Stars of all ages and evolutionary stages are seen to be surrounded by discs of matieral. during the formation of a stellar system the stars are orbited by a massive protoplanetary disc composed of interstellar gas and dust, in which planet formation occurs. Betewwen 1 and 10 Myr the protoplanetary disc disperses, leaving behind the newly formed system of planets and smaller bodies. The remaining material which has not formed into planets is referred to as a debris disc. Even though the interstellar dust grains from the protoplanetary disc have long been removed from the system, debris discs can contain large quantities of dust due to collisions between larger bodies and cometary activity. such dust can be detected by its thermal emission. This thesis focuses on observational studies at far-infrared and sub-millimetre wavelengths of debris discs and the late stages of protoplanetary disc evolution. An overview of surveys for debris discs performed to date is presented, highlighting the limitations and statistical biases. the motivation, design and sample selection for two large surveys for debris discs around nearby stars, with the Hershel space observatory and the SCUBA-2 sub-millimetre camera on the James Clerk Maxwell Telescope, are described. The combination of a uniform obstevational strategy, longer wavelengths than previous surveys, and a large, clearly chosen sample - unbiased by stellar properties - will allow robust statistical conclusions of how the incidence and properties of debris discs depend on system parameters such as stellar mass, age, metallicity, binarity and the presence of planets. As a precursor to the Hershel and SCUBA-2 surveys, a volume-limited ample of 130 A type star systems was surveyed using observations at 24 and 70 μm, which were required to determine the presence of emission from dust, were predicted by fitting model flux distributions to optical and near-infrared photometry. Debris discs were detected around 46 systems, 12 of which including the system with the largest dust mass - are new discoveries. This survey adds to the results of previous studies which show that debris disc incidence is not correleated with host star metallicity despite the wll known giant planet - metallicity correlation, This is in accordance with what is predicted from the core accretion theory of planet formation. The most signigicant result from this survey is that, contrary to results reported in a previous work, debris discs are oberall less common around binary stars. Further investigation shows that systems with separations of ~3-150 AU are especially deficient of debris, while closer binaries and the primaries of wider binaries show debris detection rates consisten with those for single stars. A sample of circumstellar discs around 29 young stellar systems with ages of 5-30 Myr were observed with the LABOCA sub-millimetre instrument on the APEX telescope at 870μm, to provide disc masses or mass upper limits in support of a large Hershel programme. These targets included the η Chamaeleontis cluster and four bright Herbig Ae/Be stars which have not previously been observed at this wavelength. All but the Herbig Ae/Be stars were not detected, and 3σ dust mass upper limits of ~ 0.1-3 M are determined, with corresponding total disc masses of ~0.03-1Mjup. These mass limits indicate that there is insufficient remaining material in these discs to form gas giant planets, and add to the prevailing view that protoplanetary discs typically disperse within 10 Myr and that gas giant planet formation must be completed before this time. A search for cold dust emimission from two of the Solar System's nearest neighbours - α Centauri AB and ε Indi - was also performed with LABOCA. In both cases no debris disc emission was detected. A bright resolved feature was detected near α Centauri AB, nowever, follow-up observations at a second epoch, two years after the initial observations, showed that the feature is not co-moving with the stars. It is argued that the feature is most likely a pre-stellar core. The stars α Centauri A and B are detected, which is one of only very few detections of main sequence stellar photospheres at sub-millimetre wavelengths.

We present multi-wavelength detections of nine candidate gravitationally lensed dusty starforming galaxies (DSFGs) selected at 218 GHz (1.4 mm) from the Atacama Cosmology Telescope (ACT) equatorial survey. Among the brightest ACT sources, these represent the subset of the total ACT sample lying in Herschel SPIRE fields, and all nine of the 218 GHz detections were found to have bright Herschel counterparts. By fitting their spectral energy distributions (SEDs) with a modified blackbody model with power-law temperature distribution, we find the sample has a median redshift of z = 4.1(-1.0)(+1.1) (68 per cent confidence interval), as expected for 218 GHz selection, and an apparent total infrared luminosity of log10(mu LIR/L-circle dot) = 13.86(-0.30)(+0.33), which suggests that they are either strongly lensed sources or unresolved collections of unlensed DSFGs. The effective apparent diameter of the sample is root mu d = 4.2(-1.0)(+1.7) kpc, further evidence of strong lensing or multiplicity, since the typical diameter of DSFGs is 1.0-2.5 kpc. We emphasize that the effective apparent diameter derives from SED modelling without the assumption of optically thin dust (as opposed to image morphology). We find that the sources have substantial optical depth (tau = 4.2(-1.9)(+3.7)) to dust around the peak in the modified blackbody spectrum (lambda(obs) <= 500 mu m), a result that is robust to model choice.

