43 research outputs found
The effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest-frame near-infrared galaxy luminosity function
We address the fundamental question of matching the rest-frame K-band
luminosity function (LF) of galaxies over the Hubble time using semi-analytic
models, after modification of the stellar population modelling. We include the
Maraston evolutionary synthesis models, that feature a higher contribution by
the Thermally Pulsating - Asymptotic Giant Branch (TP-AGB) stellar phase, into
three different semi-analytic models, namely the De Lucia and Blaizot version
of the Munich model, MORGANA and the Menci model. We leave all other input
physics and parameters unchanged. We find that the modification of the stellar
population emission can solve the mismatch between models and the observed
rest-frame K-band luminosity from the brightest galaxies derived from UKIDSS
data at high redshift. For all explored semi-analytic models this holds at the
redshifts - between 2 and 3 - where the discrepancy was recently pointed out.
The reason for the success is that at these cosmic epochs the model galaxies
have the right age (~1 Gyr) to contain a well-developed TP-AGB phase which
makes them redder without the need of changing their mass or age. At the same
time, the known overestimation of the faint end is enhanced in the K-band when
including the TP-AGB contribution. At lower redshifts (z<2) some of the
explored models deviate from the data. This is due to too short merging
timescales and inefficient 'radio-mode' AGN feedback. Our results show that a
strong evolution in mass predicted by hierarchical models is compatible with no
evolution on the bright-end of the K-band LF from z=3 to the local universe.
This means that, at high redshifts and contrary to what is commonly accepted,
K-band emission is not necessarily a good tracer of galaxy mass.Comment: 10 pages, 5 figures, accepted by MNRA
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
Poly(L-glutamic acid)-co-poly(ethylene glycol) block copolymers for protein conjugation
Poly(L-glutamic acid)-co-poly(ethylene glycol) block copolymers (PLE-PEG) are here investigated as polymers for conjugation to therapeutic proteins such as granulocyte colony stimulating factor (G-CSF) and human growth hormone (hGH). PLE-PEG block copolymers are able to stabilize and protect proteins from degradation and to prolong their residence time in the blood stream, features that are made possible thanks to PEG's intrinsic properties and the simultaneous presence of the biodegradable anionic PLE moiety. When PLE-PEG copolymers are selectively tethered to the N-terminus of G-CSF and hGH, they yield homogeneous monoconjugates that preserve the protein's secondary structure. During the current study the pharmacokinetics of PLE10-PEG20k-G-CSF and PLE20-PEG20k-G-CSF derivatives and their ability to induce granulopoiesis were, respectively, assessed in Sprague-Dawley rats and in C57BL6 mice. Our results show that the bioavailability and bioactivity of the derivatives are comparable to or better than those of PEG20k-Nter-G-CSF (commercially known as Pegfilgrastim). The therapeutic effects of PLE10-PEG20k-hGH and PLE20-PEG20k-hGH derivatives tested in hypophysectomized rats demonstrate that the presence of a negatively charged PLE block enhances the biological properties of the conjugates additionally with respect to PEG20k-Nter-hGH
Luminous Red Galaxies in Simulations: Cosmic Chronometers?
There have been a number of attempts to measure the expansion rate of the
universe at high redshift using Luminous Red Galaxies (LRGs) as "chronometers".
