About me/CV

I am a NWO Veni postdoc and University Lecturer at Leiden Observatory, and collaborate with John Peacock at the Royal Observatory in Edinburgh. Until October 2017, I was a TAC Fellow working at both UC Berkeley's astronomy department and the Lawrence Berkeley Lab. I'm interested in the formation of galaxies and the clustering of galaxies, haloes and matter, as well as the links between all of these subjects. Most of my work is based on hydro-cosmological simulations of galaxy formation, and semi-analytical models run on N-body simulations.

I started studying astronomy in Leiden in 2004, receiving my MSc degree in 2010. I obtained my PhD in 2014 after working with both Joop Schaye at Leiden Observatory, the Netherlands, as a Huygens Fellow, and Simon White at the Max Planck Institute for Astrophysics in Garching, Germany, as a CosmoComp Fellow. If you're interested, you can find my thesis here.

My full CV is available here.


While my astronomical interests are broad, my own work mostly revolves around the formation of galaxies, their evolution, and their connection to large-scale structure. I approach these topics from the theoretical side and use numerical simulations (both collisionless and hydrodynamical) to explore, for example, the effects that galaxy formation can have on the distribution of matter, and conversely what measurements of the distribution of galaxies can tell us about the physical processes that play a role in their formation. Recently, I've shown that there is a very strong correlation between the mean baryon fraction of large groups of galaxies and the suppression of structure due to galaxy formation on large scales.

The parameters that describe our Universe, including the abundances of dark matter and dark energy, determine the statistical distribution of the galaxies that inhabit it. By measuring this distribution — that is, the clustering of galaxies — we can therefore learn a lot about our Universe as a whole. However, as the precision of our measurements increases, so too must the precision of the theoretical models we use to interpret these. Consequently, understanding the complex details of how processes involved in the formation of galaxies (such as supernova and AGN feedback) affect the clustering of matter is of growing importance. A large part of my research is focused on this interplay between galaxy evolution and the large-scale properties of the Universe.

I am also interested in the future of our Universe. Together with John Peacock, I am looking into how cosmological parameters, and specifically dark energy, will influence star formation in the coming (billions of) years. This may influence how we view the cosmological coincidence problem(s).

PhD thesis

Below you will find links to my thesis, separate chapters, cover and propositions.


Since Spring 2019, I have been teaching a new computational Master's course called Numerical Recipes for Astrophysics. The course is aimed at providing students with a better understanding of algorithms and how to approach the computational problems that they might encounter during their research. Files and announcements can be found on Brightspace as well as here.

During the Fall semesters of 2017-2018 and 2018-2019 I taught the 3rd-year astronomy Bachelor's course Radiative Processes. The goals of this course are to understand how light interacts with matter and how matter can convert different kinds of energy into light, and how we extract physical information about the the object that emits this light. Here are some movies I made to show how relativity influences the propagation of electric/magnetic fields/light:

Numerical Recipes for
Astrophysics - Fall 2020

Lectures are on Thursdays, 14:15 to 16:00 on Kaltura, starting September 3rd. Exercise classes are on Mondays following lectures, from 11:15 to 13:00, also on Kaltura. A full, up-to-date schedule can be found on the Brightspace page. We follow the book "Numerical Recipes" (3rd edition); it is not required, but it generally explores topics more deeply than we do in the course. Code examples in the book are in C, but these are unimportant compared to the discussion of the theory, and so the book is still a useful source of additional information if you code in Python.

Annotated lecture slides are posted on Brightspace after every lecture, as well as a recording of the lecture. Before every lecture, the to-be-annotated slides will also be posted there, so you can use them to make your own notes.

The tutorials are not mandatory but highly advised (experience shows that not attending the tutorials has a major negative impact on your grade), and we encourage collaboration.

There will be four larger hand-in exercises, one every three lectures. The hand-in exercises have to be solved individually, and count for 40% of your grade total.

The final (theory) exam will be on January 6th, 14:00, and will count for 60% of your final grade. The exam will be on paper and not require you to write any code. A practice exam will be posted on Brightspace well in advance.

If you have any questions about a lecture or an exercise after the class is over, the TAs and myself are happy to answer them. Please use the discussion boards on the Brightspace page to ask your questions, so all students can benefit. For more personal questions, you can of course email us.


Selected papers I have worked on are listed below, including abstracts, in reverse chronological order. For a full list of my publications, including citations, see here.