We present spectral line mapping observations towards four massive star-forming regions Cepheus A, DR21S, S76E and G34.26+0.15 - with the IRAM 30-m telescope at the 2 and 3 mm bands. In total, 396 spectral lines from 51 molecules, one helium recombination line, 10 hydrogen recombination lines and 16 unidentified lines were detected in these four sources. An emission line of nitrosyl cyanide (ONCN, 14(0), 14-13(0), (13)) was detected in G34.26+0.15, as the first detection in massive star-forming regions. We found that c-C3H2 and NH2D show enhancement in shocked regions, as suggested by the evidence of SiO and/or SO emission. The column density and rotational temperature of CH3CN were estimated with the rotational diagram method for all four sources. Isotope abundance ratios of C-12/C-13 were derived using HC3N and its C-13 isotopologue, which were around 40 in all four massive star-forming regions and slightly lower than the local interstellar value (similar to 65). The N-14/N-15 and O-16/O-18 abundance ratios in these sources were also derived using the double isotopic method, which were slightly lower than in the local interstellar medium. Except for Cep A, the S-33/S-34 ratios in the other three targets were derived, which were similar to that in the local interstellar medium. The column density ratios of N(DCN)/N(HCN) and N( DCO+)/N(HCO+) in these sources were more than two orders of magnitude higher than the elemental [D]/[H] ratio, which is 1.5 x 10(-5). Our results show that the later stage sources, G34.26+0.15 in particular, present more molecular species than earlier stage sources. Evidence of shock activity is seen in all stages studied.

We present the results of the first, deep Atacama Large Millimeter Array ( ALMA) imaging covering the full similar or equal to 4.5 arcmin(2) of the Hubble Ultra Deep Field ( HUDF) imaged with Wide Field Camera 3/IR on HST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mm image reaching sigma 1.3 similar or equal to 35 mu Jy, at a resolution of similar or equal to 0.7 arcsec. From an initial list of similar or equal to 50 > 3.5 sigma peaks, a rigorous analysis confirms 16 sources with S-1.3 > 120 mu Jy. All of these have secure galaxy counterparts with robust redshifts (< z > = 2.15). Due to the unparalleled supporting data, the physical properties of the ALMA sources are well constrained, including their stellar masses ( M-*) and UV+FIR star formation rates ( SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift Universe; indeed at z = 2 our ALMA sample contains seven of the nine galaxies in the HUDF withM(*) = 2 x 10(10)M circle dot, and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S1.3 similar or equal to 10 mu Jy. We find strong evidence for a steep star-forming `main sequence' at z similar or equal to 2, with SFR. M* and a mean specific SFR similar or equal to 2.2 Gyr(-1). Moreover, we find that similar or equal to 85 per cent of total star formation at z similar or equal to 2 is enshrouded in dust, with similar or equal to 65 per cent of all star formation at this epoch occurring in high-mass galaxies ( M-* > 2 x 10(10)M circle dot), for which the average obscured: unobscured SF ratio is similar or equal to 200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at z similar or equal to 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured at z similar or equal to 4.

The far-infrared fine-structure line [C II] at 1900.5 GHz is known to be one of the brightest cooling lines in local galaxies, and therefore it has been suggested to be an efficient tracer for star formation in very high redshift galaxies. However, recent results for galaxies at z > 6 have yielded numerous non-detections in star-forming galaxies, except for quasars and submillimetre galaxies. We report the results of ALMA observations of two lensed, star-forming galaxies at z = 6.029 and z = 6.703. The galaxy A383-5.1 (star formation rate [SFR] of 3.2 M-circle dot yr(-1) and magnification of mu = 11.4 +/- 1.9) shows a line detection with L-[C II] = 8.9 x 10(6) L-circle dot, making it the lowest L-[C II] detection at z > 6. For MS0451-H (SFR = 0.4 M-circle dot yr(-1) and mu = 100 +/- 20) we provide an upper limit of L-[C II] < 3 x 10(5) L-circle dot, which is 1 dex below the local SFR-L-[C II] relations. The results are consistent with predictions for low-metallicity galaxies at z > 6; however, other effects could also play a role in terms of decreasing L-[CII]. The detection of A383-5.1 is encouraging and suggests that detections are possible, but much fainter than initially predicted.

