Showing papers by "Mike Irwin published in 1999"

Measurements of Omega and Lambda from 42 High-Redshift Supernovae

Abstract: We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04} (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat cosmology, the simplest inflationary universe model. An open, Lambda = 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(Lambda > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

A New Local Group Galaxy in Cetus

Abstract: We report the discovery of a previously uncataloged Local Group galaxy in the constellation Cetus. Faintly visible on UKST survey plates, it has a smooth, diffuse appearance and appears to be a dwarf spheroidal of type dE3.5. A color-magnitude diagram in V, V-I shows a clear giant branch but no sign of recent star formation. From the position of the tip of the giant branch, we derive a reddening-corrected distance modulus of 24.45 ± 0.15 and a metallicity of -1.9 ± 0.2. With an implied heliocentric distance of 775 ± 50 kpc, and a corresponding Local Group barycentric distance of 615 kpc, the Cetus dwarf lies well within the boundaries of the Local Group, and although it currently lacks a radial velocity measurement, it is undoubtedly a member of the Local Group. The nearest Local Group galaxies are WLM and IC 1613 at angular separations of 73 and 163 and roughly 175 and 230 kpc total distance, respectively. Although the Cetus dwarf is unlikely to be directly associated with any other Local Group galaxy, it does lie in the general direction of the extension of the Local Group toward the Sculptor Group.

NICMOS and VLA Observations of the Gravitationally Lensed Ultraluminous BAL Quasar APM 08279+5255: Detection of a Third Image

Abstract: We present a suite of observations of the recently identified ultraluminous BAL quasar APM 08279+5255, taken both in the infrared with the NICMOS high-resolution camera on board the Hubble Space Telescope and at 3.5 cm with the Very Large Array. With an inferred luminosity of ~5 × 1015 L⊙, APM 08279+5255 is apparently the most luminous system known. Extant ground-based images show that APM 08279+5255 is not pointlike, but is instead separated into two components, indicative of gravitational lensing. The much higher resolution images presented here also reveal two point sources, A and B, of almost equal brightness (fB/fA = 0.782 ± 0.010), separated by 0378 ± 0001, as well as a third, previously unknown, fainter image, C, seen between the brighter images. While the nature of C is not fully determined, several lines of evidence point to it being a third gravitationally lensed image of the quasar, rather than being the lensing galaxy. Simple models that recover the relative image configuration and brightnesses are presented. While proving to be substantially amplified, APM 08279+5255 possesses an intrinsic bolometric luminosity of ~1014 → 1015 L⊙ and remains amongst the most luminous objects known.

NICMOS and VLA Observations of the Gravitatonally Lensed Ultraluminous BAL Quasar APM~08279+5255: Detection of a Third Image

Abstract: We present a suite of observations of the recently identified ultraluminous BAL quasar APM 08279+5255, taken both in the infra-red with the NICMOS high resolution camera on board the Hubble Space Telescope, and at 3.5cm with the Very Large Array. With an inferred luminosity of ~5x10^15 Solar luminosities, APM 08279+5255 is apparently the most luminous system known. Extant ground-based images show that APM 08279+5255 is not point-like, but is instead separated into two components, indicative of gravitational lensing. The much higher resolution images presented here also reveal two point sources, A and B, of almost equal brightness (f_B/f_A=0.782 +/- 0.010), separated by 0."378 +/- 0."001, as well as a third, previously unknown, fainter image, C, seen between the brighter images. While the nature of C is not fully determined, several lines of evidence point to it being a third gravitationally lensed image of the quasar, rather than being the lensing galaxy. Simple models which recover the relative image configuration and brightnesses are presented. While proving to be substantially amplified, APM 08279+5255 possesses an intrinsic bolometric luminosity of ~10^14 to 10^15 Solar luminosities and remains amongst the most luminous objects known.

HIRES Spectroscopy of APM 08279+5255: Metal Abundances in the Lyα Forest*

Abstract: We present high-S/N echelle spectra of the recently discovered ultraluminous QSO APM 08279+5255 and use these data to reexamine the abundance of carbon in Ly? forest clouds. In agreement with previous work, we find that approximately 50% of Ly? clouds with hydrogen column densities of log N (H I) ? 14.5 have associated weak C IV absorption, with log N(C IV) 12.0, and we derive a median N(C IV)/N(H I) = 1.4 ? 10-3. The agreement with earlier estimates of this ratio may be somewhat fortuitous, however, because we show that previous analyses have probably overestimated the number of Ly? clouds that should be included in this statistic. We then investigate whether there is any C IV absorption associated with lower H I column densities by stacking 51 C IV regions corresponding to 51 Ly? lines with 13.5 ? log N(H I) ? 14.0. The coadded spectrum has a S/N 580 but shows no composite C IV absorption. In order to understand the significance of this nondetection, we have stacked together 51 theoretical C IV ?1548 lines with individual values of column density and velocity dispersion scaled appropriately from the values measured in the corresponding Ly? lines. We find that even if the typical value of N(C IV)/N(H I) = 1.4 ? 10-3 applies to these lower column density clouds, the corresponding signal in the stacked C IV region is smeared by the likely random difference in redshift between C IV and Ly? absorption and becomes very difficult to recognize. This seems to be a fundamental limitation of the stacking method, which may well explain why in the past it has led to underestimates of the metallicity of the Ly? forest. We also analyze our spectra with the pixel-by-pixel optical depth technique recently developed by Cowie & Songaila and find evidence for net C IV absorption in Ly? clouds with optical depths as low as ?(Ly?) = 0.5-2, as these authors did. However, we show with simulations that even this method requires higher sensitivities than reached up to now to be confident that the ratio N(C IV)/N(H I) remains constant down to column densities below log N(H I) 14.0. We conclude that the question of whether there is a uniform degree of metal enrichment in the Ly? forest at all column densities has yet to be fully answered. Future progress in this area will probably require concerted efforts to push further the detection limit for C IV lines in selected bright QSOs.

