Showing papers by "Mike Irwin published in 1997"
TL;DR: In this article, the authors used a light-curve width-corrected magnitudes as a function of redshift of distant (z = 0.35-0.46) supernovae to obtain a global measurement of the mass density.
Abstract: We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of more than 28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find an observational dispersion in peak magnitudes of ? -->MB=0.27; this dispersion narrows to ?MB, corr=0.19 after correcting the magnitudes using the light-curve width-luminosity relation found for nearby (z ? 0.1) Type Ia supernovae from the Cal?n/Tololo survey (Hamuy et al.). Comparing light-curve width-corrected magnitudes as a function of redshift of our distant (z = 0.35-0.46) supernovae to those of nearby Type Ia supernovae yields a global measurement of the mass density, ?M${r M}$ -->=0.88 -->+ 0.69?0.60 for a ? = 0 cosmology. For a spatially flat universe (i.e., ?M + ?? = 1), we find ?M${r M}$ -->=0.94 -->+ 0.34?0.28 or, equivalently, a measurement of the cosmological constant, ??=0.06 -->+ 0.28?0.34 ( < 0.51 at the 95% confidence level). For the more general Friedmann-Lema?tre cosmologies with independent ?M and ??, the results are presented as a confidence region on the ?M-?? plane. This region does not correspond to a unique value of the deceleration parameter q0. We present analyses and checks for statistical and systematic errors and also show that our results do not depend on the specifics of the width-luminosity correction. The results for ??-versus-?M are inconsistent with ?-dominated, low-density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.
1,272 citations
TL;DR: The Sagittarius galaxy (Sgr), the closest satellite galaxy of the Milky Way, has survived for many orbits about the Galaxy. as discussed by the authors showed that the stars of the Sgr dwarf are embedded in a constant-density dark matter halo, representing the core of a tidally limited system.
Abstract: The Sagittarius galaxy (Sgr), the closest satellite galaxy of the Milky Way, has survived for many orbits about the Galaxy. Extant numerical calculations modeled this galaxy as a system with a centrally-concentrated mass profile, following the light, and found that it should lose more than one-half of its mass every 2--4 orbits and be completely disrupted long before now. Apparently Sgr, and by implication other dSph galaxies, do not have a centrally-concentrated profile for their dark matter. We develop a model in which the stars of the Sgr dwarf are embedded in a constant-density dark matter halo, representing the core of a tidally-limited system, and show that this is consistent with its survival. We present new photometric and kinematic observations of Sgr and show these data are consistent with this explanation for the continued existence of this galaxy. Sgr is being tidally distorted and is tidally limited, but is not disrupted as yet. The corresponding minimum total mass is $10^9 \msun$, while the central mass to visual light ratio $\sim 50$ in Solar units.
Our new photographic photometry allows the detection of main-sequence stars of Sgr over an area of $22\deg \times 8\deg$. Sgr is prolate, with axis ratios $\sim$~3:1:1. For an adopted distance of $16 \pm 2 \kpc$ from the Galactic center on the opposite side of the Galaxy to the Sun, the major axis is $\gta 9 \kpc$ long and is aligned approximately normal to the plane of the Milky Way Galaxy, roughly following the coordinate line $\ell=5^\circ$.
The central velocity dispersion of giant stars which are members of Sgr is $11.4 \pm 0.7 \kms$ and is consistent with being constant over the face of the galaxy. The gradient in mean line-of-sight velocity with position along the
313 citations
144 citations
TL;DR: In this article, the authors used the radial velocities of 759 carbon stars on the periphery of the LMC to determine the rotation curve from 3 to 12 kpc.
