ALBERT

Description: Large surveys have shown that red galaxies are preferentially aligned with their haloes, while blue galaxies have a more isotropic distribution. Since haloes generally align with their filaments, this introduces a bias in the measurement of the cosmic shear from weak lensing. It is therefore vitally important to understand why this difference arises. We explore the stability of different disc orientations within triaxial haloes. We show that, in the absence of gas, the disc orientation is most stable when its spin is along the minor axis of the halo. Instead when gas cools on to a disc, it is able to form in almost arbitrary orientation, including off the main planes of the halo (but avoiding an orientation perpendicular to the halo's intermediate axis). Substructure helps gasless galaxies reach alignment with the halo faster, but has less effect on galaxies when gas is cooling on to the disc. Our results provide a novel and natural interpretation for why red, gas poor galaxies are preferentially aligned with their halo, while blue, star-forming, galaxies have nearly random orientations, without requiring a connection between galaxies’ current star formation rate and their merger history.

Description: We use a semi-analytical model for the substructure of dark matter haloes to assess the too big to fail (TBTF) problem. The model accurately reproduces the average subhalo mass and velocity functions, as well as their halo-to-halo variance, in N -body simulations. We construct thousands of realizations of Milky Way (MW)-size host haloes, allowing us to investigate the TBTF problem with unprecedented statistical power. We examine the dependence on host halo mass and cosmology, and explicitly demonstrate that a reliable assessment of TBTF requires large samples of hundreds of host haloes. We argue that previous statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using only data on the nine known MW satellite galaxies with V max  〉 15 km s –1 , demonstrate that $1.4^{+3.3}_{-1.1}$ per cent of MW-size host haloes have a subhalo population in statistical agreement with that of the MW. However, when using data on the MW satellite galaxies down to V max  = 8 km s –1 , this MW consistent fraction plummets to 〈5  x  10 –4 (at 68 per cent confidence level). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8 km s –1 , then the maximum circular velocities of MW satellites are utterly inconsistent with cold dark matter predictions, unless baryonic effects can drastically increase the spread in V max values of satellite galaxies compared to that of their subhaloes.

Description: Correlations between the star formation rates (SFRs) of nearby galaxies (so-called galactic conformity ) have been observed for projected separations up to 4 Mpc, an effect not predicted by current semi-analytic models. We investigate correlations between the mass accretion rates (d M vir /d t ) of nearby haloes as a potential physical origin for this effect. We find that pairs of host haloes ‘know about’ each others’ assembly histories even when their present-day separation is greater than thirty times the virial radius of either halo. These distances are far too large for direct interaction between the haloes to explain the correlation in their d M vir /d t . Instead, halo pairs at these distances reside in the same large-scale tidal environment, which regulates d M vir /d t for both haloes. Larger haloes are less affected by external forces, which naturally gives rise to a mass dependence of the halo conformity signal. SDSS measurements of galactic conformity exhibit a qualitatively similar dependence on stellar mass, including how the signal varies with distance. Based on the expectation that halo accretion and galaxy SFR are correlated, we predict the scale-, mass- and redshift-dependence of large-scale galactic conformity, finding that the signal should drop to undetectable levels by z 1. These predictions are testable with current surveys to z ~ 1; confirmation would establish a strong correlation between dark matter halo accretion rate and central galaxy SFR.

Description: We propose that one of the sources in the recently detected system CR7 by Sobral et al. through spectrophotometric measurements at z = 6.6 harbours a direct collapse black hole (DCBH). We argue that the LW radiation field required for direct collapse in source A is provided by sources B and C. By tracing the LW production history and star formation rate over cosmic time for the halo hosting CR7 in a CDM universe, we demonstrate that a DCBH could have formed at z ~ 20. The spectrum of source A is well fit by nebular emission from primordial gas around a BH with MBH ~4.4 x 10 6 M accreting at a 40 per cent of the Eddington rate, which strongly supports our interpretation of the data. Combining these lines of evidence, we argue that CR7 might well be the first DCBH candidate.

Description: We compare subhalo mass and velocity functions obtained from different simulations with different subhalo finders among each other, and with predictions from the new semi-analytical model presented in Paper I . We find that subhalo mass functions (SHMFs) obtained using different subhalo finders agree with each other at the level of ~20 per cent, but only at the low-mass end. At the massive end, subhalo finders that identify subhaloes based purely on density in configuration space dramatically underpredict the subhalo abundances by more than an order of magnitude. These problems are much less severe for subhalo velocity functions (SHVFs), indicating that they arise from issues related to assigning masses to the subhaloes, rather than from detecting them. Overall the predictions from the semi-analytical model are in excellent agreement with simulation results obtained using the more advanced subhalo finders that use information in six-dimensional phase-space. In particular, the model accurately reproduces the slope and host-mass-dependent normalization of both the subhalo mass and velocity functions. We find that the SHMFs and SHVFs have power-law slopes of 0.86 and 2.77, respectively, significantly shallower than what has been claimed in several studies in the literature.

Description: We investigate the ability of current implementations of galaxy group finders to recover colour-dependent halo occupation statistics. To test the fidelity of group catalogue inferred statistics, we run three different group finders used in the literature over a mock that includes galaxy colours in a realistic manner. Overall, the resulting mock group catalogues are remarkably similar, and most colour-dependent statistics are recovered with reasonable accuracy. However, it is also clear that certain systematic errors arise as a consequence of correlated errors in group membership determination, central/satellite designation, and halo mass assignment. We introduce a new statistic, the halo transition probability (HTP), which captures the combined impact of all these errors. As a rule of thumb, errors tend to equalize the properties of distinct galaxy populations (i.e. red versus blue galaxies or centrals versus satellites), and to result in inferred occupation statistics that are more accurate for red galaxies than for blue galaxies. A statistic that is particularly poorly recovered from the group catalogues is the red fraction of central galaxies as a function of halo mass. Group finders do a good job in recovering galactic conformity, but also have a tendency to introduce weak conformity when none is present. We conclude that proper inference of colour-dependent statistics from group catalogues is best achieved using forward modelling (i.e. running group finders over mock data) or by implementing a correction scheme based on the HTP, as long as the latter is not too strongly model dependent.