Exploring the effects of galaxy formation on matter clustering through a library of simulation power spectra
Marcel P. van Daalen, Ian G. McCarthy and Joop Schaye
2020, published in MNRAS [ADS] [astro-ph]
Abstract: Upcoming weak lensing surveys require a detailed theoretical understanding of the matter power spectrum in order to derive accurate and precise cosmological parameter values. While galaxy formation is known to play an important role, its precise effects are currently unknown. We present a set of 92 matter power spectra from the OWLS, cosmo-OWLS and BAHAMAS simulation suites, including different LCDM cosmologies, neutrino masses, subgrid prescriptions and AGN feedback strengths. We conduct a detailed investigation of the dependence of the relative difference between the total matter power spectra in hydrodynamical and collisionless simulations on the effectiveness of stellar and AGN feedback, cosmology and redshift. The strength of AGN feedback can greatly affect the power on a range of scales, while a lack of stellar feedback can greatly increase the effectiveness of AGN feedback on large scales. We also examine differences in the initial conditions of hydrodynamic and N-body simulations that can lead to a ∼ 1% discrepancy in the large-scale power, and furthermore show our results to be insensitive to cosmic variance. We present an empirical model capable of predicting the effect of galaxy formation on the matter power spectrum at z=0 to within 1% for k<1h/Mpc, given only the mean baryon fraction in galaxy groups. Differences in group baryon fractions can also explain the quantitative disagreement between predictions from the literature. All total and dark matter only power spectra in this library will be made publicly available at powerlib.strw.leidenuniv.nl.

A cross-correlation-based estimate of the galaxy luminosity function
Marcel P. van Daalen, Martin White
2018, published in MNRAS [ADS] [astro-ph]
Abstract: We extend existing methods for using cross-correlations to derive redshift distributions for photometric galaxies, without using photometric redshifts. The model presented in this paper simultaneously yields highly accurate and unbiased redshift distributions and, for the first time, redshift-dependent luminosity functions, using only clustering information and the apparent magnitudes of the galaxies as input. In contrast to many existing techniques for recovering unbiased redshift distributions, the output of our method is not degenerate with the galaxy bias b(z), which is achieved by modelling the shape of the luminosity bias. We successfully apply our method to a mock galaxy survey and discuss improvements to be made before applying our model to real data.

The galaxy correlation function as a constraint on galaxy formation physics
Marcel P. van Daalen, Bruno M. B. Henriques, Raul E. Angulo, Simon D. M. White
2016, published in MNRAS [ADS] [astro-ph]
Abstract: We introduce methods which allow observed galaxy clustering to be used together with observed luminosity or stellar mass functions to constrain the physics of galaxy formation. We show how the projected two-point correlation function of galaxies in a large semi-analytic simulation can be estimated to better than ~10% using only a very small subsample of the subhalo merger trees. This allows measured correlations to be used as constraints in a Monte Carlo Markov Chain exploration of the astrophysical and cosmological parameter space. An important part of our scheme is an analytic profile which captures the simulated satellite distribution extremely well out to several halo virial radii. This is essential to reproduce the correlation properties of the full simulation at intermediate separations. As a first application, we use low-redshift clustering and abundance measurements to constrain a recent version of the Munich semi-analytic model. The preferred values of most parameters are consistent with those found previously, with significantly improved constraints and somewhat shifted "best" values for parameters that primarily affect spatial distributions. Our methods allow multi-epoch data on galaxy clustering and abundance to be used as joint constraints on galaxy formation. This may lead to significant constraints on cosmological parameters even after marginalising over galaxy formation physics.

Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations
Marco Velliscig, Marcello Cacciato, Joop Schaye, Henk Hoekstra, Richard G. Bower, Robert A. Crain, Marcel P. van Daalen, Michelle Furlong, Ian G. McCarthy, Matthieu Schaller, Tom Theuns
2015, published in MNRAS [ADS] [astro-ph]
Abstract: We report results for the alignments of galaxies in the EAGLE and cosmo-OWLS simulations as a function of galaxy separation and halo mass. The combination of these hydro-cosmological simulations enables us to span four orders of magnitude in halo mass (10.7<log10(M200/[M☉/h])<15) and a large range of separations (−1<log10(r/[Mpc/h])<2). We focus on two classes of alignments: the orientations of galaxies with respect to either the directions to, or the orientations of, surrounding galaxies. We find that the strength of the alignment is a strongly decreasing function of the distance between galaxies. The orientation-direction alignment can remain significant up to ~100 Mpc, for galaxies hosted by the most massive haloes in our simulations. Galaxies hosted by more massive subhaloes show stronger alignment. At a fixed halo mass, more aspherical or prolate galaxies exhibit stronger alignments. The spatial distribution of satellites is anisotropic and significantly aligned with the major axis of the main host halo. The major axis of satellite galaxies, when all stars are considered, are preferentially aligned towards the centre of the main host halo. The predicted projected direction-orientation alignment, ϵg+(rp), is in broad agreement with recent observations when only stars within the typical observable extent of a galaxy are used to define galaxy orientations. We find that the orientation-orientation alignment is weaker than the orientation-direction alignment on all scales. Overall, the strength of galaxy alignments depends strongly on the subset of stars that are used to measure the orientations of galaxies and it is always weaker than the alignment of the dark matter haloes. Thus, alignment models that use halo orientation as a direct proxy for galaxy orientation will overestimate the impact of intrinsic alignments on weak lensing analyses.

The alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations
Marco Velliscig, Marcello Cacciato, Joop Schaye, Robert A. Crain, Richard G. Bower, Marcel P. van Daalen, Claudio Dalla Vecchia, Carlos S. Frenk, Michelle Furlong, Ian G. McCarthy, Matthieu Schaller, Tom Theuns
2015, published in MNRAS [ADS] [astro-ph]
Abstract: We report the alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations. The combination of these state-of-the-art hydro-cosmological simulations enables us to span four orders of magnitude in halo mass (11<log10(M200/[M☉/h])<15), a wide radial range (−2.3<log10(r/[Mpc/h])<1.3) and redshifts 0<z<1. The shape parameters of the dark matter, stellar and hot gas distributions follow qualitatively similar trends: they become more aspherical (and triaxial) with increasing halo mass, radius and redshift. We measure the misalignment of the baryonic components (hot gas and stars) of galaxies with their host halo as a function of halo mass, radius, redshift, and galaxy type (centrals vs satellites and early- vs late-type). Overall, galaxies align well with local distribution of the total (mostly dark) matter. However, the stellar distributions on galactic scales exhibit a median misalignment of about 45-50 degrees with respect to their host haloes. This misalignment is reduced to 25-30 degrees in the most massive haloes (13<log10(M200/[M☉/h])<15). Half of the disc galaxies in the EAGLE simulations have a misalignment angle with respect to their host haloes larger than 40 degrees. We present fitting functions and tabulated values for the probability distribution of galaxy-halo misalignment to enable a straightforward inclusion of our results into models of galaxy formations based on purely collisionless N-body simulations.

The contributions of matter inside and outside of haloes to the matter power spectrum
Marcel P. van Daalen, Joop Schaye
2015, published in MNRAS [ADS] [astro-ph]
Abstract: Halo-based models have been successful in predicting the clustering of matter. However, the validity of the postulate that the clustering is fully determined by matter inside haloes remains largely untested, and it is not clear a priori whether non-virialised matter might contribute significantly to the non-linear clustering signal. Here, we investigate the contribution of haloes to the matter power spectrum as a function of both scale and halo mass by combining a set of cosmological N-body simulations to calculate the contributions of different spherical overdensity regions, Friends-of-Friends (FoF) groups and matter outside haloes to the power spectrum. We find that matter inside spherical overdensity regions of size R200,mean cannot account for all power for 1<k<100 h/Mpc, regardless of the minimum halo mass. At most, it accounts for 95% of the power (k>20 h/Mpc). For 2<k<10 h/Mpc, haloes with mass M200,mean<10^11 M☉/h contribute negligibly to the power spectrum, and our results appear to be converged with decreasing halo mass. When haloes are taken to be regions of size R200,crit, the amount of power unaccounted for is larger on all scales. Accounting also for matter inside FoF groups but outside R200,mean increases the contribution of halo matter on most scales probed here by 5-15%. Matter inside FoF groups with M200,mean>10^9 M☉/h accounts for essentially all power for 3<k<100 h/Mpc. We therefore expect halo models that ignore the contribution of matter outside R200,mean to overestimate the contribution of haloes of any mass to the power on small scales (k>1 h/Mpc).

