178 research outputs found
Star formation in galaxies along the Pisces-Cetus Supercluster filaments
We investigate the variation of current star formation in galaxies as a
function of distance along three supercluster filaments, each joining pairs of
rich clusters, in the Pisces-Cetus supercluster, which is part of the 2dFGRS.
We find that even though there is a steady decline in the rate of star
formation, as well as in the fraction of star forming galaxies, as one
approaches the core of a cluster at an extremity of such a filament, there is
an increased activity of star formation in a narrow distance range between
3-4/h_70 Mpc, which is 1.5-2 times the virial radius of the clusters involved.
This peak in star formation is seen to be entirely due to the dwarf galaxies
(-20<M_B<-17.5). The position of the peak does not seem to depend on the
velocity dispersion of the nearest cluster, undermining the importance of the
gravitational effect of the clusters involved. We find that this enhancement in
star formation occurs at the same place for galaxies which belong to groups
within these filaments, while group members elsewhere in the 2dFGRS do not show
this effect. We conclude that the most likely mechanism for this enhanced star
formation is galaxy-galaxy harassment, in the crowded infall region of rich
clusters at the extremities of filaments, which induces a burst of star
formation in galaxies, before they have been stripped of their gas in the
denser cores of clusters. The effects of strangulation in the cores of
clusters, as well as excess star formation in the infall regions along the
filaments, are more pronounced in dwarfs since they more vulnerable to the
effects of strangulation and harassment than giant galaxies.Comment: 10 pages, 9 figures; accepted for publication in MNRA
The observed distribution function of peculiar velocities of galaxies
We give the first determination of the observed peculiar velocity
distribution function for a representative sample of galaxies (within 50 Mpc
(H=100) of the Local Group), which includes a wide range of clustering
properties. We explore in detail the effects of uncertainties in sampling and
in distance measures on the estimated distribution function. The observed
distribution function is consistent with an earlier prediction of gravitational
clustering, over the entire range of peculiar velocities, from field galaxies
to rich clusters, on scales up to 50 Mpc (H=100). In the simplest consistent
model, most of the inhomogeneous mass of the Universe is in galaxies or their
halos. We estimate the ``Mach Number'' for the bulk flow within 50 Mpc (H=100)
from us to be approximately 0.8, which includes the effect of high-dispersion
galaxies in clusters.Comment: 28 pages, gzipped postscript file, including 11 figures and 2 tables;
To appear in the Astrophysical Journal, March 199
Kernel regression estimates of time delays between gravitationally lensed fluxes
Strongly lensed variable quasars can serve as precise cosmological probes,
provided that time delays between the image fluxes can be accurately measured.
A number of methods have been proposed to address this problem. In this paper,
we explore in detail a new approach based on kernel regression estimates, which
is able to estimate a single time delay given several datasets for the same
quasar. We develop realistic artificial data sets in order to carry out
controlled experiments to test of performance of this new approach. We also
test our method on real data from strongly lensed quasar Q0957+561 and compare
our estimates against existing results.Comment: Updated to match published versio
Estimating Time Delay in Gravitationally Lensed Fluxes
We study the problem of estimating the time delay between two signals representing delayed, irregularly sampled and noisy versions of the same underlying pattern. We propose a kernel-based technique in the context of an astronomical problem, namely estimating the time delay between
two gravitationally lensed signals from a distant quasar.
We test the algorithm on several artificial data sets, and
also on real astronomical observations. By carrying out a statistical analysis of the results we present a detailed comparison of our method with the most popular methods for time delay estimation in astrophysics. Our method yields more accurate and more stable time delay estimates. Our methodology can be readily applied to current state-of-the-art optical monitoring data in astronomy, but can also be applied in other disciplines involving similar time series data
New insights into the evolution of the FR I radio galaxy 3C 270 (NGC 4261) from VLA and GMRT radio observations
We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the
nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC
4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8
GHz observations, we produce spectral index maps that reveal a constant
spectral index along the jets and a gradual steepening from the ends of the
jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted
to the integrated spectrum of the source gives an asymptotic low-frequency
index of , while JP models fitted to the
observed spectral index trend along the lobes allow us to estimate radiative
ages of Myr and Myr for the west and east lobes respectively.
Our age estimates are a factor of two lower than the 75-Myr upper limit derived
from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an
early supersonic phase in which the lobe expanded into the ISM at approximately
Mach 6 (3500 km s), and suggest that either the source underwent
multiple AGN outbursts with possible large changes in jet power, or possibly
that the source age that we find is due to a backflow that transports young
electrons from the jet tips through the lobes toward the nucleus relatively
quickly. We calculate that in the lobes the energy ratio of non-radiating to
radiating particles is indicating significant gas entrainment. If
the lobes are in pressure balance with their surroundings, the total energy
required to heat the entrained material is erg, 40% of the
total enthalpy of the lobes.Comment: 14 pages, 11 figures, 8 tables. Accepted for publication by MNRAS.
Revised throughout in response to referee's comment
Uncovering delayed patterns in noisy and irregularly sampled time series: an astronomy application
We study the problem of estimating the time delay between two signals
representing delayed, irregularly sampled and noisy versions of the same
underlying pattern. We propose and demonstrate an evolutionary algorithm for
the (hyper)parameter estimation of a kernel-based technique in the context of
an astronomical problem, namely estimating the time delay between two
gravitationally lensed signals from a distant quasar. Mixed types (integer and
real) are used to represent variables within the evolutionary algorithm. We
test the algorithm on several artificial data sets, and also on real
astronomical observations of quasar Q0957+561. By carrying out a statistical
analysis of the results we present a detailed comparison of our method with the
most popular methods for time delay estimation in astrophysics. Our method
yields more accurate and more stable time delay estimates: for Q0957+561, we
obtain 419.6 days for the time delay between images A and B. Our methodology
can be readily applied to current state-of-the-art optical monitoring data in
astronomy, but can also be applied in other disciplines involving similar time
series data.Comment: 36 pages, 10 figures, 16 tables, accepted for publication in Pattern
Recognition. This is a shortened version of the article: interested readers
are urged to refer to the published versio
The fading of two transient ultraluminous x-ray sources to below the stellar mass Eddington limit
We report new detections of the two transient ultraluminous X-ray sources (ULXs) in NGC 5128 from an ongoing series of Chandra observations. Both sources have previously been observed L (2-3) Ă âŒ10 erg s, at the lower end of the ULX luminosity range. The new observations allow us to study these sources in the luminosity regime frequented by the Galactic black hole X-ray binaries (BH XBs). We present the recent lightcurves of both ULXs. 1RXH J132519.8-430312 (ULX1) was observed at L 1 Ă 10 erg s, while CXOU J132518.2-430304 (ULX2) declined to L 2 Ă 10 erg s and then lingered at this luminosity for hundreds of days. We show that a reasonable upper limit for both duty cycles is 0.2, with a lower limit of 0.12 for ULX2. This duty cycle is larger than anticipated for transient ULXs in old stellar populations. By fitting simple spectral models in an observation with âŒ50 counts we recover properties consistent with Galactic BH XBs, but inconclusive as to the spectral state. We utilize quantile analyses to demonstrate that the spectra are generally soft, and that in one observation the spectrum of ULX2 is inconsistent with a canonical hard state at >95% confidence. This is contrary to what would be expected of an accreting intermediate mass black hole primary, which we would expect to be in the hard state at these luminosities. We discuss the paucity of transient ULXs discovered in early-type galaxies and excogitate explanations. We suggest that the number of transient ULXs scales with the giant and sub-giant populations, rather than the total number of XBs.Peer reviewe
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