arXiv subjects

Joanne C. Baker

Publications and source records attributed to Joanne C. Baker.

15 recordsLinked to original sources

arXiv2001

Associated absorption and radio source growth

Results are presented from a survey for CIV associated absorption in a complete low-frequency-selected sample of quasars. In agreement with previous work, associated absorbers are most common in steep-spectrum and lobe-dominated quasars, indicative of an anisotropic cloud distribution. Furthermore, we find the strongest CIV absorption occurs in sources of small radio size, suggesting that the absorbing clouds are destroyed or displaced as radio sources expand. Evidence for dust in the clouds is also found, such that quasars with strong absorption are systematically redder. Finally, we find no evidence for evolution in the frequency or properties of the absorbers from z~0.7 to z~3.

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arXiv2000

Tunable-filter imaging of quasar fields at z~1. I. A cluster around MRC B0450-221

Using a combination of multicolour broad- and narrow-band imaging techniques and follow-up spectroscopy, we have detected an overdensity of galaxies in the field of quasar MRC B0450-221, whose properties are consistent with a cluster at the quasar redshift z=0.9. An excess of red galaxies (V-I>2.2, I-K'>3.8) is evident within 1' of the quasar, with the colours expected for galaxies at z=0.9 that have evolved passively for 3 Gyr or more. A number of line-emitting galaxies (nine candidates with equivalent widths EW>70A) are also detected in the field using the TAURUS Tunable Filter (TTF). Three have been confirmed spectroscopically to indeed lie at z=0.9. The TTF candidates with the strongest [O II] line emission cluster in a group which lies 200-700 kpc away from the quasar and the red galaxy excess, and therefore most likely on the outskirts of the cluster. These observations are the first in a series probing quasar environments at z~1 with TTF.

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arXiv1999

Completeness and confusion in the identification of Lyman-break galaxies

We have carried out a study to simulate distant clusters of galaxies in deep ground-based optical images. We find that when model galaxies are added to deep images obtained with the William Herschel Telescope, there is considerable scatter of the recovered galaxy colours away from the model values; this scatter is larger than that expected from photometric errors and is significantly affected by confusion, due to ground-based seeing, between objects in the field. In typical conditions of $\approx$ 1-arcsec seeing, the combination of confusion and incompleteness causes a considerable underestimation of the true surface density of $z \approx 3$ galaxies. We argue that the actual surface density of $z \approx 3$ galaxies may be several times greater than that estimated by previous ground-based studies, consistent with the surface density of such objects found in the HDF.

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arXiv1999

Detection of Cosmic Microwave Background Structure in a Second Field with the Cosmic Anisotropy Telescope

We describe observations at frequencies near 15 GHz of the second 2x2 degree field imaged with the Cambridge Cosmic Anisotropy Telescope (CAT). After the removal of discrete radio sources, structure is detected in the images on characteristic scales of about half a degree, corresponding to spherical harmonic multipoles in the approximate range l= 330--680. A Bayesian analysis confirms that the signal arises predominantly from the cosmic microwave background (CMB) radiation for multipoles in the lower half of this range; the average broad-band power in a bin with centroid l=422 (theta = 51') is estimated to be Delta_T/T=2.1^{+0.4}_{-0.5} x 10^{-5}. For multipoles centred on l=615 (theta =35'), we find contamination from Galactic emission is significant, and constrain the CMB contribution to the measured power in this bin to be Delta_T/T <2.0 x 10^{-5} (1-sigma upper limit). These new results are consistent with the first detection made by CAT in a completely different area of sky. Together with data from other experiments, this new CAT detection adds weight to earlier evidence from CAT for a downturn in the CMB power spectrum on scales smaller than 1 degree. Improved limits on the values of H_0 and Omega are determined using the new CAT data.

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arXiv1999

WOMBAT & FORECAST: Making Realistic Maps of the Microwave Sky

The Wavelength-Oriented Microwave Background Analysis Team (WOMBAT) is constructing microwave maps which will be more realistic than previous simulations. Our foreground models represent a considerable improvement: where spatial templates are available for a given foreground, we predict the flux and spectral index of that component at each place on the sky and estimate uncertainties. We will produce maps containing simulated CMB anisotropy combined with expected foregrounds. The simulated maps will be provided to the community as the WOMBAT Challenge, so such maps can be analyzed to extract cosmological parameters by scientists who are unaware of their input values. This will test the efficacy of foreground subtraction, power spectrum analysis, and parameter estimation techniques and help identify the areas most in need of progress. These maps are also part of the FORECAST project, which allows web-based access to the known foreground maps for the planning of CMB missions.

