Geraint Harker

• Intro
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• Publications
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• Talks and posters
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• CV & other documents
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Who I am

I'm a Marie Curie fellow in the astrophysics group at University College London. I'm working mainly on studying the early Universe (the cosmic dawn and the epoch of reionization) with the highly-redshifted 21-cm line of hydrogen, and I'm interested more generally in statistics and inference problems in cosmology, as well as galaxy formation and the large-scale structure of the Universe. Most of this work is as part of the LOFAR Epoch of Reionization (EoR) project, along with other UCL astrophysics group members Filipe Abdalla and Emma Chapman, and the rest of the LOFAR EoR team. I originally joined LOFAR as a postdoc at the Kapteyn Institute in Groningen, working with Saleem Zaroubi. In between, I was a postdoc at the Center for Astrophysics and Space Astronomy, part of the University of Colorado Boulder, working with Jack Burns. I was part of the Lunar University Network for Astrophysics Research (LUNAR), a node of the NASA Lunar Science Institute (NLSI), which has since morphed into SSERVI. I mainly worked on foreground removal and signal extraction for sky-averaged 21-cm experiments to probe the cosmic dawn, in particular the Dark Ages Radio Explorer (DARE). My PhD work, on galaxy formation and large scale structure, was conducted at Durham University under the supervision of Shaun Cole. Before that, I studied maths at Fitzwilliam College, Cambridge, as an undergraduate and for Part III. Everywhere I've been, I've played the fabulous game of contract bridge.

Publications (see also on Google Scholar)

First-author papers

• A marked correlation function analysis of halo formation times in the Millennium Simulation — MNRAS, 367, 1039 (2006)
• Constraints on σ₈ from galaxy clustering in N-body simulations and semi-analytic models — MNRAS, 382, 1503 (2007)
• Detection and extraction of signals from the epoch of reionization using higher order one-point statistics — MNRAS, 393, 1449 (2009)
• Non-parametric foreground subtraction for 21cm epoch of reionization experiments — MNRAS, 397, 1138 (2009)
• Power spectrum extraction for redshifted 21-cm epoch of reionization experiments: the LOFAR case — MNRAS, 405, 2492 (2010)
• An MCMC approach to extracting the global 21-cm signal during the cosmic dawn from sky-averaged radio observations — MNRAS, 419, 1070 (2012)
• Selection between foreground models for global 21-cm experiments; — MNRAS Letters, 449, L21–25

