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Mike Irwin

Bio: Mike Irwin is an academic researcher from University of Cambridge. The author has contributed to research in topics: Galaxy & Milky Way. The author has an hindex of 136, co-authored 755 publications receiving 83262 citations. Previous affiliations of Mike Irwin include University of New South Wales & Lawrence Berkeley National Laboratory.


Papers
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Journal ArticleDOI
TL;DR: In this article, a set of transformation equations were developed to place the VISTA Variables in the Via Lactea (VVV) survey onto the 2MASS system, and the transformed data were subsequently employed to establish a mean reddening law of E J −H /E H −K s ǫ = 2.13± 0.04.
Abstract: The new multi-epoch near-infrared VISTA Variables in the Via Lactea (VVV) survey is sampling 562 deg2 of the Galactic bulge and adjacent regions of the disk. Accurate astrometry established for the region surveyed allows the VVV data to be merged with overlapping surveys (e.g., GLIMPSE, WISE, 2MASS, etc.), thereby enabling the construction of longer baseline spectral energy distributions for astronomical targets. However, in order to maximize use of the VVV data, a set of transformation equations are required to place the VVV JH K s photometry onto the 2MASS system. The impetus for this work is to develop those transformations via a comparison of 2MASS targets in 152 VVV fields sampling the Galactic disk. The transformation coefficients derived exhibit a reliance on variables such as extinction. The transformed data were subsequently employed to establish a mean reddening law of E J −H /E H −K s = 2.13 ± 0.04, which is the most precise determination to date and merely emphasizes the pertinence of the VVV data for determining such important parameters.

58 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present deep photometric studies of the Draco dwarf spheroidal galaxy, undertaken with the MegaCam camera at the Canada-France-Hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide field and Planetary Camera onboard the Hubble Space Telescope (HST).
Abstract: The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass-to-light ratio and compact size, holds many clues to the nature of the enigmatic dark matter. Here we present deep photometric studies of this dwarf galaxy, undertaken with the MegaCam Camera at the Canada-France-Hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide Field and Planetary Camera onboard the Hubble Space Telescope (HST). The new photometric data cover the entirety of the galaxy, and reach i' = 24.5 at 50 per cent completeness, significantly deeper than previous panoramic studies, allowing searches for tidal disturbances of much lower surface brightness than has been possible before. With these improved statistics, we find no evidence for asymmetric disturbances or tidal tails that possess more than 3 per cent of the stars found within the centre of the galaxy. We find that the central stellar density, as probed by the HST data, rises into the central 0.5 arcmin. Uncertainties in the position of the centroid of the galaxy do not allow us to determine whether the apparent flattening of the profile interior to 0.5 arcmin is reliable or not. Draco is therefore a flawless dwarf galaxy, featureless and apparently unaffected by Galactic tides.

58 citations

Journal ArticleDOI
TL;DR: In this article, the radial velocities of more than 1500 red giant branch and red clump stars towards the center of the Canis Major dwarf galaxy were obtained using the 2dF spectrograph on the Anglo-Australian Telescope.
Abstract: As part of a radial velocity survey of low Galactic latitude structures that we undertook with the 2dF spectrograph on the Anglo-Australian Telescope, we present the radial velocities of more than 1500 red giant branch and red clump stars towards the centre of the Canis Major dwarf galaxy. With a mean velocity of 72 ± 7 km s -1 at a heliocentric distance of 5.5 kpc and 114 ± 2 km s -1 at 8.5 kpc, these stars present a peculiar distance - radial velocity relation that is unlike that expected from thin or thick disc stars. Moreover, they belong to a kinematically cold population with an intrinsic dispersion that may be as low as 11 +3 -1 km s -1 . A comparison of the velocity distribution obtained in this work with previous studies shows the importance of using our new reduction pipeline and averaging the velocities obtained from different templates. The radial velocity distribution is used to select Canis Major stars in the UCAC2.0 proper motion catalogue and derive proper motions in Galactic coordinates of (μ l , μ b ) = (-3.6 ± 0.8 mas yr -1 , 1.5 ± 0.4 mas yr -1 ) for the dwarf galaxy, which after correcting for the reflex solar motion along this line of sight gives (μ' l , μ' b ) = (-6.8 ± 0.8 mas yr -1 , 0.8 ± 0.4 mas yr -1 ), corresponding to a prograde orbit with a tangential velocity of ∼235 km s -1 at the average distance of ∼7.2 kpc. All these kinematic constraints can be reproduced in simulations of the accretion of a dwarf on to the Galactic disc. Such a process could also be responsible for the Monoceros Ring that has recently been shown to encompass the Galactic disc. However, without constraints on the kinematics of the tidal arms emerging from the Canis Major dwarf, it is not yet possible to definitively prove a link between the two structures.

