arXiv subjects

C. R. Lawrence

Publications and source records attributed to C. R. Lawrence.

107 records · Page 6Linked to original sources

arXiv2007

Making Maps from Planck LFI 30GHz Data

This paper is one of a series describing the performance and accuracy of map-making codes as assessed by the Planck CTP working group. We compare the performance of multiple codes written by different groups for making polarized maps from Planck-sized, all-sky cosmic microwave background (CMB) data. Three of the codes are based on destriping algorithm, whereas the other three are implementations of a maximum-likelihood algorithm. Previous papers in the series described simulations at 100 GHz (Poutanen et al. 2006) and 217 GHz (Ashdown et al. 2006). In this paper we make maps (temperature and polarisation) from the simulated one-year observations of four 30 GHz detectors of Planck Low Frequency Instrument (LFI). We used Planck Level S simulation pipeline to produce the observed time-ordered-data streams (TOD). Our previous studies considered polarisation observations for the CMB only. For this paper we increased the realism of the simulations and included polarized galactic foregrounds to our sky model. Our simulated TODs comprised of dipole, CMB, diffuse galactic emissions, extragalactic radio sources, and detector noise. The strong subpixel signal gradients arising from the foreground signals couple to the output map through the map-making and cause an error (signal error) in the maps. Destriping codes have smaller signal error than the maximum-likelihood codes. We examined a number of schemes to reduce this error. On the other hand, the maximum-likelihood map-making codes can produce maps with lower residual noise than destriping codes.

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arXiv2006

Spitzer Observations of 3C Quasars and Radio Galaxies: Mid-Infrared Properties of Powerful Radio Sources

We have measured mid-infrared radiation from an orientation-unbiased sample of 3CRR galaxies and quasars at redshifts 0.4 < z < 1.2 with the IRS and MIPS instruments on the Spitzer Space Telescope. Powerful emission (L_24micron > 10^22.4 W/Hz/sr) was detected from all but one of the sources. We fit the Spitzer data as well as other measurements from the literature with synchrotron and dust components. The IRS data provide powerful constraints on the fits. At 15 microns, quasars are typically four times brighter than radio galaxies with the same isotropic radio power. Based on our fits, half of this difference can be attributed to the presence of non-thermal emission in the quasars but not the radio galaxies. The other half is consistent with dust absorption in the radio galaxies but not the quasars. Fitted optical depths are anti-correlated with core dominance, from which we infer an equatorial distribution of dust around the central engine. The median optical depth at 9.7 microns for objects with core-dominance factor R > 10^-2 is approximately 0.4; for objects with R < 10^-2, it is 1.1. We have thus addressed a long-standing question in the unification of FR II quasars and galaxies: quasars are more luminous in the mid-infrared than galaxies because of a combination of Doppler-boosted synchrotron emission in quasars and extinction in galaxies, both orientation-dependent effects.

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arXiv2006

Spitzer Observations of Centaurus A: Infrared Synchrotron Emission from the Northern Lobe

We present measurements obtained with the Spitzer Space Telescope in five bands from 3.6-24 microns of the northern inner radio lobe of Centaurus A, the nearest powerful radio galaxy. We show that this emission is synchrotron in origin. Comparison with ultraviolet observations from GALEX shows that diffuse ultraviolet emission exists in a smaller region than the infrared but also coincides with the radio jet. We discuss the possibility, that synchrotron emission is responsible for the ultraviolet emission and conclude that further data are required to confirm this.

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arXiv2006

Making sky maps from Planck data

We compare the performance of multiple codes written by different groups for making polarized maps from Planck-sized, all-sky cosmic microwave background (CMB) data. Three of the codes are based on a destriping algorithm; the other three are implementations of an optimal maximum-likelihood algorithm. Time-ordered data (TOD) were simulated using the Planck Level-S simulation pipeline. Several cases of temperature-only data were run to test that the codes could handle large datasets, and to explore effects such as the precision of the pointing data. Based on these preliminary results, TOD were generated for a set of four 217 GHz detectors (the minimum number required to produce I, Q, and U maps) under two different scanning strategies, with and without noise. Following correction of various problems revealed by the early simulation, all codes were able to handle the large data volume that Planck will produce. Differences in maps produced are small but noticeable; differences in computing resources are large.