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arXiv1999

The Molonglo Reference Catalog 1-Jy radio source survey IV. Optical spectroscopy of a complete quasar sample

Optical spectroscopic data are presented here for quasars from the Molonglo Quasar Sample (MQS), which forms part of a complete survey of 1-Jy radio sources from the Molonglo Reference Catalogue. The combination of low-frequency selection and complete identifications means that the MQS is relatively free from the orientation biases which affect most other quasar samples. To date, the sample includes 105 quasars and 6 BL Lac objects, 106 of which have now been confirmed spectroscopically. This paper presents a homogenous set of low-resolution optical spectra for 79 MQS quasars, the majority of which have been obtained at the Anglo-Australian Telescope. Full observational details are given and redshifts, continuum and emission-line data tabulated for all confirmed quasars.

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arXiv1998

The WOMBAT Challenge: A "Hounds and Hares" Exercise for Cosmology

The Wavelength-Oriented Microwave Background Analysis Team (WOMBAT) is constructing microwave skymaps which will be more realistic than previous simulations. Our foreground models represent a considerable improvement: where spatial templates are available for a given foreground, we predict the flux and spectral index of that component at each place on the sky and estimate the uncertainties in these quantities. We will produce maps containing simulated Cosmic Microwave Background anisotropies combined with all major expected foreground components. The simulated maps will be provided to the cosmology community as the WOMBAT Challenge, a "hounds and hares" exercise where such maps can be analyzed to extract cosmological parameters by scientists who are unaware of their input values. This exercise will test the efficacy of current foreground subtraction, power spectrum analysis, and parameter estimation techniques and will help identify the areas most in need of progress.

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arXiv1998

Deep optical imaging of the field of PC1643+4631A&B, I: Spatial distributions and the counts of faint galaxies

We present deep optical images of the PC1643+4631 field obtained at the WHT. This field contains two quasars at redshifts z=3.79 & 3.83 and a cosmic microwave background (CMB) decrement detected with the Ryle Telescope. The images are in U,G,V,R and I filters, and are complete to 25th magnitude in R and G and to 25.5 in U. The isophotal galaxy counts are consistent with the results of Metcalde et al. (1996), Hogg et al. (1997), and others. We find an excess of robust high-redshift Ly-break galaxy candidates with 25.0<R<25.5 compared with the mean number found in the fields studied by Steidel et al. -we expect 7 but find 16 - but we do not find that the galaxies are concentrated in the direction of the CMB decrement. However, we are still not sure of the distance to the system causing the CMB decrement. We have also used our images to compare the commonly used object-finding algorithms of FOCAS and SExtractor: we find FOCAS the more efficient at detecting faint objects and the better at dealing with composite objects, whereas SExtractor's morphological classification is more reliable, especially for faint objects near the resolution limit. More generally, we have also compared the flux lost using isophotal apertures on a real image with that on a noise-only image: recovery of artificial galaxies from the noise-only image significantly overestimates the flux lost from the galaxies, and we find that the corrections made using this technique suffer a systematic error of some 0.4 magnitudes.

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arXiv1998

Deep optical imaging of the field of PC1643+4631A&B, II: Estimating the colours and redshifts of faint galaxies

In an investigation of the cause of the cosmic microwave background decrement in the field of the z = 3.8 quasar pair PC1643+4631, we have carried out a study to photometrically estimate the redshifts of galaxies in deep multi-colour optical images of the field taken with the WHT. To examine the possibility that a massive cluster of galaxies lies in the field, we have attempted to recover simulated galaxies with intrinsic colours matching those of the model galaxies used in the photometric redshift estimation. We find that when such model galaxies are added to our images, there is considerable scatter of the recovered galaxy redshifts away from the model value; this scatter is larger than that expected from photometric errors and is the result of confusion, simply due to ground-based seeing, between objects in the field. We have also compared the likely efficiency of the photometric redshift technique against the colour criteria used to select z>3 galaxies via the strong colour signature of the Lyman-limit break. We find that these techniques may significantly underestimate the true surface density of z>3, due to confusion between the high-redshift galaxies and other objects near the line of sight. We argue that the actual surface density of z=3 galaxies may be as much as 6 times greater than that estimated by previous ground-based studies, and note that this conclusion is consistent with the surface density of high-redshift objects found in the HDF. Finally, we conclude that all ground-based deep field surveys are inevitably affected by confusion, and note that reducing the effective seeing in ground-based images will be of paramount importance in observing the distant universe.

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arXiv1997

Lyman-alpha absorption : links between CSS quasars and high-redshift radio galaxies

Spectroscopy of high-redshift compact steep-spectrum (CSS) quasars shows a high incidence of Ly-alpha absorption close to the quasar redshift. Associated absorption systems like these are rare in other types of radio sources, with the notable exception of high-redshift radio galaxies (HZRGs). CSS quasars and HZRGs share many properties; the hypothesis that they are intrinsically similar objects is presented and discussed.

