Galaxy formation and evolution

Abstract:

Empirical simulations based on extrapolations from well-established low-frequency (<5 GHz) surveys fail to accurately model the faint, high frequency (>10 GHz) source population; they underpredict the number of observed sources by a factor of 2 below S18GHz = 10 mJy and fail to reproduce the observed spectral index distribution. We suggest that this is because the faint radio galaxies are not modelled correctly in the simulations and show that by adding a flat-spectrum core component to the Fanaroff and Riley type-I (FRI) sources in the Square Kilometre Array (SKA) Simulated Skies, the observed 15 GHz source counts can be reproduced. We find that the observations are best matched by assuming that the fraction of the total 1.4 GHz flux density that originates from the core varies with 1.4 GHz luminosity; sources with 1.4 GHz luminosities < 1025 W Hz − 1 require a core fraction ∼0.3, while the more luminous sources require a much smaller core fraction of 5 × 10−4. The low luminosity FRI sources with high core fractions that were not included in the original simulation may be equivalent to the compact ‘FR0’ sources found in recent studies.

Abstract:

We examine dense self-gravitating stellar systems dominated by a central potential, such as nuclear star clusters hosting a central supermassive black hole. Different dynamical properties of these systems evolve on vastly different timescales. In particular, the orbital-plane orientations are typically driven into internal thermodynamic equilibrium by vector resonant relaxation before the orbital eccentricities or semimajor axes relax. We show that the statistical mechanics of such systems exhibit a striking resemblance to liquid crystals, with analogous ordered-nematic and disordered-isotropic phases. The ordered phase consists of bodies orbiting in a disk in both directions, with the disk thickness depending on temperature, while the disordered phase corresponds to a nearly isotropic distribution of the orbit normals. We show that below a critical value of the total angular momentum, the system undergoes a first-order phase transition between the ordered and disordered phases. At the critical point the phase transition becomes second-order while for higher angular momenta there is a smooth crossover. We also find metastable equilibria containing two identical disks with mutual inclinations between $90^{\circ}$ and $180^\circ$.

Abstract:

Combining large-area optical quasar surveys with the new far-infrared (FIR) Herschel-ATLAS Data Release 1, we search for an observational signature associated with the minority of quasars possessing bright FIR luminosities. We find that FIR-bright quasars show broad C IV emission-line blueshifts in excess of that expected from the optical luminosity alone, indicating particularly powerful nuclear outflows. The quasars show no signs of having redder optical colours than the general ensemble of optically selected quasars, ruling out differences in lineof- sight dustwithin the host galaxies.We postulate that these objectsmay be caught in a special evolutionary phase, with unobscured, high black hole accretion rates and correspondingly strong nuclear outflows. The high FIR emission found in these objects is then either a result of star formation related to the outflow, or is due to dust within the host galaxy illuminated by the quasar. We are thus directly witnessing coincident small-scale nuclear processes and galaxy-wide activity, commonly invoked in galaxy simulations that rely on feedback from quasars to influence galaxy evolution.