2,592 research outputs found
Selection bias in dynamically-measured super-massive black hole samples: consequences for pulsar timing arrays
Supermassive black hole -- host galaxy relations are key to the computation
of the expected gravitational wave background (GWB) in the pulsar timing array
(PTA) frequency band. It has been recently pointed out that standard relations
adopted in GWB computations are in fact biased-high. We show that when this
selection bias is taken into account, the expected GWB in the PTA band is a
factor of about three smaller than previously estimated. Compared to other
scaling relations recently published in the literature, the median amplitude of
the signal at yr drops from to
. Although this solves any potential tension between
theoretical predictions and recent PTA limits without invoking other dynamical
effects (such as stalling, eccentricity or strong coupling with the galactic
environment), it also makes the GWB detection more challenging.Comment: 6 pages 4 figures, submitted to MNRAS letter
Effects of correlation between merging steps on the global halo formation
The excursion set theory of halo formation is modified by adopting the
fractional Brownian motion, to account for possible correlation between merging
steps. We worked out analytically the conditional mass function, halo merging
rate and formation time distribution in the spherical collapse model. We also
developed an approximation for the ellipsoidal collapse model and applied it to
the calculation of the conditional mass function and the halo formation time
distribution. For models in which the steps are positively correlated, the halo
merger rate is enhanced when the accreted mass is less than , while
for the negatively correlated case this rate is reduced. Compared with the
standard model in which the steps are uncorrelated, the models with positively
correlated steps produce more aged population in small mass halos and more
younger population in large mass halos, while for the models with negatively
correlated steps the opposite is true. An examination of simulation results
shows that a weakly positive correlation between successive merging steps
appears to fit best. We have also found a systematic effect in the measured
mass function due to the finite volume of simulations. In future work, this
will be included in the halo model to accurately predict the three point
correlation function estimated from simulations.Comment: 8 pages, submitted to MNRA
Development of a flameproof elastic elastomeric fiber
Various flexible polyurethane structures containing halogen were synthesized from polyesters derived from aliphatic or aromatic polyols and dibasic acids. Aliphatic halide structures could not be used because they are unstable at the required reaction temperatures, giving of hydrogen halide which hydrolyzes the ester linkages. In contract, halogen-containing aromatic polyols were stable and satisfactory products were made. The most promising composition, a brominated neopentyl glycol capped with toluene disocyanate, was used as a conventional diisocyanate, in conjunction with hydroxy-terminated polyethers or polyesters to form elastomeric urethanes containing about 10% bromine with weight. Products made in this manner will not burn in air, have an oxygen index value of about 25, and have tensile strength values of about 5,000 psi at 450% elongation. The most efficient additives for imparting flame retardancy to Spandex urethanes are aromatic halides and the most effective of these are the bromide compounds. Various levels of flame retardancy have been achieved depending on the levels of additives used
Comparing PyMorph and SDSS photometry. II. The differences are more than semantics and are not dominated by intracluster light
The Sloan Digital Sky Survey pipeline photometry underestimates the
brightnesses of the most luminous galaxies. This is mainly because (i) the SDSS
overestimates the sky background and (ii) single or two-component Sersic-based
models better fit the surface brightness profile of galaxies, especially at
high luminosities, than does the de Vaucouleurs model used by the SDSS
pipeline. We use the PyMorph photometric reductions to isolate effect (ii) and
show that it is the same in the full sample as in small group environments, and
for satellites in the most massive clusters as well. None of these are expected
to be significantly affected by intracluster light (ICL). We only see an
additional effect for centrals in the most massive halos, but we argue that
even this is not dominated by ICL. Hence, for the vast majority of galaxies,
the differences between PyMorph and SDSS pipeline photometry cannot be ascribed
to the semantics of whether or not one includes the ICL when describing the
stellar mass of massive galaxies. Rather, they likely reflect differences in
star formation or assembly histories. Failure to account for the SDSS
underestimate has significantly biased most previous estimates of the SDSS
luminosity and stellar mass functions, and therefore Halo Model estimates of
the z ~ 0.1 relation between the mass of a halo and that of the galaxy at its
center. We also show that when one studies correlations, at fixed group mass,
with a quantity which was not used to define the groups, then selection effects
appear. We show why such effects arise, and should not be mistaken for physical
effects.Comment: 15 pages, 17 figures, accepted for publication in MNRAS. The PyMorph
luminosities and stellar masses are available at
https://www.physics.upenn.edu/~ameert/SDSS_PhotDec
The high mass end of the stellar mass function: Dependence on stellar population models and agreement between fits to the light profile
We quantify the systematic effects on the stellar mass function which arise
from assumptions about the stellar population, as well as how one fits the
light profiles of the most luminous galaxies at z ~ 0.1. When comparing results
from the literature, we are careful to separate out these effects. Our analysis
shows that while systematics in the estimated comoving number density which
arise from different treatments of the stellar population remain of order < 0.5
dex, systematics in photometry are now about 0.1 dex, despite recent claims in
the literature. Compared to these more recent analyses, previous work based on
Sloan Digital Sky Survey (SDSS) pipeline photometry leads to underestimates of
rho_*(> M_*) by factors of 3-10 in the mass range 10^11 - 10^11.6 M_Sun, but up
to a factor of 100 at higher stellar masses. This impacts studies which match
massive galaxies to dark matter halos. Although systematics which arise from
different treatments of the stellar population remain of order < 0.5 dex, our
finding that systematics in photometry now amount to only about 0.1 dex in the
stellar mass density is a significant improvement with respect to a decade ago.