58 citations

Journal ArticleDOI
TL;DR: In this article, the authors present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And IX, which appears to be the lowest surface brightness and also the faintest galaxy (MV = -8.3) found to date.
Abstract: We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And IX, which appears to be the lowest surface brightness and also the faintest galaxy (MV = -8.3) found to date. Using Keck DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion σv = 6.8 km s-1, which coupled with the low luminosity implies a very high mass-to-V-band light ratio, M/L ~ 93 M☉/L☉ (M/L > 17 M☉/L☉ at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter-dominated system.

58 citations


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TL;DR: In this paper, the mass density, Omega_M, and cosmological-constant energy density of the universe were measured using the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology project.
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.

16,838 citations

Journal ArticleDOI
TL;DR: In this article, the authors used spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 " z " 0.62.
Abstract: We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 " z " 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmo- logical parameters: the Hubble constant the mass density the cosmological constant (i.e., the (H 0 ), () M ), vacuum energy density, the deceleration parameter and the dynamical age of the universe ) " ), (q 0 ), ) M \ 1) methods. We estimate the dynamical age of the universe to be 14.2 ^ 1.7 Gyr including systematic uncer- tainties in the current Cepheid distance scale. We estimate the likely e†ect of several sources of system- atic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these e†ects appear to reconcile the data with and ) " \ 0 q 0 " 0.

16,674 citations

Journal ArticleDOI
22 Dec 2000-Science
TL;DR: An approach to solving dimensionality reduction problems that uses easily measured local metric information to learn the underlying global geometry of a data set and efficiently computes a globally optimal solution, and is guaranteed to converge asymptotically to the true structure.
Abstract: Scientists working with large volumes of high-dimensional data, such as global climate patterns, stellar spectra, or human gene distributions, regularly confront the problem of dimensionality reduction: finding meaningful low-dimensional structures hidden in their high-dimensional observations. The human brain confronts the same problem in everyday perception, extracting from its high-dimensional sensory inputs-30,000 auditory nerve fibers or 10(6) optic nerve fibers-a manageably small number of perceptually relevant features. Here we describe an approach to solving dimensionality reduction problems that uses easily measured local metric information to learn the underlying global geometry of a data set. Unlike classical techniques such as principal component analysis (PCA) and multidimensional scaling (MDS), our approach is capable of discovering the nonlinear degrees of freedom that underlie complex natural observations, such as human handwriting or images of a face under different viewing conditions. In contrast to previous algorithms for nonlinear dimensionality reduction, ours efficiently computes a globally optimal solution, and, for an important class of data manifolds, is guaranteed to converge asymptotically to the true structure.

13,652 citations

Journal ArticleDOI
TL;DR: In this article, a combination of seven-year data from WMAP and improved astrophysical data rigorously tests the standard cosmological model and places new constraints on its basic parameters and extensions.
Abstract: The combination of seven-year data from WMAP and improved astrophysical data rigorously tests the standard cosmological model and places new constraints on its basic parameters and extensions. By combining the WMAP data with the latest distance measurements from the baryon acoustic oscillations (BAO) in the distribution of galaxies and the Hubble constant (H0) measurement, we determine the parameters of the simplest six-parameter ΛCDM model. The power-law index of the primordial power spectrum is ns = 0.968 ± 0.012 (68% CL) for this data combination, a measurement that excludes the Harrison–Zel’dovich–Peebles spectrum by 99.5% CL. The other parameters, including those beyond the minimal set, are also consistent with, and improved from, the five-year results. We find no convincing deviations from the minimal model. The seven-year temperature power spectrum gives a better determination of the third acoustic peak, which results in a better determination of the redshift of the matter-radiation equality epoch. Notable examples of improved parameters are the total mass of neutrinos, � mν < 0.58 eV (95% CL), and the effective number of neutrino species, Neff = 4.34 +0.86 −0.88 (68% CL), which benefit from better determinations of the third peak and H0. The limit on a constant dark energy equation of state parameter from WMAP+BAO+H0, without high-redshift Type Ia supernovae, is w =− 1.10 ± 0.14 (68% CL). We detect the effect of primordial helium on the temperature power spectrum and provide a new test of big bang nucleosynthesis by measuring Yp = 0.326 ± 0.075 (68% CL). We detect, and show on the map for the first time, the tangential and radial polarization patterns around hot and cold spots of temperature fluctuations, an important test of physical processes at z = 1090 and the dominance of adiabatic scalar fluctuations. The seven-year polarization data have significantly improved: we now detect the temperature–E-mode polarization cross power spectrum at 21σ , compared with 13σ from the five-year data. With the seven-year temperature–B-mode cross power spectrum, the limit on a rotation of the polarization plane due to potential parity-violating effects has improved by 38% to Δα =− 1. 1 ± 1. 4(statistical) ± 1. 5(systematic) (68% CL). We report significant detections of the Sunyaev–Zel’dovich (SZ) effect at the locations of known clusters of galaxies. The measured SZ signal agrees well with the expected signal from the X-ray data on a cluster-by-cluster basis. However, it is a factor of 0.5–0.7 times the predictions from “universal profile” of Arnaud et al., analytical models, and hydrodynamical simulations. We find, for the first time in the SZ effect, a significant difference between the cooling-flow and non-cooling-flow clusters (or relaxed and non-relaxed clusters), which can explain some of the discrepancy. This lower amplitude is consistent with the lower-than-theoretically expected SZ power spectrum recently measured by the South Pole Telescope Collaboration.

