Small scale problems of the Λ\LambdaCDM model: a short review

The $\Lambda$CDM model, or concordance cosmology, as it is often called, is a paradigm at its maturity. It is clearly able to describe the universe at large scale, even if some issues remain open, such as the cosmological constant problem , the small-scale problems in galaxy formation, or the unexplained anomalies in the CMB. $\Lambda$CDM clearly shows difficulty at small scales, which could be related to our scant understanding, from the nature of dark matter to that of gravity; or to the role of baryon physics, which is not well understood and implemented in simulation codes or in semi-analytic models. At this stage, it is of fundamental importance to understand whether the problems encountered by the $\Lambda$DCM model are a sign of its limits or a sign of our failures in getting the finer details right. In the present paper, we will review the small-scale problems of the $\Lambda$CDM model, and we will discuss the proposed solutions and to what extent they are able to give us a theory accurately describing the phenomena in the complete range of scale of the observed universe.Comment: 48pp 19 figs, invited review, accepted by Galaxie

A unified solution to the small scale problems of the Λ\LambdaCDM model II: introducing parent-satellite interaction

We continue the study of the impact of baryon physics on the small scale problems of the $\Lambda$CDM model, based on a semi-analytical model (Del Popolo, 2009). Withsuch model, we show how the cusp/core, missing satellite (MSP), Too Big to Fail (TBTF) problems and the angular momentum catastrophe can be reconciled with observations, adding parent-satellite interaction. Such interaction between darkmatter (DM) and baryons through dynamical friction (DF) can sufficiently flattenthe inner cusp of the density profiles to solve the cusp/core problem. Combining, in our model, a Zolotov et al. (2012)-like correction, similarly to Brooks et al. (2013), and effects of UV heating and tidal stripping, the number of massive, luminous satellites, as seen in the Via Lactea 2 (VL2) subhaloes,is in agreement with the numbers observed in the MW, thus resolving the MSP and TBTF problems. The model also produces a distribution of the angular spin parameter and angular momentum in agreement with observations of the dwarfs studied by van den Bosch, Burkert, \\& Swaters (2001).Comment: 24pp, 5figs. arXiv admin note: text overlap with arXiv:1404.367

SPARC HSBs, and LSBs, the surface density of dark matter haloes, and MOND

In this paper, we use SPARC's HSBs, and LSBs galaxies to verify two issues. The first one is related to one claim of \citep{Donato} D09, namely: is the DM surface density (DMsd) a constant universal quantity, equal to $\log{(\rm \Sigma/M_\odot pc^{-2})}=2.15 \pm 0.2$, or does it depend on the baryon surface density of the system? The second one, is based on a MOND prediction that for HSBs the DMsd is constant, and equal to $\log{(\rm \Sigma/M_\odot pc^{-2})}=2.14$, while for LSBs the surface density is not constant and takes values that are smaller than for HSBs and D09 prediction \citep{Milgrom2009}. We find that HSBs shows a constant DMsd vs magnitude as in D09, and a constant DMsd vs $\Sigma_{\rm eff}$ as in MOND prediction, for HSBs with $\Sigma_{\rm eff}>200 L_\odot/pc^2$, and $\Sigma_{\rm eff}>300 L_\odot/pc^2$. However, the value of the DMsd is larger, $\Sigma \simeq 2.61$ (in the case of the DMsd-magnitude with $\Sigma_{\rm eff}>300 L_\odot/pc^2$), and $\Sigma \simeq 2.54$ (in the case of the surface DMsd-surface brightness with $\Sigma_{\rm eff}>200 L_\odot/pc^2$). This value slightly depends on the threshold to determine wheter a galaxy is HSB. In the case of LSBs, for $\Sigma_{\rm eff}<100 L_\odot/pc^2$, and $\Sigma_{\rm eff}<25 L_\odot/pc^2$, the surface density vs magnitude, for lower magnitudes, is approximately equal to that predicted by D09, but several galaxies, for magnitude $M>-17$, have smaller values than those predicted by D09. The DMsd vs $\Sigma_{\rm eff}$ shows a similar behavior in qualitative, but not quantitative, agreement with MOND predictions. In summary, in the case of HSBs both D09 and MOND are in qualitative, but not quantitative, agreement with the data. In the case of LSBs D09 is mainly in disagreement with the data, and MOND only in qualitative agreement with them.Comment: 31 pages, 4 figure

Mass functions from the excursion set model

Aims. We aim to study the stochastic evolution of the smoothed overdensity $\delta$ at scale $S$ of the form $\delta(S) = \int_{0}^S K(S,u)\mathrm{d}W(u)$, where $K$ is a kernel and $\mathrm{d}W$ is the usual Wiener process. Methods. For a Gaussian density field, smoothed by the top-hat filter, in real space, we used a simple kernel that gives the correct correlation between scales. A Monte Carlo procedure was used to construct random walks and to calculate first crossing distributions and consequently mass functions for a constant barrier. Results. We show that the evolution considered here improves the agreement with the results of N-body simulations relative to analytical approximations which have been proposed from the same problem by other authors. In fact, we show that an evolution which is fully consistent with the ideas of the excursion set model, describes accurately the mass function of dark matter haloes for values of $\nu \leq 1$ and underestimates the number of larger haloes. Finally, we show that a constant threshold of collapse, lower than it is usually used, it is able to produce a mass function which approximates the results of N-body simulations for a variety of redshifts and for a wide range of masses. Conclusions. A mass function in good agreement with N-body simulations can be obtained analytically using a lower than usual constant collapse threshold.Comment: 6 pages, 9 figures. A&A publishe