1,141 research outputs found
Biblioteca pessoal, biblioteca universal
A partir de uma teoria funcional da biblioteca são examinadas as características específicas da biblioteca pessoal, do ponto de vista de sua estrutura e das relações entre ela e seu criador e usuário
Morphological Number Counts and Redshift Distributions to I = 25 from the Hubble Deep Fields: Constraints on Cosmological Models from Early Type Galaxies
We combine magnitude and photometric redshift data on galaxies in the Hubble
Deep Fields with morphological classifications in order to separate out the
distributions for early type galaxies. The updated morphological galaxy number
counts down to I = 25 and the corresponding redshift distributions are used as
joint constraints on cosmological models, in particular on the values of the
density parameter Omega_{0} and normalised cosmological constant Lambda_{0}.
We find that an Einstein - de Sitter universe with simple passive evolution
gives an excellent fit to the counts and redshift data at all magnitudes. An
open, low Omega_{0}, model with no net evolution (and conservation of the
number of ellipticals), which fits the counts equally well, is somewhat less
successful, predicting slightly lower mean redshifts and, more significantly,
the lack of a high--z tail. A number conserving model with a dominant
contribution from Lambda_{0}, on the other hand, is far less successful,
predicting a much narrower distribution than seen. More complex models are
obviously possible, but we conclude that if large scale transmutation between
types does {\it not} occur, then the lambda-dominated models provide a very
poor fit to the current data.Comment: Accepted for publication in MNRA
Opacity effects and shock-in-jet modelling of low-level activity in Cygnus X-3
We present simultaneous dual-frequency radio observations of Cygnus X-3
during a phase of low-level activity. We constrain the minimum variability
timescale to be 20 minutes at 43 GHz and 30 minutes at 15 GHz, implying source
sizes of 2 to 4 AU. We detect polarized emission at a level of a few per cent
at 43 GHz which varies with the total intensity. The delay of approximately 10
minutes between the peaks of the flares at the two frequencies is seen to
decrease with time, and we find that synchrotron self-absorption and free-free
absorption by entrained thermal material play a larger role in determining the
opacity than absorption in the stellar wind of the companion. A shock-in-jet
model gives a good fit to the lightcurves at all frequencies, demonstrating
that this mechanism, which has previously been used to explain the brighter,
longer-lived giant outbursts in this source, is also applicable to these
low-level flaring events. Assembling the data from outbursts spanning over two
orders of magnitude in flux density shows evidence for a strong correlation
between the peak brightness of an event, and the timescale and frequency at
which this is attained. Brighter flares evolve on longer timescales and peak at
lower frequencies. Analysis of the fitted model parameters suggests that
brighter outbursts are due to shocks forming further downstream in the jet,
with an increased electron normalisation and magnetic field strength both
playing a role in setting the strength of the outburst.Comment: Accepted for publication in MNRAS. 15 pages, 9 figure
Substructures in WINGS clusters
We search for and characterize substructures in the projected distribution of
galaxies observed in the wide field CCD images of the 77 nearby clusters of the
WIde-field Nearby Galaxy-cluster Survey (WINGS). This sample is complete in
X-ray flux in the redshift range 0.04<z<0.07. We search for substructures in
WINGS clusters with DEDICA, an adaptive-kernel procedure. We test the procedure
on Monte-Carlo simulations of the observed frames and determine the reliability
for the detected structures. DEDICA identifies at least one reliable structure
in the field of 55 clusters. 40 of these clusters have a total of 69
substructures at the same redshift of the cluster (redshift estimates of
substructures are from color-magnitude diagrams). The fraction of clusters with
subclusters (73%) is higher than in most studies. The presence of subclusters
affects the relative luminosities of the brightest cluster galaxies (BCGs).
