100 research outputs found
The formation of young B/PS bulges in edge-on barred galaxies
We report about the fact that the stellar population that is born in the gas
inflowing towards the central regions can be vertically unstable leading to a
B/PS feature remarkably bluer that the surrounding bulge. Using new
chemodynamical simulations we show that this young population does not remain
as flat as the gaseous nuclear disc and buckles out of the plane to form a new
boxy bulge. We show that such a young B/PS bulge can be detected in colour
maps.Comment: 2 pages, 5 figures, to appear in IAU Symposium 245, Formation and
Evolution of Galaxy Bulges, M. Bureau, E. Athanassoula, and B. Barbuy (eds.),
Oxford, 16-20 July 200
Metallicity of high stellar mass galaxies with signs of merger events
We focus on an analysis of galaxies of high stellar mass and low metallicity.
We cross-correlated the Millenium Galaxy Catalogue (MGC) and the Sloan Digital
Sky Survey (SDSS) galaxy catalogue to provide a sample of MGC objects with high
resolution imaging and both spectroscopic and photometric information available
in the SDSS database. For each galaxy in our sample, we conducted a systematic
morphological analysis by visual inspection of MGC images using their
luminosity contours. The galaxies are classified as either disturbed or
undisturbed objects. We divide the sample into three metallicity regions,
within wich we compare the properties of disturbed and undisturbed objects. We
find that the fraction of galaxies that are strongly disturbed, indicative of
being merger remnants, is higher when lower metallicity objects are considered.
The three bins analysed consist of approximatively 15%, 20%, and 50% disturbed
galaxies (for high, medium, and low metallicity, respectively). Moreover, the
ratio of the disturbed to undisturbed relative distributions of the population
age indicator, Dn(4000), in the low metallicity bin, indicates that the
disturbed objects have substantially younger stellar populations than their
undisturbed counterparts. In addition, we find that an analysis of colour
distributions provides similar results, showing that low metallicity galaxies
with a disturbed morphology are bluer than those that are undisturbed. The
bluer colours and younger populations of the low metallicity, morphologically
disturbed objects suggest that they have experienced a recent merger with an
associated enhanced star formation rate. [abridged]Comment: Astronomy & Astrophysics, in pres
Clumpy Disc and Bulge Formation
We present a set of hydrodynamical/Nbody controlled simulations of isolated
gas rich galaxies that self-consistently include SN feedback and a detailed
chemical evolution model, both tested in cosmological simulations. The initial
conditions are motivated by the observed star forming galaxies at z ~ 2-3. We
find that the presence of a multiphase interstellar media in our models
promotes the growth of disc instability favouring the formation of clumps which
in general, are not easily disrupted on timescales compared to the migration
time. We show that stellar clumps migrate towards the central region and
contribute to form a classical-like bulge with a Sersic index, n > 2. Our
physically-motivated Supernova feedback has a mild influence on clump survival
and evolution, partially limiting the mass growth of clumps as the energy
released per Supernova event is increased, with the consequent flattening of
the bulge profile. This regulation does not prevent the building of a
classical-like bulge even for the most energetic feedback tested. Our Supernova
feedback model is able to establish a self-regulated star formation, producing
mass-loaded outflows and stellar age spreads comparable to observations. We
find that the bulge formation by clumps may coexit with other channels of bulge
assembly such as bar and mergers. Our results suggest that galactic bulges
could be interpreted as composite systems with structural components and
stellar populations storing archaeological information of the dynamical history
of their galaxy.Comment: Accepted for publication in MNRAS - Aug. 20, 201
GalPak3D: A Bayesian parametric tool for extracting morpho-kinematics of galaxies from 3D data
We present a method to constrain galaxy parameters directly from
three-dimensional data cubes. The algorithm compares directly the data with a
parametric model mapped in coordinates. It uses the spectral
lines-spread function (LSF) and the spatial point-spread function (PSF) to
generate a three-dimensional kernel whose characteristics are instrument
specific or user generated. The algorithm returns the intrinsic modeled
properties along with both an `intrinsic' model data cube and the modeled
galaxy convolved with the 3D-kernel. The algorithm uses a Markov Chain Monte
Carlo (MCMC) approach with a nontraditional proposal distribution in order to
efficiently probe the parameter space. We demonstrate the robustness of the
algorithm using 1728 mock galaxies and galaxies generated from hydrodynamical
simulations in various seeing conditions from 0.6" to 1.2". We find that the
algorithm can recover the morphological parameters (inclination, position
angle) to within 10% and the kinematic parameters (maximum rotation velocity)
to within 20%, irrespectively of the PSF in seeing (up to 1.2") provided that
the maximum signal-to-noise ratio (SNR) is greater than pixel
