Properties of cold gas streams for galaxy formation

The CDM "concordance" cosmological model can successfully explain the overall assembly of cosmic structures (Blumenthal et al. 1984, Coles 2005). A challenging frontier in current cosmology is to properly include the physics of the gas and galaxy formation into this paradigm. This requires the modelling of astrophysical processes, such as star formation and stellar feedback which happen on parsec scales compared to the megaparsec scales of the overall assembly of cosmic structures. They need to be simultaneously modelled as they undoubtedly influence each other. Cosmological simulations have shown that flows of cold gas can reach the centre of dark matter haloes without being shock heated (Birnboim & Dekel 2003; Keres et al. 2005; Dekel et al. 2009). From an observational point of view, strong outflows detected in high redshift galaxies are not included in the standard picture of galaxy assembly (Law et al. 2007). So cosmologists are far from a correct understanding of galaxy formation, both qualitatively and quantitatively. However, this situation will improve dramatically in the next few years, driven by the constant increase in computer power. As a result, this is the right time to address the modelling of the key processes relevant at galactic scales. My work will focus on the steady, narrow, cold gas streams that feed early galaxies. The outstanding open questions that I will address concern the general behaviour of these cold flows. It is clear that the dynamics of cold streams dominate the physics of galaxy formation and evolution since they bring the majority of gas, angular momentum and mergers into the galactic centre. By analysing large scale cosmological hydrodynamical simulations, I want to make robust statements about their number as well as their alignment with respect to the galactic disk and to the cosmic web. I will study more of their overall properties such as the total amount of inflow, their temperature, velocity and mass, as well as the number and frequency of the clumps within them and compare them against analytical predictions from the literature. I am keen to understand the underlying physical processes governing the behaviour seen in the numerical experiments. To summarise: I am in a position to launch a comprehensive study of these phenomena aiming at a fully worked out scenario for the role of cold flows in the formation of galaxies. This project proposal is a resubmission of project number M 1515-N27 "Properties of cold gas streams for galaxy formation. "

The concordance cosmological model can successfully explain the overall assembly of cosmic structures.A challenging frontier in current cosmology is to properly include the physics of the gas and galaxy formation into this paradigm. This requires the modelling of astrophysical processes, such as star formation and stellar feedback which happen on light year scales compared to the million light year scales of the overall assembly of cosmic structures. They need to be simultaneously modelled as they undoubtedly influence each other. Cosmological simulations have shown that flows of cold gas can reach the center of dark matter haloes without being shock heated. From an observational point of view, strong outflows detected in high redshift galaxies are not included in the standard picture of galaxy assembly. So cosmologists are far from a correct understanding of galaxy formation, both qualitatively and quantitatively.However, this situation improves dramatically during these years, driven by the constant increase in computer power. As a result, this was the right time to address the modelling of the key processes relevant at galactic scales.This project's work focused on the steady, narrow, cold gas streams that feed early galaxies. The outstanding open questions that it addressed concerned the general behaviour of these cold flows. It was already clear that the dynamics of cold streams dominate the physics of galaxy formation and evolution since they bring the majority of gas, angular momentum and mergers into the galactic center. By analysing large scale cosmological hydrodynamical simulations, this project made robust statements about the velocities, the accretion rates, the distribution of the accretion rates as well as the clumpiness of the accretion along streams from the cosmic web into massive galaxies at high redshift. The project found that the simulated cold stream velocities are constant with radius and that these constant inflow velocities have a parabola-like dependency on the host halo mass and a square root power-law relation with redshift. The project found that the streams keep a roughly constant accretion rate as they penetrate into the halo center. The distribution of the accretion rates can well be described by a sum of two Gaussians, the primary corresponding to smooth inflow and the secondary to mergers. The same functional form was already found for the distributions of specific star formation rates in observations. The mass fraction in the smooth component is 60 90 per cent, insensitive to redshift or halo mass. The project found that a logarithmic functional form describes the cumulative merger rates best. The parameters of this logarithmic functional form vary like a power law or a parabola-like function with mass and redshift. The project found clumps made of gas and / or stars not having their own dark matter halo.