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arXiv2006

Bayesian foreground analysis with CMB data

The quality of CMB observations has improved dramatically in the last few years, and will continue to do so in the coming decade. Over a wide range of angular scales, the uncertainty due to instrumental noise is now small compared to the cosmic variance. One may claim with some justification that we have entered the era of precision CMB cosmology. However, some caution is still warranted: The errors due to residual foreground contamination in the CMB power spectrum and cosmological parameters remain largely unquantified, and the effect of these errors on important cosmological parameters such as the optical depth tau and spectral index n_s is not obvious. A major goal for current CMB analysis efforts must therefore be to develop methods that allows us to propagate such uncertainties from the raw data through to the final products. Here we review a recently proposed method that may be a first step towards that goal.

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arXiv2006

Silicate Emission in the Spitzer IRS spectrum of FSC 10214+4724

We present the first MIR spectrum of the z=2.2856 ultraluminous, infrared galaxy FSC 10214+4724, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope. The spectrum spans a rest wavelength range of 2.3-11.5 microns, covering a number of key diagnostic emission and absorption features. The most prominent feature in the IRS spectrum is the silicate emission at rest-frame 10 microns. We also detect an unresolved emission line at a rest wavelength of 7.65 microns which we identify with [NeVI], and a slightly resolved feature at 5.6 microns identified as a blend of [Mg VII] and [Mg V]. There are no strong PAH emission features in the FSC 10214+4724 spectrum. We place a limit of 0.1 micron on the equivalent width of 6.2 micron PAH emission but see no evidence of a corresponding 7.7 micron feature. Semi-empirical fits to the spectral energy distribution suggest about 45% of the bolometric luminosity arises from cold 50 K dust, half arises from warm (190 K) dust, and the remainder, 5%, originates from hot (640 K) dust. The hot dust is required to fit the blue end of the steep MIR spectrum. The combination of a red continuum, strong silicate emission, little or no PAH emission, and no silicate absorption, makes FSC 10214+4724 unlike most other ULIRGs or AGN observed thus far with IRS. These apparently contradictory properties may be explained by an AGN which is highly magnified by the lens, masking a (dominant) overlying starburst with unusually weak PAH emission.

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arXiv2005

CMB component separation by parameter estimation

We propose a solution to the CMB component separation problem based on standard parameter estimation techniques. We assume a parametric spectral model for each signal component, and fit the corresponding parameters pixel by pixel in a two-stage process. First we fit for the full parameter set (e.g., component amplitudes and spectral indices) in low-resolution and high signal-to-noise ratio maps using MCMC, obtaining both best-fit values for each parameter, and the associated uncertainty. The goodness-of-fit is evaluated by a chi^2 statistic. Then we fix all non-linear parameters at their low-resolution best-fit values, and solve analytically for high-resolution component amplitude maps. This likelihood approach has many advantages: The fitted model may be chosen freely, and the method is therefore completely general; all assumptions are transparent; no restrictions on spatial variations of foreground properties are imposed; the results may be rigorously monitored by goodness-of-fit tests; and, most importantly, we obtain reliable error estimates on all estimated quantities. We apply the method to simulated Planck and six-year WMAP data based on realistic models, and show that separation at the muK level is indeed possible in these cases. We also outline how the foreground uncertainties may be rigorously propagated through to the CMB power spectrum and cosmological parameters using a Gibbs sampling technique.

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arXiv2004

The Spitzer Space Telescope Mission

The Spitzer Space Telescope, NASA's Great Observatory for infrared astronomy, was launched 2003 August 25 and is returning excellent scientific data from its Earth-trailing solar orbit. Spitzer combines the intrinsic sensitivity achievable with a cryogenic telescope in space with the great imaging and spectroscopic power of modern detector arrays to provide the user community with huge gains in capability for exploration of the cosmos in the infrared. The observatory systems are largely performing as expected and the projected cryogenic lifetime is in excess of 5 years. This paper summarizes the on-orbit scientific, technical and operational performance of Spitzer. Subsequent papers in this special issue describe the Spitzer instruments in detail and highlight many of the exciting scientific results obtained during the first six months of the Spitzer mission.

