X. Hernandez

Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

One of the predictions of the standard cold dark matter model 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 cold dark matter 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 (�) which has an explicit dependence with the halo dispersion velocity (v). If mx is the mass of the dark matter particle: �/mx � 4 10 25 (100 kms 1 /v) cm 2 /Gev.

A plausible Galactic spiral pattern and its rotation speed

We report calculations of the stellar and gaseous response to a Milky Way mass distribution model including a spiral pattern with a locus as traced by K-band observations, superimposed on the axisymmetric components in the plane of the disc. The stellar study extends calculations from previous work concerning the self-consistency of the pattern. The stellar response to the imposed spiral mass is studied via computations of the central family of periodic and nearby orbits as a function of the pattern rotation speed, Q p , among other parameters. A fine grid of values of Q p was explored, ranging from 12 to 25 km s -1 kpc -1 . Dynamical self-consistency is highly sensitive to Ω p , with the best fit appearing at 20 km s -1 kpc -1 . We give an account of recent independent pieces of theoretical and observational work that are dependent on the value of Ω p , all of which are consistent with the value found here: the recent star formation history of the Milky Way, local inferences of cosmic ray flux variations and Galactic abundance patterns. The gaseous response, which is also a function of Ω p , was calculated via 2D hydrodynamic simulations with the ZEUS code. For Ω p = 20 km s -1 kpc -1 , the response to a two-armed pattern is a structured pattern of four arms, with bifurcations along the arms and interarm features. The pattern qualitatively resembles the optical arms observed in our Galaxy and other galaxies. The complex gaseous pattern appears to be linked to resonances in stellar orbits. Among these, the 4:1 resonance plays an important role, as it determines the extent of the stellar spiral pattern in the self-consistency study presented here. Our findings seemingly confirm predictions by Drimmel & Spergel (2001), based on K-band data.

Chemical evolution models of local dwarf spheroidal galaxies

We calculate chemical evolution models for four dwarf spheroidal (dSph) satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently-obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal-rich selective wind that might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar populations have formed. Systems showing extended star formation histories (Carina and Leo I), however, are consistent with the idea that their tidally-limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.

Cosmological origin of the lowest metallicity halo stars

We explore the predictions of the standard hierarchical clustering scenario, regarding the numbers and metallicities of PopIII stars found within our Galaxy today. An analytic, extended Press-Schechter formalism is used to get the mass functions of halos which will host PopIII stars at a given redshift, and which will end up in the Milky Way (MW) today. Each of these is modeled as a mini galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disk instabilities, all of which determine the fraction of the baryons which are subject to star formation. Use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed and give accurate estimates of their expected numbers today. A first comparison with observational data suggests that the IMF of the first stars was increasingly high mass weighted towards high redshifts, leveling off at z ≈ 9 at a characteristic stellar mass scale m s = 10 - 15M ⊙. Details can be found in Hernandez & Ferrara (2001).

Empirical distributions of galactic λ spin parameters from the Sloan Digital Sky Survey

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter

The Outskirts of Globular Clusters as Modified Gravity Probes

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Chemical consequences of low star formation rates: stochastically sampling the initial mass function

for any real galaxy, in terms of common observational parameters. This allows a rough estimate of

A dimensional study of disc galaxies

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A natural approach to extended Newtonian gravity: tests and predictions across astrophysical scales

, which we apply to a large volume limited sample of galaxies taken from the SDSS data base. The large numbers involved (11,597) allow the derivation of reliable

Masses, Tidal Radii, and Escape Speeds in Dwarf Spheroidal Galaxies under MOND and Dark Halos Compared

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