This thesis presents my work characterising star formation in Gould Belt molecular clouds using submillimetre observations from SCUBA-2 on the James Clerk Maxwell Telescope (JCMT). I use these observations alongside data from previously published surveys using instruments including the Spitzer Space Telescope. I investigate the effect of including submillimetre data on the numbers, classifications and lifetimes of Young Stellar Objects (YSOs) in Gould Belt molecular clouds, particularly protostars. Following a literature review, I use SCUBA-2 450 and 850 μm observations to characterise star formation in the Lupus I molecular cloud. A total of eleven previously identified YSOs are detected with SCUBA-2, as well as eleven starless cores. Two cores have masses greater than the Jeans mass, and one has a virial parameter of 1.1 0.4, meaning these cores could be unstable against collapse. I use submillimetre emission to calculate disk masses, and find that one YSO has a disk mass greater than the minimum mass solar nebula. I find that Lupus I has a high percentage of both protostars and Very Low Luminosity Objects (VeLLOs). I also fit YSO Spectral Energy Distributions (SEDs) with models, allowing protostellar envelope masses and temperatures to be calculated, and interstellar extinction to be constrained for some YSOs. The signs of recent and future star formation support the hypothesis that a shock has triggered a star forming event in Lupus I. I also use SCUBA-2 data in conjunction with archival Spitzer and Herschel data to produce SEDs for five new candidate First Hydrostatic Cores (FHSCs) in Serpens South. These observations were then fit with models by the first author of this work, Alison Young. This work was able to identify two of the FHSC candidates as probable FHSCs, and constrain the rotation rate and inclination of one of them. I use JCMT Gould Belt Survey (GBS) observations of ten molecular clouds to produce an updated catalogue of protostars in these clouds. I use the FellWalker algorithm to find individual sources in the SCUBA-2 maps, and match them to the Spitzer YSO catalogue of Dunham et al. (2015). I use bolometric temperature to classify 362 out of 592 candidates as Class 0 or Class I protostars - a factor of two increase compared to the Spitzer catalogue due to improved submillimetre coverage. I find that protostellar lifetimes of 0.59 – 0.89 Myr - approximately 25 % longer than previously estimated. I also calculate protostellar luminosities, envelope masses, and envelope temperatures, and examine the distributions. Finally, I newly identify 19 protostars as VeLLOs, and increase the number of known VeLLOs in these clouds by a factor of two.

We present a novel new method for modelling magnetized molecular cloud cores using submillimetre linear polarization maps from thermal dust emission. Our PolCat modelling software builds a three-dimensional core model via the use of consecutive parametrized coordinate transformations, and produces simulated polarization maps to fit to observational datasets. We utilize the Ferret evolutionary optimizer to search the parameter space to simultaneously minimize chi-squared for the intensity and polarization position angle maps separately. We have applied PolCat to multiple test problems and several datasets from the SCUPOL Legacy Catalogue. We find that PolCat is able to distinguish between maps of twisted and non-twisted field geometries and identify twist symmetry. Preliminary fits to several datasets show that the best potential field geometries to our sample cores contain field twists. Further research using a larger number of maps is required to determine if twisted fields are commonplace in cores.

We present Herschel observations of the debris disk around γ Doradus (HD 27290, HIP 19893) from the Herschel Key Programme DEBRIS (Disc Emission via Bias-free Reconnaissance in the Infrared/Submillimetre). The disk is well-resolved with PACS at 70, 100 and 160 micron and detected with SPIRE at 250 and 350 micron. The 250 micron image is only resolved along the disk's long axis. The SPIRE 500 micron 3 σ detection includes a nearby background source. γ Dor's spectral energy distribution (SED) is sampled in the submillimetre for the first time and modelled with multiple modified-blackbody functions to account for its broad shape. Two approaches are used, both of which reproduce the SED in the same way: a model of two narrow dust rings and a model of an extended, wide dust belt. The former implies the dust rings have temperatures of ~90 and ~40 K, corresponding to blackbody radii of 25 and 135 AU, respectively. The latter model suggests the dust lies in a wide belt extending from 15 to 230 AU. The resolved images, however, show dust extending beyond ~350 AU. This is consistent with other debris disks whose actual radii are observed to be a factor of 2 - 3 times larger than the blackbody radii. Although it is impossible to determine a preferred model from the SED alone, the resolved images suggest that the dust is located in a smooth continuous belt rather than discrete narrow rings. Both models estimate that the dust mass is 6.7 x 10^{-3} Earth masses and that fractional luminosity is 2.5 x 10^{-5}. This amount of dust is within the levels expected from steady state evolution given the age of γ Dor and therefore a transient event is not needed to explain the dust mass. No asymmetries that would hint at a planetary body are evident in the disk at Herschel's resolution. However, the constraints placed on the dust's location suggest that the most likely region to find planets is within 20 AU of the star. / Graduate