11,309 citations

Journal ArticleDOI
Peter A. R. Ade1, Nabila Aghanim2, Monique Arnaud3, M. Ashdown4  +334 moreInstitutions (82)
TL;DR: In this article, the authors present a cosmological analysis based on full-mission Planck observations of temperature and polarization anisotropies of the cosmic microwave background (CMB) radiation.
Abstract: This paper presents cosmological results based on full-mission Planck observations of temperature and polarization anisotropies of the cosmic microwave background (CMB) radiation. Our results are in very good agreement with the 2013 analysis of the Planck nominal-mission temperature data, but with increased precision. The temperature and polarization power spectra are consistent with the standard spatially-flat 6-parameter ΛCDM cosmology with a power-law spectrum of adiabatic scalar perturbations (denoted “base ΛCDM” in this paper). From the Planck temperature data combined with Planck lensing, for this cosmology we find a Hubble constant, H0 = (67.8 ± 0.9) km s-1Mpc-1, a matter density parameter Ωm = 0.308 ± 0.012, and a tilted scalar spectral index with ns = 0.968 ± 0.006, consistent with the 2013 analysis. Note that in this abstract we quote 68% confidence limits on measured parameters and 95% upper limits on other parameters. We present the first results of polarization measurements with the Low Frequency Instrument at large angular scales. Combined with the Planck temperature and lensing data, these measurements give a reionization optical depth of τ = 0.066 ± 0.016, corresponding to a reionization redshift of . These results are consistent with those from WMAP polarization measurements cleaned for dust emission using 353-GHz polarization maps from the High Frequency Instrument. We find no evidence for any departure from base ΛCDM in the neutrino sector of the theory; for example, combining Planck observations with other astrophysical data we find Neff = 3.15 ± 0.23 for the effective number of relativistic degrees of freedom, consistent with the value Neff = 3.046 of the Standard Model of particle physics. The sum of neutrino masses is constrained to ∑ mν < 0.23 eV. The spatial curvature of our Universe is found to be very close to zero, with | ΩK | < 0.005. Adding a tensor component as a single-parameter extension to base ΛCDM we find an upper limit on the tensor-to-scalar ratio of r0.002< 0.11, consistent with the Planck 2013 results and consistent with the B-mode polarization constraints from a joint analysis of BICEP2, Keck Array, and Planck (BKP) data. Adding the BKP B-mode data to our analysis leads to a tighter constraint of r0.002 < 0.09 and disfavours inflationarymodels with a V(φ) ∝ φ2 potential. The addition of Planck polarization data leads to strong constraints on deviations from a purely adiabatic spectrum of fluctuations. We find no evidence for any contribution from isocurvature perturbations or from cosmic defects. Combining Planck data with other astrophysical data, including Type Ia supernovae, the equation of state of dark energy is constrained to w = −1.006 ± 0.045, consistent with the expected value for a cosmological constant. The standard big bang nucleosynthesis predictions for the helium and deuterium abundances for the best-fit Planck base ΛCDM cosmology are in excellent agreement with observations. We also constraints on annihilating dark matter and on possible deviations from the standard recombination history. In neither case do we find no evidence for new physics. The Planck results for base ΛCDM are in good agreement with baryon acoustic oscillation data and with the JLA sample of Type Ia supernovae. However, as in the 2013 analysis, the amplitude of the fluctuation spectrum is found to be higher than inferred from some analyses of rich cluster counts and weak gravitational lensing. We show that these tensions cannot easily be resolved with simple modifications of the base ΛCDM cosmology. Apart from these tensions, the base ΛCDM cosmology provides an excellent description of the Planck CMB observations and many other astrophysical data sets.

10,728 citations