Gravothermal Catastrophe in Anisotropic Spherical Systems

In this paper we investigate the gravothermal instability of spherical stellar systems endowed with a radially anisotropic velocity distribution. We focus our attention on the effects of anisotropy on the conditions for the onset of the instability and in particular we study the dependence of the spatial structure of critical models on the amount of anisotropy present in a system. The investigation has been carried out by the method of linear series which has already been used in the past to study the gravothermal instability of isotropic systems. We consider models described by King, Wilson and Woolley-Dickens distribution functions. In the case of King and Woolley-Dickens models, our results show that, for quite a wide range of amount of anisotropy in the system, the critical value of the concentration of the system (defined as the ratio of the tidal to the King core radius of the system) is approximately constant and equal to the corresponding value for isotropic systems. Only for very anisotropic systems the critical value of the concentration starts to change and it decreases significantly as the anisotropy increases and penetrates the inner parts of the system. For Wilson models the decrease of the concentration of critical models is preceded by an intermediate regime in which critical concentration increases, it reaches a maximum and then it starts to decrease. The critical value of the central potential always decreases as the anisotropy increases.Comment: 7pages, 5figures, to appear in MNRAS (figures have been replaced with their corrected versions

The Redshift Evolution of Clustering in the HDF

We present a correlation function analysis for the catalogue of photometric redshifts obtained from the Hubble Deep Field image by Fernandez-Soto et al., 1998. By dividing the catalogue into redshift bins of width $\Delta z=0.4$ we measured the angular correlation function $w(\theta)$ as a function of redshift up to $z\sim 4.8$. From these measurements we derive the trend of the correlation length $r_0$. We find that $r_0(z)$ is roughly constant with look-back time up to $z \simeq 2$, and then increases to higher values at z\simgt 2.4. We estimate the values of $r_0$, assuming $\xi(r,z)=(r/r_0(z))^{-\gamma}$, $\gamma=1.8$ and different geometries. For $\Omega_0=1$ we find $r_0(z=3)\simeq 7.00\pm 4.87 h^{-1}$ Mpc, in good agreement with the values obtained from analysis of the Lyman Break Galaxies.Comment: 6 pages, 4 postscript figures, version to appear on MNRA

Evidence for anisotropy in the distribution of short-lived gamma-ray bursts

Measurements of the two-point angular correlation function w(\theta) for 407 short gamma-ray bursts collected in the Current BATSE Catalogue reveal a ~2 \sigma deviation from isotropy on angular scales \theta ~ 2-4 degrees. Such an anisotropy is not observed in the distribution of long gamma-ray bursts and hints to the presence of repeated bursts for up to ~13% of the sources under exam. However, the available data cannot exclude the signal as due to the presence of large-scale structure. Under this assumption, the amplitude of the observed w(\theta) is compatible with those derived for different populations of galaxies up to redshifts ~0.5, result that suggests short gamma-ray bursts to be relatively local sources.Comment: 5 pages, 4 figures, submitted to MNRA

Variance and Skewness in the FIRST survey

We investigate the large-scale clustering of radio sources in the FIRST 1.4-GHz survey by analysing the distribution function (counts in cells). We select a reliable sample from the the FIRST catalogue, paying particular attention to the problem of how to define single radio sources from the multiple components listed. We also consider the incompleteness of the catalogue. We estimate the angular two-point correlation function $w(\theta)$, the variance $\Psi_2$, and skewness $\Psi_3$ of the distribution for the various sub-samples chosen on different criteria. Both $w(\theta)$ and $\Psi_2$ show power-law behaviour with an amplitude corresponding a spatial correlation length of $r_0 \sim 10 h^{-1}$Mpc. We detect significant skewness in the distribution, the first such detection in radio surveys. This skewness is found to be related to the variance through $\Psi_3=S_3(\Psi_2)^{\alpha}$, with $\alpha=1.9\pm 0.1$, consistent with the non-linear gravitational growth of perturbations from primordial Gaussian initial conditions. We show that the amplitude of variance and skewness are consistent with realistic models of galaxy clustering.Comment: 13 pages, 21 inline figures, to appear in MNRA

First Stars Contribution to the Near Infrared Background Fluctuations

We show that the emission from the first, metal-free stars inside Population III objects (PopIIIs) are needed to explain the level of fluctuations in the Near Infrared Background (NIRB) recently discovered by Kashlinsky et al. (2002), at least at the shortest wavelengths. Clustering of (unresolved) Pop IIIs can in fact account for the entire signal at almost all the ~1-30 arcsec scales probed by observations in the J band. Their contribution fades away at shorter frequencies and becomes negligible in the K band. ``Normal'', highly clustered, ~3 galaxies undergoing intense star-formation such as those found in the Hubble Deep Fields can 'fill in' this gap and provide for the missing signal. It is in fact found that their contribution to the intensity fluctuations is the dominant one at lambda=2.17 mum, while it gradually looses importance in the H and J bands. The joint contribution from these two populations of cosmic objects is able, within the errors, to reproduce the observed power spectrum in the whole Near Infrared range on small angular scales (theta < 200" for Pop III protogalaxies). Signals on larger scales detected by other experiments instead require the presence of more local sources.Comment: 6 pages, 4 figures, submitted to MNRA