1,720,964 research outputs found
An excursion set model of hierarchical clustering: ellipsoidal collapse and the moving barrier
No full textThe excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. The predictions of this approach depend on how the non-linear processes of collapse and virialization are modelled. We present simple analytic approximations that allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function, which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. This happens because low-density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high-density regions are collapsed haloes viewed at small lookback times. Although we have applied our analytic formulae only to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold
Formation times and masses of dark matter haloes
Full text linkThe most commonly used definition of halo formation is the time when the most massive progenitor of a halo first contains at least half the final mass of its parent. Reasonably accurate formulae for the distribution of formation times of haloes of fixed mass have been available for some time. We use numerical simulations of hierarchical gravitational clustering to test the accuracy of formulae for the mass at formation. We also derive and test a formula for the joint distribution of formation masses and times. The structure of a halo is expected to be related to its accretion history. Our tests show that our formulae for formation masses and times are reasonably accurate, so we expect that they will aid future analytical studies of halo structure
On the environmental dependence of halo formation
No full textA generic prediction of hierarchical gravitational clustering models is
that the distribution of halo formation times should depend relatively
strongly on halo mass, massive haloes forming more recently, and depend
only weakly, if at all, on the large-scale environment of the haloes. We
present a novel test of this assumption, which uses the statistics of
weighted or `marked' correlations, which prove to be particularly
well-suited to detecting and quantifying weak correlations with
environment. We find that close pairs of haloes form at slightly higher
redshifts than more widely separated halo pairs, suggesting that haloes
in dense regions form at slightly earlier times than haloes of the same
mass in less dense regions. The environmental trends we find are useful
for models that relate the properties of galaxies to the formation
histories of the haloes that surround them
Large-scale bias and the peak background split
No full textDark matter haloes are biased tracers of the underlying dark matter distribution. We use a simple model to provide a relation between the abundance of dark matter haloes and their spatial distribution on large scales. Our model shows that knowledge of the unconditional mass function alone is sufficient to provide an accurate estimate of the large-scale bias factor. We then use the mass function measured in numerical simulations of SCDM, OCDM and LambdaCDM to compute this bias. Comparison with these simulations shows that this simple way of estimating the bias relation and its evolution is accurate for less massive haloes as well as massive ones. In particular, we show that haloes that are less/more massive than typical M_* haloes at the time they form are more/less strongly clustered than is predicted by formulae based on the standard Press-Schechter mass function
Ellipsoidal collapse and an improved model for the number and spatial distribution of dark matter haloes
No full textThe Press-Schechter, excursion set approach allows one to make predictions about the shape and evolution of the mass function of bound objects. The approach combines the assumption that objects collapse spherically with the assumption that the initial density fluctuations were Gaussian and small. The predicted mass function is reasonably accurate, although it has fewer high-mass and more low-mass objects than are seen in simulations of hierarchical clustering. We show that the discrepancy between theory and simulation can be reduced substantially if bound structures are assumed to form from an ellipsoidal, rather than a spherical, collapse. In the original, standard, spherical model, a region collapses if the initial density within it exceeds a threshold value, δsc. This value is independent of the initial size of the region, and since the mass of the collapsed object is related to its initial size, this means that δsc is independent of final mass. In the ellipsoidal model, the collapse of a region depends on the surrounding shear field, as well as on its initial overdensity. In Gaussian random fields, the distribution of these quantities depends on the size of the region considered. Since the mass of a region is related to its initial size, there is a relation between the density threshold value required for collapse and the mass of the final object. We provide a fitting function to this δec(m) relation which simplifies the inclusion of ellipsoidal dynamics in the excursion set approach. We discuss the relation between the excursion set predictions and the halo distribution in high-resolution N-body simulations, and use our new formulation of the approach to show that our simple parametrization of the ellipsoidal collapse model represents an improvement on the spherical model on an object-by-object basis. Finally, we show that the associated statistical predictions, the mass function and the large-scale halo-to-mass bias relation, are also more accurate than the standard predictions
SMBH mass function from velocity dispersion and luminosity
No full textBlack-hole masses are tightly correlated with the stellar velocity dispersion of host galaxies, and slightly less-well correlated with the bulge luminosities; the M• -σ relation predicts fewer massive black holes than does the M• -L relation.
This is because the L-σ in currently available black-hole samples is inconsistent with that in the samples from which the distributions of L or σ are based. This suggests that current black-hole samples are biased and that the M• -σ and M• -L relations currently in the literature are also biased from their intrinsic values. Our analysis suggests that the bias in the M• -σ relation is likely to be small, whereas the M• -L relation is biased towards predicting more massive black holes for a given luminosity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An improved model for the formation times of dark matter haloes
No full textA dark matter halo is said to have formed when at least half its mass
has been assembled into a single progenitor. With this definition, it is
possible to derive a simple but useful analytic estimate of the
distribution of halo formation times. The standard estimate of this
distribution depends on the shape of the conditional mass function - the
distribution of progenitor masses of a halo as a function of time. If
the spherical collapse model is used to estimate the progenitor mass
function, then the formation times one infers systematically
underestimate those seen in numerical simulations of hierarchical
gravitational clustering. We provide estimates of halo formation which
may be related to an ellipsoidal collapse model. These estimates provide
a substantially better description of the simulations. We also provide
an alternative derivation of the formation time distribution which is
based on the assumption that haloes increase their mass through binary
mergers only. Our results are useful for models which relate halo
structure to halo formation
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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