10,828 research outputs found
Properties of Galaxy Groups in the SDSS: I.-- The Dependence of Colour, Star Formation, and Morphology on Halo Mass
Using a large galaxy group catalogue constructed from the SDSS, we
investigate the correlation between various galaxy properties and halo mass. We
split the population of galaxies in early types, late types, and intermediate
types, based on their colour and specific star formation rate. At fixed
luminosity, the early type fraction increases with increasing halo mass. Most
importantly, this mass dependence is smooth and persists over the entire mass
range probed, without any break or feature at any mass scale. We argue that the
previous claim of a characteristic feature on galaxy group scales is an
artefact of the environment estimators used. At fixed halo mass, the luminosity
dependence of the type fractions is surprisingly weak: galaxy type depends more
strongly on halo mass than on luminosity. We also find that the early type
fraction decreases with increasing halo-centric radius. Contrary to previous
studies, we find that this radial dependence is also present in low mass
haloes. The properties of satellite galaxies are strongly correlated with those
of their central galaxy. In particular, the early type fraction of satellites
is significantly higher in a halo with an early type central galaxy than in a
halo of the same mass but with a late type central galaxy. This phenomenon,
which we call `galactic conformity', is present in haloes of all masses and for
satellites of all luminosities. Finally, the fraction of intermediate type
galaxies is always ~20 percent, independent of luminosity, independent of halo
mass, independent of halo-centric radius, and independent of whether the galaxy
is a central galaxy or a satellite galaxy. We discuss the implications of all
these findings for galaxy formation and evolution.Comment: 28 pages, 15 figures. Submitted for publication in MNRA
The Expected Perimeter in Eden and Related Growth Processes
Following Richardson and using results of Kesten on First-passage
percolation, we obtain an upper bound on the expected perimeter in an Eden
Growth Process. Using results of the author from a problem in Statistical
Mechanics, we show that the average perimeter of the lattice animals resulting
from a very natural family of "growth histories" does not obey a similar bound.Comment: 11 page
Observational Evidence for an Age Dependence of Halo Bias
We study the dependence of the cross-correlation between galaxies and galaxy
groups on group properties. Confirming previous results, we find that the
correlation strength is stronger for more massive groups, in good agreement
with the expected mass dependence of halo bias. We also find, however, that for
groups of the same mass, the correlation strength depends on the star formation
rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups
decreases as the SFR of the central galaxy increases. We discuss these findings
in light of the recent findings by Gao et al (2005) that halo bias depends on
halo formation time, in that halos that assemble earlier are more strongly
biased. We also discuss the implication for galaxy formation, and address a
possible link to galaxy conformity, the observed correlation between the
properties of satellite galaxies and those of their central galaxy.Comment: 4 pages, 4 figures, Accepted for publication in ApJ Letters. Figures
3 and 4 replaced. The bias dependence on the central galaxy luminosity is
omitted due to its sensitivity to the mass mode
Active noise compensation for multichannel magnetocardiography in an unshielded environment
A multichannel high-T/sub c/-SQUID-based heart scanner for unshielded environments is under development, Outside a magnetically shielded room, sensitive SQUID measurements are possible using gradiometers. However, it is difficult to realize large-baseline gradiometers in high-T/sub c/ materials, Therefore, the authors developed two active noise compensation techniques. In the Total Field Compensation technique, a Helmholtz type coil set is placed around the sensors. One magnetometer is used as a zero detector controlling the compensation current through the coil set. For Individual Flux Compensation, the reference signal is sent to the separate SQUIDs (or their flux transformer circuits) to compensate the local environmental noise fluxes, The latter technique was tested on low-T/sub c/ rf-SQUID magnetometers, each sensor set to a field resolution SQUID magnetometers, i.e. 0.1 pT/sub RMS///spl radic/Hz. The authors were able to suppress the environmental disturbances to such an extent that magnetocardiograms could be recorded in an ordinary environment. Here the two suppression techniques are described and experimental results are presente
Optical vernier technique for in-situ measurement of the length of long Fabry-Perot cavities
We propose a method for in-situ measurement of the length of kilometer size
Fabry-Perot cavities in laser gravitational wave detectors. The method is based
on the vernier, which occurs naturally when the laser incident on the cavity
has a sideband. By changing the length of the cavity over several wavelengths
we obtain a set of carrier resonances alternating with sideband resonances.
