1,045 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
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 ,
the variance , and skewness of the distribution for the
various sub-samples chosen on different criteria. Both and
show power-law behaviour with an amplitude corresponding a spatial correlation
length of 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 , with
, 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
The Halo Distribution of 2dF Galaxies
We use the clustering results obtained by Madgwick et al. (2003) for a sample
of 96,791 2dF galaxies with redshift 0.01 \lt z \lt 0.15 to study the
distribution of late-type and early-type galaxies within dark matter haloes of
different mass. Within the framework of our models, galaxies of both classes
are found to be as spatially concentrated as the dark matter within haloes even
though, while the distribution of star-forming galaxies can also allow for some
steeper profiles, this is drastically ruled out in the case of early-type
galaxies. We also find evidence for morphological segregation, as late-type
galaxies appear to be distributed within haloes of mass scales corresponding to
groups and clusters up to about two virial radii, while passive objects show a
preference to reside closer to the halo centre. If we assume a broken power-law
of the form =(m/m_0)^{alpha_1} for m_{cut}\le m<m_0 and
=(m/m_0)^{alpha_2} at higher masses to describe the dependence of
the average number of galaxies within haloes on the halo mass, fits to the data
show that star-forming galaxies start appearing in haloes of masses
m_{cut}~10^{11}m_{sun}, much smaller than what is obtained for early-type
galaxies (m_{cut}~10^{12.6}m_{sun}). In the high-mass regime m\ge m_0,
$ increases with halo mass more slowly (alpha_2~0.7) in the case of
late-type galaxies than for passive objects which present alpha_2~1.1. We
stress that there is no degeneracy in the determination of the best functional
forms for rho(r) and , as they affect the behaviour of the
galaxy-galaxy correlation function on different scales.Comment: Revised version to appear in MNRAS, extended analysis, some new
result
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