Our results highlight the importance of using the same stellar population and
photometric models whenever low and high redshift samples are compared.Comment: 18 pages, 17 figures, accepted for publication in MNRAS. The PyMorph
luminosities and stellar masses are available at
https://www.physics.upenn.edu/~ameert/SDSS_PhotDec
On the cosmological mass function theory
This paper provides, from one side, a review of the theory of the
cosmological mass function from a theoretical point of view, starting from the
seminal paper of Press & Shechter (1974) to the last developments (Del Popolo &
Gambera (1998, 1999), Sheth & Tormen 1999 (ST), Sheth, Mo & Tormen 2001 (ST1),
Jenkins et al. 2001 (J01), Shet & Tormen 2002 (ST2), Del Popolo 2002a, Yagi et
al. 2004 (YNY)), and from another side some improvements on the multiplicity
function models in literature. ...Comment: Astronomy Reports, in prin
The virialized mass of dark matter haloes
(Abridged) Virial mass is used as an estimator for the mass of a dark matter
halo. However, the commonly used constant overdensity criterion does not
reflect the dynamical structure of haloes. Here we analyze dark matter
cosmological simulations in order to obtain properties of haloes of different
masses focusing on the size of the region with zero mean radial velocity. Dark
matter inside this region is stationary, and thus the mass of this region is a
much better approximation for the virial mass. We call this mass the static
mass to distinguish from the commonly used constant overdensity mass. We also
study the relation of this static mass with the traditional virial mass, and we
find that the matter inside galaxy-size haloes is underestimated by the virial
mass by nearly a factor of two. At redshift zero the virial mass is close to
the static mass for cluster-size haloes. The same pattern - large haloes having
M_vir > M_static - exists at all redshifts, but the transition mass M_0 = M_vir
= M_static decreases dramatically with increasing redshift. When rescaled to
the same M_0 haloes clearly demonstrate a self-similar behaviour, which in a
statistical sense gives a relation between the static and virial mass. To our
surprise we find that the abundance of haloes with a given static mass, i.e.
the static mass function, is very accurately fitted by the Press & Schechter
approximation at z=0, but this approximation breaks at higher redshifts.
Instead, the virial mass function is well fitted as usual by the Sheth & Tormen
approximation. We find an explanation why the static radius can be 2-3 times
larger as compared with the constant overdensity estimate. Applying the
non-stationary Jeans equation we find that the role of the pressure gradients
is significantly larger for small haloes.Comment: 14 pages, 16 figures, accepted for publication in MNRAS. v2:
Evolution of static mass function and some other minor changes added to match
the accepted versio
The massive end of the luminosity and stellar mass functions: Dependence on the fit to the light profile
In addition to the large systematic differences arising from assumptions
about the stellar mass-to-light ratio, the massive end of the stellar mass
function is rather sensitive to how one fits the light profiles of the most
luminous galaxies. We quantify this by comparing the luminosity and stellar
mass functions based on SDSS cmodel magnitudes, and PyMorph single-Sersic and
Sersic-Exponential fits to the surface brightness profiles of galaxies in the
SDSS. The PyMorph fits return more light, so that the predicted masses are
larger than when cmodel magnitudes are used. As a result, the total stellar
mass density at z~0.1 is about 1.2x larger than in our previous analysis of the
SDSS. The differences are most pronounced at the massive end, where the
measured number density of objects having M* > 6 x 10^{11} Msun is ~5x larger.
Alternatively, at number densities of 10^{-6} Mpc^{-3}, the limiting stellar
mass is 2x larger. The differences with respect to fits by other authors,
typically based on Petrosian-like magnitudes, are even more dramatic, although
some of these differences are due to sky-subtraction problems, and are
sometimes masked by large differences in the assumed (even after
scaling to the same IMF). Our results impact studies of the growth and assembly
of stellar mass in galaxies, and of the relation between stellar and halo mass,
so we provide simple analytic fits to these new luminosity and stellar mass
functions and quantify how they depend on morphology, as well as the binned
counts in electronic format.Comment: 8 pages, 6 figures; accepted for publication in MNRAS. Electronic
versions of Tables 2 and 3, which list the binned counts, are provided as
"ancillary" file
Probing Primordial Non-Gaussianity with Large-Scale Structure
We consider primordial non-Gaussianity due to quadratic corrections in the
gravitational potential parametrized by a non-linear coupling parameter fnl. We
study constraints on fnl from measurements of the galaxy bispectrum in redshift
surveys. Using estimates for idealized survey geometries of the 2dF and SDSS
surveys and realistic ones from SDSS mock catalogs, we show that it is possible
to probe |fnl|~100, after marginalization over bias parameters. We apply our
methods to the galaxy bispectrum measured from the PSCz survey, and obtain a
2sigma-constraint |fnl|< 1800. We estimate that an all sky redshift survey up
to z~1 can probe |fnl|~1. We also consider the use of cluster abundance to
constrain fnl and find that in order to be sensitive to |fnl|~100, cluster
masses need to be determined with an accuracy of a few percent, assuming
perfect knowledge of the mass function and cosmological parameters.Comment: 15 pages, 7 figure
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