arXiv subjects

Andrew Johnson

Publications and source records attributed to Andrew Johnson.

26 records · Page 2Linked to original sources

arXiv2016

The 2-degree Field Lensing Survey: design and clustering measurements

We present the 2-degree Field Lensing Survey (2dFLenS), a new galaxy redshift survey performed at the Anglo-Australian Telescope. 2dFLenS is the first wide-area spectroscopic survey specifically targeting the area mapped by deep-imaging gravitational lensing fields, in this case the Kilo-Degree Survey. 2dFLenS obtained 70,079 redshifts in the range z < 0.9 over an area of 731 sq deg, and is designed to extend the datasets available for testing gravitational physics and promote the development of relevant algorithms for joint imaging and spectroscopic analysis. The redshift sample consists first of 40,531 Luminous Red Galaxies (LRGs), which enable analyses of galaxy-galaxy lensing, redshift-space distortion, and the overlapping source redshift distribution by cross-correlation. An additional 28,269 redshifts form a magnitude-limited (r < 19.5) nearly-complete sub-sample, allowing direct source classification and photometric-redshift calibration. In this paper, we describe the motivation, target selection, spectroscopic observations, and clustering analysis of 2dFLenS. We use power spectrum multipole measurements to fit the redshift-space distortion parameter of the LRG sample in two redshift ranges 0.15 < z < 0.43 and 0.43 < z < 0.7 as beta = 0.49 +/- 0.15 and beta = 0.26 +/- 0.09, respectively. These values are consistent with those obtained from LRGs in the Baryon Oscillation Spectroscopic Survey. 2dFLenS data products will be released via our website http://2dflens.swin.edu.au.

astro-ph.CO↗

arXiv2016

The Detailed Science Case for the Maunakea Spectroscopic Explorer: the Composition and Dynamics of the Faint Universe

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged)

astro-ph.IM↗

arXiv2015

The 6dF Galaxy Survey: Bulk Flows on $50-70 h^{-1}$ Mpc scales

We measure the bulk flow of the local Universe using the 6dF Galaxy Survey peculiar velocity sample (6dFGSv), the largest and most homogeneous peculiar velocity sample to date. 6dFGSv is a Fundamental Plane sample of $\sim10^4$ peculiar velocities covering the whole southern hemisphere for galactic latitude $|b| > 10^\circ$, out to redshift ${z=0.0537}$. We apply the `Minimum Variance' bulk flow weighting method, which allows us to make a robust measurement of the bulk flow on scales of $50$ and $70\,h^{-1}{\rm Mpc}$. We investigate and correct for potential bias due to the lognormal velocity uncertainties, and verify our method by constructing $Λ{\rm CDM}$ 6dFGSv mock catalogues incorporating the survey selection function. For a hemisphere of radius $50\,h^{-1}{\rm Mpc}$ we find a bulk flow amplitude of $U=248\pm58\,{\rm km}\,{\rm s}^{-1}$ in the direction $(l,b) = (318^\circ\pm20^\circ,40^\circ\pm13^\circ)$, and for $70\,h^{-1}{\rm Mpc}$ we find $U=243\pm58\,{\rm km}\,{\rm s}^{-1}$, in the same direction. Our measurement gives us a constraint on $σ_8$ of $1.01^{+1.07}_{-0.58}$. Our results are in agreement with other recent measurements of the direction of the bulk flow, and our measured amplitude is consistent with a $Λ{\rm CDM}$ prediction.

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arXiv2015

Probing bulk flow with nearby SNe Ia data

We test the isotropy of the local Universe using low redshift supernova data from various catalogs and the non-parametric method of smoothed residuals. Using a recently developed catalog which combines supernova data from various surveys, we show that the isotropic hypothesis of a Universe with zero velocity perturbation can be rejected with moderate significance, with $p$-value $\sim 0.07$ out to redshift $z < 0.045$. We estimate the direction of maximal anisotropy on the sky for various pre-existing catalogs and show that it remains relatively unaffected by the light curve fitting procedure. However the recovered direction is biased by the underlying distribution of data points on the sky. We estimate both the uncertainty and bias in the direction by creating mock data containing a randomly oriented bulk flow and using our method to reconstruct its direction. We conclude that the inhomogeneous nature of the data introduces a directional bias in galactic latitude of approximately $|Δb_{\rm max}| \sim 18^{\circ}$ for the supernova catalog considered in this work, and after correcting for this effect we infer the direction of maximum anisotropy as $(b,\ell) = (20^{\circ}, 276^{\circ}) \pm (12^{\circ},29^{\circ})$ in galactic coordinates. Finally we compare the anisotropic signal in the data to mock realisations in which large scale velocity perturbations are consistently accounted for at the level of linear perturbation theory. We show that including the effect of the velocity perturbation in our mock catalogs degrades the significance of the anisotropy considerably, with $p$-value increasing to $\sim 0.29$. One can conclude from our analysis that there is a moderate deviation from isotropy in the supernova data, but the signal is consistent with a large scale bulk velocity expected within $Λ$CDM.

