J. P. Leahy

The cosmic microwave background power spectrum out to ℓ= 1400 measured by the Very Small Array

We have observed the cosmic microwave background (CMB) in three regions of sky using the Very Small Array (VSA) in an extended configuration with antennas of beamwidth 2 degrees at 34 GHz. Combined with data from previous VSA observations using a more compact array with larger beamwidth, we measure the power spectrum of the primordial CMB anisotropies between angular multipoles l = 160 - 1400. Such measurements at high l are vital for breaking degeneracies in parameter estimation from the CMB power spectrum and other cosmological data. The power spectrum clearly resolves the first three acoustic peaks, shows the expected fall off in power at high l and starts to constrain the position and height of a fourth peak.

Chandra X-Ray Detection of the Radio Hot Spots of 3C 295

An observation of the radio galaxy 3C 295 during the calibration phase of the Chandra X-Ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hot spots, and from the previously known cluster gas. We discuss the possible emission processes for the hot spots and argue that a synchrotron self-Compton (SSC) model is preferred for most or all of the observed X-ray emission. SSC models with near-equipartition fields thus explain the X-ray emission from the hot spots in the two highest surface brightness FR II radio galaxies, Cygnus A and 3C 295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

A multiband study of Hercules A — II. Multifrequency Very Large Array imaging

We have mapped the powerful radio galaxy Hercules A at six frequencies spanning 1295 to 8440 MHz using the Very Large Array (VLA) in all four configurations. Here we discuss the structure revealed in total intensity, spectral index, polarization, and projected magnetic field. n n n nOur observations clearly reveal the relation between the bright jets, prominent rings, bulbous outer lobes and faint bridge that make up the radio source. The jets and rings form a coherent structure with a dramatically flatter spectrum than the surrounding lobes and bridge, strongly suggesting that they represent a recently renewed outburst from the active nucleus. The spectrum of the lobes is also steeper than in typical radio sources, and steepens further towards the centre. The compact core is optically thin and also has a remarkably steep spectrum (α≃−1.2). There is some evidence that the old lobe material has been swept up and compressed ahead of the new outburst. We interpret the dramatic asymmetry in the bright structure, and more subtle differences between diffuse lobe structures, in terms of relativistic beaming combined with front-to-back light-travel delays, which mean that we view the two lobes at different stages of the outburst. n n n nAfter correcting for Faraday rotation the projected magnetic field closely follows the edge of the lobes, the jets and the rings; the field pattern in the two lobes is broadly similar. We confirm a strong asymmetry in depolarization and Faraday rotation, with the jet side being the less depolarized and with the flatter spectrum, consistent with general correlations between these asymmetries. The spectral index asymmetry is clearly present in the ‘old’ lobe material and so, at least in this case, is not caused by beaming; but it can be understood in terms of the light-travel delay.

First results from the Very Small Array – II. Observations of the cosmic microwave background

We have observed the cosmic microwave background temperature fluctuations in eight fields covering three separated areas of sky with the Very Small Array at 34 GHz. A total area of 101 square degrees has been imaged, with sensitivity on angular scales 3. ◦ 6–0. ◦ 4 (equivalent to angular multipoles l=150–900). We describe the field selection and observing strategy for these observations. In the full-resolution images (with synthesised beam of FWHM ≃ 17 arcmin) the thermal noise is typically 45 � K and the CMB signal typically 55 � k. The noise levels in each field agree well with the expected thermal noise level of the telescope, and there is no evidence of any residual systematic features. The same CMB features are detected in separate, overlapping observations. Discrete radio sources have been detected using a separate 15 GHz survey and their effects removed using pointed follow-up observations at 34 GHz. We estimate that the residual confusion noise due to unsubtracted radio sources is less than 14 mJy beam −1 (15 � K in the full-resolution images), which added in quadrature to the thermal noise increases the noise level by 6 %. We estimate that the rms contribution to the images from diffuse Galactic emission is less than 6 � K. We also present images which are convolved to maximise the signal-to-noise of the CMB features and are co-added in overlapping areas, in which the signal-to-noise of some individual CMB features exceeds 8.

