Abundance Of Dark Matter Halos Research Articles

THE VELOCITY FUNCTION IN THE LOCAL ENVIRONMENT FROM {lambda}CDM AND {lambda}WDM CONSTRAINED SIMULATIONS

Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 keV warm dark matter (WDM), we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function (MF) within 20 h(-1) Mpc of the Local Group is similar to 2 times larger than the universal MF in the 10(9) - 10(13) h(-1) M(circle dot) mass range. Imposing the field of view of the ongoing Hi blind survey Arecibo Legacy Fast ALFA (ALFALFA) in our simulations, we predict that the velocity function (VF) in the Virgo-direction region (VdR) exceeds the universal VF by a factor of 3. Furthermore, employing a scheme to translate the halo VF into a galaxy VF, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the VF in the 80-300 km s(-1) velocity range, having a value similar to 10 times larger than the universal VF in the VdR. In the low-velocity regime, 35-80 km s(-1), the WDM simulation reproduces the observed flattening of the VF. In contrast, the simulation with CDM predicts a steep rise in the VF toward lower velocities; for V(max) = 35 km s(-1), it forecasts similar to 10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the CDM paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

THE VELOCITY FUNCTION IN THE LOCAL ENVIRONMENT FROM ΛCDM AND ΛWDM CONSTRAINED SIMULATIONS

Using constrained simulations of the local universe for generic cold dark matter (CDM) and for 1 keV warm dark matter (WDM), we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function (MF) within 20 h−1 Mpc of the Local Group is ∼2 times larger than the universal MF in the 109–1013 h−1 M☉ mass range. Imposing the field of view of the ongoing H i blind survey Arecibo Legacy Fast ALFA (ALFALFA) in our simulations, we predict that the velocity function (VF) in the Virgo-direction region (VdR) exceeds the universal VF by a factor of 3. Furthermore, employing a scheme to translate the halo VF into a galaxy VF, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the VF in the 80–300 km s-1 velocity range, having a value ∼10 times larger than the universal VF in the VdR. In the low-velocity regime, 35–80 km s-1, the WDM simulation reproduces the observed flattening of the VF. In contrast, the simulation with CDM predicts a steep rise in the VF toward lower velocities; for Vmax = 35 km s-1, it forecasts ∼10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the CDM paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.

The impact of early dark energy on non-linear structure formation

We study non-linear structure formation in high-resolution simulations of Early Dark Energy (EDE) cosmologies and compare their evolution with the standard LCDM model. Extensions of the spherical top-hat collapse model predict that the virial overdensity and linear threshold density for collapse should be modified in EDE model, yielding significant modifications in the expected halo mass function. Here we present numerical simulations that directly test these expectations. Interestingly, we find that the Sheth & Tormen formalism for estimating the abundance of dark matter halos continues to work very well in its standard form for the Early Dark Energy cosmologies, contrary to analytic predictions. The residuals are even slightly smaller than for LCDM. We also study the virial relationship between mass and dark matter velocity dispersion in different dark energy cosmologies, finding excellent agreement with the normalization for Lambda as calibrated by Evrard et al.(2008). The earlier growth of structure in EDE models relative to LCDM produces large differences in the mass functions at high redshift. This could be measured directly by counting groups as a function of the line-of-sight velocity dispersion, skirting the ambiguous problem of assigning a mass to the halo. Using dark matter substructures as a proxy for member galaxies, we demonstrate that even with 3-5 members sufficiently accurate measurements of the halo velocity dispersion function are possible. Finally, we determine the concentration-mass relationship for our EDE cosmologies. Consistent with the earlier formation time, the EDE halos show higher concentrations at a given halo mass. We find that the magnitude of the difference in concentration is well described by the prescription of Eke et al.(2001) for estimating halo concentrations.