From the measurement of the separation between the carrier and a sideband
resonance we determine the length of the cavity. We apply the technique to the
measurement of the length of a Fabry-Perot cavity in the Caltech 40m
Interferometer and discuss the accuracy of the technique.Comment: LaTeX 2e, 12 pages, 4 figure
Mixing in anharmonic potential well
We prove phase-space mixing for solutions to Liouvilleâs equation for integrable systems. Under a natural non-harmonicity condition, we obtain weak convergence of the distribution function with rate âštimeâ©â1. In one dimension, we also study the case where this condition fails at a certain energy, showing that mixing still holds but with a slower rate. When the condition holds and functions have higher regularity, the rate can be faster
Properties of Galaxy Groups in the SDSS: II.- AGN Feedback and Star Formation Truncation
Successfully reproducing the galaxy luminosity function and the bimodality in
the galaxy distribution requires a mechanism that can truncate star formation
in massive haloes. Current models of galaxy formation consider two such
truncation mechanisms: strangulation, which acts on satellite galaxies, and AGN
feedback, which predominantly affects central galaxies. The efficiencies of
these processes set the blue fraction of galaxies as function of galaxy
luminosity and halo mass. In this paper we use a galaxy group catalogue
extracted from the Sloan Digital Sky Survey (SDSS) to determine these
fractions. To demonstrate the potential power of this data as a benchmark for
galaxy formation models, we compare the results to the semi-analytical model
for galaxy formation of Croton et al. (2006). Although this model accurately
fits the global statistics of the galaxy population, as well as the shape of
the conditional luminosity function, there are significant discrepancies when
the blue fraction of galaxies as a function of mass and luminosity is compared
between the observations and the model. In particular, the model predicts (i)
too many faint satellite galaxies in massive haloes, (ii) a blue fraction of
satellites that is much too low, and (iii) a blue fraction of centrals that is
too high and with an inverted luminosity dependence. In the same order, we
argue that these discrepancies owe to (i) the neglect of tidal stripping in the
semi-analytical model, (ii) the oversimplified treatment of strangulation, and
(iii) improper modeling of dust extinction and/or AGN feedback. The data
presented here will prove useful to test and calibrate future models of galaxy
formation and in particular to discriminate between various models for AGN
feedback and other star formation truncation mechanisms.Comment: 16 pages, 5 figures, submitted to MNRA
Deep radio images of the HEGRA and Whipple TeV sources in the Cygnus OB2 region
Context. The modern generation of Cherenkov telescopes has revealed a new
population of gamma-ray sources in the Galaxy. Some of them have been
identified with previously known X-ray binary systems while other remain
without clear counterparts a lower energies. Our initial goal here was
reporting on extensive radio observations of the first extended and yet
unidentified source, namely TeV J2032+4130. This object was originally detected
by the HEGRA telescope in the direction of the Cygnus OB2 region and its nature
has been a matter of debate during the latest years.
Aims. We aim to pursue our radio exploration of the TeV J2032+4130 position
that we initiated in a previous paper but taking now into account the latest
results from new Whipple and MILAGRO TeV telescopes.
Methods. Our investigation is mostly based on interferometric radio
observations with the Giant Metre Wave Radio Telescope (GMRT) close to Pune
(India) and the Very Large Array (VLA) in New Mexico (USA). We also conducted
near infrared observations with the 3.5 m telescope and the OMEGA2000 camera at
the Centro Astronomico Hispano Aleman (CAHA) in Almeria (Spain).
Results. We present deep radio maps centered on the TeV J2032+4130 position
at different wavelengths. In particular, our 49 and 20 cm maps cover a field of
view larger than half a degree that fully includes the Whipple position and the
peak of MILAGRO emission. Our most important result here is a catalogue of 153
radio sources detected at 49 cm within the GMRT antennae primary beam with a
full width half maximum (FWHM) of 43 arc-minute. Moreover, our
multi-configuration VLA images reveal the non-thermal extended emission
previously reported by us with improved angular resolution.Comment: 10 pages, 8 figures, 1 online catalogue. Accepted for publication in
Astronomy & Astrophysic
Structural effect on the static spin and charge correlations in LaBaSrCuO
We report the results of elastic neutron scattering measurements performed on
1/8-hole doped LaBaSrCuO single crystals with
{\it x}=0.05, 0.06, 0.075 and 0.085. In the low-temperature less-orthorhombic
(LTLO, {\it Pccn} symmetry) phase, the charge-density-wave (CDW) and
spin-density-wave (SDW) wavevectors were found to tilt in a low-symmetric
direction with one-dimensional anisotropy in the CuO plane, while they
were aligned along the high-symmetry axis in the low-temperature tetragonal
(LTT, {\it P}4/{\it ncm} symmetry) phase. The coincident direction of two
wavevectors suggests a close relation between CDW and SDW orders. The SDW
wavevector systematically deviates from the Cu-O bond direction in the LTLO
phase upon Sr substitution and the tilt angle in the LTLO phase is smaller than
that in the low-temperature orthorhombic phase (LTO, {\it B}{\it mab} symmetry)
with comparable in-plane orthorhombic distortion. These results demonstrate a
correlation between the corrugated pattern of CuO plane and the
deviations.Comment: 6 pages, 7figure
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