Showing papers by "Mike Irwin published in 1996"
TL;DR: In this paper, a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters has been developed, based on the initial seven of >28 supernova discovered to date in the Supernova Cosmology Project.
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 >28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find a dispersion in peak magnitudes of sigma_{M_B} = 0.27 this dispersion narrows to sigma_{M_B,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 Calan/Tololo survey (Hamuy et al. 1996). Comparing lightcurve-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, Omega_M = 0.88^{+0.69}_{-0.60} for a Lambda = 0 cosmology. For a spatially flat universe (i.e., Omega_M +Omega_Lambda = 1), we find Omega_M = 0.94 ^{+0.34}_{-0.28} or, equivalently, a measurement of the cosmological constant, Omega_Lambda = 0.06 ^{+0.28}_{-0.34} (<0.51 at the 95% confidence level). For the more general Friedmann-Lemaitre cosmologies with independent Omega_M and Omega_Lambda, the results are presented as a confidence region on the Omega_M-Omega_Lambda plane. This region does not correspond to a unique value of the deceleration parameter q_0. 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 Omega_Lambda-versus-Omega_M are inconsistent with Lambda-dominated, low density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.
1,151 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
253 citations
TL;DR: In this article, it was shown that the cosmological mass density in neutral gas does not rise as steeply prior to z ∼ 2 as indicated by previous studies, and there is evidence in the observed Ωg for a flattening at z ≥ 2 and a possible turnover at z ∼ 3, which points to an epoch at z ≳ 3 prior to which the highest column density damped systems are still forming.
Abstract: Although observationally rare, damped Lyα absorption systems dominate the mass density of neutral gas in the Universe. 11 high-redshift damped Lyα systems covering 2.8 ≤z ≤ 4.4 were discovered in 26 QSOs from the APM z > 4 QSO survey, extending these absorption system surveys to the highest redshifts currently possible. Combining our new data set with previous surveys, we find that the cosmological mass density in neutral gas, Ωg, does not rise as steeply prior to z ∼ 2 as indicated by previous studies. There is evidence in the observed Ωg for a flattening at z ∼ 2 and a possible turnover at z ∼ 3. When combined with the decline at z > 3.5 in number density per unit redshift of damped systems with column densities log NHI ≥ 21 atom cm^(−2), these results point to an epoch at z ≳ 3 prior to which the highest column density damped systems are still forming. We find that, over the redshift range 2 < z < 4, the total mass in neutral gas is marginally comparable to the total visible mass in stars in present-day galaxies. However, if one considers the total mass visible in stellar discs alone, i.e. excluding galactic bulges, the two values are comparable. We are observing a mass of neutral gas that is comparable to the mass of visible disc stars. Lanzetta, Wolfe & Turnshek found that Ω(z ≈ 3.5) was twice Ω(z ≈ 2), implying that a much larger amount of star formation must have taken place between z = 3.5 and 2 than is indicated by metallicity studies. This created a ‘cosmic G-dwarf problem’. The more gradual evolution of Ωg that we find alleviates this. These results have profound implications for theories of galaxy formation.
188 citations
TL;DR: In this article, the authors presented a large survey for low surface brightness galaxies in the nearby universe (z less than or similar to 0.1) using automated plate measuring machine scans of survey plates from the UK Schmidt Telescope and from follow-up observations at radio and optical wavelengths.
Abstract: Data are presented for 693 galaxies identified in a large new survey for low surface brightness galaxies in the nearby universe (z less than or similar to 0.1). The survey covers 786 square degrees centered on the equator, and it extends significantly the surface brightness range of galaxy surveys in which there are a substantial number of galaxies with redshifts. The data are derived from the Automated Plate Measuring machine scans of survey plates from the UK Schmidt Telescope and from follow-up observations at radio and optical wavelengths. Accurate positions, total B magnitudes, surface brightness parameters, and angular sizes are tabulated for each galaxy. Radial velocities, optical luminosities, and neutral hydrogen masses are listed for a subset of the sample. Finding charts are also presented for those objects having a large enough angular size that the scans from survey plates provide some morphological information. The selection function and the luminosity function that can be derived from the survey are discussed in two companion papers.
178 citations
TL;DR: The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m_r ~ 19, resulted in the discovery of 31 quasars with z ≳ 4.4 (eight with z > 3.5).