Abstract: Radial velocities of 759 carbon stars on the periphery of the LMC are used to determine the rotation curve from 3 to 12 kpc. After a peak of 42 km s-1 at 4 kpc, the velocities decline to 35 km s-1 at 6.5 kpc, suggesting that 90% of the LMC's matter lies inside this radius. The rising velocities seen at larger radii are explained by particle-particle N-body simulations as arising from tidal interactions between LMC and both the SMC and the Galaxy. For an inclination of 33° the mass contained within a 5 kpc radius is 6.2 ± 0.9 × 109 M☉. An upper limit on the LMC halo mass is determined.
82 citations
TL;DR: In this article, a luminosity function for low surface brightness (LSB) galaxies identi-ed in the Automatic Plate Measuring Facility survey of Impey et al. is presented.
Abstract: We present a luminosity function for low surface brightness (LSB) galaxies identi-ed in the Automatic Plate Measuring Facility survey of Impey et al. These galaxies have central surface brightnesses (k(0)) in B in the range 22.0 " k(0) " 25.0. Using standard maximum likelihood estimators, we determine that the best--t Schechter function parameters for this luminosity function (LF) are a \( 1.42, M* \( 18.34, and /* \ 0.0036, assuming km s~1 Mpc~1. We compare the luminosity and number den- H 0 \ 100 h 100 sities derived from this luminosity function with those obtained from other recent -eld galaxy studies and -nd that surveys that do not take account of the observational selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conser- vative estimates, our derivation of the LF for LSB galaxies suggests that the CfA Redshift Survey has missed at least one-third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help to close the gap between local and moderate-redshift galaxy counts. Subject headings: galaxies: fundamental parameters E galaxies: luminosity function, mass function E galaxies: statistics
75 citations
TL;DR: In this paper, the authors presented the current status of a survey of cool carbon stars in the halo of the Magellanic Clouds, using the duPont Telescope, Las Campanas, and the UK Schmidt Telescope BJ and R survey plates.
Abstract: We present the current status of our ongo- ing cool carbon star survey in the halo of the Magellanic Clouds. Candidate cool carbon stars were identied from APM measures of pairs of UK Schmidt Telescope BJ and R survey plates. Intermediate resolution spectroscopy on the duPont Telescope, Las Campanas, was used to simul- taneously verify the nature of the candidates, late M-type giants or AGB carbon stars, and to derive their radial velocity. Coordinates, nding charts and radial velocity data for 392 spectroscopically identied cool carbon stars distributed out to angular distances of 10 degrees from the Cloud optical centres are given. Radial velocities were also obtained for 133 known carbon stars in the Large Magellanic Cloud, in the inter-Cloud region, in the wing of the Small Magellanic Cloud and in a few SMC star clusters. These intermediate-age carbon stars dene ideal kinematic test particles to investigate recent dynamical interactions between the Galaxy-LMC-SMC system and in particular the origin of the morphological disturbances seen in the SMC and parts of the LMC, the origin of the Magellanic Stream and the total mass of the LMC.
57 citations
TL;DR: In this paper, a luminosity function for low surface brightness (LSB) galaxies identified in the APM survey of Impey et al. was derived and compared to those of other recent studies and found that surveys which do not take account of the observation selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe.
Abstract: We present a luminosity function for low surface brightness (LSB) galaxies identified in the APM survey of Impey et al 1996. These galaxies have central surface brightnesses in B in the range 22 < mu < 25. Using standard maximum-likelihood estimators, we determine that the best-fit Schechter function parameters for this luminosity function (LF) are alpha = -1.42, M* = -18.34, and phi* = 0.0036, assuming H_0 = 100. We compare these results to those of other recent studies and find that surveys which do not take account of the observation selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conservative estimates, our derivation of the LF for LSB galaxies suggests that the CfA redshift survey has missed at least one third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help close the gap between local and moderate-redshift galaxy counts.
54 citations
TL;DR: In this article, photometry and spectra of the individual images of the quadruple gravitational lens system Q2237+0305 were compared, showing evidence for significant changes in the emission line to continuum ratio of the strong ultraviolet CIV, CIII, and MgII lines.
