Do On-Line Labor Market Intermediaries Matter? The Impact of AlmaLaurea on the University-to-Work Transition

This paper evaluates the impact of the availability of electronic labor markets on the university-to-work transition. In particular, we analyze the effect of the intermediation activity carried on by the inter-university consortium, AlmaLaurea, on graduates' labor market outcomes. The different timing of universities' enrollment in AlmaLaurea allows us to apply the difference-in-differences method to a repeated cross section data set. If the usual assumption concerning parallel outcomes holds, AlmaLaurea reduces the individual unemployment probability and improves matching quality. Interestingly, we also find that on-line intermediaries foster graduates' geographic mobility.

Cold uniform spherical collapse revisited

We report results of a study of the Newtonian dynamics of N self-gravitating particles which start in a quasi-uniform spherical configuration, without initial velocities. These initial conditions would lead to a density singularity at the origin at a finite time when N \rightarrow \infty, but this singularity is regulated at any finite N (by the associated density fluctuations). While previous studies have focussed on the behaviour as a function of N of the minimal size reached during the contracting phase, we examine in particular the size and energy of the virialized halo which results. We find the unexpected result that the structure decreases in size as N increases, scaling in proportion to N^{-1/3}, a behaviour which is associated with an ejection of kinetic energy during violent relaxation which grows in proportion to N^{1/3}. This latter scaling may be qualitatively understood, and if it represents the asymptotic behaviour in N implies that this ejected energy is unbounded above. We discuss also tests we have performed which indicate that this ejection is a mean-field phenomenon (i.e. a result of collisionless dynamics).Comment: 10 pages, 9 figures; proceedings of "Invisible Universe" conference, Paris, July 200

Breaking the self-averaging properties of spatial galaxy fluctuations in the Sloan Digital Sky Survey - Data Release Six

Statistical analyses of finite sample distributions usually assume that fluctuations are self-averaging, i.e. that they are statistically similar in different regions of the given sample volume. By using the scale-length method, we test whether this assumption is satisfied in several samples of the Sloan Digital Sky Survey Data Release Six. We find that the probability density function (PDF) of conditional fluctuations, filtered on large enough spatial scales (i.e., r>30 Mpc/h), shows relevant systematic variations in different sub-volumes of the survey. Instead for scales r<30 Mpc/h the PDF is statistically stable, and its first moment presents scaling behavior with a negative exponent around one. Thus while up to 30 Mpc/h galaxy structures have well-defined power-law correlations, on larger scales it is not possible to consider whole sample average quantities as meaningful and useful statistical descriptors. This situation is due to the fact that galaxy structures correspond to density fluctuations which are too large in amplitude and too extended in space to be self-averaging on such large scales inside the sample volumes: galaxy distribution is inhomogeneous up to the largest scales, i.e. r ~ 100 Mpc/h, probed by the SDSS samples. We show that cosmological corrections, as K-corrections and standard evolutionary corrections, do not qualitatively change the relevant behaviors. Finally we show that the large amplitude galaxy fluctuations observed in the SDSS samples are at odds with the predictions of the standard LCDM model of structure formation.(Abridged version).Comment: 32 pages, 28 figures, accepted for publication in Astronomy and Astrophysics. A higher resolution version is available at http://pil.phys.uniroma1.it/~sylos/fsl_highlights.html . Version v2 has been corrected to match the published on

Fractal correlations in the CfA2-South redshift survey

We report our analysis of the properties of galaxy clustering for a new redshift sample of galaxies, the CfA2-South catalog, using statistical methods which do not rely on the assumption of homogeneity. We find that, up to ~ 20 Mpc/h, which is the largest scale to which correlation properties can be reliably inferred, the galaxy clustering is scale-invariant and characterized by a fractal dimension D=1.9 \pm 0.1. Further there is no statistical evidence for homogeneity at any of the larger scales (up to ~150 Mpc/h) probed more weakly by the catalog. These results means that characteristic ``correlation lengths'' for the clustering of galaxies derived using standards methods of analysis are not meaningful. Further the results are very consistent with those obtained from many other catalogs using the methods adopted here, which show the D =2 fractal continuing to beyond 100 Mpc/h. The incompleteness of the relevant data conjectured by various authors to give rise to such behaviour is therefore proved to have no significant effect (up to 20 Mpc/h) on the measured correlations.Comment: 18 pages, latex, 1 postscript figure, also available at http://www.phys.uniroma1.it/DOCS/PIL/pil.html Accepted for publication in Astrophysical Journal Letter