182 research outputs found
The role of galaxy formation in the structure and dynamics of dark matter halos
The structure and dynamics of dark matter halos, as predicted by the hierarchical clustering scenario, are at odds with the properties inferred from the observations at galactic scales. My Thesis addresses this problem by taking an evolutionary approach. I analysed in detail the many and different observational evidences of a discrepancy the predicted halo equilibrium state and the one inferred from the measurable properties of disk galaxies, as well as of the scaling relations existing between the angular momentum, geometry and mass distribution of the luminous and dark components, and realized that they all seem to point towards the same conclusion: the baryons hosted inside the halo, by collapsing and assembling to form the galaxy, perturb the halo equilibrium structure and made it evolve into new configurations. From the theoretical point of view, the behaviour of dark matter halos as collisionless systems of particles makes their equilibrium structure and mass distribution extremely sensitive to perturbations of their inner dynamics. The galaxy formation occurring inside the halos is a tremendous event, and the dynamical coupling between the baryons and the dark matter during the protogalaxy collapse represents a perturbation of the halo dynamical structure large enough to trigger a halo evolution, according to the relative mass and angular momentum of the two components. My conclusion is that the structure and dynamics of dark matter halos, as well as the origin of the connection between the halo and galaxy properties, are to be understood in in terms of a joint evolution of the baryonic and dark components, originating at the epoch of the collapse and formation of the galaxy
Hierarchical models of high redshift galaxies with thermally pulsing asymptotic giant branch stars: comparison with observations
In a recent paper we presented the first semi-analytic model of galaxy
formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of
stellar evolution has been fully implemented. Here we address the comparison
with observations, and show how the TP-AGB recipe affects the performance of
the model in reproducing the colours and near-IR luminosities of high-redshift
galaxies. We find that the semi-analytic model with the TP-AGB better matches
the colour-magnitude and colour-colour relations at z ~ 2, both for
nearly-passive and for star-forming galaxies. The model with TP-AGB produces
star-forming galaxies with red V-K colours, thus revising the unique
interpretation of high-redshift red objects as 'red & dead'. We also show that
without the TP-AGB the semi-analytic model fails at reproducing the observed
colours, a situation that cannot be corrected by dust reddening. We also
explore the effect of nebular emission on the predicted colour-magnitude
relation of star-forming galaxies, to conclude that it does not play a
significant role in reddening their colours, at least in the range of
star-formation rates covered by the model. Finally, the rest-frame K-band
luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This
indicates that the AGN feedback recipe that is adopted to regulate the
high-mass end of the luminosity function should be sophisticated to take the
effect of the stellar populations into account at high redshifts.Comment: 10 pages, 8 figures; effects of nebular emission included; accepted
for publication on MNRA
The Theoretical Astrophysical Observatory: Cloud-Based Mock Galaxy Catalogues
We introduce the Theoretical Astrophysical Observatory (TAO), an online
virtual laboratory that houses mock observations of galaxy survey data. Such
mocks have become an integral part of the modern analysis pipeline. However,
building them requires an expert knowledge of galaxy modelling and simulation
techniques, significant investment in software development, and access to high
performance computing. These requirements make it difficult for a small
research team or individual to quickly build a mock catalogue suited to their
needs. To address this TAO offers access to multiple cosmological simulations
and semi-analytic galaxy formation models from an intuitive and clean web
interface. Results can be funnelled through science modules and sent to a
dedicated supercomputer for further processing and manipulation. These modules
include the ability to (1) construct custom observer light-cones from the
simulation data cubes; (2) generate the stellar emission from star formation
histories, apply dust extinction, and compute absolute and/or apparent
magnitudes; and (3) produce mock images of the sky. All of TAO's features can
be accessed without any programming requirements. The modular nature of TAO
opens it up for further expansion in the future.Comment: 17 pages, 11 figures, 2 tables; accepted for publication in ApJS. The
Theoretical Astrophysical Observatory (TAO) is now open to the public at
https://tao.asvo.org.au/. New simulations, models and tools will be added as
they become available. Contact [email protected] if you have data you
would like to make public through TAO. Feedback and suggestions are very
welcom
Semi-Analytic Galaxy Evolution (SAGE): Model Calibration and Basic Results
This paper describes a new publicly available codebase for modelling galaxy
formation in a cosmological context, the "Semi-Analytic Galaxy Evolution"
model, or SAGE for short. SAGE is a significant update to that used in Croton
et al. (2006) and has been rebuilt to be modular and customisable. The model
will run on any N-body simulation whose trees are organised in a supported
format and contain a minimum set of basic halo properties. In this work we
present the baryonic prescriptions implemented in SAGE to describe the
formation and evolution of galaxies, and their calibration for three N-body
simulations: Millennium, Bolshoi, and GiggleZ. Updated physics include: gas
accretion, ejection due to feedback, and reincorporation via the galactic
fountain; a new gas cooling--radio mode active galactic nucleus (AGN) heating
cycle; AGN feedback in the quasar mode; a new treatment of gas in satellite
galaxies; and galaxy mergers, disruption, and the build-up of intra-cluster
stars. Throughout, we show the results of a common default parameterization on
each simulation, with a focus on the local galaxy population.Comment: 15 pages, 9 figures, accepted for publication in ApJS. SAGE is a
publicly available codebase for modelling galaxy formation in a cosmological
context, available at https://github.com/darrencroton/sage Questions and
comments can be sent to Darren Croton: [email protected]
Star formation rates and masses of z ~ 2 galaxies from multicolour photometry
Fitting synthetic spectral energy distributions (SED) to the multi-band
photometry of galaxies to derive their star formation rates (SFR), stellar
masses, ages, etc. requires making a priori assumptions about their star
formation histories (SFH). A widely adopted parameterization of the SFH, the
so-called tau-models where SFR goes as e^{-t/tau) is shown to lead to
unrealistically low ages when applied to star forming galaxies at z ~ 2, a
problem shared by other SFHs when the age is left as a free parameter in the
fitting. This happens because the SED of such galaxies, at all wavelengths, is
dominated by their youngest stellar populations, which outshine the older ones.
