Exploring the faint source population at 15.7 GHz Imogen Helen Whittam Trinity Hall University of Cambridge June 2014 A dissertation submitted for the degree of Doctor of Philosophy Contents Contents iii Summary vii Declaration ix Acknowledgements xi 1 Introduction1 1.1 Extragalactic radio source types ......................... 2 1.2 Source counts................................... 8 1.3 High-frequency radio surveys........................... 9 1.4 Models of the high-frequency source population................. 11 1.5 Multi-wavelength studies............................. 13 1.6 The Lockman Hole................................ 14 1.7 Thesis outline................................... 15 2 Deriving the radio properties of 10C sources in the Lockman Hole 17 2.1 Sample selection ................................. 17 2.2 Deriving the source parameters.......................... 24 2.3 Summary ..................................... 32 3 Analysis of the radio properties of 10C sources in the Lockman Hole 33 3.1 Sample analysis.................................. 34 3.2 Comparison with samples selected at 1.4 GHz.................. 45 3.3 Comparison with the SKADS Simulated Sky .................. 48 3.4 Conclusions.................................... 53 4 Milliacrsecond properties of 10C sources in the Lockman Hole 55 4.1 Introduction.................................... 55 iii 4.2 Sample definition and properties......................... 56 4.3 Properties of the 10C sources........................... 59 4.4 Properties of VLBA sources ........................... 69 4.5 Conclusions.................................... 71 5 Multi-wavelength data, photometric redshifts and derived radio properties 73 5.1 Summary of multi-wavelength data used..................... 74 5.2 Matching the catalogues ............................. 78 5.3 Radio sample used – ‘Sample W’......................... 82 5.4 Photometric redshift fitting............................ 87 5.5 Radio to optical ratio ............................... 99 5.6 Source properties.................................102 5.7 Comparison with the SKADS Simulated Sky ..................107 5.8 Conclusions....................................110 6 Investigating the nature of 10C radio galaxies 111 6.1 Methods of distinguishing between high-excitation and low-excitation radio galaxies......................................112 6.2 Identifying HERGs and LERGs .........................116 6.3 Comparing and combining the different methods . 119 6.4 Summary .....................................124 7 The properties of high-excitation and low-excitation 10C radio galaxies 127 7.1 Properties of HERGs and LERGs.........................128 7.2 Comparing the properties of the 10C sample with other studies . 135 7.3 Conclusions....................................138 8 Deep 15.7-GHz observations of the Lockman Hole and AMI001 fields 139 8.1 Observations and data reduction .........................139 8.2 The source catalogue...............................140 8.3 Checking the catalogue..............................148 8.4 Source counts...................................153 8.5 Discussion.....................................160 8.6 Conclusions....................................162 9 Conclusions 163 9.1 Properties of the 15.7 GHz sources........................163 9.2 Discussion of the 15.7 GHz radio sky ......................164 iv 9.3 Further work ...................................165 A Source catalogues 167 A.1 Radio properties..................................167 A.2 Multi-wavelength properties ...........................175 A.3 Redshift values ..................................181 B Confused sources 185 B.1 Comments on individual sources.........................185 B.2 Properties of possible counterparts for confused sources . 189 References 191 v Summary A sample of 296 faint (> 0:5 mJy) radio sources is selected from an area of the Tenth Cam- bridge (10C) survey at 15.7 GHz in the Lockman Hole. The 10C survey is complete to 0.5 mJy at 15.7 GHz and has a resolution of 30 arcsec. By matching this catalogue to several lower fre- quency surveys (e.g. including a deep GMRT survey at 610 MHz, a WSRT survey at 1.4 GHz, NVSS, FIRST and WENSS) I have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to a source in one or more of these surveys. I have found a significant increase in the proportion of flat spectrum sources at flux densities below ≈1 mJy – the median spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. Thus a population of faint, flat spectrum sources is emerging at flux densities . 