The Cosmological Mass Function with 1D Gravity

The cosmological mass function problem is analyzed in full detail in the case of 1D gravity, with analytical, semi-analytical and numerical techniques. The extended Press & Schechter theory is improved by detailing the relation between smoothing radius and mass of the objects. This is done by introducing in the formalism the concept of a growth curve for the objects. The predictions of the extended Press & Schechter theory are compared to large N-body simulations of flat expanding 1D universes with scale-free power spectra of primordial perturbations. The collapsed objects in the simulations are located with a clump-finding algorithm designed to find regions that have undergone orbit crossing or that are in the multi-stream regime (these are different as an effect of the finite size of the multi-stream regions). It is found that the semi-analytical mass function theory, which has no free parameters, is able to recover the properties of collapsed objects both statistically and object by object. In particular, the predictions of regions in orbit crossing are optimized by the use of Gaussian filtering, while the use of sharp k-space filtering apparently allows to reproduce the larger multi-stream regions. The mass function theory does not reproduce well the clumps found with the standard friends-of-friends algorithm; however, the performance of this algorithm has not been thoroughly tested in the 1D cosmology. Our preliminary analyses of the 3D case confirms that the techniques developed in this paper are precious in understanding the cosmological mass function problem in 3D.Comment: 25 pages, revtex, postscript figures included, in press on Physical Review

The effect of AGN feedback on the halo mass function

[Abridged.] We investigate baryon effects on the halo mass function (HMF), with emphasis on the role played by AGN feedback. Halos are identified with both Friends-of-Friends (FoF) and Spherical Overdensity (SO) algorithms. We embed the standard SO algorithm into a memory-controlled frame program and present the {\bf P}ython spher{\bf I}c{\bf A}l {\bf O}verdensity code --- {\small PIAO}. For both FoF and SO halos, the effect of AGN feedback is that of suppressing the HMFs to a level even below that of Dark Matter simulations. The ratio between the HMFs in the AGN and in the DM simulations is $\sim 0.8$ at overdensity $\Delta_c=500$, a difference that increases at higher overdensity $\Delta_c=2500$, with no significant redshift and mass dependence. A decrease of the halo masses ratio with respect to the DM case induces the decrease of the HMF in the AGN simulation. The shallower inner density profiles of halos in the AGN simulation witnesses that mass reduction is induced by the sudden displacement of gas induced by thermal AGN feedback. We provide fitting functions to describe halo mass variations at different overdensities, which can recover the HMFs with a residual random scatter $\lt 5$ per cent for halo masses larger than $10^{13} ~h^{-1}{\rm M_\odot}$.Comment: 16 pages, 11 figures. Matches to MNRAS published version, typo corrected in the fitting functio

How galaxies lose their angular momentum

The processes are investigated by which gas loses its angular momentum during the protogalactic collapse phase, leading to disk galaxies that are too compact with respect to the observations. High-resolution N-body/SPH simulations in a cosmological context are presented including cold gas and dark matter. A halo with quiet merging activity since z~3.8 and with a high spin parameter is analysed that should be an ideal candidate for the formation of an extended galactic disk. We show that the gas and the dark matter have similar specific angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1. All the gas involved in the merger loses a substantial fraction of its specific angular momentum due to tidal torques and falls quickly into the center. Dynamical friction plays a minor role,in contrast to previous claims. In fact, after this event a new extended disk begins to form from gas that was not involved in the 5:1 merger event and that falls in subsequently. We argue that the angular momentum problem of disk galaxy formation is a merger problem: in cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1 are expected to occur in almost all galaxies. We suggest that energetic feedback processes could in principle solve this problem, however only if the heating occurs at the time or shortly before the last substantial merger event. Good candidates for such a coordinated feedback would be a merger-triggered star burst or central black hole heating. If a large fraction of the low angular momentum gas would be ejected as a result of these processes, late-type galaxies could form with a dominant extended disk component, resulting from late infall, a small bulge-to-disk ratio and a low baryon fraction, in agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution figures to the author

The baryon fraction in hydrodynamical simulations of galaxy clusters

We study the baryon mass fraction in a set of hydrodynamical simulations of galaxy clusters performed using the Tree+SPH code GADGET-2. We investigate the dependence of the baryon fraction upon the radiative cooling, star formation, feedback through galactic winds, conduction and redshift. Both the cold stellar component and the hot X-ray emitting gas have narrow distributions that, at large cluster-centric distances r>R500, are nearly independent of the physics included in the simulations. Only the non-radiative runs reproduce the gas fraction inferred from observations of the inner regions (r ~ R2500) of massive clusters. When cooling is turned on, the excess star formation is mitigated by the action of galactic winds, but yet not by the amount required by observational data. The baryon fraction within a fixed overdensity increases slightly with redshift, independent of the physical processes involved in the accumulation of baryons in the cluster potential well. In runs with cooling and feedback, the increase in baryons is associated with a larger stellar mass fraction that arises at high redshift as a consequence of more efficient gas cooling. For the same reason, the gas fraction appears less concentrated at higher redshift. We discuss the possible cosmological implications of our results and find that two assumptions generally adopted, (1) mean value of Yb = fb / (Omega_b/Omega_m) not evolving with redshift, and (2) a fixed ratio between f_star and f_gas independent of radius and redshift, might not be valid. In the estimate of the cosmic matter density parameter, this implies some systematic effects of the order of Delta Omega_m/Omega_m < +0.15 for non-radiative runs and Delta Omega_m/Omega_m ~ +0.05 and < -0.05 for radiative simulations.Comment: 10 pages, to appear in MNRA

Applying logistic regression analysis to identify patient’s satisfaction predictors with general practitioner assistance: evidence from four Italian regions.

In the last years the interest for patient experience with health care services largely increased. Several surveys have been conducted in order to observe if health care systems answer to the overall patient needs. In 2000 World Health Organization challenged modern health care providers to ensure responsiveness to patients, i.e. to deliver also non-health assistance (respect for dignity, confidentiality, prompt attention, quality of amenities, access to social support networks, choice of provider, etc.). Poor evidence is available in Italy about connections between perceived quality and the capability of the healthcare system to respond to patients’ needs. This work aims at investigating patient experience with General Practitioner (GP) assistance and at measuring the impact of personal and organizational characteristics on overall satisfaction and on willingness to recommend. In 2009 a sample survey was conducted in four regions of Italy (Tuscany, Piedmont, Umbria, and Liguria). About 15.000 citizens answered to a large questionnaire related to Primary Care services, including a section dedicated to General Practitioner (GP) assistance. A logistic regression analysis was applied to analyze which are the predictors of overall satisfaction with GP, focusing mainly on variables related to patient’s expectations, continuity of care and organizational aspects (e.g. scheduled access, waiting time, health case history, etc.) and if there are differences across the four Italian Regions. Econometric analysis has been carried out through both ordered logistic regression and generalised ordered logit models. The inhabitants of the four Italian Regions refer a nice experience with GP assistance: more than 85% of them judged excellent or good the overall service. Generally, in some regions patient expectations affect more the willingness to recommend GP to friends or family members than the overall judgement on service. Besides, the findings provide convincing evidence that GP is a nodal point in the continuity of care process .: patient satisfaction, general practitioner, organizational aspects, continuity of care

Vedere nel paziente una persona. Una nuova prospettiva per i servizi sanitari

Disponibile al link: http://saluteinternazionale.info/2009/03/vedere-nel-paziente-una-persona-una-nuova-prospettiva-per-i-servizi-sanitari