Abstract: The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m_r ~ 19, resulted in the discovery of 31 quasars with z ≳ 4. High signal-to-noise optical spectrophotometry at 5 A resolution has been obtained for the 28 quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high-redshift Lyman-limit systems, damped Lyα absorbers, and metal absorption systems (e.g., C IV and Mg II). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission- and absorption-line characteristics, with five exhibiting broad absorption lines and one with extremely strong emission lines (BR 2248 - 1242). Eleven candidate damped Lyα absorption systems have been identified covering the redshift range 2.8 ≤ z ≤ 4.4 (eight with z > 3.5). An analysis of the measured redshifts of the high-ionization emission lines with the low-ionization lines shows them to be blueshifted by 430 ± 60 km s^(-1). In a previous paper (by Storrie-Lombardi et al.) we discussed the redshift evolution of the Lyman limit systems cataloged here. In subsequent papers we will discuss the properties of the Lyα forest absorbers and the redshift and column density evolution of the damped Lyα absorbers.
128 citations
TL;DR: In this paper, the first measurement of the rate of Type Ia supernovae at high redshift was presented, using a large subset of data from the Supernova Cosmology Project as described in more detail at this meeting.
Abstract: We present the first measurement of the rate of Type Ia supernovae at high redshift. The result is derived using a large subset of data from the Supernova Cosmology Project as described in more detail at this meeting by Perlmutter et al. (1996). We present our methods for estimating the numbers of galaxies and the number of solar luminosities to which the survey is sensitive, the supernova detection efficiency and hence the control time. We derive a rest-frame Type la supernova rate at z~ 0.4 of 0.82 −0.37 +0.54 −0.32 +0.42 h 2 SNu where the first uncertainty is statistical and the second includes systematic effects.
100 citations
TL;DR: The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m(R)~19, resulted in the discovery of thirty-one quasars with z ≥ 4.
Abstract: The APM multicolor survey for bright z > 4 objects, covering 2500 deg^2 of sky to m(R)~19, resulted in the discovery of thirty-one quasars with z > 4. High signal-to-noise optical spectrophotometry at 5A resolution has been obtained for the twenty-eight quasars easily accessible from the northern hemisphere. These spectra have been surveyed to create new samples of high redshift Lyman-limit systems, damped Lyman-alpha absorbers, and metal absorption systems (e.g. CIV and MgII). In this paper we present the spectra, together with line lists of the detected absorption systems. The QSOs display a wide variety of emission and absorption line characteristics, with 5 exhibiting broad absorption lines and one with extremely strong emission lines (BR2248-1242). Eleven candidate damped Ly-alpha absorption systems have been identified covering the redshift range 2.8 3.5). An analysis of the measured redshifts of the high ionization emission lines with the low ionization lines shows them to be blueshifted by 430+/-60 km/s. In a previous paper (Storrie-Lombardi et. al. 1994) we discussed the redshift evolution of the Lyman limit systems catalogued here. In subsequent papers we will discuss the properties of the Ly-alpha forest absorbers and the redshift and column density evolution of the damped Ly-alpha absorbers.
95 citations
TL;DR: In this article, the first measurement of the rate of Type Ia supernovae at high redshift was presented using a large subset of data from the Supernova Cosmology Project.
Abstract: We present the first measurement of the rate of Type Ia supernovae at high redshift. The result is derived using a large subset of data from the Supernova Cosmology Project. Three supernovae were discovered in a surveyed area of 1.7 square degrees. The survey spanned a $\sim 3$ week baseline and used images with $3\sigma$ limiting magnitude of $R\sim 23$. We present our methods for estimating the numbers of galaxies and the number of solar luminosities to which the survey is sensitive, and the supernova detection efficiency which is used to determine the control time, the effective time for which the survey is sensitive to a Type Ia event. We derive a rest-frame Type Ia supernova rate at $z\sim0.4$ of $0.82\ {^{+0.54}_{-0.37}}\ {^{+0.37}_{-0.25}} $ $h^2$ SNu (1 SNu = 1 SN per century per $10^{10}$\Lbsun), where the first uncertainty is statistical and the second includes systematic effects. For the purposes of observers, we also determine the rate of SNe, per sky area surveyed, to be $ 34.4\ {^{+23.9}_{-16.2}}$ SNe\ $\rm year^{-1} deg^{-2}$ for SN magnitudes in the range $21.3 < R < 22.3$.
87 citations
TL;DR: In this paper, the authors studied the evolution of the number density per unit redshift of the damped systems by fitting the sample with the customary power law N(z) = N_o(1 + z)^γ.
Abstract: Eleven candidate damped Lyα absorption systems were identified in 27 spectra of the quasars from the APM z ≳ 4 survey covering the redshift range 2.8 ≤ z_(absorption) ≤ 4.4 (eight with z_(absorption) > 3.5). High-resolution echelle spectra (0.8-A FWHM) have been obtained for three quasars, including two of the highest redshift objects in the survey. Two damped systems have confirmed H i column densities of N_(HI) ≥ 10^(20.3) atom cm^(−2), with a third falling just below this threshold. We have discovered the highest redshift damped Lyα absorber known at z = 4.383 in QSO BR 1202 — 0725.