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arXiv2015

Searching for Modified Gravity: Scale and Redshift Dependent Constraints from Galaxy Peculiar Velocities

We present measurements of both scale- and time-dependent deviations from the standard gravitational field equations. These late-time modifications are introduced separately for relativistic and non-relativistic particles, by way of the parameters $G_{\rm matter}(k,z)$ and $G_{\rm light}(k,z)$ using two bins in both scale and time, with transition wavenumber $0.01$ Mpc$^{-1}$ and redshift 1. We emphasize the use of two dynamical probes to constrain this set of parameters, galaxy power spectrum multipoles and the direct peculiar velocity power spectrum, which probe fluctuations on different scales. The multipole measurements are derived from the WiggleZ and BOSS Data Release 11 CMASS galaxy redshift surveys and the velocity power spectrum is measured from the velocity sub-sample of the 6-degree Field Galaxy Survey. We combine with additional cosmological probes including baryon acoustic oscillations, Type Ia SNe, the cosmic microwave background (CMB), lensing of the CMB, and the temperature--galaxy cross-correlation. Using a Markov Chain Monte Carlo likelihood analysis, we find the inferred best-fit parameter values of $G_{\rm matter}(k,z)$ and $G_{\rm light}(k,z)$ to be consistent with the standard model at the $95\%$ confidence level. Furthermore, accounting for the Alcock-Paczynski effect, we perform joint fits for the expansion history and growth index gamma; we measure $γ= 0.665 \pm 0.0669$ ($68\%$ C.L) for a fixed expansion history, and $γ= 0.73^{+0.08}_{-0.10}$ ($68\%$ C.L) when the expansion history is allowed to deviate from $Λ$CDM. With a fixed expansion history the inferred value is consistent with GR at the $95\%$ C.L; alternatively, a $2σ$ tension is observed when the expansion history is not fixed, this tension is worsened by the combination of growth and SNe data.

astro-ph.CO↗

arXiv2014

The 6dF Galaxy Velocity Survey: Cosmological constraints from the velocity power spectrum

We present scale-dependent measurements of the normalised growth rate of structure $fσ_{8}(k, z=0)$ using only the peculiar motions of galaxies. We use data from the 6-degree Field Galaxy Survey velocity sample (6dFGSv) together with a newly-compiled sample of low-redshift $(z < 0.07)$ type Ia supernovae. We constrain the growth rate in a series of $Δk \sim 0.03 h{\rm Mpc^{-1}}$ bins to $\sim35\%$ precision, including a measurement on scales $>300 h^{-1}{\rm Mpc}$, which represents one of the largest-scale growth rate measurement to date. We find no evidence for a scale dependence in the growth rate, or any statistically significant variation from the growth rate as predicted by the {\it Planck} cosmology. Bringing all the scales together, we determine the normalised growth rate at $z=0$ to $\sim15\%$ in a manner {\it independent} of galaxy bias and in excellent agreement with the constraint from the measurements of redshift-space distortions from 6dFGS. We pay particular attention to systematic errors. We point out that the intrinsic scatter present in Fundamental-Plane and Tully-Fisher relations is only Gaussian in logarithmic distance units; wrongly assuming it is Gaussian in linear (velocity) units can bias cosmological constraints. We also analytically marginalise over zero-point errors in distance indicators, validate the accuracy of all our constraints using numerical simulations, and demonstrate how to combine different (correlated) velocity surveys using a matrix `hyper-parameter' analysis. Current and forthcoming peculiar velocity surveys will allow us to understand in detail the growth of structure in the low-redshift universe, providing strong constraints on the nature of dark energy.

astro-ph.CO↗

arXiv2013

Are peculiar velocity surveys competitive as a cosmological probe?

Peculiar velocity surveys, which measure galaxy peculiar velocities directly from standard candles in addition to redshifts, can provide strong constraints on the linear growth rate of cosmological large-scale structure at low redshift. The improvement originates from the physical relationship between galaxy density and peculiar velocity, which substantially reduces cosmic variance. We present the results of Fisher matrix forecasts of correlated fields of galaxy density and velocity. Peculiar velocity can improve the growth rate constraints by about a factor of two compared to density alone, if we can use all the information for wavenumber k < 0.2 h/Mpc. Future peculiar velocity surveys, TAIPAN, and the all-sky HI surveys, WALLABY and WNSHS, can measure the growth rate, f*sigma8 to 3 per cent at z ~ 0.025. Although the velocity subsample is about an order of magnitude smaller than the redshift sample from the same survey, it improves the constraint by 40 per cent compared to the same survey without velocity measurements. Peculiar velocity surveys can also measure the growth rate as a function of wavenumber with 15-30 per cent uncertainties in bins with widths 0.01 h/Mpc in the range 0.02 h/Mpc < k < 0.1 h/Mpc, which is a large improvement over galaxy density only. Such measurements on very large scales can detect signatures of modified gravity or non-Gaussianity through scale-dependent growth rate or galaxy bias. We use N-body simulations to improve the modelling of auto- and cross-power spectra of galaxy density and peculiar velocity by introducing a new redshift-space distortion term to the velocity, which has been neglected in previous studies. The velocity power spectrum has a damping in redshift space, which is larger than that naively expected from the similar effect in the galaxy power spectrum.

astro-ph.CO↗

arXiv2011

CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey - The Hubble Space Telescope Observations, Imaging Data Products and Mosaics

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at $z\sim1.5-8$, and to study Type Ia SNe beyond $z>1.5$. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers \sim125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of \sim800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

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