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Register a new userThe Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations
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Ricardo A. Flores
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(Abridged) We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat LCDM universe. We present a simple analytical model for the prediction of cluster-scale X-ray observations, approximating clusters as isothermal systems in hydrostatic equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We test the model against high-resolution, hydrodynamic cluster simulations to gauge its reliability. We find that this simple prescription does a good job of predicting the distribution of cluster X-ray ellipticities compared to the simulations as long as one focuses on cluster regions that are less sensitive to recent mergers. Based on this simple model, the distribution of cluster-size halo shapes expected in the concordance LCDM cosmology implies an X-ray ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +- 0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include the mass dependence of halo shape to make comparisons to observational samples that contain many, very massive clusters. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with observations. In particular, we find remarkably good agreement between two recent ROSAT samples and LCDM predictions that DO NOT include gas cooling. We also test how well our analytical model can predict Sunyaev-Zeldovich decrement maps and find that it is less successful although still useful; the model does not perform as well as a function of flux level in this case because of the changing triaxiality of dark matter haloes as a function of radial distance. Both this effect and the changing alignment of isodensity shells of dark matter haloes leave an imprint on cluster gas...
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One of the predictions of the standard CDM is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting CDM as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (sigma) which has an explicit dependence with the halo dispersion velocity (v). If m_x is the mass of the dark matter particle: sigma/m_x ~4 10^-25 (v/100 km s^-1)^-1 cm^2/Gev.
In the present paper, we improve the Extended Secondary Infall Model (ESIM) of Williams et al. (2004) to obtain further insights on the cusp/core problem. The model takes into account the effect of ordered and random angular momentum, dynamical friction and baryon adiabatic contraction in order to obtain a secondary infall model more close to the collapse reality. The model is applied to structures on galactic scales (normal and dwarf spiral galaxies) and on cluster of galaxies scales. The results obtained suggest that angular momentum and dynamical friction are able, on galactic scales, to overcome the competing effect of adiabatic contraction eliminating the cusp. The NFW profile can be reobtained, in our model only if the system is constituted just by dark matter and the magnitude of angular momentum and dynamical friction are reduced with respect to the values predicted by the model itself. The rotation curves of four LSB galaxies from de Blok & Bosma (2002) are compared to the rotation curves obtained by the model in the present paper obtaining a good fit to the observational data. On scales smaller than $simeq 10^{11} h^{-1} M_{odot}$ the slope $alpha simeq 0$ and on cluster scales we observe a similar evolution of the dark matter density profile but in this case the density profile slope flattens to $alpha simeq 0.6$ for a cluster of $simeq 10^{14} h^{-1} M_{odot}$. The total mass profile, differently from that of dark matter, shows a central cusp well fitted by a NFW model.
In this paper, in the framework of the secondary infall model, the correlation between the central surface density and the halo core radius of galaxy, and cluster of galaxies, dark matter haloes was analyzed, this having recently been studied on a wide range of scales. We used Del Popolo (2009) secondary infall model taking into account ordered and random angular momentum, dynamical friction, and dark matter (DM) adiabatic contraction to calculate the density profile of haloes, and then these profiles are used to determine the surface density of DM haloes. The main result is that $r_ast$ (the halo characteristic radius) is not an universal quantity as claimed by Donato et al. (2009) and Gentile et al. (2009). On the contrary, we find a correlation with the halo mass $M_{200}$ in agreement with Cardone & Tortora (2010), Boyarsky at al. (2009) and Napolitano et al. (2010), but with a significantly smaller scatter, namely $0.16 pm 0.05$. We also consider the baryon column density finding this latter being indeed a constant for low mass systems such as dwarfs, but correlating with mass with a slope $alpha= 0.18 pm 0.05$. In the case of the surface density of dark matter for a system composed only of dark matter, as in dissipationless simulations, we get $alpha=0.20 pm 0.05$. These results leave little room for the recently claimed universality of (dark and stellar) column density.
In the previous papers in this series, we have measured the stellar and hi content in a sample of edge-on galaxies. In the present paper, we perform a simultaneous rotation curve and vertical force field gradient decomposition for five of these edge-on galaxies. The rotation curve decomposition provides a measure of the radial dark matter potential, while the vertical force field gradient provide a measure of the vertical dark matter potential. We fit dark matter halo models to these potentials. Using our hi self-absorption results, we find that a typical dark matter halo has a less dense core ($0.094pm0.230$,M$_odot$/pc$^3$) compared to an optically thin hi model ($0.150pm0.124$,M$_odot$/pc$^3$). The HI self-absorption dark matter halo has a longer scale length $R_c$ of $1.42pm 3.48$,kpc, versus $1.10pm 1.81$,kpc for the optically thin HI model. The median halo shape is spherical, at $q=1.0pm0.6$ (self-absorbing hi), while it is prolate at $q=1.5pm0.6$ for the optically thin. Our best results were obtained for ESO,274-G001 and UGC,7321, for which we were able to measure the velocity dispersion in Paper III. These two galaxies have drastically different halo shapes, with one oblate and one strongly prolate. Overall, we find that the many assumptions required make this type of analysis susceptible to errors.
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