6,247 research outputs found
The role of active galactic nuclei in galaxy formation
We use Monte-Carlo Markov chain techniques to constrain acceptable parameter
regions for the Munich L-Galaxies semi-analytic galaxy formation model.
Feedback from active galactic nuclei (AGN) is required to limit star-formation
in the most massive galaxies. However, we show that the introduction of tidal
stripping of dwarf galaxies as they fall into and merge with their host systems
can lead to a reduction in the required degree of AGN feedback. In addition,
the new model correctly reproduces both the metallicity of large galaxies and
the fraction of intracluster light.Comment: Monster's Fiery Breath Conference Proceedings, 4 page
Exact Solutions for Boson-Fermion Stars in (2+1) dimensions
We solve Einstein equations coupled to a complex scalar field with infinitely
large self-interaction, degenerate fermions, and a negative cosmological
constant in dimensions. Exact solutions for static boson-fermion stars
are found when circular symmetry is assumed. We find that the minimum binding
energy of boson-fermion star takes a negative value if the value of the
cosmological constant is sufficiently small.Comment: 19 pages, 5 figures, RevTeX 3.0, second revised versio
Quasi-complete intersection homomorphisms
Extending a notion defined for surjective maps by Blanco, Majadas, and
Rodicio, we introduce and study a class of homomorphisms of commutative
noetherian rings, which strictly contains the class of locally complete
intersection homomorphisms, while sharing many of its remarkable properties.Comment: Final version, to appear in the special issue of Pure and Applied
Mathematics Quarterly dedicated to Andrey Todorov. The material in the first
four sections has been reorganized and slightly expande
Monte Carlo Markov Chain parameter estimation in semi-analytic models of galaxy formation
We present a statistical exploration of the parameter space of the De Lucia and Blaizot version of the Munich semi-analytic (SA) model built upon the Millennium dark matter simulation. This is achieved by applying a Monte Carlo Markov Chain method to constrain the six free parameters that define the stellar and black hole mass functions at redshift zero. The model is tested against three different observational data sets, including the galaxy K-band luminosity function, B - V colours and the black hole-bulge mass relation, separately and combined, to obtain mean values, confidence limits and likelihood contours for the best-fitting model. Using each observational data set independently, we discuss how the SA model parameters affect each galaxy property and find that there are strong correlations between them. We analyse to what extent these are simply reflections of the observational constraints, or whether they can lead to improved understandings of the physics of galaxy formation.
When all the observations are combined, we find reasonable agreement between the majority of the previously published parameter values and our confidence limits. However, the need to suppress dwarf galaxy formation requires the strength of the supernova feedback to be significantly higher in our best-fitting solution than in previous work.
To balance this, we require the feedback to become ineffective in haloes of lower mass than before, so as to permit the formation of sufficient high-luminosity galaxies: unfortunately, this leads to an excess of galaxies around L*. Although the best fit is formally consistent with the data, there is no region of parameter space that reproduces the shape of galaxy luminosity function across the whole magnitude range.
For our best fit, we present the model predictions for the bJ-band luminosity and stellar mass functions. We find a systematic disagreement between the observed mass function and the predictions from the K-band constraint, which we explain in light of recent works that suggest uncertainties of up to 0.3 dex in the mass determination from stellar population synthesis models.
We discuss modifications to the SA model that might simultaneously improve the fit to the observed mass function and reduce the reliance on excessive supernova feedback in small haloes
Gravitational wave generation in hybrid quintessential inflationary models
We investigate the generation of gravitational waves in the hybrid
quintessential inflationary model. The full gravitational-wave energy spectrum
is calculated using the method of continuous Bogoliubov coefficients. The
post-inflationary kination period, characteristic of quintessential
inflationary models, leaves a clear signature on the spectrum, namely, a peak
at high frequencies. The maximum of the peak is firmly located at the MHz-GHz
region of the spectrum and corresponds to . This
peak is substantially smaller than the one appearing in the gravitational-wave
energy spectrum of the original quintessential inflationary model, therefore
avoiding any conflict with the nucleosynthesis constraint on
\Omega_\Omega_{GW}.Comment: 10 pages, 11 figures, one reference adde
Diffusion anomaly and dynamic transitions in the Bell-Lavis water model
In this paper we investigate the dynamic properties of the minimal Bell-Lavis
(BL) water model and their relation to the thermodynamic anomalies. The
Bell-Lavis model is defined on a triangular lattice in which water molecules
are represented by particles with three symmetric bonding arms interacting
through van der Waals and hydrogen bonds. We have studied the model diffusivity
in different regions of the phase diagram through Monte Carlo simulations. Our
results show that the model displays a region of anomalous diffusion which lies
inside the region of anomalous density, englobed by the line of temperatures of
maximum density (TMD). Further, we have found that the diffusivity undergoes a
dynamic transition which may be classified as fragile-to-strong transition at
the critical line only at low pressures. At higher densities, no dynamic
transition is seen on crossing the critical line. Thus evidence from this study
is that relation of dynamic transitions to criticality may be discarded
Supermassive black holes as the regulators of star formation in central galaxies
We present a relationship between the black hole mass, stellar mass, and star
formation rate of a diverse group of 91 galaxies with dynamically-measured
black hole masses. For our sample of galaxies with a variety of morphologies
and other galactic properties, we find that the specific star formation rate is
a smoothly decreasing function of the ratio between black hole mass and stellar
mass, or what we call the specific black hole mass. In order to explain this
relation, we propose a physical framework where the gradual suppression of a
galaxy's star formation activity results from the adjustment to an increase in
specific black hole mass and, accordingly, an increase in the amount of
heating. From this framework, it follows that at least some galaxies with
intermediate specific black hole masses are in a steady state of partial
quiescence with intermediate specific star formation rates, implying that both
transitioning and steady-state galaxies live within this region known as the
"green valley." With respect to galaxy formation models, our results present an
important diagnostic with which to test various prescriptions of black hole
feedback and its effects on star formation activity.Comment: 15 pages, 4 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
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