Characterisation of Herschel-selected strong lens candidates through HST and sub-mm/mm observations

Dusty star-forming galaxies (DSFGs) are a population of high-redshift, heavily dust-obscured sources that experience strong episodes of star formation. These galaxies emit most of their radiation in the sub-mm wavelengths due to the dust heated by the UV radiation produced by the newly formed stars. DSFGs play a key role in the early mass assembly of the Universe. Large samples of DSFGs have been gathered and characterized in the literature. However, spatially resolved analyses are still rare due to the high redshift of these sources. A possible way to address these issues is to rely on the boost in flux and angular resolution produced by strong gravitational lensing. Lensed DSFGs can be identified using a flux density cut in wide area sub-mm surveys like those carried out with Herschel, thanks to the lack of many intrinsically bright unlensed DSFGs. Moreover, the negative K-correction of such sub-mm observations aids in observing large volumes, increasing the likelihood of the chance alignment between the lens and background source. However, the angular resolution of Herschel is not good enough to resolve the lensing features of background sources. Hence, there is a need for high-resolution observations. In the past decade, the ISM properties of lensed DSFGs have been studied in great detail. However, one parameter the lensed sources are still missing is the stellar mass, which is fundamental in understanding how evolved these sources are. This thesis focuses on a sample of 281 lensed DSFG candidates identified through Herschel and followed up with the HST in the near-IR. These kinds of follow-up observations can help achieve three goals: confirm the lensing nature of the candidate, characterize the photometric properties and morphology of the lenses, and possibly detect and reconstruct the surface-brightness distribution of the background sources. We begin the analysis of our sample by classifying the observed candidates according to their likelihood of being lensed. We combine a visual inspection of the HST images and available sub-mm observations, finding 130 systems that are either lensed or show a possible lensing galaxy. For these, we model and subtract the candidate lens surface-brightness distribution. After the lens subtraction, we repeat the visual inspection of the residuals, producing a final lens classification. We confirm 65 systems to be lensed 30 new discoveries. We characterize the lens morphology, finding that 90% are single galaxies with only a few group lenses. Most lensing galaxies are consistent with early-type galaxies from morphological and photometric analysis. We successfully perform lens modelling and source reconstruction of 23 lensed DSFGs well detected by HST and lensed by isolated galaxies. We model all the lenses as singular isothermal ellipsoids. We find that the Einstein radii of the lenses and magnifications of the background sources are consistent with previous studies. However, the background-source circularized radii are 3 times smaller than those measured in the sub-mm for a similar sample. This finding might indicate the onset of inside-out quenching processes, clearing the dust from the central regions and causing the rest-frame optical emission to pass through. We compare our lenses with those in the SLACS survey, confirming that our sub-mm selection is more effective at picking up fainter and more diffuse galaxies and group lenses. Lastly, we test a combined Keck and Spitzer analysis of ID141, disentangling the heavily blended lensing galaxies and lensed DSFG in the low-resolution Spitzer observations. We perform SED modelling to measure the stellar mass of the lensed DSFG, finding that it is a massive main sequence galaxy. This sample represents the first step towards systematically characterising the near-IR properties and stellar masses of the gravitationally lensed DSFGs.