1,332 research outputs found
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 we
measured the angular correlation function as a function of redshift
up to . From these measurements we derive the trend of the
correlation length . We find that is roughly constant with
look-back time up to , and then increases to higher values at
z\simgt 2.4. We estimate the values of , assuming
, and different geometries. For
we find 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
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
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
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
The large scale clustering of radio sources
The observed two-point angular correlation function, w(theta), of mJy radio
sources exhibits the puzzling feature of a power-law behaviour up to very large
(almost 10 degrees) angular scales which cannot be accounted for in the
standard hierarchical clustering scenario for any realistic redshift
distribution of such sources. After having discarded the possibility that the
signal can be explained by a high density local source population, we find no
alternatives to assuming that - at variance with all the other extragalactic
populations studied so far, and in particular with optically selected quasars -
radio sources responsible for the large-scale clustering signal were
increasingly less clustered with increasing look-back time, up to at least z=1.
The data are accurately accounted for in terms of a bias function which
decreases with increasing redshift, mirroring the evolution with cosmic time of
the characteristic halo mass, M_{star}, entering the non linear regime. In the
framework of the `concordance cosmology', the effective halo mass controlling
the bias parameter is found to decrease from about 10^{15} M_{sun}/h at z=0 to
the value appropriate for optically selected quasars, 10^{13} M_{sun}/h, at
z=1.5. This suggests that, in the redshift range probed by the data, the
clustering evolution of radio sources is ruled by the growth of large-scale
structure, and that they are associated with the densest environments
virializing at any cosmic epoch. The data provide only loose constraints on
radio source clustering at z>1 so we cannot rule out the possibility that at
these redshifts the clustering evolution of radio sources enters a different
regime, perhaps similar to that found for optically selected quasars. The
dependence of w(theta) on cosmological parameters is also discussed.Comment: 9 pages, 8 figures. Accepted for publication on MNRA
On the correlation of Short Gamma--Ray Bursts and Clusters of galaxies
We cross correlate Gamma--Ray Bursts and X--Ray selected clusters of galaxies
at z\leq0.45. We find a positive 2\sigma signal for the angular
cross--correlation function w_{bc}(\theta) on scales \theta\leq 3 deg between
short GRBs and clusters. Conversely, no correlation is found between clusters
and the population of long GRBs. The comparison with the cluster
autocorrelation function shows that short GRBs do not trace the cluster
distribution as not all short GRBs are found in clusters. A higher signal in
w_{bc}(\theta) is found if we only consider the cluster population up to z=0.1.
By comparing the short burst autocorrelation function with model predictions we
then constrain short bursts to mostly originate within \sim 270 Mpc (i.e. z\leq
0.06). Our analysis also reveals that short GRBs are better correlated with
``normal'' galaxies. The double compact object merger model for short GRBs
would associate them preferentially to early--type galaxies but the present
statistics do not allow us to exclude that at least a fraction of these events
might also take place in late--type galaxies, in agreement with recent
evidences.Comment: 5 pages, 5 figures, accepted for publication in MNRA
Radio Properties of FIRST Radio Sources at 1 mJy
This paper presents a detailed analysis of the radio properties for the
sample of faint radio sources introduced in Magliocchetti et al. (2000). The
sample comprises mainly intrinsically low-power sources whose majority (\simgt
70 per cent) is made of FR I radio galaxies. These objects show some degree
(at 1 confidence level) of luminosity evolution, which is also needed
to correctly reproduce the total number and shape of the counts distribution at
1.4 GHz. Analysis of the de-evolved local radio luminosity function shows a
good agreement between data and model predictions for this class of sources.
Particular care has been devoted to the issue of 'lined' galaxies (i.e. objects
presenting in their spectra a continuum typical of early-type galaxies plus
emission lines of different nature), which appear as an intermediate class of
sources between AGN-dominated and starburst galaxies. Different evolutionary
behaviours are seen between the two sub-populations of lined and non-lined
low-power radio galaxies, the first class indicating a tendency for the radio
luminosity to decrease with look-back time, the second one showing positive
evolution. We note that different evolutionary properties also seem to
characterize BL Lacs selected in different bands, so that one might envisage an
association between lined FR I and the sub-class of BL Lacs selected in the
X-ray band.Comment: 9 pages, 6 figures, to appear in MNRA
- …