KECK HIRES Spectroscopy of APM 08279+5255*

Abstract: With an optical R‐band magnitude of 15.2, the recently discovered \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $z=3.911$ \end{document} broad absorption line quasar APM 08279+5255 is an exceptionally bright high‐redshift source. Its brightness has allowed us to acquire a high signal‐to‐noise ratio (∼80), high‐resolution (∼6 km s−1) spectrum using the HIRES echelle spectrograph on the 10 m Keck I telescope. Given the quality of the data, these observations provide an unprecedented view of associated and intervening abs...

Light curves of the trans-Neptunian objects 1996 TP66 and 1994 VK8

Abstract: We have obtained a number of CCD images of two trans-Neptunian objects, 1994 VK8 and 1996 TP66, over two nights. The changes in magnitude of these objects have been examined, in a search for periodic variation. In the case of 1996 TP66, nothing other than random noise can be found to within the errors of ∼0.04 mag. Although a periodic signal is found for 1994 VK8, it appears to be an artefact, as the same frequency appears in the variation of sky brightness and is probably due to the sampling of the data. However, 1994 VK8 does exhibit a variation of ∼0.5 mag. This would suggest either significant non-sphericity or a change in surface composition over a large area. In either case 1994 VK8 warrants further investigation.

Proper Motions of Local Group Satellites

Abstract: Many direct attempts to measure the proper motions of Galactic satellite galaxies are either in progress or have recently been completed. In this review I will summarise and examine the current state of knowledge in this field and will also highlight several indirect methods that can be used as alternative estimators or constraints on the proper motions of satellite galaxies throughout the Local Group. A good example of this latter approach is provided by the Sagittarius dwarf galaxy, where ongoing tidal disruption of the system has left a giant arc of debris tracing out its orbital path.

Metal Abundances in the Lyman Alpha Forest

Abstract: We present high S/N echelle spectra of the recently discovered ultraluminous QSO APM 08279+5255 and use these data to re-examine the abundance of Carbon in Ly alpha forest clouds. In agreement with previous work, we find that approximately half of the Ly alpha clouds whose hydrogen column densities log N(HI) > 14.5 have associated weak CIV absorption with log N(CIV) > 12, and we derive a median N(CIV)/N(H I) = 1.4 x 10^-3. We then investigate whether there is any CIV absorption in weaker Ly alpha clouds with 13.5 < log N(HI) < 14.0. Two contrasting techniques are compared - firstly by stacking individual CIV lines to produce a high S/N (=580) composite spectrum and secondly by constructing pixel-by-pixel optical depth distributions, a technique recently developed by Cowie & Songaila (1998). By simulating stacks of synthetic absorption lines, the composite spectrum technique is found to be severely hampered by random redshift offsets between CIV lines and their corresponding HI. Whilst the optical depth method showed evidence for CIV asborption even at HI optical depths as small as 0.5 - 2, simulations showed that even higher S/N ratios are required in order to determine whether the CIV/HI ratio remains constant at these low column densities.

Metal abundances in the Lyman-alpha forest

Abstract: We present high S/N echelle spectra of the recently discovered ultraluminous QSO APM 08279+5255 and use these data to re-examine the abundance of Carbon in Ly alpha forest clouds. In agreement with previous work, we find that approximately half of the Ly alpha clouds whose hydrogen column densities log N(HI) > 14.5 have associated weak CIV absorption with log N(CIV) > 12, and we derive a median N(CIV)/N(H I) = 1.4 x 10^-3. We then investigate whether there is any CIV absorption in weaker Ly alpha clouds with 13.5 < log N(HI) < 14.0. Two contrasting techniques are compared - firstly by stacking individual CIV lines to produce a high S/N (=580) composite spectrum and secondly by constructing pixel-by-pixel optical depth distributions, a technique recently developed by Cowie & Songaila (1998). By simulating stacks of synthetic absorption lines, the composite spectrum technique is found to be severely hampered by random redshift offsets between CIV lines and their corresponding HI. Whilst the optical depth method showed evidence for CIV asborption even at HI optical depths as small as 0.5 - 2, simulations showed that even higher S/N ratios are required in order to determine whether the CIV/HI ratio remains constant at these low column densities.