849 research outputs found
Halo abundances within the cosmic web
We investigate the dependence of the mass function of dark-matter haloes on
their environment within the cosmic web of large-scale structure. A dependence
of the halo mass function on large-scale mean density is a standard element of
cosmological theory, allowing mass-dependent biasing to be understood via the
peak-background split. On the assumption of a Gaussian density field, this
analysis can be extended to ask how the mass function depends on the
geometrical environment: clusters, filaments, sheets and voids, as classified
via the tidal tensor (the Hessian matrix of the gravitational potential). In
linear theory, the problem can be solved exactly, and the result is
attractively simple: the conditional mass function has no explicit dependence
on the local tidal field, and is a function only of the local density on the
filtering scale used to define the tidal tensor. There is nevertheless a strong
implicit predicted dependence on geometrical environment, because the local
density couples statistically to the derivatives of the potential. We compute
the predictions of this model and study the limits of their validity by
comparing them to results deduced empirically from -body simulations. We
have verified that, to a good approximation, the abundance of haloes in
different environments depends only on their densities, and not on their tidal
structure. In this sense we find relative differences between halo abundances
in different environments with the same density which are smaller than 13%.
Furthermore, for sufficiently large filtering scales, the agreement with the
theoretical prediction is good, although there are important deviations from
the Gaussian prediction at small, non-linear scales. We discuss how to obtain
improved predictions in this regime, using the 'effective-universe' approach.Comment: 14 pages, 6 figures. Revision matching journal versio
Fourier analysis of luminosity-dependent galaxy clustering
We extend the Fourier transform based method for the analysis of galaxy
redshift surveys of Feldman, Kaiser & Peacock (1994: FKP) to model
luminosity-dependent clustering. In a magnitude limited survey, galaxies at
high redshift are more luminous on average than galaxies at low redshift.
Galaxy clustering is observed to increase with luminosity, so the inferred
density field is effectively multiplied by an increasing function of radius.
This has the potential to distort the shape of the recovered power spectrum. In
this paper we present an extension of the FKP analysis method to incorporate
this effect, and present revised optimal weights to maximize the precision of
such an analysis. The method is tested and its accuracy assessed using mock
catalogues of the 2-degree field galaxy redshift survey (2dFGRS). We also show
that the systematic effect caused by ignoring luminosity-dependent bias was
negligible for the initial analysis of the 2dFGRS of Percival et al. (2001).
However, future surveys, sensitive to larger scales, or covering a wider range
of galaxy luminosities will benefit from this refined method.Comment: 9 pages, 4 figures, accepted for publication in MNRA
Overmerging and M/L ratios in phenomenological galaxy formation models
We show that the discrepancy between the Tully-Fisher relation and the
luminosity function predicted by most phenomenological galaxy formation models
is mainly due to overmerging of galaxy haloes. We have circumvented this
overmerging problem, which is inherent in both the Press-Schechter formalism
and dissipationless N-body simulations, by including a specific galaxy halo
formation recipe into an otherwise standard N-body code. This numerical
technique provides the merger trees which, together with simplified gas
dynamics and star formation physics, constitute our implementation of a
phenomenological galaxy formation model. Resolving the overmerging problem
provides us with the means to match both the I-band Tully-Fisher relation and
the B and K band luminosity functions within an EdS sCDM structure formation
scenario. It also allows us to include models for chemical evolution and
starbursts, which improves the match to observational data and renders the
modelling more realistic. We show that the inclusion of chemical evolution into
the modelling requires a significant fraction of stars to be formed in short
bursts triggered by merging events.Comment: 15 pages, 7 figures, to be published in MNRA
Extended Hernquist-Springel formalism for cosmic star formation
We present a revised and extended version of the analytic model for cosmic
star formation originally given by Hernquist & Springel in 2003. The key
assumption of this formalism is that star formation proceeds from cold gas, at
a rate that is limited by an internal consumption timescale at early times, or
by the rate of generation of gas via cooling at late times. These processes are
analysed as a function of the mass of dark matter haloes and integrated over
the halo population. We modify this approach in two main ways to make it more
general: (1) halo collapse times are included explicitly, so that the behaviour
is physically reasonable at late times; (2) allowance is made for a
mass-dependent baryon fraction in haloes, which incorporates feedback effects.
This model reproduces the main features of the observed baryonic Tully-Fisher
relationship, and is consistent with observational estimates of the baryon mass
fraction in the intergalactic medium. With minimal adjustment of parameters,
our approach reproduces the observed history of cosmic star formation within a
factor of two over the redshift range . This level of agreement is
comparable to that achieved by state-of-the-art cosmological simulations. Our
simplified apparatus has pedagogical value in illuminating the results of such
detailed calculations, and also serves as a means for rapid approximate
exploration of non-standard cosmological models.Comment: submitted to MNRA
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