Welcome to my thesis website! Below you will find an overview of my thesis. It can be regarded as a table of contents. There are the abstracts of all chapters, including links to the separate chapters. Clicking the image of the cover will reveal the whole document as one. My 'stellingen' (propositions) can be found here.
In the navigation bar on the right you find the links to my other websites.
If you want to contact me, the contact details are below the navigation bar.
Chapter 1: Introduction
No summary here...
Chapter 2: Physical properties of simulated galaxies from varying input physics
We investigate the baryonic properties, such as stellar mass, (specific) star formation rate, gas consumption time scale, and gas fraction, of haloes at redshift two using a large set of high-resolution cosmological simulations from the OWLS project. We vary the sub-grid models for radiative cooling, reionization, the pressure of the unresolved multiphase ISM, star formation, feedback from massive stars and AGN, as well as the cosmology, box size and numerical resolution. While reionization and metal-line cooling are important for low- and high- mass haloes, respectively, galactic winds driven by feedback from star formation and/or accreting black holes determine the main properties of galaxies. The star formation rate is regulated through the ejection of gas by galactic winds. The gas fraction, and thus the star formation rate, adjusts until the (time averaged) rate at which energy/momentum are injected is sufficient to balance the accretion, which is itself determined by cosmology and cooling. Consequently, the assumed star formation law affects the gas fractions, but not the star formation rates. The predictions are sensitive to variations in the sub-grid implementation of galactic outflows, even if the energy per unit stellar mass is fixed. Feedback becomes inefficient if the initial wind velocity falls below a minimum value that increases with the pressure of the ISM and hence with halo mass. In galaxies from which winds do not escape, the pile up of newly formed metals results in catastrophic cooling and strong star formation. Our results suggests that a wide range of stellar mass functions could be produced by varying the initial wind velocity and mass loading with halo mass. In fact, even without such tuning many of our models predict stellar mass functions that agree with the observations. Reproducing the high values of the observed specific star formation rate appears, however, to be more difficult. In particular, the efficient feedback required to reproduce the mass function results in much lower specific star formation rates than observed.
Chapter 3: Disentangling galaxy environment and host halo mass
The properties of observed galaxies and dark matter haloes in simulations depend on their environment. The term “environment” has, however, been used to describe a wide variety of measures that may or may not correlate with each other. Useful measures of environment include, for example, the distance to the N th nearest neighbour, the number density of objects within some distance, or, for the case of galaxies, the mass of the host dark matter halo. Here we use results from the Millennium simulation and a semi-analytic model for galaxy formation to quantify the relation between different measures of environment and halo mass. We show that most of the environmental parameters used in the observational literature are in effect measures of halo mass. The strongest correlation between environmental density and halo mass arises when the number of objects is counted out to a distance of 1.5 – 2 times the virial radius of the host halo and when the galaxies/haloes are required to be relatively bright/massive. For observational studies this virial radius is not easily determined, but the number of neighbours out to 1 – 2 Mpc/h gives a similarly strong correlation with halo mass. For the distance to the N th nearest neighbour the (anti-)correlation with halo mass is nearly as strong provided N is bigger than about 2. We demonstrate that this environmental parameter can be made insensitive to halo mass if it is constructed from dimensionless quantities. This can be achieved by scaling both the minimum luminosity/mass of neighbours as well as the distance to the nearest galaxy/halo to the properties of the object that the environment is determined for. We show how such a halo mass independent environmental parameter can be defined for both observational and numerical studies. The results presented here will help future studies to disentangle the effects of halo mass and external environment on the properties of galaxies and dark matter haloes.
Chapter 4: Simulated galaxy luminosity functions: dust attenuation and comparison to virtual observations
We investigate the luminosity function (LF) resulting from cosmological hydrodynamical simulations with vary- ing input physics, with and without an estimate for dust attenuation. We find that in simulations in which the supernova (SN) feedback is inefficient in massive galaxies, due to too low a wind velocity, a ‘bump’ in the lu- minosity function appears due to the overproduction of luminous galaxies. Invoking efficient feedback in these massive galaxies (either through the use of a momentum-driven wind prescription in which the energy in the winds increases with galaxy mass, a top-heavy IMF for star formation at high pressure or AGN feedback) re- sults in a monotonically decreasing LF. Dust attenuation, implemented by assuming that the optical depth scales with the metallicity-weighted column density, is more efficient in galaxies with less efficient feedback, as there is more (high metallicity) gas available in such galaxies. With efficient feedback, little gas is left in the galaxies, reducing the effect of attenuation to close to zero. In low luminosity galaxies the column densities and optical depth are in general lower. From virtual observations we find that the LF as obtained using techniques used for observations results in LFs very similar to those obtained directly from halo catalogues. Nevertheless, for large PSFs (corresponding to typical ground-based seeing conditions) very deep observations may result in shallower faint-end slopes of the LF, due to the preferential removal of low-surface brightness galaxies.
Chapter 5: Variations in integrated galactic initial mass functions due to sampling method and cluster mass function
Stars are thought to be formed predominantly in clusters. The star clusters are formed according to a cluster initial mass function (CMF) similar to the stellar initial mass function (IMF). Both the IMF and the CMF can be approximated by (broken) power-laws, which favour low-mass objects. The numerous low-mass clusters will lack high mass stars compared to the underlying IMF, since the most massive star cannot be more massive than its host cluster. If the integrated galactic initial mass function (IGIMF, i.e. the total stellar mass function of all stars in a galaxy) originates from stars formed in star clusters, the IGIMF could be steeper than the IMF in clusters. We investigate how well constrained this steepening is and how it depends on the choice of sampling method and CMF. We investigate the observability of the IGIMF effect in terms of galaxy photometry and metallicities. We study various ways to sample the stellar IMF within star clusters and build up the IGIMF from these clusters. We compare analytic sampling to several implementations of random sampling of the IMF and different CMFs. We implement different IGIMFs into the galev evolutionary synthesis package to obtain colours and metallicities for galaxies. Choosing different ways of sampling the IMF results in different IGIMFs. Depending on the lower cluster mass limit and the slope of the cluster mass function, the steepening varies between very strong and negligible. We find the size of the effect is continuous as a function of the power-law slope of the CMF if the CMF extends to masses smaller than the maximum stellar mass. The number of O-stars detected by GAIA will, if some uncertain factors are better understood, help to judge the importance of the IGIMF effect. The impact of different IGIMFs on integrated galaxy photometry is small, within the intrinsic scatter of observed galaxies. Observations of gas fractions and metallicities could rule out at least the most extreme sampling methods, if other sources of error are sufficiently understood. As we still do not understand the details of star formation and the sampling of the stellar IMF in clusters, one sampling method cannot be favoured over another. Also, the CMF at very low cluster masses is not well constrained observationally. These uncertainties therefore need to be taken into account when using an IGIMF, with severe implications for galaxy evolution models and interpretations of galaxy observations.
The summary is in Dutch. Deze zou leesbaar moeten zijn voor een groot publiek. Ik tracht hierin het onderzoek beschreven in dit proefschrift uit te leggen, in simpelere termen.