High sensitivity measurements of the CMB power spectrum with the extended Very Small Array

We present deep Ka-band (ν ≈ 33 GHz) observations of the cosmic microwave background (CMB) made with the extended Very Small Array (VSA). This configuration produces a naturally weighted synthesized FWHM beamwidth of ∼11 arcmin, which covers anrange of 300 to 1500. On these scales, foreground extragalactic sources can be a major source of contamination to the CMB anisotropy. This problem has been alleviated by identifying sources at 15 GHz with the Ryle Telescope and then monitoring these sources at 33 GHz using a single-baseline interferometer collocated with the VSA. Sources with flux densities20 mJy at 33 GHz are subtracted from the data. In addition, we calculate a statistical correction for the small residual contribution from weaker sources that are below the detection limit of the survey. The CMB power spectrum corrected for Galactic foregrounds and extragalactic point sources is presented. A totalrange of 150-1500 is achieved by combining the complete extended array data with earlier VSA data in a compact configuration. Our resolution of �� ≈ 60 allows the first three acoustic peaks to be clearly delineated. This is achieved by using mosaiced observations in seven regions covering a total area of 82 deg 2 . There is good agreement with the Wilkinson Microwave Anisotropy Probe (WMAP) data up to � = 700 where WMAP data run out of resolution. For highervalues out to � = 1500, the agreement in power spectrum amplitudes with other experiments is also very good despite differences in frequency and observing technique.

Cosmological parameter estimation and Bayesian model comparison using Very Small Array data

We constrain the basic cosmological parameters using the first observations by the Very Small Array (VSA) in its extended configuration, together with existing cosmic microwave background data and other cosmological observations. We estimate cosmological parameters for four different models of increasing complexity. In each case, careful consideration is given to implied priors and the Bayesian evidence is calculated in order to perform model selection. We find that the data are most convincingly explained by a simple flat ACDM cosmology without tensor modes. In this case, combining just the VSA and COBE data sets yields the 68 per cent confidence intervals Ω b h 2 = 0.034 + 0 . 0 0 7 - 0 . 0 0 7 , Ω d m h 2 = 0.18 + 0 . 0 6 - 0 . 0 4 , h = 0.72 + 0 . 1 5 - 0 . 1 3 , n s = 1.07 + 0 . 0 6 - 0 . 0 6 and σ 8 = 1.17 + 0 . 2 5 - 0 . 2 0 . The most general model considered includes spatial curvature, tensor modes, massive neutrinos and a parametrized equation of state for the dark energy. In this case, by combining all recent cosmological data, we find, in particular, a 95 per cent limit on the tensor-to-scalar ratio R < 0.63 and on the fraction of massive neutrinos f v < 0.11; we also obtain the 68 per cent confidence interval w = -1.06 + 0 . 2 0 - 0 . 2 5 on the equation of state of dark energy.

X-Ray Emission from a Radio Hot Spot in 3C 390.3: Evidence for the Deflection of a Radio Jet by a Neighboring Galaxy

By summing a large number of ROSAT High Resolution Imager (HRI) observations of the variable radio galaxy 3C 390.3, we demonstrate that the X-ray emission associated with the northern radio lobe (reported by Prieto) can be identified with hot spot B. None of the other hot spots have been detected. We present evidence that the anomalous X-ray emission is the consequence of a strong shock produced where the northern radio jet impinges onto an external galaxy.

