Theoretical galaxy formation models are an established and powerful tool for interpreting the astrophysical significance of observational data, particularly galaxy surveys. Such models have been utilised with great success by optical surveys such as 2dFGRS and SDSS, but their application to radio surveys of cold gas in galaxies has been limited. In this chapter we describe recent developments in the modelling of the cold gas properties in the models, and how these developments are essential if they are to be applied to cold gas surveys of the kind that will be carried out with the SKA. By linking explicitly a galaxy's star formation rate to the abundance of molecular hydrogen in the galaxy rather than cold gas abundance, as was assumed previously, the latest models reproduce naturally many of the global atomic and molecular hydrogen properties of observed galaxies. We review some of the key results of the latest models and highlight areas where further developments are necessary. We discuss also how model predictions can be most accurately compared with observational data, what challenges we expect when creating synthetic galaxy surveys in the SKA era, and how the SKA can be used to test models of dark matter.Advancing Astrophysics with the Square Kilometre Array

The ultraviolet (UV) bright accretion disc in active galactic nuclei (AGN) should give rise to line driving, producing a powerful wind which may play an important role in AGN feedback as well as in producing structures like the broad line region. However, coupled radiationhydrodynamics codes are complex and expensive, so we calculate the winds instead using a non-hydrodynamical approach (the Q framework). The original Q model assumed the initial conditions in the wind, and had only simple radiation transport. Here, we present an improved version which derives the wind initial conditions and has significantly improved ray-tracing to calculate the wind absorption self consistently given the extended nature of the UV emission. We also correct the radiation flux for relativistic effects, and assess the impact of this on the wind velocity. These changes mean the model is more physical, so its predictions are more robust. We find that, even when accounting for relativistic effects, winds can regularly achieve velocities (0.1 − 0.5) 𝑐, and carry mass loss rates which can be up to 30% of the accreted mass for black hole masses of 10 7−9 M , and mass accretion rates of 50% of the Eddington rate. Overall, the wind power scales as a power law with the black hole mass accretion rate, unlike the weaker scaling generally assumed in current cosmological simulations that include AGN feedback. The updated code, Q 3, is publicly available in GitHub †.

We present predictions for the evolution of radio emission from Active Galactic Nuclei (AGNs). We use a model that follows the evolution of Supermassive Black Hole (SMBH) masses and spins, within the latest version of the galform semi-analytic model of galaxy formation. We use a Blandford-Znajek type model to calculate the power of the relativistic jets produced by black hole accretion discs, and a scaling model to calculate radio luminosities. First, we present the predicted evolution of the jet power distribution, finding that this is dominated by objects fuelled by hot halo accretion and an ADAF accretion state for jet powers above 10 32 W at z = 0, with the contribution from objects fuelled by starbursts and in a thin disc accretion state being more important for lower jet powers at z = 0 and at all jet powers at high redshifts (z 3). We then present the evolution of the jet power density from the model. The model is consistent with current observational estimates of jet powers from radio luminosities, once we allow for the significant uncertainties in these observational estimates. Next, we calibrate the model for radio emission to a range of observational estimates of the z = 0 radio luminosity function. We compare the evolution of the model radio luminosity function to observational estimates for 0 < z < 6, finding that the predicted evolution is similar to that observed. Finally, we explore recalibrating the model to reproduce luminosity functions of core radio emission, finding that the model is in approximate agreement with the observations.

Active galactic nuclei (AGN) jets play an important role as a feedback mechanism that quenches the growth of massive galaxies. These jets have so far been simulated almost exclusively with grid-based codes. In this work we present results from hydrodynamical tests of AGN jets simulated with the SWIFT code and its implementation of smoothed particle hydrodynamics (SPH). We reach numerical resolutions of more than a million particles per jet, unprecedented for an SPH code. In all cases we find broad agreement between our jets and theoretical predictions for the jet and lobe lengths and widths. At very low resolutions, typical of lowresolution cosmological simulations (𝑚 gas ≈ 10 7 M ), the shape of the lobes is close to selfsimilar ones at a level of 15% accuracy. This indicates that the basics of jet lobe physics can be captured even at such resolutions (≈ 500 particles per jet). The jets first evolve ballistically, and then transition to a self-similar phase, during which the kinetic and thermal energies in the lobes and the shocked ambient medium are constant fractions of the total injected energy. In our standard simulation, two thirds of the initially injected energy is transferred to the ambient medium by the point the jets are turned off, mainly through a bow shock. Of that, three quarters is in thermal form, indicating that the bow shock thermalizes efficiently. We find that jet launching velocity and power, in addition to determining the temperature, mass and overall length of the jets, also play an important role in the resolution of the jets.

Abstract. The local space density of galaxies as a function of their basic structural parameters -like luminosity, surface brightness and scalesizeis still poorly known. Our poor knowledge is mainly the result of strong selection biases against low surface brightness and small scalesize galaxies in any optically selected sample. We show that in order to correct for selection biases one has to obtain accurate surface photometry and distance estimates for a large ( ∼ >1000) sample of galaxies. We derive bivariate space density distributions in the (scalesize, surface brightness)-plane and the (luminosity, scalesize)-plane for a sample of ∼1000 local Sb-Sdm spiral galaxies. We present a parameterization of these bivariate distributions, based on a Schechter type luminosity function and a log-normal scalesize distribution at a given luminosity. We show how surface brightness limits and (1+z) 4 cosmological redshift dimming can influence interpretation of luminosity function determinations and deep galaxy counts.