Down to L ~ 10^11.2 L_Sun, our observed differential distribution of subcluster
luminosities is consistent with the theoretical prediction of the differential
mass function of substructures in cosmological simulations.Comment: A&A accepted - figure 6 is available from
http://adlibitum.oats.inaf.it/ramella/WINGSfig
The red-sequence of 72 WINGS local galaxy clusters
We study the color-magnitude red sequence and blue fraction of 72 X-ray
selected galaxy clusters at z=0.04-0.07 from the WINGS survey, searching for
correlations between the characteristics of the red sequence and the
environment. We consider the slope and scatter of the red sequence, the number
ratio of red luminous-to-faint galaxies, the blue fraction and the fractions of
ellipticals, S0s and spirals that compose the red sequence. None of these
quantities correlate with the cluster velocity dispersion, X-ray luminosity,
number of cluster substructures, BCG prevalence over next brightest galaxies
and spatial concentration of ellipticals. Instead, the properties of the red
sequence depend strongly on local galaxy density. Higher density regions have a
lower RS scatter, a higher luminous-to-faint ratio, a lower blue fraction, and
a lower spiral fraction on the RS. Our results highlight the prominent effect
of the local density in setting the epoch when galaxies become passive and join
the red sequence, as opposed to the mass of the galaxy host structure.Comment: Accepted for publication in A&
Tidal Dwarf Galaxies at Intermediate Redshifts
We present the first attempt at measuring the production rate of tidal dwarf
galaxies (TDGs) and estimating their contribution to the overall dwarf
population. Using HST/ACS deep imaging data from GOODS and GEMS surveys in
conjunction with photometric redshifts from COMBO-17 survey, we performed a
morphological analysis for a sample of merging/interacting galaxies in the
Extended Chandra Deep Field South and identified tidal dwarf candidates in the
rest-frame optical bands. We estimated a production rate about 1.4 {\times}
10^{-5} per Gyr per comoving volume for long-lived TDGs with stellar mass 3
{\times} 10^{8-9} solar mass at 0.5<z<1.1. Together with galaxy merger rates
and TDG survival rate from the literature, our results suggest that only a
marginal fraction (less than 10%) of dwarf galaxies in the local universe could
be tidally-originated. TDGs in our sample are on average bluer than their host
galaxies in the optical. Stellar population modelling of optical to
near-infrared spectral energy distributions (SEDs) for two TDGs favors a burst
component with age 400/200 Myr and stellar mass 40%/26% of the total,
indicating that a young stellar population newly formed in TDGs. This is
consistent with the episodic star formation histories found for nearby TDGs.Comment: 9 pages, 5 figures, Accepted for publication in Astrophysics & Space
Scienc
Optical and radio variability of the BL Lac object AO 0235+16: a possible 5-6 year periodicity
New optical and radio data on the BL Lacertae object AO 0235+16 have been
collected in the last four years by a wide international collaboration, which
confirm the intense activity of this source. The optical data also include the
results of the Whole Earth Blazar Telescope (WEBT) first-light campaign
organized in November 1997. The optical spectrum is observed to basically
steepen when the source gets fainter. We have investigated the existence of
typical variability time scales and of possible correlations between the
optical and radio emissions by means of visual inspection, Discrete Correlation
Function analysis, and Discrete Fourier Transform technique. The major radio
outbursts are found to repeat quasi-regularly with a periodicity of about 5.7
years; this period is also in agreement with the occurrence of some of the
major optical outbursts, but not all of them.Comment: to be published in A&
Dwarf Galaxies with Gentle Star Formation and the Counts of Galaxies in the Hubble Deep Field
In this paper the counts and colors of the faint galaxies observed in the
Hubble Deep Field are fitted by means of simple luminosity evolution models
which incorporate a numerous population of fading dwarfs. The observed color
distribution of the very faint galaxies allows now to put constrains on the
star formation history in dwarfs. It is shown that the star-forming activity in
these small systems has to proceed in a gentle way, i.e. through episodes each
one lasting much longer than a simple instantaneous burst of star formation. By
allowing dwarfs to form stars in this gentle way the number of predicted red
remnants is severely reduced, in good agreement with the observations. Then, if
the faint counts are to be fitted by means of dwarfs, the simple model for
dwarfs forming stars in single, very short episodes is challenged, and a more
complex star formation history has to be invoked. Recent observational evidence
supporting this new dwarf models are also discussed.Comment: 19 pages, 11 postscript figures, Latex. To appear in Ap
Rings and bars: unmasking secular evolution of galaxies
Secular evolution gradually shapes galaxies by internal processes, in
contrast to early cosmological evolution which is more rapid. An important
driver of secular evolution is the flow of gas from the disk into the central
regions, often under the influence of a bar. In this paper, we review several
new observational results on bars and nuclear rings in galaxies. They show that
these components are intimately linked to each other, and to the properties of
their host galaxy. We briefly discuss how upcoming observations, e.g., imaging
from the Spitzer Survey of Stellar Structure in Galaxies (S4G), will lead to
significant further advances in this area of research.Comment: Invited review at "Galaxies and their Masks", celebrating Ken
Freeman's 70-th birthday, Sossusvlei, Namibia, April 2010. To be published by
Springer, New York, editors D.L. Block, K.C. Freeman, & I. Puerari; minor
change
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