Investigating strong gravitational lensing with infrared space missions : AKARA, Spitzer and Herschel

Hopwood, Rosalind Helen Bevis (2011). Investigating strong gravitational lensing with infrared space missions : AKARA, Spitzer and Herschel. PhD thesis The Open University.



This thesis addresses the use of strong gravitational lensing to facilitate deep midinfrared observations of star-forming galaxies and to study highly dust obscured sub-millimetre galaxies (SMG). 15 um extragalactic number counts were taken from ultra deep AKARI mapping of the gravitational lensing cluster Abell 2218, which is the deepest image taken by any facility at this wavelength. Via strong gravitational lensing these data probe beyond the AKARI blank field confusion limit. By de-magnifying the extracted source catalogue and performing careful photometric de-blending, using multi-wavelength positional priors, galaxy number counts down to ~0.01 mJy were achieved. These counts are ~3x deeper than previous results and resolve 70-100% of the cosmic infrared background at 15/.Lm,giving a new stronger lower limit of 2.0±0.3 nWm-2 Hz-1. These data are sampling the normal star forming population that dominates the peak epoch of star formation. Stacking analysis of the AKARI 15 um source positions at Herschel/SPIRE wavelengths, show that a deep 15 um galaxy population resolves around 40% of the 250 um cosmic infrared background, but is less representative of galaxy populations sampled at longer wavelengths, where the background is dominated by sub-mm galaxies. This thesis also focuses on strong galaxy-galaxy lensing events and the decoupling of the lens and source photometry in order to estimate redshifts and constrain physical characteristics. The first sample of bright sub-mm gravitational lenses, selected at SPIRE 500 um by the Herschel Astrophysical Terahertz Large Area Survey, are investigated with light profile and SED fitting, to derive physical characteristics. For two of the five lenses, it was possible to disentangle the lens and background galaxy components in highly photometrically blended Spitzer data, after observations by the Submillimeter Array revealed the lensed structure. The lensed background galaxies are highly dust obscured SMG with intrinsic infrared luminosities < ultra luminous infrared galaxies (ULIRGs), and constrain the number density of un-lensed ULIRGs and hyper luminous infrared galaxies to < 3.2 deg-2.

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