. (1989), and Mathewson et al. (a986, 1988) leaves little doubt that forces other than gravity play a role in the dynamics of the H I.

The Acceleration of the Universe:. Measurements of Cosmological Parameters from Type Ia Supernovae

Abstract: The fate of the Universe, infinite expansion or a ldquo;big crunchrdquo;, can be determined by measuring the redshifts and brightness of very distant supernovae. These provide a record of changes in the expansion rate of the Universe over the past several billion years. The mass density, ?M, and cosmological-constant energy density ??, are measured from a data-set consisting of 42 high-redshift Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these supernovae, at redshifts between 0.18 and 0.83, are fit jointly with a set of supernovae from the Cal?n/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. We find ?Mflat = 0.28+0.09-0.08 (1? statistical)+0.05-0.04 (identified systematics). The data are strongly inconsistent with a ? = 0 flat cosmology, the simplest inflationary universe model. An open, ? = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(?>0) = 99%, including the identified systematic uncertainties. Thus, the Universe is found to be accelerating, i.e., q0 = ?M/2 - ?? < 0. The best-fit age of the universe relative to the Hubble time is t0flat = 14.9+1.4-1.1 (0.63/h) Gyr for a flat cosmology.

The optical velocity of the Antlia dwarf galaxy

Abstract: We present the results of a Very Large Telescope observing programme carried out in service mode using fors1 on ANTU (UT1) in long slit mode to determine the optical velocities of nearby low surface brightness galaxies. Outlying Local Group galaxies are of paramount importance in placing constraints on the dynamics and thus on both the age and the total mass of the Local Group. Optical velocities are also necessary to determine if the observations of H i gas in and around these systems are the result of gas associated with these galaxies or a chance superposition with high-velocity H i clouds or the Magellanic Stream. The data were of a sufficient signal-to-noise ration to enable us to obtain a reliable result in one of the galaxies we observed – Antlia – for which we have found an optical heliocentric radial velocity of 351±15 km s−1.

The Evolution and Space Density of Damped Lyman-alpha Galaxies

Abstract: The results of a new spectroscopic survey of 66 $z \simgt 4$ quasars for Damped Lyman-alpha absorption systems are presented. The search led to the discovery of 30 new DLA candidates which are analysed in order to compute the comoving mass density of neutral gas in a non-zero lambda Universe. The possible sources of uncertainty are discussed and the implications of our results for the theories of galaxy formation and evolution are emphasized. A subsequent paper will present details of the calculations summarised here and a more extensive explanation of the consequences of our observations for the understanding of the nature of DLAs.

The Second APM UKST Colour Survey for z>4 Quasars

Abstract: We present the spectra, positions, and finding charts for 31 bright (R 4.5. The majority are in the southern sky (dec 2.5) colours from UK or POSSII Schmidt Plates scanned at the Automated Plate Measuring facility in Cambridge. Low resolution (~> 10A) spectra were obtained to identify the quasars, primarily at the Las Campanas Observatory. The highest redshift quasar in our survey is at z ~ 4.8 (R = 18.7) and its spectrum shows a damped Lyman-alpha absorption system at z = 4.46. This is currently the highest redshift damped Lyman-alpha absorber detected. Five of these quasars exhibit intrinsic broad absorption line features. Combined with the previously published results from the first part of the APM UKST survey we have now surveyed a total of ~8000 squ deg of sky i.e. 40% of the high galactic latitude (|b|>30 deg) sky, resulting in 59 optically selected quasars in the redshift range 3.85 to 4.78; 49 of which have z>=4.00.