1 mJy. The spectral index distribution of this sample of sources selected at 15.7 GHz is compared to those of two samples selected at 1.4 GHz from FIRST and NVSS. I find that there is a sig- nificant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4 GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al.; this simulation fails to reproduce the observed spec- tral index distribution and significantly under predicts the number of sources in the faintest flux density bin. I conclude that it is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies, rather than the emergence of a new population of starforming galaxies. I have used recent Very Long Baseline Interferometry (VLBI) observations by Middleberg et al. with a resolution of ≈ 10 mas to investigate the properties of these faint 10C sources in the Lockman Hole and find that 33 out of the 51 10C sources in the VLBI field (65 percent) are detected by the VLBI observations. The high brightness temperature of these VLBI-detected sources rules out the possibility that this faint, high frequency population is dominated by starbursting or starforming sources and indicates that they must be Active Galactic Nuclei. The sources in the Lockman Hole 10C sample are matched to optical, infrared and X- ray data available in the field. A complete sample of 96 sources with high-resolution radio information available is defined; multi-wavelength counterparts are identified for 80 out of the vii 96 sources in this sample, for which is it possible to derive photometric redshifts. The radio- to-optical ratios of these sources show that the 10C sample is almost completely dominated by radio galaxies. 59/80 sources have luminosities greater than the FRI/FRII dividing luminosity. The nature of these radio galaxies is investigated, using the multi-wavelength data to split the sources into high-excitation and low-excitation radio galaxies (HERGs and LERGs respect- ively). This shows that 34 sources are probably HERGs and 33 are probably LERGs, with 29 which could not be classified at this stage. The properties of these HERGs and LERGs are compared and I find that the HERGs tend to be found at higher redshifts, have flatter spectra, higher flux densities and smaller linear sizes. This study is extended to lower flux densities using new, very deep, observations made with the Arcminute Microkelvin Imager in two fields. I use these observations to extend the 15.7-GHz source count down to 0.1 mJy, a factor of five deeper than the 10C count. These new deeper counts are consistent with the extrapolation of the fit to the 10C count, and do not show any evidence for an upturn. There is therefore no evidence for a new population (e.g. of starforming sources) contributing to the 15.7 GHz source count above 0.1 mJy, and suggesting that the faint, high-frequency population continues to be dominated by radio galaxies. Recent models of the high-frequency source counts under-predict the number of sources observed by a factor of two, consistent with the fact that these models fail to include the dominance of the cores and the faintness of the extended structures of these sources. viii Declaration This dissertation is the result of work carried out in the Cavendish Laboratory betweeen Octo- ber 2010 and June 2014. The work in this thesis in my own except where stated otherwise. no part of this dissertation has been submitted for a degree, diploma or other qualification at this or any other university. The total length of this dissertation does not exceed 60,000 words. Imogen Whittam, Cavendish Laboratory, June 2014 ix Acknowledgements First, I’d like to thank my supervisor Julia for her constant time and support and for reading endless drafts of the thesis. Thank you to Dave for always being there to answer my many questions and for introducing me to the dark arts of aips (and, of course, for his help with the LATEX!). I couldn’t imagine a better pair to guide me through my PhD. Thank you to the other members of the AP group for providing stimulating tea and coffee conversations, and to Liz Waldram for her advice with the source counts. I’d also like to thank the members of the AMI consortium, particularly Clare and Michel, for providing me with the data for the last chapter and helping me explore the mysteries of AMI data. Thanks must go to the Science and Technology Facilities Council for providing the stu- dentship which has made this thesis possible. I’d also like to thank my college, Trinity Hall, for their invaluable support throughout my undergraduate and graduate studies. It has been a rewarding and stimulating community to be part of for the last eight years. To my fellow students, Alex, Michel, Sam, Clare, Rena, Yvette, Tamela, Becky, Dave, Luke, Marion and many more, thank you for providing many entertaining lunchtimes and trips to the pub. I’ve learnt a lot from all of you, particularly the large gaps in my film-viewing history. Special thanks must go to Sam, for accompanying me in my descent into insanity in the last few months. Looks like we made it out the other side! Thanks must go to everyone at CUYC, CURTC and THBC for providing many welcome distractions. I have spent many happy hours on some form of water or on the real tennis court, which as well as endless fun and many friends, has provided me with the space sometimes needed from my studies.