The method generally assumes that stars in LRGs are all formed at the same
time. In this paper, we quantify the uncertainties on the measurement of H(z)
which arise when one considers more realistic, extended star formation
histories. In selecting galaxies from the Millennium Simulation for this study,
we show that using rest-frame criteria significantly improves the homogeneity
of the sample and that H(z) can be recovered to within 3% at z~0.42 even when
extended star formation histories are considered. We demonstrate explicitly
that using Single Stellar Populations to age-date galaxies from the
semi-analytical simulations provides insufficient accuracy for this experiment
but accurate ages are obtainable if the complex star formation histories
extracted from the simulation are used. We note, however, that problems with
SSP-fitting might be overestimated since the semi-analytical models tend to
over predict the late-time star-formation in LRGs. Finally, we optimize an
observational program to carry out this experiment.Comment: 11 pages, 10 figures. Accepted to MNRAS
The chemical evolution of galaxies within the IGIMF theory: the [alpha/Fe] ratios and downsizing
The chemical evolution of galaxies is investigated within the framework of
the star formation rate (SFR) dependent integrated galactic initial mass
function (IGIMF). We study how the global chemical evolution of a galaxy and in
particular how [alpha/Fe] abundance ratios are affected by the predicted
steepening of the IGIMF with decreasing SFR. We use analytical and
semi-analytical calculations to evaluate the mass-weighted and
luminosity-weighted [alpha/Fe] ratios in early-type galaxies of different
masses. The models with the variable IGIMF produce a [alpha/Fe] vs. velocity
dispersion relation which has the same slope as the observations of massive
galaxies, irrespective of the model parameters, provided that the star
formation duration inversely correlates with the mass of the galaxy
(downsizing). These models also produce steeper [alpha/Fe] vs. sigma relations
in low-mass early-type galaxies and this trend is consistent with the
observations. Constant IMF models are able to reproduce the [alpha/Fe] ratios
in large elliptical galaxies as well, but they do not predict this change of
slope for small galaxies. In order to obtain the best fit between our results
and the observations, the downsizing effect (i.e. the shorter duration of the
star formation in larger galaxies) must be milder than previously thought.Comment: 13 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
Modelling element abundances in semi-analytic models of galaxy formation
We update the treatment of chemical evolution in the Munich semi-analytic model, L-GALAXIES. Our new implementation includes delayed enrichment from stellar winds, supernovae type II (SNe-II) and supernovae type Ia (SNe-Ia), as well as metallicity-dependent yields and a reformulation of the associated supernova feedback. Two different sets of SN-II yields and three different SN-Ia delay-time distributions (DTDs) are considered, and eleven heavy elements (including O, Mg and Fe) are self-consistently tracked. We compare the results of this new implementation with data on a) local, star-forming galaxies, b) Milky Way disc G dwarfs, and c) local, elliptical galaxies. We find that the z=0 gas-phase mass-metallicity relation is very well reproduced for all forms of DTD considered, as is the [Fe/H] distribution in the Milky Way disc. The [O/Fe] distribution in the Milky Way disc is best reproduced when using a DTD with less than or equal to 50 per cent of SNe-Ia exploding within ~400 Myrs. Positive slopes in the mass-[alpha/Fe] relations of local ellipticals are also obtained when using a DTD with such a minor `prompt' component. Alternatively, metal-rich winds that drive light alpha elements directly out into the circumgalactic medium also produce positive slopes for all forms of DTD and SN-II yields considered. Overall, we find that the best model for matching the wide range of observational data considered here should include a power-law SN-Ia DTD, SN-II yields that take account of prior mass loss through stellar winds, and some direct ejection of light alpha elements out of galaxies
On the identification of merger debris in the {\it Gaia} Era
We model the formation of the Galactic stellar halo via the accretion of
satellite galaxies onto a time-dependent semi-cosmological galactic potential.
Our goal is to characterize the substructure left by these accretion events in
a close manner to what may be possible with the {\it Gaia} mission. We have
created a synthetic {\it Gaia} Solar Neighbourhood catalogue by convolving the
6D phase-space coordinates of stellar particles from our disrupted satellites
with the latest estimates of the {\it Gaia} measurement errors, and included
realistic background contamination due to the Galactic disc(s) and bulge. We
find that, even after accounting for the expected observational errors, the
resulting phase-space is full of substructure. We are able to successfully
isolate roughly 50% of the different satellites contributing to the `Solar
Neighbourhood' by applying the Mean-Shift clustering algorithm in
energy-angular momentum space. Furthermore, a Fourier analysis of the space of
orbital frequencies allows us to obtain accurate estimates of time since
accretion for approximately 30% of the recovered satellites.Comment: 13 pages, 11 figures, submitted to MNRA
Chemical enrichment of galaxy clusters from hydrodynamical simulations
We present cosmological hydrodynamical simulations of galaxy clusters aimed
at studying the process of metal enrichment of the intra--cluster medium (ICM).