Though there has been a considerable amount of work investigating the early stages of low-mass star formation in recent years, the general theory is only broadly understood and several open questions remain. Specifically, the dominant physical mechanisms which connect large-scale molecular cloud structures, intermediate-scale filamentary gas flows, and small-scale collapsing prestellar envelopes in the interstellar medium are poorly constrained. Even for an individual forming protostar, the evolution of the mass accretion rate from the envelope onto the central object is debated with little observational evidence to help guide the theoretical framework. In addition, with the development of new technology such as the continuum imaging instrument in operation at the James Clerk Maxwell Telescope (JCMT), the Submillimetre Common User Bolometer Array 2 (SCUBA-2), the best practices for data reduction and image calibration for ground-based, submillimetre wavelength observations are still being investigated. In this dissertation, I address facets of these open questions in five main projects with an overarching focus on the flow of material from the largest to the smallest scales in a molecular cloud. By performing synthetic observations of a numerical simulation of a turbulent molecular cloud, I investigate the nature of prestellar envelopes and find evidence of larger mass reservoirs that form filamentary structures and feed cluster formation. Then, after robustly investigating and suggesting improvements for ground-based, submillimetre data reduction techniques, I continue to probe the connection between larger and smaller scales by characterising structure fragmentation in the Southern Orion A Molecular Cloud from the perspective of 850 m continuum data. Finally, I follow star forming material to even smaller scales by exploring the evolution of the mass accretion rate onto individual protostars. This examination has required designing and implementing unprecedented spatial alignment and flux calibration techniques at 850 m. Using these newly calibrated images, I am able to identify several candidate sources that show evidence for submillimetre variability, suggesting changes in protostellar accretion rates over several year timescales. / Graduate

The most intensely star-forming galaxies lie at z ∼ 2 and are thought to be the progenitors of the most massive galaxies today, yet study of this important population has been hampered by vast quantities of dust, making them almost invisible in the optical and ultraviolet (UV) regimes, and by the low sensitivity and angular resolution of many infrared (IR) facilities. Chapter 2 describes the use of the flux and angular extent boost provided by strong gravitational lensing in the detailed study of individual high-redshift dusty star-forming galaxies (DSFGs). The low number density of such systems has been overcome by recent wide area far-infrared (FIR)–mm surveys, and a sample of candidate systems which are bright enough to study with single-dish FIR telescopes are assembled from these surveys. The chapter further describes spectra of these galaxies obtained using the the Spectral and Photometric Imaging REceiver (SPIRE; Griffin et al. 2010) Fourier transform spectrometer (FTS) on board the Herschel Space Observatory (Pilbratt et al. 2010), exploiting the increased flux densities to search for FIR atomic and ionic spectral lines: important coolants of warm gas surrounding star-formation regions. Chapter 3 describes the first “blind” redshift obtained using Herschel, via the detection of [C ii] 158 μm in one of our spectra. Confirmation of this redshift was provided by detection of CO lines with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and the Plateau de Bure Interferometer (PdBI), and along with multi-wavelength photometric follow-up, allowed a characterisation of the galaxy, indicating both a merger-driven starburst and an active galactic nucleus (AGN) within the system. Chapter 4 describes the first detection of a massive outflow of molecular gas at high-redshift. Stacking five repeat spectra of the Cosmic Eyelash, one of the best-studied strongly lensed DSFGs, one of the massive star-forming clumps is shown to drive this outflow, albeit likely at a velocity lower than that required to become unbound from the hosting gravitational potential well. Chapter 5 describes line measurements and spectral energy distribution (SED) fitting from the full set of spectra and Herschel PACS mini scan maps. The spectra are stacked to search for faint lines, and compared to a detailed interstellar medium (ISM) model to determine average physical properties of the star-forming gas. Photodissociation region (PDR) properties are found to be similar to those derived using other models, however a cosmic ray ionisation rate of 103 times that of the Milky Way, expected in galaxies of this type cannot reproduce the observed line ratios, in particular the low [O i] 63 μm flux. Chapter 6 finally describes the conclusions drawn from the work presented in this thesis and how these data and analysis add to our knowledge and interpretation of high-redshift DSFGs.