28 research outputs found
Properties of Galaxies and Groups: Nature versus Nurture
Due to the inherently nonlinear nature of gravity cosmological N-body
simulations have become an invaluable tool when the growth of structure is
being studied and modelled closer to the present epoch. Large simulations with
high dynamical range have made it possible to model the formation and growth of
cosmic structure with unprecedented accuracy. Moreover, galaxies, the basic
building blocks of the Universe, can also be modelled in cosmological context.
However, despite all the simulations and successes in recent decades, there are
still many unanswered questions in the field of galaxy formation and evolution.
One of the longest standing issue being the significance of the formation place
and thus initial conditions to a galaxy's evolution in respect to environment,
often formulated simply as "nature versus nurture" like in human development
and psychology. Unfortunately, our understanding of galaxy evolution in
different environments is still limited, albeit, for example, the
morphology-density relation has shown that the density of the galaxy's local
environment can affect its properties. Consequently, the environment should
play a role in galaxy evolution, however despite the efforts, the exact role of
the galaxy's local environment to its evolution remains open. This thesis
introduction discusses briefly the background cosmology, cosmological N-body
simulations and semi-analytical models. The second part is reserved for groups
of galaxies, whether they are gravitationally bound, and what this may imply
for galaxy evolution. The third part of the thesis concentrates on describing
results of a case study of isolated field elliptical galaxies. The final
chapter discusses another case study of luminous infra-red galaxies.Comment: Introduction part of a PhD thesi
Are the nearby groups of galaxies gravitationally bound objects?
We have compared numerical simulations to observations for the nearby (< 40
Mpc) groups of galaxies (Huchra & Geller 1982 and Ramella et al. 2002). The
group identification is carried out using a group-finding algorithm developed
by Huchra and Geller (1982). Using cosmological N-body simulation code with the
LambdaCDM cosmology, we show that the dynamical properties of groups of
galaxies identified from the simulation data are, in general, in a moderate,
within 2sigma, agreement with the observational catalogues of groups of
galaxies. As simulations offer more dynamical information than observations, we
used the N-body simulation data to calculate whether the nearby groups of
galaxies are gravitationally bound objects by using their virial ratio. We show
that in a LambdaCDM cosmology about 20 per cent of nearby groups of galaxies,
identified by the same algorithm as in the case of observations, are not bound,
but merely groups in a visual sense. This is quite significant, specifically
because estimations of group masses in observations are often based on an
assumption that groups of galaxies found by the friends-of-friends algorithm
are gravitationally bound objects. Simulations with different resolutions show
the same results. We also show how the fraction of gravitationally unbound
groups varies when the apparent magnitude limit of the sample and the value of
the cosmological constant is changed. In general, a larger value of the
Omega_Lambda generates slightly more unbound groups.Comment: 13 figures and 7 tables, Accepted 2007 September 19. Received 2007
September 19; in original form 2007 April
Young, metal-enriched cores in early-type dwarf galaxies in the Virgo cluster based on colour gradients
Early-type dwarf galaxies are not simply featureless, old objects, but were
found to be much more diverse, hosting substructures and a variety of stellar
population properties. To explore the stellar content of faint early-type
galaxies, and to investigate in particular those with recent central star
formation, we study colours and colour gradients within one effective radius in
optical (g-r) and near-infrared (i-H) bands for 120 Virgo cluster early types
with -19 mag < < -16 mag. Twelve galaxies turn out to have blue cores,
when defined as g-r colour gradients larger than 0.10 mag/, which
represents the positive tail of the gradient distribution. For these galaxies,
we find that they have the strongest age gradients, and that even outside the
blue core, their mean stellar population is younger than the mean of ordinary
faint early types. The metallicity gradients of these blue-cored early-type
dwarf galaxies are, however, in the range of most normal faint early types,
which we find to have non-zero gradients with higher central metallicity. The
blue central regions are consistent with star formation activity within the
last few 100 Myr. We discuss that these galaxies could be explained by
environmental quenching of star formation in the outer galaxy regions, while
the inner star formation activity has continued
Formation, Evolution and Properties of Isolated Field Elliptical Galaxies
[Abridged] We study the properties, evolution and formation mechanisms of
isolated field elliptical galaxies. We create a mock catalogue of isolated
field elliptical galaxies from the Millennium Simulation Galaxy Catalogue, and
trace their merging histories. The formation, identity and assembly redshifts
of simulated isolated and non-isolated elliptical galaxies are studied and
compared. Observational and numerical data are used to compare age, mass, and
the colour-magnitude relation. Our results, based on simulation data, show that
almost seven per cent of all elliptical galaxies brighter than -19mag in B-band
can be classified as isolated field elliptical galaxies. Isolated field
elliptical galaxies show bluer colours than non-isolated elliptical galaxies
and they appear younger, in a statistical sense, according to their mass
weighted age. Isolated field elliptical galaxies also form and assemble at
lower redshifts compared to non-isolated elliptical galaxies. About 46 per cent
of isolated field elliptical galaxies have undergone at least one major merging
event in their formation history, while the same fraction is only about 33 per
cent for non-isolated ellipticals. The mean time of the last major merging is z
= 0.6 or 6 Gyrs ago for isolated ellipticals, while non-isolated ellipticals
experience their last major merging significantly earlier at z = 1.1 or 8 Gyrs
ago. After inspecting merger trees of simulated isolated field elliptical
galaxies, we conclude that three different, yet typical formation mechanisms
can be identified: solitude, coupling and cannibalism. Our results also predict
a previously unobserved population of blue, dim and light galaxies that fulfill
observational criteria to be classified as isolated field elliptical galaxies.