Description: The connection between galaxies and dark matter haloes is often inferred from data using probabilistic models, such as the halo occupation distribution (HOD). Conventional HOD formulations assume that only halo mass governs the galaxy–halo connection. Violations of this assumption, known as galaxy assembly bias , threaten the HOD programme. We introduce decorated HODs , a new, flexible class of models designed to account for assembly bias. Decorated HODs minimally expand the parameter space and maximize the independence between traditional and novel HOD parameters. We use decorated HODs to quantify the influence of assembly bias on clustering and lensing statistics. For SDSS-like samples, the impact of assembly bias on galaxy clustering can be as large as a factor of 2 on r  ~ 200 kpc scales and ~15 per cent in the linear regime. Assembly bias can either enhance or diminish clustering on large scales, but generally increases clustering on scales r 1 Mpc. We performed our calculations with halotools , an open-source, community-driven python package for studying the galaxy–halo connection ( http://halotools.readthedocs.org ). We conclude by describing the use of decorated HODs to treat assembly bias in otherwise conventional likelihood analyses.

Description: The empirical model of Lu et al. is updated with recent data of galaxy stellar mass functions (SMFs). The model predicts that the slope of galaxy SMFs at z  〉 2 should be quite steep at the low-mass end, beyond the current detection limit, and it is a strong prediction that can be tested against future observations. The model is used to investigate the galaxy star formation and assembly or merger histories in detail. Most of the stars in cluster centrals, corresponding to brightest cluster galaxies in observations, formed earlier than z   2 but have been assembled much later. Typically, they have experienced 5 major mergers since their star formation was quenched. Milky Way mass galaxies have had on-going star formation without significant mergers since z   2, and are thus free of significant (classic) bulges produced by major mergers. Dwarf galaxies in haloes with M h  〈 10 11 h –1 M or M *  〈 10 9 M have experienced a star formation burst at z  〉 2, followed by a nearly constant star formation rate after z  = 1, and the stellar age decreases with stellar mass, contrary to the ‘downsizing’ trend for more massive galaxies. Major mergers are not uncommon during the early burst phase and may result in the formation of old spheroids in dwarf galaxies. We also characterize the stellar population of halo stars in different haloes.

Description: We simultaneously constrain cosmology and galaxy bias using measurements of galaxy abundances, galaxy clustering and galaxy–galaxy lensing taken from the Sloan Digital Sky Survey. We use the conditional luminosity function (which describes the halo occupation statistics as a function of galaxy luminosity) combined with the halo model (which describes the non-linear matter field in terms of its halo building blocks) to describe the galaxy–dark matter connection. We explicitly account for residual redshift-space distortions in the projected galaxy–galaxy correlation functions, and marginalize over uncertainties in the scale dependence of the halo bias and the detailed structure of dark matter haloes. Under the assumption of a spatially flat, vanilla cold dark matter (CDM) cosmology, we focus on constraining the matter density, m , and the normalization of the matter power spectrum, 8 , and we adopt 7-year Wilkinson Microwave Anisotropy Probe ( WMAP 7) priors for the spectral index, n , the Hubble parameter, h , and the baryon density, b . We obtain that m = 0.278 + 0.023 – 0.026 and 8 = 0.763 + 0.064 – 0.049 (95 per cent CL). These results are robust to uncertainties in the radial number density distribution of satellite galaxies, while allowing for non-Poisson satellite occupation distributions results in a slightly lower value for 8 (0.744 + 0.056 – 0.047 ). These constraints are in excellent agreement (at the 1 level) with the cosmic microwave background constraints from WMAP . This demonstrates that the use of a realistic and accurate model for galaxy bias, down to the smallest non-linear scales currently observed in galaxy surveys, leads to results perfectly consistent with the vanilla CDM cosmology.

Description: We quantify the accuracy with which the cosmological parameters characterizing the energy density of matter ( m ), the amplitude of the power spectrum of matter fluctuations ( 8 ), the energy density of neutrinos ( ) and the dark energy equation of state ( w 0 ) can be constrained using data from large galaxy redshift surveys. We advocate a joint analysis of the abundance of galaxies, galaxy clustering, and the galaxy–galaxy weak-lensing signal in order to simultaneously constrain the halo occupation statistics (i.e. galaxy bias) and the cosmological parameters of interest. We parametrize the halo occupation distribution of galaxies in terms of the conditional luminosity function and use the analytical framework of the halo model described in Cacciato et al. (our companion Paper III), to predict the relevant observables. By performing a Fisher matrix analysis, we show that a joint analysis of these observables, even with the precision with which they are currently measured from the Sloan Digital Sky Survey, can be used to obtain tight constraints on the cosmological parameters, fully marginalized over uncertainties in galaxy bias. We demonstrate that the cosmological constraints from such an analysis are nearly uncorrelated with the halo occupation distribution constraints, thus, minimizing the systematic impact of any imperfections in modelling the halo occupation statistics on the cosmological constraints. In fact, we demonstrate that the constraints from such an analysis are both complementary to and competitive with existing constraints on these parameters from a number of other techniques, such as cluster abundances, cosmic shear and/or baryon acoustic oscillations, thus paving the way to test the concordance cosmological model.