Where a galaxy is located has a strong effect on its properties. The dense cluster environment is home to a large population of red, quiescent elliptical galaxies, whereas blue, star-forming, spiral galaxies are common in lower-density environments. This difference is intricately linked to the ability of the galaxy to form new stars and therefore ultimately to the fuel for star formation, the atomic and molecular gas. In this thesis, I use two large JCMT surveys to explore the effects of environment on the atomic gas, molecular gas, and star formation properties of a large sample of nearby gas-rich galaxies. From the NGLS and follow-up studies, I select a sub-sample of 98 HI-flux selected spiral galaxies. I measure their total molecular gas mass using the CO J=3-2 line and combine this data with measurements of their total atomic gas mass using the 21-cm line and star formation rate using attenuation-corrected H-alpha luminosity. I find an enhancement in the mean H2 mass and a higher H2-to-HI ratio for the Virgo Cluster sample. Virgo Cluster galaxies also have longer molecular gas depletion times (H2/SFR), which suggests that they are forming stars at a lower rate relative to their molecular gas reservoirs than non-Virgo galaxies. Next, I collect VLA 21 cm line maps from the VIVA survey and follow-up VLA studies of selected galaxies in the NGLS. I measure the surface density maps of the atomic gas, molecular gas, and star formation rate in order to determine radial trends. I find that the H2 distribution is enhanced near the centre for Virgo Cluster galaxies, along with a steeper total gas (HI + H2) radial profile. I suggest that this is due to the effects of moderate ram pressure stripping, which would strip away low-density gas in the outskirts while enhancing high-density gas near the centre. There are no trends with radius for the molecular gas depletion times, but the longer depletion times for the Virgo Cluster sample is still present. Finally, I use 850 micron continuum observations for 105 star-forming galaxies and CO J=2-1 line observations for 35 galaxies in the initial data release (DR1) of the JINGLE survey. I match the JINGLE galaxies to a SDSS group catalogue and measure environmental parameters such as the host halo mass, environment density, and location in phase space. I find that the molecular gas masses estimated from the 850 μm and CO J=2-1 line observations are well-correlated. The H2-to-HI ratio and the molecular gas depletion times do not appear to vary with stellar mass. I did not find any significant variation with environment in the DR1 sample, but I will apply this framework to the full JINGLE sample once the complete dataset is available. / Thesis / Doctor of Philosophy (PhD)

We investigate the uses of gravitational lensing for analysing the dark matter haloes around galaxies, comparing galaxies within groups and clusters to those in the field. We consider two cases: when only photometric redshift data is available, and when spectroscopic redshift data is available for a sufficiently large sample of galaxies. For the case of data with photometric redshifts, we analyse the CFHTLenS dataset. This dataset is derived from the CFHTLS-Wide survey, and encompasses 154 deg^2 of high-quality shape data. Using the photometric redshifts to estimate local density, we divide the sample of lens galaxies with stellar masses in the range 10^9 Msun to 10^10.5 Msun into those likely to lie in high-density environments (HDE) and those likely to lie in low-density environments (LDE). Through comparison with galaxy catalogues extracted from the Millennium Simulation, we show that the sample of HDE galaxies should primarily (~61%) consist of satellite galaxies in groups, while the sample of LDE galaxies should consist of mostly (~87%) non-satellite (field and central) galaxies. Comparing the lensing signals around samples of HDE and LDE galaxies matched in stellar mass, we show that the subhaloes of HDE galaxies are less massive than those around LDE galaxies by a factor 0.65+/-0.12, significant at the 2.9 sigma level. A natural explanation is that the haloes of satellite galaxies are stripped through tidal effects in the group environment. Our results are consistent with a typical tidal truncation radius of ~40 kpc. For the case of data with spectroscopic redshifts, we analyse the GAMA-I and the ongoing GAMA-II surveys. We demonstrate the possibility of detecting tidal stripping of dark matter subhaloes within galaxy groups using weak gravitational lensing. We have run ray-tracing simulations on galaxy catalogues from the Millennium Simulation to generate mock shape catalogues. The ray-tracing catalogues assume a halo model for galaxies and groups, using various models with different distributions of mass between galaxy and group haloes to simulate different stages of group evolution. Using these mock catalogues, we forecast the lensing signals that will be detected around galaxy groups and satellite galaxies, as well as test two different methods for isolating the satellites' lensing signals. A key challenge is to determine the accuracy to which group centres can be identified. We show that with current and ongoing surveys, it will possible to detect stripping in groups of mass 10^12 Msun to 10^15 Msun.

We investigate the uses of gravitational lensing for analysing the dark matter haloes around galaxies, comparing galaxies within groups and clusters to those in the field. We consider two cases: when only photometric redshift data is available, and when spectroscopic redshift data is available for a sufficiently large sample of galaxies. For the case of data with photometric redshifts, we analyse the CFHTLenS dataset. This dataset is derived from the CFHTLS-Wide survey, and encompasses 154 deg^2 of high-quality shape data. Using the photometric redshifts to estimate local density, we divide the sample of lens galaxies with stellar masses in the range 10^9 Msun to 10^10.5 Msun into those likely to lie in high-density environments (HDE) and those likely to lie in low-density environments (LDE). Through comparison with galaxy catalogues extracted from the Millennium Simulation, we show that the sample of HDE galaxies should primarily (~61%) consist of satellite galaxies in groups, while the sample of LDE galaxies should consist of mostly (~87%) non-satellite (field and central) galaxies. Comparing the lensing signals around samples of HDE and LDE galaxies matched in stellar mass, we show that the subhaloes of HDE galaxies are less massive than those around LDE galaxies by a factor 0.65+/-0.12, significant at the 2.9 sigma level. A natural explanation is that the haloes of satellite galaxies are stripped through tidal effects in the group environment. Our results are consistent with a typical tidal truncation radius of ~40 kpc. For the case of data with spectroscopic redshifts, we analyse the GAMA-I and the ongoing GAMA-II surveys. We demonstrate the possibility of detecting tidal stripping of dark matter subhaloes within galaxy groups using weak gravitational lensing. We have run ray-tracing simulations on galaxy catalogues from the Millennium Simulation to generate mock shape catalogues. The ray-tracing catalogues assume a halo model for galaxies and groups, using various models with different distributions of mass between galaxy and group haloes to simulate different stages of group evolution. Using these mock catalogues, we forecast the lensing signals that will be detected around galaxy groups and satellite galaxies, as well as test two different methods for isolating the satellites' lensing signals. A key challenge is to determine the accuracy to which group centres can be identified. We show that with current and ongoing surveys, it will possible to detect stripping in groups of mass 10^12 Msun to 10^15 Msun.