Co-authored papers

• Foreground simulations for the LOFAR Epoch of Reionization experiment — Vibor Jelić et al., MNRAS, 389, 1319 (2008)
• Fast Large-Scale Reionization Simulations — Rajat Thomas et al., MNRAS, 393, 32 (2009); version with high resolution figures here
• The LOFAR EoR Data Model: (I) Effects of Noise and Instrumental Corruptions on the 21-cm Reionization Signal-Extraction Strategy — Panos Labropoulos et al. (2009)
• Foregrounds for observations of the cosmological 21 cm line: I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field — Gianni Bernardi et al., A&A, 500, 965 (2009); version with high resolution figures here
• Foregrounds for observations of the cosmological 21 cm line: II. Westerbork observations of the fields around 3C196 and the North Celestial Pole — Gianni Bernardi et al., A&A, 522, id.A67 (2010); version with high resolution figures here
• Probing the First Stars and Black Holes in the Early Universe with the Dark Ages Radio Explorer (DARE) — Jack Burns et al., Adv. Space Res., 49, 433 (2012)
• Foreground Removal using FastICA: A Showcase of LOFAR-EoR — Emma Chapman et al., MNRAS, 423, 3 (2012)
• Imaging neutral hydrogen on large scales during the Epoch of Reionization with LOFAR — Saleem Zaroubi et al., MNRAS, 425, 2964 (2012)
• The LOFAR radio environment — André Offringa et al., A&A, 549, id.A11 (2013)
• Prospects for detecting the 21 cm forest from the diffuse intergalactic medium with LOFAR — Benedetta Ciardi et al., MNRAS, 428, 1755 (2013)
• Initial deep LOFAR observations of epoch of reionization windows. I. The north celestial pole — Sarod Yatawatta et al., A&A, 550, id.A136 (2013)
• The scale of the problem: recovering images of reionization with Generalized Morphological Component Analysis — Emma Chapman et al., MNRAS, 429, 165 (2013)
• LOFAR insights into the epoch of reionization from the cross-power spectrum of 21 cm emission and galaxies — Rob Wiersma et al., MNRAS, 432, 2615 (2013)
• Probing reionization with LOFAR using 21-cm redshift space distortions — Hannes Jensen et al., MNRAS, 435, 460 (2013)
• The brightness and spatial distribution of terrestrial radio sources — André Offringa et al., MNRAS, 435, 584 (2013)
• Interpreting the Global 21-cm Signal from High Redshifts. I. Model Independent Constraints — Jordan Mirocha, Geraint J. A. Harker & Jack O. Burns, ApJ, 777, 118 (2013)
• Constraining the epoch of reionization with the variance statistic: simulations of the LOFAR case — Ajinkya Patil et al., MNRAS, 443, 1113 (2014)
• Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field — Vibor Jelić et al., A&A, 568, id.A101 (2014)
• Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz — Harish Vedantham et al., MNRAS, 450, 2291 (2015)
• Effects of the ionosphere on ground-based detection of the global signal from the Cosmic Dawn and the Dark Ages — Abhirup Datta et al., ApJ, submitted (arXiv/1409.0513)
• Polarization leakage in Epoch of Reionization windows: I. LOFAR observations of the 3C196 field — Khan Asad et al., MNRAS, 451, 3709 (2015)
• Simulating the 21cm forest detectable with LOFAR and SKA in the spectra of high-z GRBs — Benedetta Ciardi et al., MNRAS, 453, 101 (2015)

Conference Proceedings

• Barium Sulphate Precipitation In Porous Rock Through Dispersive Mixing — A. W. Woods & G. Harker, International Symposium on Oilfield Scale, Jan. 2003
• Cosmic Dawn and Epoch of Reionization Foreground Removal with the SKA — Emma Chapman et al., Advancing Astrophysics with the SKA, June 2014

Talks and posters

Talks

Invited

• Foreground removal for 21-cm experiments — conference on 'Understanding Galactic and extragalactic foregrounds: A road to success for cosmological experiments', Zadar, Croatia, May 2011.
• Data analysis and foreground removal for DARE and its prototypes — USNC-URSI National Radio Science Meeting, Boulder, Colorado, Jan. 2014.

Contributed

• Talks about the environmental dependence of halo formation times from two Virgo Consortium meetings in 2005:
  • Munich (early results);
  • Durham (an update).
• Kapteyn Institute colloquium on my thesis work, Groningen, Summer 2006
• Five talks given at LOFAR EoR plenary meetings:
  • Oct. 2007 (Groningen) — Redshift space distortions during the EoR;
  • Apr. 2008 (Groningen) — EoR signal extraction using higher-order one-point statistics;
  • Nov. 2008 (Dwingeloo) — Non-parametric foreground fitting;
  • May 2009 (Groningen) — Progress on power spectrum extraction.
  • Apr. 2010 (Groningen) — Update on power spectra.
• EoR signal extraction using skewness — Hamburg LOFAR workshop ('Astrophysics with E-LOFAR'), Sep. 2008.
• Extracting the 21cm power spectrum from the EoR — conference on 'The Cosmic Evolution of Helium and Hydrogen', Ringberg Castle, Germany, Mar. 2009.
• Extracting a signal from the epoch of reionization with LOFAR — colloquium in Boulder, Colorado, June 2009.
• Foreground removal — Aspen Center for Physics summer workshop, 'Astrophysics and Cosmology with the 21cm Background', Aspen, Colorado, June 2010.
• Extracting information about high-redshift cosmology from a lunar-orbiting dipole — NLSI Lunar Science Forum, NASA Ames Research Center, California, July 2010.
• Modelling the foregrounds and the system response for DARE — workshop on 'Robotic Science from the Moon', Boulder, Colorado, Oct. 2010.
• Signal extraction for sky-averaged 21-cm experiments — USNC-URSI National Radio Science Meeting, Boulder, Colorado, Jan. 2012.
• Signal extraction for sky-averaged 21-cm experiments — 219th AAS meeting, Austin, Texas, Jan. 2012.
• Watching the Cosmic Dawn from the Moon: How and Why (slides; recording) — 2013 Lunar Science Virtual Forum
• Telerobotic exploration and radio astrophysics from the far side of the Moon — European Lunar Symposium, London, May 2014.
• Signal and foreground inference for global 21-cm observations — Olympian Symposium on Cosmology and the Epoch of Reionization, Greece, May 2015.
• What can we really learn from global 21-cm observations of the Cosmic Dawn? — RAS National Astronomy Meeting, Llandudno, Wales, July 2015.

