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Measuring the Quasar Luminosity Function Below the Detection Threshold

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Measuring the Quasar Luminosity Function Below the Detection Threshold University of the Western Cape Masters' Thesis Measuring the quasar luminosity function below the detection threshold Supervisors: Prof Mario Santos Author: Dr Jonathan Zwart Eliab Malefahlo Prof Matt Jarvis Prof Roy Maartens A thesis submitted in fulfilment of the requirements for the degree of Msc Astronomy in the Astronomy group Department of Physics and Astronomy March 2016 Declaration of Authorship I, Eliab Malefahlo, declare that this thesis titled, 'Measuring the quasar luminosity function below the detection threshold' and the work presented in it are my own. I confirm that: This work was done wholly or mainly while in candidature for a research degree at this University. Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated. Where I have consulted the published work of others, this is always clearly attributed. Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. I have acknowledged all main sources of help. Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself. Signed: Date: ii University of the Western Cape Abstract FACULTY NATURAL SCIENCE Department of Physics and Astronomy Msc Astronomy Measuring the quasar luminosity function below the detection threshold by Eliab Malefahlo The radio emission of radio-quiet active galactic nuclei (AGN) is thought to be from star formation and AGN related emission. I investigate these sources using 1.4 GHz radio data from FIRST and three optical quasars samples from the SDSS: (i) a volume-limited sample in the redshift range 0:2 < z < 0:4 defined by Mi < −23 (ii) magnitude-limited sample in the redshift range 1:8 < z < 2:5 defined by mr ≤ 18:5 and (iii) a uniform sample in the redshift range 0:2 < z < 3:5 (divided into 12 redshift bins). I constructed radio source counts and radio luminosity functions (RLFs) using the optical quasars detected in FIRST, which are consistent with literature results obtained using SDSS and NVSS quasars. There are differences at the low fluxs end because of the different resolutions of FIRST and NVSS. I applied a median stack method to the 12 redshift bins of the uniform sample and found that the median flux decreases from 182 µJy in the lowest redshift bin to 39 µJy and the highest redshift bin. This is because the high redshift quasars although more luminous than their low redshift counterparts, they are much further away so they have lower fluxes. I probed the quasar radio source counts to lower levels using reconstructed source counts obtained by applying the Bayesian stack technique. The reconstructed radio source counts were then used to constructed the quasar RLF to lower levels, where I found: (i) for z < 1 the constructed quasar RLF has the same slope as the detected quasars, suggesting that like the detect quasars their radio emission is dominated by AGN related emission (ii) above z = 1 the constructed RLF steepens with redshift, which suggests the strong link between accretion rate and radio jet power is gradually breaking down towards faint optical luminosities at high redshift. Acknowledgements I would firstly like to thank my living God for the life and health He has and continues to bless me with, for granting me this opportunity, the power and strength to complete this work. Ke ya moleboga Modimo wa Sethepu, Mambo wamazimambo, Senatla sa dinatla Ntate Moemedi. I would like to thank my superiors; Mario, Jon (JZ), Matt and Roy for being there and always willing to help, going through the concepts step by step and letting me figure things on my own were needed. This work would not be possible without them. Thanks you JZ for all those hours of explaining the code and proofreading my work thanks chief. I would like to thank my parents for their love and continued support. Thanks Matt Prescott for all those LFs sessions. I would also like to thank my fellow students in the Astro group at UWC for their support, especially Rudi-Lee and my home boy Emanuel. Lastly, I would like to thank Neville for all the advice and motivation. I thank the National astrophysics and space science program (NASSP) and SKA Africa for financial assistance during my studies. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by CENTRA/IST. Computations were performed at the cluster \Baltasar-Sete-S´ois"and supported by the H2020 ERC Consolidator Grant "Matter and strong field gravity: New frontiers in Einstein's theory" grant agreement no. MaGRaTh-646597. iv Contents Declaration of Authorship ii Abstract iii Acknowledgements iv Contents v 1 Introduction 2 1.1 Cosmological context ..................................... 