These simulations have been performed by implementing a detailed model of
chemical evolution in the Tree-SPH \gd code. This model allows us to follow the
metal release from SNII, SNIa and AGB stars, by properly accounting for the
lifetimes of stars of different mass, as well as to change the stellar initial
mass function (IMF), the lifetime function and the stellar yields. As such, our
implementation of chemical evolution represents a powerful instrument to follow
the cosmic history of metal production. The simulations presented here have
been performed with the twofold aim of checking numerical effects, as well as
the impact of changing the model of chemical evolution and the efficiency of
stellar feedback.Comment: to appear on MNRA
Observed versus modelled u,g,r,i,z-band photometry of local galaxies - Evaluation of model performance
We test how well available stellar population models can reproduce observed
u,g,r,i,z-band photometry of the local galaxy population (0.02<=z<=0.03) as
probed by the SDSS. Our study is conducted from the perspective of a user of
the models, who has observational data in hand and seeks to convert them into
physical quantities. Stellar population models for galaxies are created by
synthesizing star formations histories and chemical enrichments using single
stellar populations from several groups (Starburst99, GALAXEV, Maraston2005,
GALEV). The role of dust is addressed through a simplistic, but observationally
motivated, dust model that couples the amplitude of the extinction to the star
formation history, metallicity and the viewing angle. Moreover, the influence
of emission lines is considered (for the subset of models for which this
component is included). The performance of the models is investigated by: 1)
comparing their prediction with the observed galaxy population in the SDSS
using the (u-g)-(r-i) and (g-r)-(i-z) color planes, 2) comparing predicted
stellar mass and luminosity weighted ages and metallicities, specific star
formation rates, mass to light ratios and total extinctions with literature
values from studies based on spectroscopy. Strong differences between the
various models are seen, with several models occupying regions in the
color-color diagrams where no galaxies are observed. We would therefore like to
emphasize the importance of the choice of model. Using our preferred model we
find that the star formation history, metallicity and also dust content can be
constrained over a large part of the parameter space through the use of
u,g,r,i,z-band photometry. However, strong local degeneracies are present due
to overlap of models with high and low extinction in certain parts of color
space.Comment: MNRAS accepted. 18 pages, incl. 15 figure
Stellar Population Trends in S0 Galaxies
We present stellar population age and metallicity trends for a sample of 59
S0 galaxies based on optical SDSS and NIR J & H photometry. When combined with
optical g and r passband imaging data from the SDSS archive and stellar
population models, we obtain radial age and metallicity trends out to at least
5 effective radii for most of the galaxies in our sample. The sample covers a
range in stellar mass and light concentration. We find an average central
light-weighted age of ~ 4 Gyr and central metallicity [Z/H] ~ 0.2 dex. Almost
all galaxies show a negative metallicity gradient from the center out, with an
average value of Delta[Z/H]/Delta(log(r/Re)) = -0.6. An age increase, decrease,
and minimal change with radius is observed for 58%, 19%, and 23%, respectively,
for a mean age gradient of Delta(age)/Delta(log(r/Re)) = 2.3 Gyr dex^{-1}. For
14 out of 59 galaxies, the light-weighted age of the outer region is greater
than 10 Gyr. We find that galaxies with both lower mass and lower concentration
have younger light-weighted ages and lower light-weighted metallicities. This
mass-metallicity relation extends into the outer regions of our S0 galaxies.
Our results are consistent with the formation of S0 galaxies through the
transformation of spiral galaxy disks. Determining the structural component
that makes up the outer region of galaxies with old outksirts is a necessary
step to understand the formation history of S0 galaxies.Comment: accepted to MNRA