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arXiv2004

Spitzer/IRS Observations of the Redshift 3.91 quasar APM 08279+5255

The Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope (SST) has been used to obtain low and moderate resolution spectra of the dust and gas-rich quasar APM08279+5255 (z=3.91). Broad Paschen $α$ and $β$ recombination lines of hydrogen were detected at wavelengths of 9.235 and 6.315microns, as well as a strong, red continuum that is a smooth power law over the observed (rest frame) wavelength range 5.3-35microns (1.08 - 7.1microns). The observed P$α$/P$β$ line flux ratio of 1.05$\pm$0.2 is far from the case B value of ~2 and simple models of high density, high optical depth ionized gas regions (~1.8). This deviation is opposite in sense to the expected effect of reddening. No evidence is found in the spectrum for either the 3.3micron or 6.2micron emission features usually attributed to aromatic hydrocarbons in gas rich galaxies in the local Universe. This is consistent with the high luminosity AGN nature of APM08279+5255.

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arXiv2004

XMM-Newton observations of three high-redshift radio galaxies

We present the results of XMM-Newton observations of three high-redshift powerful radio galaxies 3C 184, 3C 292 and 3C 322. Although none of the sources lies in as rich an X-ray-emitting environment as is seen for some powerful radio galaxies at low redshift, the environments provide sufficient pressure to confine the radio lobes. The weak gas emission is particularly interesting for 3C 184, where a gravitational arc is seen, suggesting the presence of a massive cluster. Here Chandra data complement the XMM-Newton measurements by spatially separating X-rays from the extended atmosphere, the nucleus and the small-scale radio source. For 3C 292 the X-ray-emitting gas has a temperature of ~2 keV and luminosity of 6.5E43 erg/s, characteristic of a poor cluster. In all three cases, structures where the magnetic-field strength can be estimated through combining measurements of radio-synchrotron and inverse-Compton-X-ray emission, are consistent with being in a state of minimum total energy. 3C 184 and 3C 292 (and possibly 3C 322) have a heavily absorbed component of nuclear emission of N_H ~ $ few 10^{23} cm^{-2}.

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arXiv2002

Prospecting for Heavy Elements with Future Far-IR/Submillimeter Observatories

To understand the cosmic history of element synthesis it will be important to obtain extinction-free measures of the heavy element contents of high-redshift objects and to chart two monumental events: the collapse of the first metal-free clouds to form stars, and the initial seeding of the universe with dust. The information needed to achieve these objectives is uniquely available in the far-infrared/submillimeter (FIR/SMM) spectral region. Following the Decadal Report and anticipating the development of the Single Aperture Far-IR (SAFIR) telescope and FIR/SMM interferometry, we estimate the measurement capabilities of a large-aperture, background-limited FIR/SMM observatory and an interferometer on a boom, and discuss how such instruments could be used to measure the element synthesis history of the universe.

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arXiv2002

2016+112: A Gravitationally Lensed Type-II Quasar

A single-screen model of the gravitational lens system 2016+112 is proposed, that explains recent Hubble Space Telescope} (HST) infrared (NICMOS-F160W) observations and new high-resolution European VLBI Network (EVN) 5-GHz radio observations, presented in this paper. In particular, we find that a massive `dark' structure at the lens position, previously suggested by X-ray, optical and spectroscopic observations of the field around 2016+112, is not necessarily required to accommodate the strong lensing constraints. A massive structure to the north-west of the lens system, suggested from a weak-lensing analysis of the field, is included in the model. The lensed source is an X-ray bright active galaxy at z=3.273 with a central bright optical continuum core and strong narrow emission lines, suggestive of a type-II quasar. The EVN 5-GHz radio maps show a radio-jet structure with at least two compact subcomponents. We propose that the diamond caustic crosses the counter-jet of the radio source, so that part of the counter-jet, host galaxy and narrow-line emission regions are quadruply imaged. The remainder of the radio source, including the core, is doubly imaged. Our lens model predicts a very high magnification (mu~300) at the bightness peaks of the inner two radio components of complex C. If the jet exhibits relativistic velocities on micro-arsecond scales, it might result in apparent hyperluminal motion. However, the lack of strong radio variability and the peaked radio spectrum imply that these motions need not be present in the source. Our model furthermore implies that the optical spectrum of C' can only show features of the AGN and its host galaxy.