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arXiv1997

Tunable filter imaging: structure forming around a quasar at z=0.9

Preliminary results are described for emission-line imaging of the field of a quasar at z=0.9 with a unique new instrument --- the TAURUS Tunable Filter (TTF). At least fourteen [O II]-emitting galaxy candidates are found within +/- 750 km/s of the quasar redshift with a significance of >3 sigma. Another eight candidates (>3 sigma) are also found with relative velocities +/- 750-1500 km/s from the quasar, indicative of a large velocity dispersion in the field. Together with the existence of a group of red galaxies in the field identified through broad-band imaging, we suggest that the TTF data are probing cluster merging which is ongoing at z=0.9.

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arXiv1997

The Radio-Optical Correlation in Steep-Spectrum Quasars

Using complete samples of steep-spectrum quasars, we present evidence for a correlation between radio and optical luminosity which is not caused by selection effects, nor caused by an orientation dependence (such as relativistic beaming), nor a byproduct of cosmic evolution. We argue that this rules out models of jet formation in which there are no parameters in common with the production of the optical continuum. This is arguably the most direct evidence to date for a close link between accretion onto a black hole and the fuelling of relativistic jets. The correlation also provides a natural explanation for the presence of aligned optical/radio structures in only the most radio luminous high-redshift galaxies.

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arXiv1996

Detection of a CMB decrement towards the z = 3.8 quasar pair PC1643+4631A&B

In a 15-GHz Ryle-Telescope observation of PC1643+4631 A & B, a pair of quasars at redshifts z = 3.79 and 3.83 separated by 198 arcsec on the sky, we find a decrement in the cosmic microwave background (CMB) of -380 +/- 64 uJy in a 110 arcsec x 175 arcsec beam. Assuming this to be a Sunyaev-Zel'dovich effect due to an intervening cluster, the minimum magnitude of the central temperature decrement is 560 uK. A serendipitous ROSAT observation shows that there is no X-ray-luminous cluster in the direction of the decrement at z < 1. The implied gas mass is >~ 2 x 10^{14} solar masses (assuming a temperature of ~ 5 keV), indicating a total mass of > 10^{15} solar masses. This result demonstrates the existence of a massive system too distant to be detected by its emission, but which can be found via its imprint on the CMB.

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arXiv1996

Optical and infrared investigation towards the z=3.8 quasar pair PC1643+4631A&B

In a companion paper (Paper I: Jones et al. 1996) we report the discovery of a cosmic microwave background decrement, indicative of a distant cluster with mass ~10^{15} solar masses, towards the quasar pair PC1643+4631A&B (z=3.79,3.83, separation 198''). To search for the cluster responsible, we have obtained R-, J- and K-band images of the field and have also carried out optical spectroscopy of selected objects in it. No such cluster is evident in these images. Assuming the cluster causing the decrement is similar to massive clusters already known, our magnitude limits imply that it must lie about or beyond z=1. This provides independent support for the X-ray-based distance argument of Paper I. The cluster must gravitationally lens objects behind it; for a cluster z around 1-2, the Einstein-ring radius for sources at z ~= 3.8 is ~ 100''. Simple modelling, producing simultaneously the S-Z effect and the lensing, shows that the source positions of quasars A and B lie within ~10'' of each other and may indeed be coincident. The two quasar spectra are found to be remarkably similar apart from their one-percent redshift difference. Assuming A and B are images of a single quasar, we present a possible explanation of this difference.

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arXiv1996

Origin of the viewing-angle dependence of the optical continuum emission in quasars

The orientation-dependence of the optical continuum emission in radio-loud quasars is investigated using a new, complete sample of low-frequency-selected quasars, the Molonglo Quasar Sample (MQS). The optical continuum is found to be highly anisotropic, brightening continuously from lobe- to core-dominated quasars by 3-5 mag. It is argued that aspect-dependent extinction, rather than relativistic boosting as has been previously proposed, provides the simplest explanation consistent with the data. The reddening hypothesis is supported by both the steeper optical slopes and the larger Balmer decrements found in lobe-dominated quasars, as well as the stronger anisotropy seen at blue wavelengths. The dust responsible is shown to be physically associated with the quasar, lying mostly at radii between the broad and narrow-line regions in a clumpy distribution. Such a geometry is reminiscent of a torus. However, substantial numbers of dust clouds must lie within the torus opening angle, contributing to an increasing average optical depth with increasing viewing angle away from the jet axis.

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