This separate population comprises about 26 per cent of all IfEs.Comment: Accepted for publication in MNRAS, 19 pages, 16 figure
Physical properties of Herschel selected galaxies in a semi-analytic galaxy formation model
[Abridged] We make use of a semi-analytic cosmological model that includes
simple prescriptions for dust attenuation and emission to make predictions for
the observable and physical properties of galaxies that may be detected by the
recently launched Herschel Space Observatory in deep fields such as
GOODS-Herschel. We compare our predictions for differential galaxy number
counts in the PACS (100 & 160) and SPIRE (250, 350, and 500 micron) bands with
available observations. We find very good agreement with the counts in the PACS
bands, for the overall counts and for galaxies binned by redshift at z< 2. At z
> 2 our model underpredicts the number of bright galaxies by a factor of ten.
The agreement is much worse for all three SPIRE bands, and becomes
progressively worse with increasing wavelength. We discuss a number of possible
reasons for these discrepancies, and hypothesize that the effect of blending on
the observational flux estimates is likely to be the dominant issue. We note
that the PACS number counts are relatively robust to changes in the dust
emission templates, while the predicted SPIRE number counts are more template
dependent. We present quantitative predictions for the relationship between the
observed PACS 160 and SPIRE 250 micron fluxes and physical quantities such as
halo mass, stellar mass, cold gas mass, star formation rate, and total infrared
(IR) luminosity, at different redshifts. We also present quantitative
predictions for the correlation between PACS 160 micron flux and the
probability that a galaxy has experienced a recent major or minor merger.
Although our models predict a strong correlation between these quantities, such
that more IR-luminous galaxies are more likely to be merger-driven, we find
that more than half of all high redshift IR-luminous galaxies detected by
Herschel are able to attain their high star formation rates without enhancement
by a merger.Comment: Accepted for publication in MNRA
Intranight polarization variability in radio-loud and radio-quiet AGN
(Abriged) Intranight polarization variability in AGN has not been studied
extensively so far. Studying the variability in polarization makes it possibly
to distinguish between different emission mechanisms. Thus it can help
answering the question if intranight variability in radio-loud and radio-quiet
AGN is of the same or of fundamentally different origin. In this paper we
investigate intranight polarization variability in AGN. Our sample consists of
28 AGN at low to moderate redshifts (0.048 < z < 1.036), 12 of which are
radio-quiet quasars (RQQs) and 16 are radio-loud blazars. The subsample of
blazars consists of eight flat-spectrum radio-quasars (FSRQs) and eight BL Lac
objects. We find clear differences between the two samples. A majority of the
radio-loud AGN show moderate to high degrees of polarization, more than half of
them also show variability in polarization. There seems to be a dividing line
for polarization intranight variability at P~5 per cent over which all objects
vary in polarization. Only two out of 12 radio-quiet quasars show polarized
emission, both at levels of P<1 per cent. The lack of polarization intranight
variability in radio-quiet AGN points towards accretion instabilities being the
cause for intranight flux variability whereas the high duty cycle of
polarization variability in radio-loud objects is more likely caused by
instabilities in the jet or changes of physical conditions in the jet plasma.Comment: Accepted for Publication in MNRAS (17 pages, 14 figures, 4 tables
PlatoSim: an end-to-end PLATO camera simulator for modelling high-precision space-based photometry
Context. PLAnetary Transits and Oscillations of stars (PLATO) is the ESA M3 space mission dedicated to detect and characterise transiting exoplanets including information from the asteroseismic properties of their stellar hosts. The uninterrupted and high-precision photometry provided by space-borne instruments such as PLATO require long preparatory phases. An exhaustive list of tests are paramount to design a mission that meets the performance requirements and as such simulations are an indispensable tool in the mission preparation.
Aims. To accommodate PLATOs need of versatile simulations prior to mission launch that at the same time describe innovative yet complex multi-telescope design accurately, in this work we present the end-to-end PLATO simulator specifically developed for that purpose, namely PlatoSim. We show, step-by-step, the algorithms embedded into the software architecture of PlatoSim that allow the user to simulate photometric time series of charge-coupled device (CCD) images and light curves in accordance to the expected observations of PLATO.
Methods. In the context of the PLATO payload, a general formalism of modelling, end-to-end, incoming photons from the sky to the final measurement in digital units is discussed. According to the light path through the instrument, we present an overview of the stellar field and sky background, the short- and long-term barycentric pixel displacement of the stellar sources, the cameras and their optics, the modelling of the CCDs and their electronics, and all main random and systematic noise sources.
Results. We show the strong predictive power of PlatoSim through its diverse applicability and contribution to numerous working groups within the PLATO mission consortium. This involves the ongoing mechanical integration and alignment, performance studies of the payload, the pipeline development, and assessments of the scientific goals.
Conclusions. PlatoSim is a state-of-the-art simulator that is able to produce the expected photometric observations of PLATO to a high level of accuracy. We demonstrate that PlatoSim is a key software tool for the PLATO mission in the preparatory phases until mission launch and prospectively beyond
Science Frontiers In Galaxy Evolution: Deep-Wide Surveys
Astro2010: The Astronomy and Astrophysics Decadal Survey : Science White Papers no 79. Available online : http://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=24