The Galaxy and beyond with the Arcminute Microkelvin Imager

The Arcminute Microkelvin Imager (AMI) is a dual-array radio interferometer sited at Lord's Bridge, near Cambridge. Although it was designed specifically for studying galaxy clusters via observations of the Sunyaev-Zel'dovich (SZ) effect, it is also an ideal instrument for Galactic science. This thesis describes science programmes investigating both Galactic objects and galaxy clusters that I have carried out with AMI. A new data analysis pipeline is described which has been developed to allow the automated processing of data taken by AMI in drift-scan mode, pointing the telescope at a fixed azimuth and elevation and observing the sky that drifts past. This is a very efficient mode for large-scale surveys, but the different character of the data has required innovative algorithms for effective processing. The AMI Galactic Plane Survey uses drift-scanning to cover the northern Galactic plane between |b| < approximately 5 degrees. It is the first Galactic plane survey at cm-wave frequencies to achieve crucial mJy-sensitivity levels at arcminute-scale resolution over a wide area, and as such provides a unique opportunity to investigate hitherto unusual objects such as ultra- and hyper-compact HII regions. I describe my work on the survey strategy and its implementation and on some of the science I have extracted so far including the follow-up of candidate hyper-compact HII regions. The recently-released Planck satellite results include the largest catalogue of SZ-selected clusters of galaxies to date. I describe the AMI follow-up programme to observe the clusters within the AMI observation limits, and present the first results from the programme including an interesting discrepancy between the cluster parameters according to AMI and Planck. Since the two instruments are observing the same physical process, this indicates a fundamental problem with the 'universal' pressure profile currently favoured for modelling clusters. In an attempt to address the discrepancy, I use simulations to investigate the effect of allowing the shape of the pressure profile to vary. The derived parameter constraints are found to vary when clusters are not simulated and recovered with the same model; the effects are dependent on angular size, worsening for larger clusters. I also assess the potential for using AMI data to constrain the cluster shape parameters, and conclude that weak constraints on the shape parameters are possible with a careful choice of prior.This work was supported by a Cambridge Commonwealth Trust/Cavendish Laboratory Rutherford Scholarship, and the Royal Society of New Zealand

Extending the Planetary Mass Function to Earth Mass by Microlensing at Moderately High Magnification

A measurement by microlensing of the planetary mass function of planets with masses ranging from 5M_E to 10M_J and orbital radii from 0.5 to 10 AU was reported recently. A strategy for extending the mass range down to (1-3)M_E is proposed here. This entails monitoring the peaks of a few tens of microlensing events with moderately high magnifications with 1-2m class telescopes. Planets of a few Earth masses are found to produce deviations of ~ 5% to the peaks of microlensing light curves with durations ~ (0.7-3)hr in events with magnification ~ 100 if the projected separation of the planet lies in the annular region (0.85-1.2)r_E. Similar deviations are produced by Earth mass planets in the annular region (0.95-1.05)r_E. It is possible that sub-Earths could be detected very close to the Einstein ring if they are sufficiently abundant, and also planetary systems with more than one low mass planet.Comment: 12 pages, 20 figures (in press) MNRAS (2013

The highest frequency detection of a radio relic : 16 GHz AMI observations of the 'Sausage' cluster

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters. © 2014 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We observed the cluster CIZA J2242.8+5301 with the Arcminute Microkelvin Imager at 16 GHz and present the first high radio-frequency detection of diffuse, non-thermal cluster emission. This cluster hosts a variety of bright, extended, steep-spectrum synchrotron-emitting radio sources, associated with the intracluster medium, called radio relics. Most notably, the northern, Mpc-wide, narrow relic provides strong evidence for diffusive shock acceleration in clusters. We detect a puzzling, flat-spectrum, diffuse extension of the southern relic, which is not visible in the lower radio-frequency maps. The northern radio relic is unequivocally detected and measures an integrated flux of 1.2 ± 0.3 mJy. While the low-frequency (<2 GHz) spectrum of the northern relic is well represented by a power law, it clearly steepens towards 16 GHz. This result is inconsistent with diffusive shock acceleration predictions of ageing plasma behind a uniform shock front. The steepening could be caused by an inhomogeneous medium with temperature/density gradients or by lower acceleration efficiencies of high energy electrons. Further modelling is necessary to explain the observed spectrum.Peer reviewe

28 -- 40 GHz variability and polarimetry of bright compact sources in the QUIJOTE cosmological fields

We observed 51 sources in the Q-U-I JOint TEnerife (QUIJOTE) cosmological fields which were brighter than 1 Jy at 30 GHz in the Planck Point Source Catalogue (version 1), with the Very Large Array at 28 -- 40 GHz, in order to characterise their high-radio-frequency variability and polarization properties. We find a roughly log-normal distribution of polarization fractions with a median of 2%, in agreement with previous studies, and a median rotation measure (RM) of $\approx$ 1110 rad m$^{-2}$ with one outlier up to $\approx$ 64000 rad m$^{-2}$ which is among the highest RMs measured in quasar cores. We find hints of a correlation between the total intensity flux density and median polarization fraction. We find 59% of sources are variable in total intensity, and 100% in polarization at $3\sigma$ level, with no apparent correlation between total intensity variability and polarization variability. This indicates that it will be difficult to model these sources without simultaneous polarimetric monitoring observations and they will need to be masked for cosmological analysis.Comment: 17 pages, 14 figures, accepted to MNRA

Physical modelling of galaxy clusters detected by the Planck satellite

We present a comparison of mass estimates for $54$ galaxy cluster candidates from the second Planck catalogue (PSZ2) of Sunyaev-Zel'dovich sources. We compare the mass values obtained with data taken from the Arcminute Microkelvin Imager (AMI) radio interferometer system and from the Planck satellite. The former of these uses a Bayesian analysis pipeline that parameterises a cluster in terms of its physical quantities, and models the dark matter & baryonic components of a cluster using NFW and GNFW profiles respectively. Our mass estimates derived from Planck data are obtained from the results of the Bayesian detection algorithm PowellSnakes (PwS), are based on the methodology detailed in the PSZ2 paper, and produce two sets of mass estimates; one estimate is calculated directly from the angular radius $\theta$ - integrated Comptonisation parameter $Y$ posterior distributions, and the other uses a `slicing function' to provide information on $\theta$ based on X-ray measurements and previous Planck mission samples. We find that for $37$ of the clusters, the AMI mass estimates are lower than both values obtained from Planck data. However the AMI and slicing function estimates are within one combined standard deviation of each other for $31$ clusters. We also generate cluster simulations based on the slicing-function mass estimates, and analyse them in the same way as we did the real AMI data. We find that inclusion in the simulations of radio-source confusion & CMB noise and measurable radio-sources causes AMI mass estimates to be systematically low.This work was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing (HPC) Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council ..⁠. Kamran Javid acknowledges an STFC studentship. Yvette Perrott acknowledges support from a Trinity College Junior Research Fellowship

AMI observations of unmatched Planck ERCSC LFI sources at 15.75 GHz

The Planck Early Release Compact Source Catalogue includes 26 sources with no obvious matches in other radio catalogues (of primarily extragalactic sources). Here we present observations made with the Arcminute Microkelvin Imager Small Array (AMI SA) at 15.75 GHz of the eight of the unmatched sources at declination > +10 degrees. Of the eight, four are detected and are associated with known objects. The other four are not detected with the AMI SA, and are thought to be spurious.Comment: 6 pages, 5 figures, 4 table