AGN-driven helium reionization and the incidence of extended HeIII regions at redshift z>3

We use hydrodynamic simulations post-processed with the radiative-transfer code RADAMESH to assess recent claims that the low HeII opacity observed in z>3 quasar spectra may be incompatible with models of HeII reionization driven by the observed population of active galactic nuclei (AGNs). In particular, building upon our previous work, we consider an early population of sources and start the radiative-transfer calculation at redshifts z>=5. Our model faithfully reproduces the emissivity of optically selected AGNs as inferred from measurements of their luminosity function. We find that HeII reionization is very extended in redshift ({\Delta} z>=2) and highly spatially inhomogeneous. In fact, mock spectra extracted from the simulations show a large variability in the evolution of the HeII effective optical depth within chunks of size {\Delta} z=0.04. Regions with low opacity ({\tau}^{eff}_{HeII}<3) can be found at high redshift, in agreement with the most recent observations of UV-transmitting quasars. At the highest redshift currently probed by observations (z~3.4), our updated model predicts a much lower HeII effective optical depth than previous simulations in the literature relieving most of the tension with the current data, that, however, still persists at about the (Gaussian) 1{\sigma} to 2{\sigma} level. Given the very small number of observed lines of sight, our analysis indicates that current data cannot rule out a purely AGN-driven scenario with high statistical significance.Comment: 12 pages, 8 figures. Matches version accepted for publication in MNRA

The formation of CDM haloes II: collapse time and tides

We use two cosmological simulations of structure formation in the LambdaCDM scenario to study the evolutionary histories of dark-matter haloes and to characterize the Lagrangian regions from which they form. We focus on haloes identified at redshift z_id=0 and show that the classic ellipsoidal collapse model systematically overestimates their collapse times. If one imposes that halo collapse takes place at z_id, this model requires starting from a significantly lower linear density contrast than what is measured in the simulations at the locations of halo formation. We attempt to explain this discrepancy by testing two key assumptions of the model. First, we show that the tides felt by collapsing haloes due to the surrounding large-scale structure evolve non-linearly. Although this effect becomes increasingly important for low-mass haloes, accounting for it in the ellipsoidal collapse model only marginally improves the agreement with N-body simulations. Second, we track the time evolution of the physical volume occupied by forming haloes and show that, after turnaround, it generally stabilizes at a well-defined redshift, z_c>z_id, contrary to the basic assumption of extended Press-Schechter theory based on excursion sets. We discuss the implications of this result for understanding the origin of the mass-dependence and scatter in the linear threshold for halo formation. Finally, we show that, when tuned for collapse at z_c, a modified version of the ellipsoidal collapse model that also accounts for the triaxial nature of protohaloes predicts their linear density contrast in an unbiased way.Comment: 15 pages, 11 figures, MNRAS in pres

Properties of Dark Matter Haloes in Clusters, Filaments, Sheets and Voids

Using a series of high-resolution N-body simulations of the concordance cosmology we investigate how the formation histories, shapes and angular momenta of dark-matter haloes depend on environment. We first present a classification scheme that allows to distinguish between haloes in clusters, filaments, sheets and voids in the large-scale distribution of matter. This method is based on a local-stability criterion for the orbits of test particles and closely relates to the Zel'dovich approximation. Applying this scheme to our simulations we then find that: i) Mass assembly histories and formation redshifts strongly depend on environment for haloes of mass M<M* (haloes of a given mass tend to be older in clusters and younger in voids) and are independent of it for larger masses; ii) Low-mass haloes in clusters are generally less spherical and more oblate than in other regions; iii) Low-mass haloes in clusters have a higher median spin than in filaments and present a more prominent fraction of rapidly spinning objects; we identify recent major mergers as a likely source of this effect. For all these relations, we provide accurate functional fits as a function of halo mass and environment. We also look for correlations between halo-spin directions and the large-scale structures: the strongest effect is seen in sheets where halo spins tend to lie within the plane of symmetry of the mass distribution. Finally, we measure the spatial auto-correlation of spin directions and the cross-correlation between the directions of intrinsic and orbital angular momenta of neighbouring haloes. While the first quantity is always very small, we find that spin-orbit correlations are rather strong especially for low-mass haloes in clusters and high-mass haloes in filaments.Comment: 13 pages, 13 figures. Version accepted for publication in MNRAS (references added). Version with high-resolution figures available at http://www.exp-astro.phys.ethz.ch/hahn/pub/HPCD06.pd

The halo distribution of 2dF galaxies

We use the clustering results obtained by Madgwick et al. (2003) for a sample of 96 791 galaxies from the 2dF Galaxy Redshift Survey with redshift 0.01 < z < 0.15 to study the distribution of late-type and early-type galaxies within dark matter haloes of different mass. Within the framework of our models, galaxies of both classes are found to be as spatially concentrated as the dark matter within haloes, even though, while the distribution of star-forming galaxies can also allow for some steeper profiles, this is ruled out drastically in the case of early-type galaxies. We also find evidence for morphological segregation, as late-type galaxies appear to be distributed within haloes of mass-scales corresponding to groups and clusters up to about two virial radii, whereas passive objects show a preference to reside closer to the halo centre. If we assume a broken power law of the form 〈Ngal〉(m) = (m/m0) for mcut≤m < m0 and 〈Ngal〉(m) = (m/m0) at higher masses to describe the dependence of the average number of galaxies within haloes on the halo mass, fits to the data show that star-forming galaxies start appearing in haloes of masses mcut≃ 1011 M⊙, much smaller than what is obtained for early-type galaxies (mcut≃ 1012.6 M⊙). In the high-mass regime m≥m0, 〈Ngal〉 increases with halo mass more slowly (α2≃ 0.7) in the case of late-type galaxies than for passive objects which present α2≃ 1.1. The above results imply that late-type galaxies dominate the 2dF counts at all mass-scales. We stress that — at variance with previous statements — there is no degeneracy in the determination of the best functional forms for ρ(r) and 〈Ngal〉, as they affect the behaviour of the galaxy-galaxy correlation function on different scale