125 research outputs found
Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies
We investigate the evolution of star-forming gas-rich disks, using a 3D
chemodynamical model including a dark halo, stars, and a two-phase interstellar
medium with feedback processes from the stars. We show that galaxy evolution
proceeds along very different routes depending on whether it is the gas disk or
the stellar disk which first becomes unstable, as measured by the respective
Q-parameters. This in turn depends on the uncertain efficiency of energy
dissipation of the cold cloud component from which stars form. When the cold
gas cools efficiently and drives the instability, the galactic disk fragments
and forms a number of massive clumps of stars and gas. The clumps spiral to the
center of the galaxy in a few dynamical times and merge there to form a central
bulge component in a strong starburst. When the kinetic energy of the cold
clouds is dissipated at a lower rate, stars form from the gas in a more
quiescent mode, and an instability only sets in at later times, when the
surface density of the stellar disk has grown sufficiently high. The system
then forms a stellar bar, which channels gas into the center, evolves, and
forms a bulge whose stars are the result of a more extended star formation
history. We investigate the stability of the gas-stellar disks in both regimes,
as well as the star formation rates and element enrichment. We study the
morphology of the evolving disks, calculating spatially resolved colours from
the distribution of stars in age and metallicity, including dust absorption. We
then discuss morphological observations such as clumpy structures and chain
galaxies at high redshift as possible signatures of fragmenting, gas-rich
disks. Finally, we investigate abundance ratio distributions as a means to
distinguish the different scenarios for bulge formation.Comment: 16 pages, Latex, 14 figures, to appear in Astronomy and Astrophysics,
Version with high quality images available at
http://www.astro.unibas.ch/leute/ai.shtm
The Formation of a Disk Galaxy within a Growing Dark Halo
We present a dynamical model for the formation and evolution of a massive
disk galaxy, within a growing dark halo whose mass evolves according to
cosmological simulations of structure formation. The galactic evolution is
simulated with a new 3D chemo-dynamical code, including dark matter, stars and
a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark
halo in a LCDM universe and we follow the evolution until the present epoch.
The energy release by massive stars and SNe prevents a rapid collapse of the
baryonic matter and delays the maximum star formation until z=1. The galaxy
forms radially from inside-out and vertically from halo to disk. The first
galactic component that forms is the halo, followed by the bulge, the disk-halo
transition region, and the disk. At z=1, a bar begins to form which later turns
into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk
stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge
and inner disk (G-dwarf problem). The mean rotation and the distribution of
orbital eccentricities for all stars as a function of metallicity are not very
different from those observed in the solar neighbourhood, showing that
homogeneous collapse models are oversimplified. The approach presented here
provides a detailed description of the formation and evolution of an isolated
disk galaxy in a LCDM universe, yielding new information about the kinematical
and chemical history of the stars and the ISM, but also about the evolution of
the luminosity, the colours and the morphology of disk galaxies.Comment: 23 pages, LaTeX, 18 figures, A&A accepted, a high resolution version
of the paper can be found at http://www.astro.unibas.ch/leute/ms.shtm
Exoplanet Imaging Data Challenge, phase II: Characterization of exoplanet signals in high-contrast images
Today, there exists a wide variety of algorithms dedicated to high-contrast
imaging, especially for the detection and characterisation of exoplanet
signals. These algorithms are tailored to address the very high contrast
between the exoplanet signal(s), which can be more than two orders of magnitude
fainter than the bright starlight residuals in coronagraphic images. The
starlight residuals are inhomogeneously distributed and follow various
timescales that depend on the observing conditions and on the target star
brightness. Disentangling the exoplanet signals within the starlight residuals
is therefore challenging, and new post-processing algorithms are striving to
achieve more accurate astrophysical results. The Exoplanet Imaging Data
Challenge is a community-wide effort to develop, compare and evaluate
algorithms using a set of benchmark high-contrast imaging datasets. After a
first phase ran in 2020 and focused on the detection capabilities of existing
algorithms, the focus of this ongoing second phase is to compare the
characterisation capabilities of state-of-the-art techniques. The
characterisation of planetary companions is two-fold: the astrometry (estimated
position with respect to the host star) and spectrophotometry (estimated
contrast with respect to the host star, as a function of wavelength). The goal
of this second phase is to offer a platform for the community to benchmark
techniques in a fair, homogeneous and robust way, and to foster collaborations.Comment: Submitted to SPIE Astronomical Telescopes + Instrumentation 2022,
Adaptive Optics Systems VIII, Paper 12185-
Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants
We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope
for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to
measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant
stars observed in 3 fields along the minor axis of the Galactic bulge and at
latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the
globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other
three fields along the major axis. Our results confirm, with large number