Thus, the SED of such galaxies conveys little information on the beginning of
star formation. To cope with this problem, we explore a variety of SFHs, such
as constant SFR and inverted-tau models - with SFR as e^{+t/tau) - along with
various priors on age, including assuming that star formation started at high
redshift in all the galaxies. We find that inverted-tau models with such latter
assumption give SFRs and extinctions in excellent agreement with the values
derived using only the UV part of the SED. These models are also shown to
accurately recover the SFRs and masses of mock galaxies at z ~ 2 constructed
from semi-analytic models. All other explored SFH templates do not fulfil these
two test. In particular, direct-tau models with unconstrained age in the
fitting procedure overstimate SFRs and underestimate stellar mass, and would
exacerbate an apparent mismatch between the cosmic evolution of the volume
densities of SFR and stellar mass. We conclude that for high-redshift star
forming galaxies an exponentially increasing SFR with a high formation redshift
is preferable to other forms of the SFH so far adopted in the literature.Comment: 19 pages, 28 figures, Monthly Notices of the Royal Astronomical
Society in pres
The hierarchical build-up of the Tully-Fisher relation
We use the semi-analytic model GalICS to predict the Tully-Fisher relation in
the B, I and for the first time, in the K band, and its evolution with
redshift, up to z~1. We refined the determination of the disk galaxies rotation
velocity, with a dynamical recipe for the rotation curve, rather than a simple
conversion from the total mass to maximum velocity. The new recipe takes into
account the disk shape factor, and the angular momentum transfer occurring
during secular evolution leading to the formation of bulges. This produces
model rotation velocities that are lower by ~20-25% for the majority of the
spirals. We implemented stellar population models with a complete treatment of
the TP-AGB, which leads to a revision of the mass-to-light ratio in the
near-IR. I/K band luminosities increase by ~0.3/0.5 mags at redshift z=0 and by
~0.5/1 mags at z=3. With these two new recipes in place, the comparison between
the predicted Tully-Fisher relation with a series of datasets in the optical
and near-IR, at redshifts between 0 and 1, is used as a diagnostics of the
assembly and evolution of spiral galaxies in the model. At 0.4<z<1.2 the match
between the new model and data is remarkably good, especially for later-type
spirals (Sb/Sc). At z=0 the new model shows a net improvement in comparison
with its original version of 2003, and in accord with recent observations in
the K band, the model Tully-Fisher also shows a morphological differentiation.
However, in all bands the z=0 model Tully-Fisher is too bright. We argue that
this behaviour is caused by inadequate star formation histories in the model
galaxies at low redshifts. The star-formation rate declines too slowly, due to
continuous gas infall that is not efficiently suppressed. An analysis of the
model disk scale lengths, at odds with observations, hints to some missing
physics in the modeling of disk formation inside dark matter halos.Comment: Accepted for publication on MNRAS. 2 new plots, 1 new section, and
extended discussion. 21 pages, 11 figures in tota
The impact of TP-AGB stars on hierarchical galaxy formation models
The spectro-photometric properties of galaxies in galaxy formation models are
obtained by combining the predicted history of star formation and mass
accretion with the physics of stellar evolution through stellar population
models. In the recent literature, significant differences have emerged
regarding the implementation of the Thermally-Pulsing Asymptotic Giant Branch
phase of stellar evolution. The emission in the TP-AGB phase dominates the
bolometric and near-IR spectrum of intermediate-age (~1 Gyr) stellar
populations, hence it is crucial for the correct modeling of the galaxy
luminosities and colours. In this paper for the first time, we incorporate a
full prescription of the TP-AGB phase in a semi-analytic model of galaxy
formation. We find that the inclusion of the TP-AGB in the model spectra
dramatically alters the predicted colour-magnitude relation and its evolution
with redshift. When the TP-AGB phase is active, the rest-frame V-K galaxy
colours are redder by almost 2 magnitudes in the redshift range z~2-3 and by 1
magnitude at z~1. Very red colours are produced in disk galaxies, so that the
V-K colour distributions of disk and spheroids are virtually undistinguishable
at low redshifts. We also find that the galaxy K-band emission is more than 1
magnitude higher in the range z~1-3. This may alleviate the difficulties met by
the hierarchical clustering scenario in predicting the red galaxy population at
high redshifts. The comparison between simulations and observations have to be
revisited in the light of our results.Comment: 6 pages, 4 figures. Accepted for publication on MNRAS Letter
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