Defence: 28 March 2012 at 16:15h
Academiegebouw, Rapenburg 73, Leiden
Party: 28 March 2012 at 21:00h
Scheltema, Marktsteeg 1, Leiden
Full thesis (200 pages)
Thesis propositions (in English and Dutch)
Thesis cover (and bookmark)
Chapter 1: Introduction
Galaxy formation has undergone fast development in the past few decades, both theoretically and observationally. The number of observed galaxies is expanding rapidly and observations are pushing to lower masses and higher redshifts. Diffuse gas has been observed in absorption against bright background objects and in emission around galaxies. Simulations predict that the spatial distribution of the intergalactic medium, the cosmic web, has a profound impact on the evolution of galaxies. Gas accretion provides the fuel for star formation, which is inhibited by outflows powered by supernova explosions and active galactic nuclei. Star formation and feedback produce and distribute metals in the surrounding medium, which aids the process of galaxy formation. To understand the assembly of galaxies, we need to understand how they are fuelled.
Chapter 2: The rates and modes of gas accretion onto galaxies and their gaseous haloes
We study the rate at which gas accretes onto galaxies and haloes and investigate whether the accreted gas was shocked to high temperatures before reaching a galaxy. For this purpose we use a suite of large cosmological, hydrodynamical simulations from the OWLS project. We improve on previous work by considering a wider range of halo masses and redshifts, by distinguishing accretion onto haloes and galaxies, by including important feedback processes, and by comparing simulations with different physics. The specific rate of gas accretion onto haloes is, like that for dark matter, only weakly dependent on halo mass. For halo masses Mhalo>>10^11 Msun it is relatively insensitive to feedback processes. In contrast, accretion rates onto galaxies are determined by radiative cooling and by outflows driven by supernovae and active galactic nuclei. Galactic winds increase the halo mass at which the central galaxies grow the fastest by about two orders of magnitude to Mhalo~10^12 Msun. Gas accretion is bimodal, with maximum past temperatures either of order the virial temperature or <~10^5 K. The fraction of gas accreted on to haloes in the hot mode is insensitive to feedback and metal-line cooling. It increases with decreasing redshift, but is mostly determined by halo mass, increasing gradually from less than 10% for ~10^11 Msun to greater than 90% at 10^13 Msun. In contrast, for accretion onto galaxies the cold mode is always significant and the relative contributions of the two accretion modes are more sensitive to feedback and metal-line cooling. The majority of stars present in any mass halo at any redshift were formed from gas accreted in the cold mode, although the hot mode contributes typically over 10% for Mhalo>~10^11 Msun. Galaxies, but not necessarily their gaseous haloes, are predominantly fed by gas that did not experience an accretion shock when it entered the host halo.
Chapter 3: The drop in the cosmic star formation rate below redshift 2 is caused by a change in the mode of gas accretion and by active galactic nucleus feedback
The cosmic star formation rate is observed to drop sharply after redshift z=2. We use a large, cosmological, smoothed particle hydrodynamics simulation to investigate how this decline is related to the evolution of gas accretion and to outflows driven by active galactic nuclei (AGN). We find that the drop in the star formation rate follows a corresponding decline in the global cold-mode accretion rate density onto haloes, but with a delay of order the gas consumption time scale in the interstellar medium. Here we define cold-mode (hot-mode) accretion as gas that is accreted and whose temperature has never exceeded (did exceed) 10^5.5 K. In contrast to cold-mode accretion, which peaks at z~3, the hot mode continues to increase to z~1 and remains roughly constant thereafter. By the present time, the hot mode strongly dominates the global accretion rate onto haloes. Star formation does not track hot-mode halo accretion because most of the hot halo gas never accretes onto galaxies. AGN feedback plays a crucial role by preferentially preventing gas that entered haloes in the hot mode from accreting onto their central galaxies. Consequently, in the absence of AGN feedback, gas accreted in the hot mode would become the dominant source of fuel for star formation and the drop off in the cosmic star formation rate would be much less steep.
