Astrophysics

About

Before rebranding myself as a data scientist and joining the Civil Service, my career was in the field of Astrophysics, first in research and later in publishing. This page details some of the professional highlights of that period.


Research

Large-scale structure with radio galaxies

The two-point correlation function (2pCF) describes the excess clustering of pairs of galaxies as a function of the separation distance, compared with a random, unclustered source distribution. In the majority of galaxy surveys, complete redshift information is not available to construct a three-dimensional clustering measure, so we must make use of the angular 2pCF.

While individual galaxy redshifts are not required, a redshift distribution of these sources allows us to convert angular clustering statistics into spatial information using the Limber equation, under certain assumptions. With the source redshift distribution and spatial correlation length (in Mpc/h), we can infer the linear mass bias - the degree to which the galaxies’ structure traces that of the underlying dark matter.

FIRST radio sources in the GAMA survey

Cross-matching with optical counterparts taken from GAMA and SDSS/UKIDSS, I assigned individual spectroscopic and photometric redshifts, respectively, to FIRST 1.4 GHz radio galaxies. The ~4,000 matches yield a bias measure for >1 mJy radio sources at z~0.5.

Taking a redshift distribution from the SKADS simulations, I can assume a distribution for the FIRST survey as a whole, or subtract the cross-matched distribution to infer that of the unmatched radio sources. As a result, I estimated the bias for the FIRST sources over a wider area at z~1.2 and for the unmatched sources in the GAMA regions (z~1.5).

Using only the GAMA survey, and spectroscopic redshifts of ~1,600 cross-matched radio sources, great detail can be found in the cross-correlation function (above left) and auto-correlation of GAMA galaxies (above right), further refining the bias estimates at low redshift (z<0.6).

Radio sources in the VIDEO survey

Deeper VLA radio observations complement deep zYJHK photometry in VIDEO over the 1 square degree XMM-LSS3 field. I can obtain an accurate measure of the bias of the more numerous, but less strongly clustered infra-red sources. The angular cross-correlation function between radio and infra-red then serves to determine a measure of the radio bias with respect to that infra-red population, and by extension the dark matter, out to z~2.2.

Cross-correlation with the CMB

Allison et al. 2015

Spectroscopy of >100 mJy radio sources

I have attended long-slit spectroscopic observing runs on the William Herschel Telescope in La Palma (3 nights, February ’11) and the ESO New Technology Telescope in Chile (5 nights, March ’12), targeting radio-loud galaxies in the Herschel/GAMA 9h/12h/15h fields with poor or no available redshifts.

On the NTT run, having checked for double/multiple sources and targeted expected host galaxy positions, we observed all feasible radio sources above 100 mJy (at 1.4 GHz) in the 9h and 15h fields. I carried out the IRAF data reduction on these ~80 targets, finding redshifts for ~30 quasars and narrow-line radio galaxies up to a maximum of z=3.07.

Photometric redshift estimation

Almosallam et al. 2016


Publications

I have published two papers as lead author and several others as co-author. Most of these were published in Monthly Notices of the Royal Astronomical Society (MNRAS), where I subsequently worked as an Assistant Editor and Deputy Press Officer for the Royal Astronomical Society (see below).