and that the ratio of the galaxy half-light radius to seeing radius is greater
than about 1.5. One can use such an algorithm to constrain simultaneously the
kinematics and morphological parameters of (nonmerging) galaxies observed in
nonoptimal seeing conditions. The algorithm can also be used on adaptive-optics
(AO) data or on high-quality, high-SNR data to look for nonaxisymmetric
structures in the residuals.Comment: 16 pages, 10 figures, accepted to publication in AJ, revised version
after proofs corrections. Algorithm available at http://galpak.irap.omp.e
Properties of simulated Milky Way-mass galaxies in loose group and field environments
We test the validity of comparing simulated field disk galaxies with the
empirical properties of systems situated within environments more comparable to
loose groups, including the Milky Way's Local Group. Cosmological simulations
of Milky Way-mass galaxies have been realised in two different environment
samples: in the field and in environments with similar properties to the Local
Group. Apart from the environments of the galaxies, the samples are kept as
homogeneous as possible with equivalent ranges in last major merger time, halo
mass and halo spin. Comparison of these two samples allow for systematic
differences in the simulations to be identified. Metallicity gradients, disk
scale lengths, colours, magnitudes and age-velocity dispersion relations are
studied for each galaxy in the suite and the strength of the link between these
and environment of the galaxies is studied. The bulge-to-disk ratio of the
galaxies show that these galaxies are less spheroid dominated than many other
simulated galaxies in literature with the majority of both samples being disk
dominated. We find that secular evolution and mergers dominate the spread of
morphologies and metallicity gradients with no visible differences between the
two environment samples. In contrast with this consistency in the two samples
there is tentative evidence for a systematic difference in the velocity
dispersion-age relations of galaxies in the different environments. Loose group
galaxies appear to have more discrete steps in their velocity dispersion-age
relations. We conclude that at the current resolution of cosmological galaxy
simulations field environment galaxies are sufficiently similar to those in
loose groups to be acceptable proxies for comparison with the Milky Way
provided that a similar assembly history is considered.Comment: 16 pages, 11 figures, abstract abridged for arXiv. Accepted for
publication in Astronomy & Astrophysic
The UV, Lyman α, and dark matter halo properties of high-redshift galaxies
We explore the properties of high-redshift Lyman alpha emitters (LAEs), and their link with the Lyman-break galaxy (LBG) population, using a semi-analytic model of galaxy formation that takes into account resonant scattering of Lyα photons in gas outflows. We can reasonably reproduce the abundances of LAEs and LBGs from zâ3 to 7, as well as most UV luminosity functions (LFs) of LAEs. The stronger dust attenuation for (resonant) Lyα photons compared to UV continuum photons in bright LBGs provides a natural interpretation to the increase of the LAE fraction in LBG samples, XLAE, towards fainter magnitudes. The redshift evolution of XLAE seems however very sensitive to UV magnitudes limits and equivalent width (EW) cuts. In spite of the apparent good match between the statistical properties predicted by the model and the observations, we find that the tail of the Lyα EW distribution (EWâł100âĂ
) cannot be explained by our model, and we need to invoke additional mechanisms. We find that LAEs and LBGs span a very similar dynamical range, but bright LAEs are âŒ4times rarer than LBGs in massive haloes. Moreover, massive haloes mainly contain weak LAEs in our model, which might introduce a bias towards low-mass haloes in surveys which select sources with high-EW cuts. Overall, our results are consistent with the idea that LAEs and LBGs make a very similar galaxy population. Their apparent differences seem mainly due to EW selections, UV detection limits, and a decreasing Lyα to UV escape fraction ratio in high star formation rate galaxie
The mass-metallicity relation of interacting galaxies
We study the mass-metallicity relation of galaxies in pairs and in isolation
taken from the SDSS-DR4 using the stellar masses and oxygen abundances derived
by Tremonti et al. (2004). Close galaxy pairs, defined by projected separation
r_p < 25kpc/h and radial velocity Delta_V < 350 km/s, are morphologically
classified according to the strength of the interaction signs. We find that
only for pairs showing signs of strong interactions, the mass-metallicity
relation differs significantly from that of galaxies in isolation. In such
pairs, the mean gas-phase oxygen abundances of galaxies with low stellar masses
(Mstar ~< 10^9 Msun/h) exhibit an excess of 0.2 dex. Conversely, at larger
masses (Mstar >~ 10^10 Msun/h) galaxies have a systematically lower
metallicity, although with a smaller difference (-0.05 dex). Similar trends are
obtained if g-band magnitudes are used instead of stellar masses. In minor
interactions, we find that the less massive member is systematically enriched,
while a galaxy in interaction with a comparable stellar mass companion shows a
metallicity decrement with respect to galaxies in isolation. We argue that