Webinars

• I delivered a LUNAR webinar in combination with a CASA/JILA Friday lunch talk. A recording is available online here. Or, you can just look at the slides. This was part of the LUNAR webinar series which I organized.

Posters

• The Millennium Simulation, for the 2005 Royal Society Summer Science Meeting (credit for the poster's images goes to Volker Springel).
• Posters shown at the RAS National Astronomy Meeting:
  • 2005 (Birmingham) — Halo formation and environment in the Millennium Simulation;
  • 2006 (Leicester) — Cosmological constraints from models of galaxy clustering.
• Statistics for signal extraction from the Epoch of Reionization — 24th IAP Colloquium ('Light from the Young Universe'), Paris, July 2008.
• Power Spectrum Extraction for the LOFAR EoR Experiment, with applications to a future lunar array — The High Redshift Universe: A Multi-Wavelength View, Aspen, Feb. 2010.
• Searching for the one in a million: extracting the cosmic signal from Dark Ages Radio Explorer data — NLSI Lunar Science Forum, NASA Ames Research Center, California, July 2011.
• MCMC signal extraction for 21-cm global signal experiments — 220th AAS meeting, Anchorage, Alaska, June 2012.
• A flexible, Monte Carlo approach to modeling data from DARE and its ground-based prototype — NLSI Lunar Science Forum, NASA Ames Research Center, California, July 2012.
• Seeing the cosmic dawn from lunar orbit with the sky-averaged 21-cm signal — Galaxy Evolution Over Five Decades, Cambridge, UK, Sep. 2013.
• Inferring the properties of the first stars and galaxies from a radiometer in lunar orbit — European Lunar Symposium, London, May 2014.
• Global 21-cm signal modelling and foreground separation — Accurate Astrophysics. Correct Cosmology, London July 2015.
• Global 21-cm signal modelling and foreground separation — IAU General Assembly, Honolulu, Hawaii, August 2015

Internal talks and seminars

Lectures

Other documents

• CV.
• My PhD thesis is titled Connecting Galaxy Formation and Galaxy Clustering (see also scanned version at Durham University library). Chapters 2 and 4 are mostly reproduced in (or superseded by) the papers above, but Chapter 3 contains some results about the effect of environmental dependence on clustering statistics which can't be found elsewhere.
• Notes on a limited implementation of Wp smoothing (Martin Mächler's page on the real deal may be found here).
• Technical stuff (basically the appendix to my thesis) on rescaling N-body simulations.

Some results (not of general interest)

• Old plots about constraints on σ₈.
• Clustering in the ghsmc_800 simulation.
• Some puzzling results on redshift space distortions.
• LOFAR power spectrum recovery estimates.
• Looking at the trade-off between depth and area for EoR power spectrum recovery.