A. C. Readhead

Publications (7)23.07 Total impact

• Article: A centimetre-wave excess over free-free emission in planetary nebulae

  S. Casassus, L. -A. Nyman, C. Dickinson, T. J. Pearson, A. C. Readhead
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  ABSTRACT: This paper has been withdrawn by the author(s), as it is superseded by 0708.2385.
  07/2007;
• Source

  Available from: Brian Mason

  Article: Vela X at 31 GHz

  A. S. Hales, S. Casassus, H. Alvarez, J May, L. Bronfman, A. C. Readhead, T. J. Pearson, B. S. Mason, R. Dodson
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  ABSTRACT: We present observations of the Vela X region at 31 GHz using the Cosmic Background Imager (CBI). We find a strong compact radio source (5.9'x4.1', FWHM) about the Vela pulsar, which we associate with the Vela pulsar wind nebula (PWN) recently discovered at lower radio-frequencies. The CBI's 4' resolution for a 45' field of view allows the PWN to be studied in the large-scale context of Vela X. Filamentary structure in Vela X, which stands out in lower frequency maps, is very low-level at 31 GHz. By combining the 10 CBI channels, which cover 26-36 GHz, and 8.4 GHz archive data, we study the spectral energy distribution (SED) of the PWN and the brightest filaments. Our results show that the spectral index alpha (F_{nu} propto nu^alpha) of the PWN is flat, or even marginally positive, with a value of alpha_{8.4}^{31}=0.10+-0.06, while the Vela X filamentary structure has a negative spectral index of alpha_{8.4}^{31}=-0.28+-0.09. The SED inhomogeneity observed in Vela X suggests different excitation processes between the PWN and the filaments. We investigate whether the PWN's flat spectrum is a consequence of variability or truly reflects the SED of the object. The peak of the PWN at 31 GHz is 80''+-20'' south-west of the peak at 8.4 GHz. This shift is confirmed by comparing the 31 GHz CBI image with higher resolution 5 GHz Australia Telescope Compact Array observations, and is likely to be due to SED variations within the PWN. We also investigate the nature of the Vela X filamentary structure. Comment: Accepted for publication in The Astrophysical Journal. 25 pages, 9 figures
  The Astrophysical Journal 05/2004; · 6.02 Impact Factor
• Source

  Available from: Brian Mason

  Article: CMB observations with the Cosmic Background Imager (CBI) Interferometer

  C. R. Contaldi, J. R. Bond, D. Pogosyan, B. S. Mason, S. T. Myers, T. J. Pearson, U. -L. Pen, S. Prunet, A. C. Readhead, M. I. Ruetalo, J. L. Sievers, J. W. Wadsley, P. J. Zhang
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  ABSTRACT: We review the recently published results from the CBI's first season of observations. Angular power spectra of the CMB were obtained from deep integrations of 3 single fields covering a total of 3 deg^2 and 3 shallower surveys of overlapping (mosaiced) fields covering a total of 40 deg^2. The observations show a damping of the anisotropies at high-l as expected from the standard scenarios of recombination. We present parameter estimates obtained from the data and discuss the significance of an excess at l>2000 observed in the deep fields.
  11/2002;
• Source

  Available from: Brian Mason

  Article: Cosmological Parameters from CMB measurements with the CBI

  C. R. Contaldi, J. R. Bond, D. Pogosyan, B. S. Mason, S. T. Myers, T. J. Pearson, U. -L. Pen, S. Prunet, A. C. Readhead, M. I. Ruetalo, J. L. Sievers, J. W. Wadsley, P. J. Zhang
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  ABSTRACT: We derive cosmological parameters from the CBI measurements of the Cosmic Microwave Background (CMB) angular power spectrum. Our results provide an independent confirmation of the standard $\Omega_{\rm tot}=1$ $\Lambda$CDM model within the adiabatic, inflationary paradigm. Above $\ell=2000$ the observations show evidence of power in excess of that expected in the standard models. We use hydrodynamical simulations to show how Sunyaev-Zeldovich Effect (SZE) may account for the excess power for models with fluctuation amplitude $\sigma_8\sim 1$ which is in the high end of the range allowed by the primary CMB observations.
  09/2002;
• Source

  Available from: arxiv.org

  Article: Limits on the cosmological abundance of supermassive compact objects from a search for multiple imaging in compact radio sources.

  P N Wilkinson, D R Henstock, I W Browne, A G Polatidis, P Augusto, A C Readhead, T J Pearson, W Xu, G B Taylor, R C Vermeulen
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  ABSTRACT: Using very long baseline interferometry we have searched a sample of 300 compact radio sources for examples of multiple imaging produced by gravitational lensing; no multiple images were found with separations in the angular range 1.5--50 milliarcsec. This null result allows us to place a limit on the cosmological abundance of intergalactic supermassive compact objects in the mass range approximately 10(6)M( middle dot in circle) to approximately 10(8)M( middle dot in circle); such objects cannot make up more than approximately 1% of the closure density ( 95% confidence). A uniformly distributed population of supermassive black holes forming soon after the big bang does not, therefore, contribute significantly to the dark matter content of the Universe.
  Physical Review Letters 02/2001; 86(4):584-7. · 7.37 Impact Factor
• Source

  Available from: pnas.org

  Article: Evolution of powerful extragalactic radio sources.

  A C Readhead
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  ABSTRACT: Observations of complete flux density limited samples of powerful extragalactic radio sources by very-long-baseline interferometry enable us to study the evolution of these objects over the range of linear scales from 1 parsec to 15 kiloparsees (1 parsec = 3.09 x 10(18) cm). The observations are consistent with the unifying hypothesis that compact symmetric objects evolve into compact steep-spectrum doubles, which in turn evolve into large-scale Fanaroff-Riley class II objects. It is suggested that this is the primary evolutionary track of powerful extragalactic radio sources. In this case there must be significant luminosity evolution in these objects, but little velocity evolution, as they expand from 1 parsec to several hundred kiloparsecs in overall size.
  Proceedings of the National Academy of Sciences 01/1996; 92(25):11447-50. · 9.68 Impact Factor
• Source

  Available from: caltech.edu

  Article: A New Component in the Radio Continua of PNe

  S. Casassus, L.-Å. Nyman, T. J. Pearson, A. C. Readhead
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  ABSTRACT: A byproduct of experiments designed to map the CMB is the recent detection of a new component of foreground galactic emission. The anomalous foreground at 10–30 GHz, unexplained by traditional emission mechanisms, correlates with 100 mum dust emission, and is thus presumably due to dust.Is the anomalous foreground ubiquitous in the Galaxy? I will present evidence obtained with the CBI and SIMBA+SEST supporting the existence of the new component in the ISM at large, and in specific objects, in the form of a 31 GHz excess over free-free emission in PNe.