The clustering of baryonic matter. II: halo model and hydrodynamic simulations
Cosimo Fedeli, Elisabetta Semboloni, Marco Velliscig, Marcel P. van Daalen, Joop Schaye, Henk Hoekstra
2014, published in JCAP [ADS] [astro-ph]
Abstract: We recently developed a generalization of the halo model in order to describe the spatial clustering properties of each mass component in the Universe, including hot gas and stars. In this work we discuss the complementarity of the model with respect to a set of cosmological simulations including hydrodynamics of different kinds. We find that the mass fractions and density profiles measured in the simulations do not always succeed in reproducing the simulated matter power spectra, the reason being that the latter encode information from a much larger range in masses than that accessible to individually resolved structures. In other words, this halo model allows one to extract information on the growth of structures from the spatial clustering of matter, that is complementary with the information coming from the study of individual objects. We also find a number of directions for improvement of the present implementation of the model, depending on the specific application one has in mind. The most relevant one is the necessity for a scale dependence of the bias of the diffuse gas component, which will be interesting to test with future detections of the Warm-Hot Intergalactic Medium. This investigation confirms the possibility to gain information on the physics of galaxy and cluster formation by studying the clustering of mass, and our next work will consist of applying the halo model to use future high-precision cosmic shear surveys to this end.

The impact of galaxy formation on the total mass, profiles and abundance of haloes
Marco Velliscig, Marcel P. van Daalen, Joop Schaye, Ian G. McCarthy, Marcello Cacciato, Amandine M. C. Le Brun, Claudio Dalla Vecchia
2014, published in MNRAS [ADS] [astro-ph]
Abstract: We use cosmological hydrodynamical simulations to investigate how the inclusion of physical processes relevant to galaxy formation (star formation, metal-line cooling, stellar winds, supernovae and feedback from Active Galactic Nuclei, AGN) change the properties of haloes, over four orders of magnitude in mass. We find that gas expulsion and the associated dark matter (DM) expansion induced by supernova-driven winds are important for haloes with masses M_200<10^13 M☉/h, lowering their masses by up to 20% relative to a DM-only model. AGN feedback, which is required to prevent overcooling, has a significant impact on halo masses all the way up to cluster scales (M_200~10^15 M☉/h). Baryonic physics changes the total mass profiles of haloes out to several times the virial radius, a modification that cannot be captured by a change in the halo concentration. The decrease in the total halo mass causes a decrease in the halo mass function of about 20%. This effect can have important consequences for abundance matching technique as well as for most semi-analytic models of galaxy formation. We provide analytic fitting formulae, derived from simulations that reproduce the observed baryon fractions, to correct halo masses and mass functions from DM-only simulations. The effect of baryonic physics (AGN feedback in particular) on cluster number counts is about as large as changing the cosmology from WMAP7 to Planck, even when a moderately high mass limit of M_500~10^14 M☉/h is adopted. Thus, for precision cosmology the effects of baryons must be accounted for.

The impact of baryonic processes on the two-point correlation functions of galaxies, subhaloes and matter
Marcel P. van Daalen, Joop Schaye, Ian G. McCarthy, C. M. Booth, Claudio Dalla Vecchia
2014, published in MNRAS [ADS] [astro-ph]
Abstract: The observed clustering of galaxies and the cross-correlation of galaxies and mass provide important constraints on both cosmology and models of galaxy formation. Even though the dissipation and feedback processes associated with galaxy formation are thought to affect the distribution of matter, essentially all models used to predict clustering data are based on collisionless simulations. Here, we use large hydrodynamical simulations to investigate how galaxy formation affects the autocorrelation functions of galaxies and subhaloes, as well as their cross-correlation with matter. We show that the changes due to the inclusion of baryons are not limited to small scales and are even present in samples selected by subhalo mass. Samples selected by subhalo mass cluster ~10% more strongly in a baryonic run on scales r>1Mpc/h, and this difference increases for smaller separations. While the inclusion of baryons boosts the clustering at fixed subhalo mass on all scales, the sign of the effect on the cross-correlation of subhaloes with matter can vary with radius. We show that the large-scale effects are due to the change in subhalo mass caused by the strong feedback associated with galaxy formation and may therefore not affect samples selected by number density. However, on scales r<r_vir significant differences remain after accounting for the change in subhalo mass. We conclude that predictions for galaxy-galaxy and galaxy-mass clustering from models based on collisionless simulations will have errors greater than 10% on sub-Mpc scales, unless the simulation results are modified to correctly account for the effects of baryons on the distributions of mass and satellites.