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arXiv2001

Chandra measurements of 3C 220.1's X-ray core and cluster

We report results of an 18 ks exposure with the ACIS instrument on Chandra of the powerful z = 0.62 radio galaxy 3C 220.1. The X-ray emission separates into cluster gas of emission-weighted kT ~ 5 keV and 0.7-12 keV luminosity (to a radius of 45 arcsec) 5.6 x 10^44 ergs s^-1, and unresolved emission (coincident with the radio core). While the extended X-ray emission is clearly thermal in nature, a straightforward cooling-flow model, even in conjunction with a point-source component, is a poor fit to the radial profile of the X-ray emission. This is despite the fact that the measured properties of the gas suggest a massive cooling flow of ~ 130 M_\odot yr^-1, and the data show weak evidence for a temperature gradient. The central unresolved X-ray emission has a power-law spectral energy index α~ 0.7 and 0.7-12 keV luminosity 10^45 ergs s^-1, and any intrinsic absorption is relatively small. The two-point spectrum of the core emission between radio and X-ray energies has α_rx = 0.75. Since this is a flatter spectrum than seen in other sources where the X-ray emission is presumed to be radio-related, regions close to the AGN in this source may dominate the central X-ray output, as is believed to be the case for lobe-dominated quasars. Simple unification models would be challenged if this were found to be the case for a large fraction of high-power radio galaxies.

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arXiv2000

A Measurement of Anisotropy in the Cosmic Microwave Background on 7-22 Arcminute Scales

We report a measurement of anisotropy in the cosmic microwave background radiation (CMBR) on 7-22 arcminute scales. Observations of 36 fields near the North Celestial Pole (NCP) were made at 31.7 and 14.5 GHz, using the 5.5-meter and 40-meter telescopes at the Owens Valley Radio Observatory (OVRO) from 1993 to 1996. Multi-epoch VLA observations at 8.5 and 15 GHz allow removal of discrete source contamination. After point-source subtraction, we detect significant structure, which we identify with emission from a combination of a steep-spectrum foreground and the CMBR. The foreground component is found to correlate with IRAS 100 micron dust emission. Lack of H-alpha emission near the NCP suggests that this foreground is either high-temperature thermal bremsstrahlung (T_e ~ 10^6 K), flat-spectrum synchrotron or an exotic component of dust emission. On the basis of low-frequency maps of the NCP, we can restrict the spectral index of the foreground to beta >= -2.2. Although the foreground signal dominates at 14.5 GHz, the extracted CMBR component contributes 88\% of the variance at 31.7 GHz, yielding an rms fluctuation amplitude of 82^{+12.1}_{-9.1} microKelvin, including 4.3% calibration uncertainty and 12% sample variance (68% confidence). The equivalent broadband amplitude is dT_{l_e} = 59^{+8.6}_{-6.5} microKelvin at l_e = 589. This measurement, when combined with small angular-scale upper limits obtained at the OVRO, indicates that the CMBR angular power spectrum decreases between l ~ 600 and l ~ 2000 and is consistent with flat cosmological models.

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arXiv1999

Bayesian Approach to Foreground Removal

Our ability to extract the maximal amount of information from future observations at gigahertz frequencies depends on our ability to separate the underlying cosmic microwave background (CMB) from galactic and extragalactic foregrounds. We review the separation problem and its formulation within Bayesian inference, give examples of specific solutions with particular choices of prior density, and finally comment on the generalization of Bayesian methods to a multi-resolution framework. We propose a strategy for the regularization of solutions allowing a spatially varying spectral index, and discuss possible computational approaches such as multi-scale stochastic relaxation.

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arXiv1998

Extended and compact X-ray emission from the powerful radio galaxy 3C220.1

We report on ROSAT HRI observations of the z=0.61 radio galaxy 3C220.1. The X-ray emission from this object consists of an extended component, which we attribute to luminous cluster emission, and a compact central source. The compact component is too bright to be modelled as a cooling flow under some plausible assumptions for the hot gas temperature and distribution; we suggest instead that it is directly related to the core of the radio source. The X-ray flux of the compact component is consistent with the prediction of Worrall et al (1994) that all powerful radio galaxies should have a central jet-related X-ray emission component that is proportional in strength to the radio core flux density. Other observations of distant 3CR radio sources are consistent with this model.

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arXiv1996

The Lensing Galaxy in MG1549+3047

We have measured a velocity dispersion for the foreground galaxy in this gravitationally lensed system. Our dispersion confirms the prediction from lens models, provided that the source is distant enough ($z_{_S}>0.2$). Current interpretations of lensing statistics depend sensitively on how the optical and mass dispersions are related. For $z_{_S}>0.5$, our observations favor $σ_{mass}/σ_{opt}\simeq1$, but our uncertainties prevent us from clearly distinguishing between dynamical models. We could not obtain a unique mass profile from lensing constraints, but models with a constant mass-to-light ratio are possible. We also found unusual rotation in the lensing galaxy, and confirmed its redshift.

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