2 1.1.1 Evolution of the Universe .............................. 2 1.1.2 Galaxy formation and evolution ........................... 4 1.2 Galactic processes ...................................... 5 1.2.1 Star formation .................................... 6 1.2.2 Active galactic nuclei ................................ 6 1.2.3 Feedback ....................................... 8 1.2.4 Synchrotron radiation ................................ 9 1.2.5 Far-Infrared Radio correlation ............................ 9 1.3 Radio sources ......................................... 10 1.3.1 Radio-loud ...................................... 11 1.3.2 Radio-quiet ...................................... 11 1.4 Quasars ............................................ 11 1.5 Large radio surveys ...................................... 12 1.6 Outline ............................................ 13 2 Data 15 2.1 SDSS ............................................. 15 2.1.1 SDSS I ........................................ 15 2.1.2 SDSS II ........................................ 15 2.1.3 SDSS III ....................................... 17 2.1.4 Quasar target selection: SDSS I and II ....................... 18 2.1.5 Quasar target selection: SDSS III .......................... 19 2.1.6 Spectroscopy ..................................... 21 2.1.7 Quasar catalogue ................................... 23 v List of Tables 1 2.2 Sample............................................. 23 2.2.1 Sample 1: Volume-limited sample .......................... 23 2.2.2 Sample 2: Magnitude-limited sample ........................ 23 2.2.3 Sample 3: Uniform sample ............................. 24 2.2.4 Completeness ..................................... 24 2.3 FIRST ............................................. 25 2.4 Matching SDSS with FIRST ................................. 26 2.4.1 Catalogue sources .................................. 26 2.4.2 FIRST cutouts .................................... 27 2.5 Catalogue and extracted fluxes ............................... 27 2.5.1 Snapshot bias ..................................... 28 2.5.2 Extraction position .................................. 29 2.6 Sample area .......................................... 29 2.6.1 BOSS overlap ..................................... 29 2.6.2 Legacy overlap .................................... 31 3 Above the detection threshold: Radio-loud quasars 32 3.1 Source counts ......................................... 32 3.1.1 Sample 1 ....................................... 35 3.1.2 Sample 2 ....................................... 36 3.2 Radio luminosity function .................................. 36 4 Below the detection threshold: stacking and radio-quiet quasars 40 4.1 Stacking ............................................ 41 4.2 Sample 3: Uniform sample .................................. 41 4.3 Simple averaging ....................................... 42 4.4 Bayesian framework ..................................... 43 4.4.1 Bayes' theorem .................................... 43 4.4.2 Nested sampling ................................... 45 4.4.3 Models considered here ............................... 45 4.4.4 Likelihood ....................................... 46 4.4.5 Priors ......................................... 47 4.4.6 Tests and simulations ................................ 48 4.5 Results ............................................. 48 4.5.1 Posterior distributions ................................ 51 4.5.2 Source counts ..................................... 51 4.5.3 Luminosity functions ................................. 52 5 Conclusions and future work 61 5.1 Future work .......................................... 63 Bibliography 64 Chapter 1 Introduction In this thesis, I use a Bayesian stacking (Bayestack) technique to extend measurements of the quasar radio luminosity function (QRLF) and source counts of optically selected sources below the detection threshold of currently completed large-area radio surveys. I use the results to investigate the role of star formation and active galactic nuclear accretion (AGN) activity on the radio emission of quasars, in particular the radio-quiet quasars. First, I explore the importance of these two sources of emissions for galaxy formation and evolution. 1.1 Cosmological context 1.1.1 Evolution of the Universe The Big Bang theory is the most accepted evolutionary model of the Universe (see Peebles and Ratra [2003] for an overview). The theory states that the Universe originated from a singularity with infinite temperature and density, expanding to what is observed today (assuming general relativity holds in the early Universe; Hawking and Ellis [1973]).
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