The dynamics and ISM properties of high-redshift dusty star-forming galaxies

Abstract

In this thesis we present a range of observations of submillimetre galaxies (SMGs), a subclass of dust-obscured star-forming galaxies (DSFGs) at redshifts of z~1-5. SMGs are among the most actively star forming sources ever observed, believed to contribute significantly to the star-formation rate density (SFRD) at its peak, so-called 'cosmic noon', at z~2. Given their extreme nature, SMGs provide a strong test of galaxy formation and evolution models. Advancements in instrumentation, in particular with the Submillimetre Common-User Bolometer Area 2 (SCUBA-2) and the Atacama Large (sub-)Millimeter Array (ALMA), have driven significant progress in SMGs studies over the last decade. We have now identified samples of hundreds of SMGs in survey fields with a plethora of photometric coverage, such as the Cosmic Evolution Survey (COSMOS), the UKIDSS Ultra Deep Survey (UDS) and the Extended Chandra Deep Field Survey (ECDFS). Indeed, the main motivation of this thesis is to exploit these samples of SMGs, with a particular focus on the molecular and ionised gas properties, using state-of-the-art instrumentation such as ALMA and the Northern Extended Millimeter Array (NOEMA) for the former, and the K-band Multi-Object Spectrograph (KMOS) mounted on the Very Large Telescope for the latter.

Firstly, in Chapter 2 we present CO observations of 47 SMGs, providing one of the largest and highest quality samples of its kind. With this study we demonstrate the capability of ALMA and NOEMA to undertake blind redshift scans in the 3mm waveband, and in doing so add significantly to the number of SMGs with spectroscopic redshifts, which prior to the work presented in this thesis was small. We also exploit the multi-wavelength coverage of the samples, together with the robust new spectroscopic redshifts, to model their spectral energy distributions (SEDs) with the MAGPHYS code and subsequently estimate key physical properties such as stellar masses and star-formation rates.

Perhaps more importantly, this survey has allowed us to characterise the molecular gas content in the SMG population, along with its excitation properties, results from which we present in Chapter 3. We also show that the gas depletion timescale in SMGs remains constant, and given that SMGs are significant contributors to the star-formation rate density (SFRD) at z~2, the global evolution of star-formation in SMGs appears to coincide with the evolution of the molecular gas content, as opposed to any variation in star-formation efficiency. We provide a new test of the SMG population as descendants of massive local early-type galaxies, using the derived CO linewidths and baryonic masses.

In Chapter 4 we present our Large Programme with KMOS which, when completed, will have observed ~400 SMGs in the COSMOS, UDS and ECDFS fields. Expanding on the work of Chapters 2 and 3 this is designed to further add to the catalog of SMGs with spectroscopic redshifts by detecting the H_alpha and/or [OIII] emission, which probes ionised gas and can also be used to estimate star-formation rates. We detail the target selection and observing strategy of this survey, before presenting early results for 43 emission line-detected sources, including the H_alpha-derived star-formation rates, the mass-metallicity relation and BPT diagram. We also compare the H_alpha, rest-frame optical/near-infrared and dust sizes where available, finding median radii of R_e = 3.6+/-0.3 kpc, R_Halpha = 4.2+/-0.4 kpc and R_dust = 1.2+/-0.3 kpc. Additionally, the sample are consistent with a median Sersic index of n=1, i.e. with an exponential disc-like light profile.

The integral field spectrograph (IFS) capabilities of KMOS allow us to spatially resolve the H_alpha/[OIII] emission when it is sufficiently bright and extended, and this provides valuable diagnostics of the galaxy kinematics. Therefore, in Chapter 5 we present resolved H_alpha/[OIII] velocity and velocity dispersion maps for 36 SMGs, from which we derive rotation curves and dispersion profiles. We compare the derived kinematics of our SMGs with less active galaxies at lower redshifts, and divide the sample into 28 'ordered' sources with clear velocity gradients, and rotation curves which can be modelled as Freeman disks, and eight 'disordered' sources with much more messy velocity maps, from which little reliable kinematic information can be obtained. We measure a median rotational velocity of v_rot = 190+/-20 km/s and a median intrinsic velocity dispersion of sigma_0 = 87+/-5 km/s from the 'ordered' subset, both of which are significantly higher than the less actively star-forming galaxies to which we compare. The median ratio of rotational velocity to intrinsic velocity dispersion in the 'ordered' sample is v_rot/sigma_0 = 2.2+/-0.5, indicating that our sources are somewhat rotationally supported, and we therefore suggest that our SMG sample likely represents 'scaled-up' versions of more 'normal' star-forming galaxies, rather than merger-dominated systems.