Dark matter halo abundances, clustering and assembly histories at high redshift

We use a suite of high-resolution N-body simulations to study the properties, abundance and clustering of high mass halos at high redshift, including their mass assembly histories and mergers. We find that the analytic form which best fits the abundance of halos depends sensitively on the assumed definition of halo mass, with common definitions of halo mass differing by a factor of two for these low concentration, massive halos. A significant number of massive halos are undergoing rapid mass accretion, with major merger activity being common. We compare the mergers and mass accretion histories to the extended Press-Schechter formalism. We consider how major merger induced star formation or black hole accretion may change the distribution of photon production from collapsed halos, and hence reionization, using some simplified examples. In all of these, the photon distribution for a halo of a given mass acquires a large scatter. If rare, high mass halos contribute significantly to the photon production rates, the scatter in photon production rate can translate into additional scatter in the sizes of ionized bubbles.

Non-Markov excursion set model of dark matter halo abundances

The excursion set model provides a convenient theoretical framework to derive dark matter halo abundances. This paper generalizes the model by introducing a more realistic merging and collapse process. A new parameter regulates the influence of the environment and thus the coherence (non-Markovianity) of the merging and the collapse of individual mass shells. The model mass function also includes the effects of an ellipsoidal collapse. Analytic approximations of the halo mass function are derived for scale-invariant power spectra with the slopes . The mass function can be compared with the results obtained from the “Hubble volume” simulations. A significant detection of non-Markovian effects is found for an assumed accuracy of the simulated mass function of 10%.

Non-Markov excursion set model of dark matter halo abundances

The excursion set model provides a convenient theoretical framework to derive dark matter halo abundances. This paper generalizes the model by introducing a more realistic merging and collapse process. A new parameter regulates the influence of the environment and thus the coherence (non-Markovianity) of the merging and the collapse of individual mass shells. The model mass function also includes the effects of an ellipsoidal collapse. Analytic approximations of the halo mass function are derived for scale-invariant power spectra with the slopes n = 0, −1, −2. The n = −2 mass function can be compared with the results obtained from the Hubble volume simulations. A significant detection of non-Markovian effects is found for an assumed accuracy of the simulated mass function of 10%.

Large-scale bias and the peak background split

Dark matter haloes are biased tracers of the underlying dark matter distribution. We use a simple model to provide a relation between the abundance of dark matter haloes and their spatial distribution on large scales. Our model shows that knowledge of the unconditional mass function alone is sufficient to provide an accurate estimate of the large-scale bias factor. We then use the mass function measured in numerical simulations of SCDM, OCDM and ΛCDM to compute this bias. Comparison with these simulations shows that this simple way of estimating the bias relation and its evolution is accurate for less massive haloes as well as massive ones. In particular, we show that haloes that are less/more massive than typical M* haloes at the time they form are more/less strongly clustered than is predicted by formulae based on the standard Press–Schechter mass function.

Damped Lya systems at high redshift and models of protogalactic discs

We employ observationally determined intrinsic velocity widths and column densities of damped Lyman alpha (Lyα) systems at high redshift to investigate the distribution of baryons in protogalaxies within the context of a standard cold dark matter (CDM) model. We proceed under the assumption that damped Lyα systems represent a population of cold, rotationally supported, protogalactic discs, and that the abundance of dark matter haloes is well approximated by a CDM model with critical density and vanishing cosmological constant. Using conditional cross-sections to observe a damped system with a given velocity width and column density, we compare observationally inferred velocity width and column density distributions to the corresponding theoretically determined distributions for a variety of disc parameters and CDM normalizations. In general, we find that the observations cannot be reproduced by the models for most disc parameters and CDM normalizations. Whereas the column density distribution favours small discs with large neutral gas fraction, the velocity width distribution favours large and thick discs with small neutral gas fraction. The possible resolutions of this problem in the context of this CDM model may be (1) an increased contribution of rapidly rotating discs within massive dark matter haloes to damped Lya absorption, or (2) the abandoning of simple disc models within this CDM model for damped Lya systems at high redshift. Here the first possibility may be achieved by supposing that damped Lya system formation occurs only in haloes with fairly large circular velocities, and the second possibility may result from a large contribution of mergers and double discs to damped Lya absorption at high redshift.