Dr Sam Lindsay

Assistant Editor (MNRAS), Royal Astronomical Society

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Below is a short description of my PhD research. To find out more, please view my Publications or read about the Data I have used. 


Large-scale structure in 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-GAMA matched source ACF

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).


2D spatial cross-correlation of radio sources with the full GAMA redshift catalogue2D spatial correlation function of GAMA galaxies

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).

VLA-VIDEO (Radio-IR) cross-correlation function

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.


Spectroscopy of  >100 mJy radio sources


Radio galaxy spectrum taken with the NTT, Chile

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.