metal-rich starbursts triggered by a more massive component, and inflows of low
metallicity gas induced by comparable or less massive companion galaxies,
provide a natural scenario to explain our findings.Comment: Accepted for publication in MNRAS Letter
Photometric and dynamic evolution of an isolated disc galaxy simulation
We present a detailed analysis of the evolution of a simulated isolated disc
galaxy. The simulation includes stars, gas, star formation and simple chemical
yields. Stellar particles are split in two populations: the old one is present
at the beginning of the simulation and is calibrated according to various ages
and metallicities; the new population borns in the course of the simulation and
inherits the metallicity of the gas particles. The results have been calibrated
in four wavebands with the spectro-photometric evolutionary model GISSEL2000
(Bruzual & Charlot 1993). Dust extinction has also been taken into account. A
rest-frame morphological and bidimensional photometric analysis has been
performed on simulated images, with the same tools as for observations. The
effects of the stellar bar formation and the linked star formation episode on
the global properties of the galaxy (mass and luminosity distribution, colours,
isophotal radii) have been analysed. In particular, we have disentangled the
effects of stellar evolution from dynamic evolution to explain the cause of the
isophotal radii variations. We show that the dynamic properties (e.g. mass) of
the area enclosed by any isophotal radius depends on the waveband and on the
level of star formation activity. It is also shown that the bar isophotes
remain thinner than mass isodensities a long time (> 0.7 Gyr) after the maximum
of star formation rate. We show that bar ellipticity is very wavelength
dependent as suggested by real observations. Effects of dust extinction on
photometric and morphological measurements are systematically quantified.Comment: 14 pages, 16 figures (13 in eps, 3 in jpg format). Accepted for
publication in A&
The length of stellar bars in SB galaxies and N-body simulations
We have investigated the accuracy and reliability of six methods used to
determine the length of stellar bars in galaxies or N-body simulations. All
these methods use ellipse fitting and Fourier decomposition of the surface
brightness. We have applied them to N-body simulations that include stars, gas,
star formation, and feedback. Stellar particles were photometrically calibrated
to make B and K-band mock images. Dust absorption is also included. We discuss
the advantages and drawbacks of each method, the effects of projection and
resolution, as well as the uncertainties introduced by the presence of dust.
The use of N-body simulations allows us to compare the location of Ultra
Harmonic Resonance (UHR or 4/1) and corotation (CR) with measured bar lengths.
We show that the minimum of ellipticity located just outside the bulk of the
bar is correlated with the corotation, whereas the location of the UHR can be
approximated using the phase of the fitted ellipses or the phase of the m=2
Fourier development of the surface brightness. We give evidence that the
classification of slow/fast bars, based on the ratio R = Rcr/Rbar could
increase from 1 (fast bar) to 1.4 (slow bar) just by a change of method. We
thus conclude that one has to select the right bar-length estimator depending
on the application, since these various estimators do not define the same
physical area.Comment: Major revision, A&A in pres
Understanding the escape of LyC and Lyα photons from turbulent clouds
Understanding the escape of Lyman continuum (LyC) and Lyman alpha (Lya)
photons from molecular clouds is one of the keys to constraining the
reionization history of the Universe and the evolution of galaxies at high
redshift. Using a set of radiation-hydrodynamic simulations with adaptive mesh
refinement, we investigate how photons propagate and escape from turbulent
clouds with different masses, star formation efficiencies (SFEs), and
metallicities, as well as with different models of stellar spectra and
supernova feedback. We find that the escape fractions in both LyC and Lya are
generally increasing with time if the cloud is efficiently dispersed by
radiation and supernova feedback. When the total SFE is low (1% of the cloud
mass), 0.1-5% of LyC photons leave the metal-poor cloud, whereas the fractions
increase to 20-70% in clouds with a 10% SFE. LyC photons escape more
efficiently if gas metallicity is lower, if the upper mass limit in the stellar
initial mass function is higher, if binary interactions are allowed in the
evolution of stars, or if additional strong radiation pressure, such as Lya
pressure, is present. As a result, the number of escaping LyC photons can
easily vary by a factor of on cloud scales. The escape fractions of Lya
photons are systemically higher (60-80%) than those of LyC photons despite
large optical depths at line centre (). Scattering of Lya
photons is already significant on cloud scales, leading to double-peaked
profiles with peak separations of during
the initial stage of the cloud evolution, while it becomes narrower than
in the LyC bright phase. Comparisons
with observations of low-redshift galaxies suggest that Lya photons require
further interactions with neutral hydrogen to reproduce their velocity offset
for a given LyC escape fraction
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