Chapter 4: Properties of gas in and around galaxy haloes
We study the properties of gas inside and around galaxy haloes as a function of radius and halo mass. For this purpose, we use a suite of large cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations project. The properties of cold- and hot-mode gas, which we separate depending on whether the temperature has been higher than 10^5.5 K while it was extragalactic, are clearly distinguishable in the outer parts of massive haloes (virial temperatures >> 10^5 K. The differences between cold- and hot-mode gas resemble those between inflowing and outflowing gas. The cold-mode gas is mostly confined to clumpy filaments that are in pressure equilibrium with the diffuse, hot-mode gas. Besides being colder and denser, cold-mode gas typically has a much lower metallicity and is much more likely to be infalling. However, the spread in the properties of the gas is large, even for a given mode and a fixed radius and halo mass, which makes it impossible to make strong statements about individual gas clouds. Metal-line cooling causes a strong cooling flow near the central galaxy, which makes it hard to distinguish gas accreted through the cold and hot modes in the inner halo. Stronger feedback results in larger outflow velocities and pushes hot-mode gas to larger radii. The gas properties evolve as expected from virial arguments, which can also account for the dependence of many gas properties on halo mass. We argue that cold streams penetrating hot haloes are observable as high-column density HI Lyman-alpha absorption systems in sightlines near massive foreground galaxies.
Chapter 5: Cold accretion flows and the nature of high column density HI absorption at redshift 3
Simulations predict that galaxies grow primarily through the accretion of gas that has not gone through an accretion shock near the virial radius and that this cold gas flows towards the central galaxy along dense filaments and streams. There is, however, little observational evidence for the existence of these cold flows. We use a large, cosmological, hydrodynamical simulation that has been post-processed with radiative transfer to study the contribution of cold flows to the observed z=3 column density distribution of neutral hydrogen, which our simulation reproduces. We find that nearly all of the HI absorption arises in gas that has remained colder than 10^5.5 K, at least while it was extragalactic. In addition, the majority of the HI is rapidly falling towards a nearby galaxy, with non-negligible contributions from outflowing and static gas. Above a column density of N_HI = 10^17 cm^-2, most of the absorbers reside inside haloes, but the interstellar medium only dominates for N_HI > 10^21 cm^-2. Haloes with total mass below 10^10 Msun dominate the absorption for 10^17
Chapter 6: Soft X-ray and ultra-violet metal-line emission from the gas around galaxies
The gas around galaxies is diffuse and much fainter than the galaxies themselves. A large fraction of the gas has temperatures between 10^4.5 and 10^7 K. If the gas has metallicities above 0.1 Zsun, it will cool primarily through metal-line emission. With current and upcoming instruments we may be able to detect the halo gas in emission, either directly or statistically. We stack the galaxies in several large cosmological, hydrodynamical simulations and calculate the expected metal-line surface brightness as a function of radius from the centre. We then compare it to the capabilities of current and future facilities. At low redshift, proposed X-ray telescopes can detect O VIII emission out to the virial radius of groups and clusters (assuming a detection limit of 0.1 photon/s/cm^2/sr). C VI, N VII, O VII, and Ne X can also be detected to smaller radii, 0.1-0.5 Rvir. At high redshift it will be possible to observe rest-frame UV lines, C III, C IV, O VI, Si III, and Si IV, out to 10-20 per cent of the virial radius in haloes larger than 10^11 Msun with upcoming instruments (assuming a detection limit of 10^-20 erg/s/cm^2/arcsec^2). Most of these lines have surface brightnesses a factor of a few higher at low redshift and could therefore easily be detected with the next generation UV space telescope. The metal-line emission is, in general, biased towards regions of high density and high metallicity and also towards the temperature where the emissivity peaks. This bias varies with radius, halo mass, redshift and between different emission lines. We can quantify the clumpiness of the emitting gas with respect to the underlying density using the clumping factor. The clumping factor is highest in the most massive haloes, because for these haloes the mass-weighted temperature is much higher than the peak emissivity temperature and the emission is thus dominated by cold, dense clumps. The X-ray-flux-weighted properties are similar for all metal lines considered, whereas the UV-flux-weighted properties vary strongly between metal lines.