The effects of halo alignment and shape on the clustering of galaxies
Marcel P. van Daalen, Raul E. Angulo, Simon D. M. White
2012, published in MNRAS [ADS] [astro-ph]
Abstract: We investigate the effects of halo shape and its alignment with larger scale structure on the galaxy correlation function. We base our analysis on the galaxy formation models of Guo et al. (2011), run on the Millennium Simulations. We quantify the importance of these effects by randomizing the angular positions of satellite galaxies within haloes, either coherently or individually, while keeping the distance to their respective central galaxies fixed. We find that the effect of disrupting the alignment with larger scale structure is a ~2 per cent decrease in the galaxy correlation function around r=1.8 Mpc/h. We find that sphericalizing the ellipsoidal distributions of galaxies within haloes decreases the correlation function by up to 20 per cent for r<1 Mpc/h and increases it slightly at somewhat larger radii. Similar results apply to power spectra and redshift-space correlation functions. Models based on the Halo Occupation Distribution, which place galaxies spherically within haloes according to a mean radial profile, will therefore significantly underestimate the clustering on sub-Mpc scales. In addition, we find that halo assembly bias, in particular the dependence of clustering on halo shape, propagates to the clustering of galaxies. We predict that this aspect of assembly bias should be observable through the use of extensive group catalogues.

Quantifying the effect of baryon physics on weak lensing tomography
Elisabetta Semboloni, Henk Hoekstra, Joop Schaye, Marcel P. van Daalen, Ian G. McCarthy
2011, published in MNRAS [ADS] [astro-ph]
Abstract: We use matter power spectra from cosmological hydrodynamic simulations to quantify the effect of baryon physics on the weak gravitational lensing shear signal. The simulations consider a number of processes, such as radiative cooling, star formation, supernovae and feedback from active galactic nuclei (AGN). Van Daalen et al. (2011) used the same simulations to show that baryon physics, in particular the strong feedback that is required to solve the overcooling problem, modifies the matter power spectrum on scales relevant for cosmological weak lensing studies. As a result, the use of power spectra from dark matter simulations can lead to significant biases in the inferred cosmological parameters. We show that the typical biases are much larger than the precision with which future missions aim to constrain the dark energy equation of state, w_0. For instance, the simulation with AGN feedback, which reproduces X-ray and optical properties of groups of galaxies, gives rise to a ∼40% bias in w_0. We also explore the effect of baryon physics on constraints on Ω_m, σ_8, the running of the spectral index, the mass of the neutrinos and models of warm dark matter. We demonstrate that the modification of the power spectrum is dominated by groups and clusters of galaxies, the effect of which can be modelled. We consider an approach based on the popular halo model and show that simple modifications can capture the main features of baryonic feedback. Despite its simplicity, we find that our model, when calibrated on the simulations, is able to reduce the bias in w_0 to a level comparable to the size of the statistical uncertainties for a Euclid-like mission. While observations of the gas and stellar fractions as a function of halo mass can be used to calibrate the model, hydrodynamic simulations will likely still be needed to extend the observed scaling relations down to halo masses of 10^12 M☉/h.

The effects of galaxy formation on the matter power spectrum: A challenge for precision cosmology
Marcel P. van Daalen, Joop Schaye, C. M. Booth, Claudio Dalla Vecchia
2011, published in MNRAS [ADS] [astro-ph]
Abstract: Upcoming weak lensing surveys, such as LSST, EUCLID, and WFIRST, aim to measure the matter power spectrum with unprecedented accuracy. In order to fully exploit these observations, models are needed that, given a set of cosmological parameters, can predict the non-linear matter power spectrum at the level of 1% or better for scales corresponding to comoving wave numbers 0.1<k<10 h/Mpc. We have employed the large suite of simulations from the OWLS project to investigate the effects of various baryonic processes on the matter power spectrum. In addition, we have examined the distribution of power over different mass components, the back-reaction of the baryons on the CDM, and the evolution of the dominant effects on the matter power spectrum. We find that single baryonic processes are capable of changing the power spectrum by up to several tens of per cent. Our simulation that includes AGN feedback, which we consider to be our most realistic simulation as, unlike those used in previous studies, it has been shown to solve the overcooling problem and to reproduce optical and X-ray observations of groups of galaxies, predicts a decrease in power relative to a dark matter only simulation ranging, at z=0, from 1% at k~0.3 h/Mpc to 10% at k~1 h/Mpc and to 30% at k~10 h/Mpc. This contradicts the naive view that baryons raise the power through cooling, which is the dominant effect only for k>70 h/Mpc. Therefore, baryons, and particularly AGN feedback, cannot be ignored in theoretical power spectra for k>0.3 h/Mpc. It will thus be necessary to improve our understanding of feedback processes in galaxy formation, or at least to constrain them through auxiliary observations, before we can fulfil the goals of upcoming weak lensing surveys.


  • Room OG 538
    Leiden Observatory
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  • +31 71 527 5812
  • daalen [at] strw.leidenuniv.nl