The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z > 3. We combine this high-redshift sample with other quasar samples covering the redshift range 0.008 21.2. The breaks are more pronounced at higher redshift. The column density distribution function for the data with log N_(HI) ≥ 20.3 is better fitted with the form f (N) = (f_*/N_*) (N/N_*)^(−β)exp (— N/N_*) with log N_* = 21.63 ± 0.35, β = 1.48 ± 0.30, and f_* = 1.77 × 10^(−2). We study the evolution of the number density per unit redshift of the damped systems by fitting the sample with the customary power law N(z) = N_o(1 + z)^γ. For a population with no intrinsic evolution in the product of the absorption cross-section and comoving spatial number density this will give γ = 1/2 (Ω = 1) or γ = 1 (Ω = 0). The best maximum-likelihood fit for a single power law is γ = 1.3 ± 0.5 and N_o = 0.041_(−0.02)^(+0.03), consistent with no intrinsic evolution even though the value of γ is also consistent with that found for the Lyman limit systems where evolution is detected at a significant level. However, redshift evolution is evident in the higher column density systems with an apparent decline in N(z) for z > 3.5.
67 citations
TL;DR: In this paper, the amplification probability distributions for a large sample of microlensing light curves were presented and a temporal analysis for this light-curve sample was explored using the first-order structure function.
Abstract: Microlensing at cosmological distances can induce violent fluctuations in the observed light curves of macro lensed quasars. In a previous publication the amplification probability distributions for a large sample of microlensing light curves were presented. This paper extends these earlier results and presents a temporal analysis for this light-curve sample. The temporal properties are explored using the first-order structure function. The results of this study show that (1) the asymptotic global curvature of the structure function depends on the macrolensing parameters (J' * and y and not on the detail of the mass distribution, (2) the characteristic timescale for variability due to microlensing is oc.J , where is the average mass of the micro lensing objects, and again does not depend on the detailed shape of the mass distribution, and (3) the structure function time-scale does not vary with shear angle as a purely geometric transformation, but does possess an underlying symmetry solely dependent on the macro lensing parameters. A comparison of simulations of the quadruple lens, 2237 + 0305, with the currently available temporal light curves suggests that the average mass distribution of the lensing objects in 2237 + 0305 is within the range 0.1 < 10 Mo'
61 citations
TL;DR: In this article, the authors present optical spectroscopic data for a complete sample of 161 S_5GHz >= 0.2 Jy, flat-spectrum radio sources, which were observed as part of a survey for high redshift, radio-loud quasars.
Abstract: We present optical spectroscopic data for a complete sample of 161 S_5GHz >= 0.2 Jy, flat-spectrum radio sources. The sources were observed as part of a survey for high redshift, radio-loud quasars, and were selected for spectroscopic follow-up based on criteria of red optical colour and unresolved optical counterpart, as measured from APM scans of POSS-I plates. 13 objects from the spectroscopic sample were found to be radio-loud quasars with z > 3, of which two were previously known. We give positions, E (red) magnitudes, O-E colours, 5GHz radio fluxes, radio spectral indices, optical spectra and redshifts where possible for the spectroscopic sample. We also give finding charts for the z > 3 QSOs. The highest redshift object found is a QSO with z = 4.30 (GB1508+5714, the subject of an earlier Letter). The sample also contains a z = 3.05 QSO, GB1759+7539, which is optically very luminous (E=16.1). In addition, spectra are given for 18 S_5GHz >= 0.2 Jy, flat-spectrum radio sources that do not form part of the complete sample.
TL;DR: In this article, a near-infrared photometry is presented of 75 late-type giants in the direction of the Sagittarius dwarf galaxy, and the most luminous star which is a confirmed member of the dwarf galaxy is a carbon star with Mbol ~ −4.5.
TL;DR: In this article, the authors observed 19 giant stars in the Draco dwarf spheroidal (dSph) galaxy and obtained good quality spectra for 17 of them, assuming dynamical equilibrium, isotropy in the velocity dispersion and a negligible contribution from binary stars.
Abstract: We have observed 19 giant stars in the Draco dwarf spheroidal (dSph) galaxy and obtained good quality spectra for 17 of these. The velocity dispersion of the sample, calculated by the maximum-likelihood method, is 10.5 ±~:; km S-1. The mean velocity is 293.8 ± ~:~ km s -1. The data were taken at a single epoch, so there is no direct information about possible binary star contamination. Assuming dynamical equilibrium, isotropy in the velocity dispersion and a negligible contribution from binary stars, the core mass-to-light ratio is 166 ±i~~ MolLo and the total mass-tolight ratio is 145 ± ~~6 MolLo, consistent with the presence of large quantities of dark matter.