Year Title Authors (Journal)
2016 The galaxy-halo connection in the VIDEO survey at 0.5<z<1.7 P. W. Hatfield, S. N. Lindsay, M. J. Jarvis, B.Häußler, M.Vaccari and A. Verma (MNRAS, 459, 2618)
2016 A sparse Gaussian process framework for photometric redshift estimation Ibrahim A. Almosallam, Sam N. Lindsay, Matt J. Jarvis and Stephen J. Roberts (MNRAS, 455, 2387)
2015 The Atacama Cosmology Telescope: measuring radio galaxy bias through cross-correlation with lensing Rupert Alison, Sam N. Lindsay, Blake D. Sherwin, Francesco de Bernardis, J. Richard Bond, Erminia Calabrese, Mark J. Devlin, Joanna Dunkley, Patricio Gallardo,Shawn Henderson, Adam D. Hincks, Renee Hlozek, Matt Jarvis, Arthur Kosowsky, Thibaut Louis, Mathew Madhavacheril, Jeff McMahon, Kavilan Moodley, Sigurd Naess, Laura Newburgh, Michael D. Niemack, Lyman A. Page, Bruce Partridge, Neelima Sehgal, David N. Spergel, Suzanne T. Staggs, Alexander van Engelen and Edward J. Wollack (MNRAS, 451, 849)
2015 Cosmology with SKA Radio Continuum Surveys Matt J. Jarvis, David Bacon, Chris Blake, Michael L. Brown, Sam N. Lindsay, Alvise Raccanelli, Mario Santos and Dominik Schwarz (PoS)
2014 Evolution in the bias of faint radio sources to z~2.2 S. N. Lindsay, M. J. Jarvis and K. McAlpine (MNRAS, 440, 2322)
2014 Galaxy and Mass Assembly (GAMA):The evolution of bias in the radio source population to z~1.5 S. N. Lindsay, M. J. Jarvis, M. G. Santos, M. J. I. Brown, S. M. Croom, S. P. Driver, A. M. Hopkins, J. Liske, J. Loveday, P. Norberg and A. S. G. Robotham (MNRAS, 440, 1527)
2012 Cosmological measurements with forthcoming radio continuum surveys A. Raccanelli, G.-B. Zhao, D. J. Bacon, M. J. Jarvis, W. J. Percival, R. P. Norris, H. Röttgering, F. B. Abdalla, C. M. Cress, J.-C. Kubwimana, S. Lindsay, R. C. Nichol, M. G. Santos and D. J. Schwarz (MNRAS, 424, 801)

PhD Thesis

The work published in my two 2014 papers was covered in more detail alongside some other work in my PhD thesis by the title of “Tracing Large-Scale Structure with Radio Sources”. It can be downloaded freely here at the University of Hertfordshire Research Archive.


Presentations

  • Contributed talk at the Clustering Measurements of Active Galactic Nuclei conference
    July 14th-18th 2014, ESO (Garching), Germany

    Evolution in the Bias of Radio Sources

  • Contributed talk at the ‘Radio Cosmology’ session of the National Astronomy Meeting
    June 23rd-26th 2014, University of Portsmouth, UK

    Evolution in the Bias of Radio Sources

  • Internal seminar talk by final-year PhD students
    Oct 9th 2013, University of Hertfordshire, UK

    Tracing Large-Scale Structure with Radio Sources

  • Contributed talk at the Radio Astronomy in the LSST Era conference
    May 6th-8th 2013, NRAO/University of Virginia, USA

    Evolution in the Bias in the Radio Population

  • Poster presentation at the Ripples in the Cosmos conference
    July 22nd-26th 2013, Durham University, UK

    Evolution of Bias in Radio Galaxies


Press

While employed by the Royal Astronomical Society in 2016/17, I was often contacted by the press for comments or clarifications on astronomy-related news items. The following table contains links (where available) to any contributions that were broadcast.

Date Organisation Activity Topic Link
24/7/15 ITV News Video clip NASA New Horizons images of Pluto ITV.com
11/8/15 Sky News Radio interview Perseid meteor shower
12/8/15 The Telegraph Print interview Perseid meteor shower Telegraph.co.uk
14/8/15 BBC London 94.9 Radio interview Greenwich meridian mp3
27/9/15 BBC World Service Radio interview Lunar eclipse mp3
27/9/15 BBC World News TV interview Lunar eclipse
28/9/15 Sky News TV interview (OB) Lunar eclipse
28/9/15 BBC World Service Radio interview Lunar eclipse mp3
28/9/15 BBC World News TV interview (incl. photos) Lunar eclipse video
28/10/15 BBC News Print interview Conjunction of Venus, Mars & Jupiter bbc.co.uk/news
20/1/16 The Press and Journal Print interview Alignment of 5 planets pressandjournal.co.uk
21/1/16 ITV News Video clip Evidence for 9th planet facebook.com/itvnews
11/2/16 Al Jazeera TV interview Detection of gravitational waves
10/8/16 Vice.com Print interview Perseid meteor shower (photography) vice.com
12/8/16 BBC News Print interview (incl. photos) Perseid meteor shower bbc.co.uk/news