Abstract: We present both photometry and spectra of the individual images of the quadruple gravitational lens system Q2237+0305. Comparison of spectra obtained at two epochs, separated by $\sim~3\,$years, shows evidence for significant changes in the emission line to continuum ratio of the strong ultraviolet CIV~$\lambda$1549, CIII]~$\lambda$1909 and MgII~$\lambda$2798 lines. The short, $\sim~1\,$day, light--travel time differences between the sight lines to the four individual quasar images rule out any explanation based on intrinsic variability of the source. The spectroscopic differences thus represent direct detection of microlensing--induced spectroscopic differences in a quasar. The observations allow constraints to be placed on the relative spatial scales in the nucleus of the quasar, with the ultra--violet continuum arising in a region of $\la~0.05~{\rm pc}$ in extent, while the broad emission line material is distributed on scales much greater than this.
54 citations
TL;DR: In this article, optical and near-IR photometry of the few brightest KBOs and Centaurs shows a diversity from neutral to extremely red colours, and no compelling evidence for a correlation of colours with orbital zones, and consequently no clear mechanism to explain this diversity in terms of the irradiation mantle model.
Abstract: The recent discoveries of over 40 new objects with orbits beyond 30 au represent the first sampling of a reservoir of objects lying beyond Neptune, known as the Kuiper Belt (or Edgeworth-Kuiper Belt), which may be a source of short-period comets and Centaurs (objects whose orbits cross those of the giant planets). There are very few observations from which to derive physical properties of these Kuiper Belt objects (KBOs) due to their faintness and the concentration on discovery rather than follow-up, although optical and near-IR photometry of the few brightest KBOs and Centaurs shows a diversity from neutral to extremely red colours. We present new BVRI photometry of five KBOs (1994JQ(1), 1995DC(2), 1994JR(1), 1995DA(2) and one undesignated new KBO) and the Centaur object 1995DW(2). With the current small sample, we find no compelling evidence for a correlation of colours with orbital zones, and consequently no clear mechanism to explain this diversity in terms of the irradiation mantle model.
47 citations
TL;DR: In this paper, the discovery of a new member of the Local Group in the constellation of Antlia is reported, which appears to be a typical dwarf spheroidal galaxy of type dE3 with no apparent young blue stars or unusual features.
Abstract: We report the discovery of new member of the Local Group in the constellation of Antlia. Optically the system appears to be a typical dwarf spheroidal galaxy of type dE3.5 with no apparent young blue stars or unusual features. A color-magnitude diagram in I, V-I shows the tip of the red giant branch, giving a distance modulus of 25.3 +/- 0.2 (1.15 Mpc +/- 0.1) and a metallicity of -1.6 +/- 0.3. Although Antlia is in a relatively isolated part of the Local Group it is only 1.2 degrees away on the sky from the Local Group dwarf NGC3109, and may be an associated system.
34 citations
TL;DR: In this article, the first seven supernovae (SN Ia's) of the Calan/Tololo survey were used as standard candles without a light-curve width correction.
Abstract: The Supernova Cosmology Project has discovered over 28 supernovae (SNs) at 0.35 < z < 0.65 in an ongoing program that uses Type Ia SNs (SN Ia's) as high-redshift distance indicators. Here we present measurements of the ratio between the locally observed and global Hubble constants, HL0/HG0, based on the first seven SNs of this high-redshift data set compared with 18 SNs at z ≤ 0.1 from the Calan/Tololo survey. If ΩM ≤ 1, then light-curve width corrected SN magnitudes yield HL0/HG0 < 1.10 (95% confidence level) in both a Λ = 0 and a flat universe. The analysis using the SN Ia's as standard candles without a light-curve width correction yields similar results. These results rule out the hypothesis that the discrepant ages of the Universe derived from globular clusters and recent measurements of the Hubble constant are attributable to a locally underdense bubble. Using the Cepheid-distance-calibrated absolute magnitudes for SN Ia's of Sandage et al., we can also measure the global Hubble constant, HG0. If ΩM ≥ 0.2, we find that HG0 < 70 km s-1 Mpc-1 in a Λ = 0 universe and HG0 < 78 km s-1 Mpc-1 in a flat universe, correcting the distant and local SN apparent magnitudes for light-curve width. Lower results for HG0 are obtained if the magnitudes are not width-corrected.