ROSAT observations of 3C 388: A test of minimum energy

We have made X-ray observations of the radio galaxy 3C 388 with the ROSAT HRI, resolving the thermal gas that confines the lobes of this powerful (FR II) radio source. The gas pressure is substantially higher than the minimum pressures in the radio lobes, implying that the latter is an underestimate by an order of magnitude or more. We discuss how this can be reconciled with recent observations that find inverse-Compton emission from radio lobes at the level predicted from conventional minimum-energy arguments. We suggest that the excess pressure is due to low-energy electrons and positrons and/or protons, so that magnetic field strengths are below equipartition. If this calibration of minimum-energy arguments applies to radio lobes in general, the power of radio jets has been underestimated by the same factor, implying that jet kinetic energy flux is close to or exceeds bolometric luminosity in radio-loud active galactic nuclei (AGNs). Most of the X-rays from 3C 388 are emitted by the thermal gas, which is well centered on the host cD galaxy, itself the center of a small cluster. This component is well fitted by a standard β model. The ends of the radio lobes are beyond the ≈33 kpc core radius, so the density is declining roughly as a power law with ρ ∝ R-1.6. On most models, this is not a fast enough decline to allow a constant expansion speed for the source. A compact X-ray source with 0.1-2.4 keV luminosity of 5 × 1035 W lies at the center. This is probably the AGN, although a galaxy-scale cooling-flow core cannot be ruled out.

Polarized radio filaments outside the Galactic plane

We used data from the WMAP satellite at 23, 33 and 41 GHz to study the diffuse polarized emission over the entire sky. The emission originates mostly from filamentary structures with well-ordered magnetic fields. Some of these structures have been known for decades in radio continuum maps. Their origin is not clear and there are many filaments that are visible for the first time. We have identified and studied 11 filaments. The polarization fraction of some of them can be as high as 40 per cent, which is a signature of a well-ordered magnetic field. The polarization spectral indices, averaged over 18 regions in the sky is ?? = -3.06 �� 0.02, consistent with synchrotron radiation. There are significant variations in ?? over the sky (?�?? ?�� 0.2). We explore the link between the large-scale filaments and the local interstellar medium, using the model of an expanding shell in the solar vicinity. We compared observed polarization angles with the predictions from the model and found good agreement. This strongly suggests that many large-scale filaments and loops are nearby structures. This is important in the context of the Galactic magnetic field as these structures are normally included in global models, neglecting the fact that they might be local. We also studied the level of contamination added by the diffuse filaments to the CMB (cosmic microwave background) polarization power spectra. We conclude that, even though these filaments present low radio brightness, a careful removal will be necessary for future all-sky CMB polarization analysis.

Observations of the neutral gas and dust in the radio galaxy 3C 305

We present MERLIN and Hubble Space Telescope (HST) observations of the central region of the nearby radio galaxy 3C 305 and use them to study the gas and dust in this object. The MERLIN observations are of neutral hydrogen (H I) absorption against the strong non-thermal 20 cm continuum seen towards the central 4 kpc of 3C 305. Our ∼0.2 arcsec (160 pc) resolution observations show that the H I absorption is highly localized against the south-western radio-emission with column densities ∼1.9 x 10 2 1 cm - 2 . The absorption is broad (full width at half maximum, FWHM, of 145 ′ 26 km s - 1 ) and redshifted by 130 km s - 1 relative to the systemic velocity. The HST images in multiple optical and infrared filters (430 nm, 702 nm, [O III] 500.7 nm, [Fe II] 1.64 μm and K-band polarization) are presented. Evidence is seen for coincidence of the [Fe n] emission with the knot at the end of the radio jet, which is evidence for the presence of shocks. We compare the optical and radio images in order to investigate the relationship between the dust and neutral gas distributions. An unresolved (0.07 arcsec) nucleus is detected in H and K and its properties are consistent with a quasar reddened by A V > 4. We propose that the absorption arises in a region of neutral gas and dust. Its structure is complex but is broadly consistent with an inclined disc of gas and dust encircling, but not covering, the active galactic nucleus. A comparison of the neutral gas observations and previous emission-line observations suggests that both the neutral and ionized gas are undergoing galactic rotation towards the observer in the north-east and away from the observer in the south-west. We propose that the outflow giving rise to the radio emission has a component towards the observer in the north-east and away from the observer in the south-west. Unfortunately, as we do not detect radio emission from the compact nucleus, we cannot set limits to neutral hydrogen absorption from a circumnuclear obscuring torus.