TL;DR: In this article, the authors discuss the techniques employed for identifying low surface brightness galaxies for the Automated Plate Measuring (APM) survey and discuss the uncertainties associated with the calibrations.
Abstract: We discuss the techniques employed for identifying low surface brightness galaxies for the Automated Plate Measuring (APM) survey. We present the method of calibrating the photometry performed from the APM scans and discuss the uncertainties associated with the calibrations. We also review the possible sources of incompleteness in the survey, and we present the results of a program of simulations to estimate the completeness as a function of galaxy parameters. Finally, we discuss the implications of the survey results and show, in particular, that the observed distribution of central surface brightnesses among all galaxies in one survey held is almost flat for mu(B)(0) > 23 mag arcsec-(2).
01 Jan 1996
TL;DR: In this article, the authors present the first direct spectroscopic evidence for this effect, providing a limit on the scale of the continuum and the broad line emission regions at the center of a source quasar (2237+0305).
Abstract: The degree of microlensing induced amplification is dependent upon the size of a source. As quasar spectra consist of the sum of emission from different regions this scale dependent amplification can produce spectral differences between the images of a macrolensed quasar. This paper presents the first direct spectroscopic evidence for this effect, providing a limit on the scale of the continuum and the broad line emission regions at the center of a source quasar (2237+0305). Lack of centroid and profile differences in the emission lines indicate that substructure in the broad emission line region is > 0.05 parsecs.
01 Nov 1996
TL;DR: In this article, the first results from a systematic search for high redshift Type Ia supernovae were reported, with redshift z = 0.35 − 0.46.
Abstract: Alhough there is little doubt at present that the redshift of distant galaxies is due to an expansion of the universe, we present in this paper a direct confirmation for the cosmological expansion. This work is based on the first results from a systematic search for high redshift Type Ia supernovae. We discovered over twenty seven SNe, before or at maximum light. In this paper we report on the first seven of these, with redshift z = 0.35 – 0.46. Type Ia SNe are known to be a homogeneous group of SNe, to first order, with very similar light curves, spectra and peak luminosities. In this paper we report that the light curves we observe are all broadened (time dilated) as expected from the expanding universe hypothesis. Small variations from the expected 1 + z broadening of the light curve widths can be attributed to a width-brightness correlation that has been observed for nearby SNe ( z
Journal Article•
TL;DR: A preliminary search for stars that may have formed coevally with the apparently young halo B-type star PHL 346 has been performed with the 2dF multifibre spectrograph on the Anglo-Australian Telescope (AAT).
Abstract: A preliminary search for stars that may have formed coevally with the apparently young halo B-type star PHL 346 has been performed with the 2dF multifibre spectrograph on the Anglo-Australian Telescope (AAT). Candidates were selected for spectroscopy from APM scans of B and R Schmidt plates centred on PHL 346. A total of 476 stars of spectral type A or F were found; radial velocity estimates and more accurate spectral type assignments narrowed the number of possible coeval candidates to 6 A-type and 14 F-type stars. A statistical analysis of these results using a comparison with a control field suggests that the number of A-type or F-type candidate stars around PHL 346 is not unexpected, and that they need not be associated with PHL 346. A number of ways to improve the project are suggested.
01 Nov 1996
TL;DR: Goldhaber et al. as mentioned in this paper used a systematic search-and-study technique for high-redshift supernovae to measure the cosmological parameters of the supernova cosmology project.
Abstract: We have developed a new tool for measurement of the cosmological parameters: a systematic search-and-study technique for high-redshift supernovae. In the first years of this Supernova Cosmology Project, we have discovered over 27 supernovae. Using a “batch” search strategy, almost all were discovered before maximum light and were observed over the peak of their light curves. The spectra and light curves indicate that almost all were Type Ia supernovae at redshifts z = 0.35 – 0.65. These high-redshift supernovae can provide a distance indicator and “standard clock” to study the cosmological parameters q 0 , Λ, Ω 0 , and H 0 . This presentation and the following presentation of Goldhaber et al . (1996), will discuss observation strategies and rates, analysis and calibration issues, the sources of measurement uncertainty, and the cosmological implications, including bounds on q 0 , of the first 7 high-redshift supernovae from our ongoing search. This is one of the most direct approaches to cosmological measurements, and could become a standard method in the field.
Journal Article•