TL;DR: In this article, the Supernova Cosmology Project has discovered over twenty-eight supernovae (SNe) at 0.35 = 0.2, and they find that H_0^G < 70 km/s/Mpc in a Lambda=0 universe and H_ 0^G > 78 km/m/MPC in a flat universe, correcting the distant and local SN apparent magnitudes for light curve width.
Abstract: The Supernova Cosmology Project has discovered over twenty-eight supernovae (SNe) at 0.35 = 0.2, we find that H_0^G < 70 km/s/Mpc in a Lambda=0 universe and H_0^G < 78 km/s/Mpc in a flat universe, correcting the distant and local SN apparent magnitudes for light curve width. Lower results for H_0^G are obtained if the magnitudes are not width corrected.
TL;DR: In this paper, the discovery of a new member of the Local Group in the constellation of Antlia is reported, which appears to be a typical dwarf spheroidal galaxy of type dE3 with no apparent young blue stars or unusual features.
Abstract: We report the discovery of new member of the Local Group in the constellation of Antlia. Optically the system appears to be a typical dwarf spheroidal galaxy of type dE3.5 with no apparent young blue stars or unusual features. A color-magnitude diagram in I, V-I shows the tip of the red giant branch, giving a distance modulus of 25.3 +/- 0.2 (1.15 Mpc +/- 0.1) and a metallicity of -1.6 +/- 0.3. Although Antlia is in a relatively isolated part of the Local Group it is only 1.2 degrees away on the sky from the Local Group dwarf NGC3109, and may be an associated system.
TL;DR: In this paper, the authors demonstrate the use of a variant of Principal Component Analysis (PCA) for discrimination problems in astronomy, and demonstrate that the proposed method is robust to signal to noise (S/N) degradation.
Abstract: We demonstrate the use of a variant of Principal Component Analysis (PCA) for discrimination problems in astronomy. This variant of PCA is shown to provide the best linear discrimination between data classes. As a test case, we present the problem of discrimination between K giant and K dwarf stars from intermediate resolution spectra near the Mg `b' feature. The discrimination procedure is trained on a set of 24 standard K giants and 24 standard K dwarfs, and then used to perform giant - dwarf classification on a sample of approximately 1500 field K stars of unknown luminosity class which were initially classified visually. For the highest S/N spectra, the automated classification agrees very well (at the 90 - 95% level) with the visual classification. Most importantly, however, the automated method is found to classify stars in a repeatable fashion, and, according to numerical experiments, is very robust to signal to noise (S/N) degradation.
TL;DR: In this article, the authors demonstrate the use of a variant of Principal Component Analysis (PCA) for discrimination problems in astronomy, and demonstrate that the proposed method is robust to signal to noise (S/N) degradation.
Abstract: We demonstrate the use of a variant of Principal Component Analysis (PCA) for discrimination problems in astronomy. This variant of PCA is shown to provide the best linear discrimination between data classes. As a test case, we present the problem of discrimination between K giant and K dwarf stars from intermediate resolution spectra near the Mg `b' feature. The discrimination procedure is trained on a set of 24 standard K giants and 24 standard K dwarfs, and then used to perform giant - dwarf classification on a sample of approximately 1500 field K stars of unknown luminosity class which were initially classified visually. For the highest S/N spectra, the automated classification agrees very well (at the 90 - 95% level) with the visual classification. Most importantly, however, the automated method is found to classify stars in a repeatable fashion, and, according to numerical experiments, is very robust to signal to noise (S/N) degradation.