Nature of Dark Matter from the Astrophysics of High Redshift Star-forming Galaxies

In the late 1970s, Vera Rubin showed that the flat rotation curves are ubiquitous in local spiral galaxies, and concluded that it is due the presence of invisible matter, the so-called `Dark Matter'. She remarked- “galaxies are surrounded by a dark matter halo that extends much further than their visible matter”. These observational results were later supported by theoretical models of structure formation. Thereafter dark matter became an essential building block of current cosmological models, which dominates the formation and evolution of all structures in the Universe. Recent observations of high redshift galaxy rotation curves show remarkable differences in dark matter profiles compared to the local Universe. Therefore, there is an urgent need to repeat the `classical' observational study of Rubin et al. at high redshift with a large dataset. In search of dark matter and its astrophysical nature at high redshift, I study the shape of the rotation curves of z~1 star-forming disc-like galaxies. Thereby dark matter fraction, structural properties of dark matter halo, and fundamental scaling relations in the progenitors of local disc galaxies. For this purpose, I exploit $344$ star-forming galaxies of KMOS Redshift One Spectroscopic Survey (KROSS) from publicly available data. I first aimed to disentangle the prevailing observational and physical uncertainties of high redshift observations, namely beam smearing and pressure support. To this end, the kinematics of the KROSS sample is derived using 3D forward modelling with $^{3D}$BAROLO, which accounts for beam smearing in 3D space and provide: $Halpha$ kinematic maps, surface density, rotation, and dispersion curves. For the purpose of this work, I analysed and investigated 256 rotation dominated disc-like galaxies from the main sample, covering the redshift range $z=0.57-1.04$. The main entity, i.e. rotation curves, are then corrected for pressure support using the pressure gradient correction. I have found that only a combination of the three techniques (3D-kinematic modelling + 3D-beam smearing correction + pressure gradient correction) yields the accurate rotation curves of high redshift galaxies. The rotation curves are then dynamically probed to determine the dark matter fraction. Then, in seek of the distribution of the dark matter profile, I dynamically mass modelled the rotation curves, employing Burkert and NFW halo profiles. The outcomes of the latter are cross-checked with previous studies, and state-of-the-art cosmological simulations: EAGLE and Illustris TNG. In this way, I was able to gain new insights into the structural properties of dark matter halos and current state of fundamental scaling relations of star-forming galaxies at z~1. I confirm that the outer rotation curves of star-forming disc-like galaxies at $zsim 1$ are similar to the outer rotation curves of local star-forming disc galaxies. Statistically, the latter results indicate that the {em total mass} within the outer region ($sim 5$ to $20$ kpc) remains the same at $zsim 1$ and $zapprox 0$, while the {em stellar mass} distribution, defined by the stellar disc radii, varies as a function of the {em total mass} (or circular velocity). I show that star-forming galaxies at $0.7 leq z leq 1$ have outer discs dominated by dark matter ($sim 5$ to $20$ kpc). Only a small fraction ($sim 5%$) of star-forming galaxies at $zsim 1$ have a low dark matter fraction within the effective radius. The dynamical mass modelling of rotation curves and their various comparisons show that the Burkert halo profile fits our sample best. The results of the dark matter structural properties ($r_{_0}$ and $ ho_{_0}$) show that the dark matter cores at $zsim 1$ are on average a factor of 0.3 smaller and more than an order of magnitude denser than those of local star-forming galaxies. As for the fundamental scaling relations, I find that star-forming disc-like systems at $zsim 1$ exhibit the same mass-size relations as local disc galaxies, as well as a similar scaling of the specific stellar angular momentum with stellar mass. On the other hand, I found a significant evolution in the slope of the stellar Tully-Fisher relation at $zsim 1$, while its zero-point remains the same as that of the local disc galaxies. To conclude, I have shown that dark matter is ubiquitous in star-forming disc-like galaxies at z~1. A preliminary study of dark matter halo structures indicates smaller and denser dark matter cores at z~1, which suggests that the predictions concerning the transformation of cusps into cores, which are observed in hydrodynamical simulations, indeed occur in nature. That is, the dark matter responds to the baryonic processes that dynamically heat-up the dark matter particles, leading to a disruption and displacement in its initial distribution. The latter can be interpreted as, transforming the original dense inner dark matter density (cusp) into a sparse distribution (core). Most importantly, with this study I attempted to provide the first empirical evidence of {em gravitational potential perturbations} in the inner region of galaxies, linking the dark and luminous matter properties, and constraining the dark matter halo structural properties. If my results are verified with high quality data, they will have implications for theories of galaxy formation and evolution, as well as for the nature of dark matter particle itself.