27,519 research outputs found
Comparison of simple mass estimators for slowly rotating elliptical galaxies
We compare the performance of mass estimators for elliptical galaxies that
rely on the directly observable surface brightness and velocity dispersion
profiles, without invoking computationally expensive detailed modeling. These
methods recover the mass at a specific radius where the mass estimate is
expected to be least sensitive to the anisotropy of stellar orbits. One method
(Wolf et al. 2010) uses the total luminosity-weighted velocity dispersion and
evaluates the mass at a 3D half-light radius , i.e., it depends on the
GLOBAL galaxy properties. Another approach (Churazov et al. 2010) estimates the
mass from the velocity dispersion at a radius where the surface
brightness declines as , i.e., it depends on the LOCAL properties. We
evaluate the accuracy of the two methods for analytical models, simulated
galaxies and real elliptical galaxies that have already been modeled by the
Schwarzschild's orbit-superposition technique. Both estimators recover an
almost unbiased circular speed estimate with a modest RMS scatter (). Tests on analytical models and simulated galaxies indicate that the local
estimator has a smaller RMS scatter than the global one. We show by examination
of simulated galaxies that the projected velocity dispersion at could
serve as a good proxy for the virial galaxy mass. For simulated galaxies the
total halo mass scales with as with RMS scatter
.Comment: 19 pages, 14 figures, 4 tables, accepted for publication in MNRA
Time-frequency analysis of locally stationary Hawkes processes
Locally stationary Hawkes processes have been introduced in order to
generalise classical Hawkes processes away from stationarity by allowing for a
time-varying second-order structure. This class of self-exciting point
processes has recently attracted a lot of interest in applications in the life
sciences (seismology, genomics, neuro-science,...), but also in the modelling
of high-frequency financial data. In this contribution we provide a fully
developed nonparametric estimation theory of both local mean density and local
Bartlett spectra of a locally stationary Hawkes process. In particular we apply
our kernel estimation of the spectrum localised both in time and frequency to
two data sets of transaction times revealing pertinent features in the data
that had not been made visible by classical non-localised approaches based on
models with constant fertility functions over time.Comment: Bernoulli journal, A Para{\^i}tr
zCOSMOS 10k-bright spectroscopic sample: Exploring mass and environment dependence in early-type galaxies
Aims. We present the analysis of the U â V rest-frame color distribution and some spectral features as a function of mass and environment for a sample of early-type galaxies up to z = 1 extracted from the zCOSMOS spectroscopic survey. This analysis is used to place constraints on the relative importance of these two properties in controlling galaxy evolution.
Methods. We used the zCOSMOS 10k-bright sample, limited to the AB magnitude range 15 < I < 22.5, from which we extracted two different subsamples of early-type galaxies. The first sample (âred galaxiesâ) was selected using a photometric classification (2098 galaxies), while in the second case (âETGsâ) we combined morphological, photometric, and spectroscopic properties to obtain a more reliable sample of elliptical, red, passive, early-type galaxies (981 galaxies). The analysis is performed at fixed mass to search for any dependence of the color distribution on environment, and at fixed environment to search for any mass dependence.
Results. In agreement with the low redshift results of the SDSS, we find that the color distribution of red galaxies is not strongly dependent on environment for all mass bins, exhibiting only a weak trend such that galaxies in overdense regions (log_(10)(1+Î) ~ 1.2) are redder than galaxies in underdense regions (log_(10)(1+Î) ~ 0.1), â©with a difference of = 0.027±0.008 mag. On the other hand, the dependence on mass is far more significant, and we find that the average colors of massive galaxies (log_(10)(M/M_â) ~ 10.8) are redder by = 0.093±0.007 mag than low-mass galaxies (log_(10)(M/M_â) ~ 10) â©throughout the entire redshift range. We study the color-mass (U â V)_(rest) â S_M ·log_(10)(M/M_â) relation, finding a mean slope = 0.12±0.005, while the color-environment (U â V)_(rest) â S_ÎŽ · log_(10)(1+Î) relation is flatter, with a slope always smaller than S_ÎŽ â 0.04. â©
The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results: we study the 4000 Ă
â break and the equivalent width of the HÎŽ Balmer line, finding for D4000 a dependence on mass ( =0.11±0.02 between log_(10)(M/M_â) ~ 10.2 and log_(10)(M/M_â) ~ 10.8), and a much weaker dependence on environment ( = 0.05±0.02 between high and low environment quartiles). The same is true for the equivalent width of HÎŽ, for which we measure a difference of ÎEW0(HÎŽ) = 0.28±0.08 Ă
â across the same mass range and no significant dependence on environment. By analyzing the lookback time of early-type galaxies, we support the possibility of a downsizing scenario, in which massive galaxies with a stronger D4000 and an almost constant equivalent width of HÎŽ formed their mass at higher redshift than lower mass ones. We also conclude that the main driver of galaxy evolution is the galaxy mass, the environment playing a subdominant role
Halo assembly bias and the tidal anisotropy of the local halo environment
We study the role of the local tidal environment in determining the assembly
bias of dark matter haloes. Previous results suggest that the anisotropy of a
halo's environment (i.e, whether it lies in a filament or in a more isotropic
region) can play a significant role in determining the eventual mass and age of
the halo. We statistically isolate this effect using correlations between the
large-scale and small-scale environments of simulated haloes at with
masses between . We
probe the large-scale environment using a novel halo-by-halo estimator of
linear bias. For the small-scale environment, we identify a variable
that captures the in a region of radius
around the halo and correlates strongly with halo bias
at fixed mass. Segregating haloes by reveals two distinct
populations. Haloes in highly isotropic local environments
() behave as expected from the simplest, spherically
averaged analytical models of structure formation, showing a
correlation between their concentration and large-scale
bias at masses. In contrast, haloes in anisotropic,
filament-like environments () tend to show a
correlation between bias and concentration at any mass. Our
multi-scale analysis cleanly demonstrates how the overall assembly bias trend
across halo mass emerges as an average over these different halo populations,
and provides valuable insights towards building analytical models that
correctly incorporate assembly bias. We also discuss potential implications for
the nature and detectability of galaxy assembly bias.Comment: 19 pages, 15 figures; v2: revised in response to referee comments,
added references and discussion, conclusions unchanged. Accepted in MNRA
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