statistics, the chemical similarity between the Galactic bulge and thick disk,
which are both enhanced in alpha elements when compared to the thin disk. In
the same context, we analyze [alpha/Fe] vs. [Fe/H] trends across different
bulge regions. The most metal rich stars, showing low [alpha/Fe] ratios at b=-4
disappear at higher Galactic latitudes in agreement with the observed
metallicity gradient in the bulge. Metal-poor stars ([Fe/H]<-0.2) show a
remarkable homogeneity at different bulge locations. We have obtained further
constrains for the formation scenario of the Galactic bulge. A metal-poor
component chemically indistinguishable from the thick disk hints for a fast and
early formation for both the bulge and the thick disk. Such a component shows
no variation, neither in abundances nor kinematics, among different bulge
regions. A metal-rich component showing low [alpha/Fe] similar to those of the
thin disk disappears at larger latitudes. This allows us to trace a component
formed through fast early mergers (classical bulge) and a disk/bar component
formed on a more extended timescale.Comment: 13 pages, 17 figures. Accepted for publication in Astronomy and
Astrophysic
Colour Evolution of Disk Galaxy Models from z=4 to z=0
We calculate synthetic UBVRIJHKLM images, integrated spectra and colours for
the disk galaxy formation models of Samland & Gerhard (2002), from redshift z=4
to z=0. Two models are considered, an accretion model based on LambdaCDM
structure formation simulations, and a classical collapse model in a dark
matter halo. Both models provide the star formation history and dynamics of the
baryonic component within a three-dimensional chemo-dynamical description. To
convert to spectra and colours, we use the latest, metallicity-calibrated
spectral library of Westera et al. (2002), including internal absorption. As a
first application, we compare the derived colours with Hubble Deep Field North
bulge colours and find good agreement. With our model, we disentangle
metallicity effects and absorption effects on the integrated colours, and find
that absorption effects are dominant for redshift z < 1.5. Furthermore, we
confirm the quality of m_K as a mass tracer, and find indications for a
correlation between (J-K) and metallicity gradients.Comment: 29 pages, LaTeX, 17 figures, revised version, submitted to A&A, a
high resolution version can be found at
http://www.astro.unibas.ch/leute/ms.shtm
Implications of O and Mg abundances in metal-poor halo stars for stellar iron yields
Inhomogeneous chemical evolution models of galaxies which try to reproduce
the scatter seen in element-to-iron ratios of metal-poor halo stars are heavily
dependent on theoretical nucleosynthesis yields of core-collapse supernovae.
Hence inhomogeneous models present themselves as a test for stellar nucleosyn-
thesis calculations. Applying an inhomogeneous chemical evolution model to our
Galaxy reveals a number of shortcomings of existing theoretical nucleosynthesis
yields. One problem is the predicted scatter in [O/Fe] and [Mg/Fe] which is too
large compared to the one observed in metal-poor halo stars. This can be either
due to the O or Mg yields or due to the Fe yields (or both). However, O and Mg
are alpha-elements that are produced mainly during hydrostatic burning and thus
are not affected by the theoretical uncertainties afflicting the collapse and
explosion of a massive star. Stellar iron yields, on the other hand, depend
heavily on the choice of the mass-cut between ejecta and proto neutron star and
are therefore very uncertain. We present Fe yield distributions as function of
progenitor mass that are consistent with the abundance distribution of metal-
poor halo stars and are in agreement with observed Ni yields of SNe II with
known progenitor masses. The iron yields of lower-mass SNe II (in the range
10-20 Msol) are well constrained by those observations. Present observations,
however, do not allow to determine a unique solution for higher-mass SNe.
Nevertheless, the main dependence of the stellar iron yield as function of
progenitor mass may be derived and can be used as constraint for future
supernova/hypernova models. The results are of importance for the earliest
stages of galaxy formation when the ISM is dominated by chemical
inhomogeneities and the instantaneous mixing approximation is not valid.Comment: 16 pages, 15 figures, submitted to A&A, for higher quality figures
contact the author ([email protected]
An imaged 15Mjup companion within a hierarchical quadruple system
Since 2019, the direct imaging B-star Exoplanet Abundance Study (BEAST) at
SPHERE@VLT has been scanning the surroundings of young B-type stars in order to
ascertain the ultimate frontiers of giant planet formation. Recently, the
Myr HIP 81208 was found to host a close-in (~50 au) brown dwarf
and a wider (~230 au) late M star around the central 2.6Msun primary. Alongside
the continuation of the survey, we are undertaking a complete reanalysis of
archival data aimed at improving detection performances so as to uncover
additional low-mass companions. We present here a new reduction of the
observations of HIP 81208 using PACO ASDI, a recent and powerful algorithm
dedicated to processing high-contrast imaging datasets, as well as more
classical algorithms and a dedicated PSF-subtraction approach. The combination
of different techniques allowed for a reliable extraction of astrometric and
photometric parameters. A previously undetected source was recovered at a short
separation from the C component of the system. Proper motion analysis provided
robust evidence for the gravitational bond of the object to HIP 81208 C.
Orbiting C at a distance of ~20 au, this 15Mjup brown dwarf becomes the fourth
object of the hierarchical HIP 81208 system. Among the several BEAST stars
which are being found to host substellar companions, HIP 81208 stands out as a
particularly striking system. As the first stellar binary system with
substellar companions around each component ever found by direct imaging, it
yields exquisite opportunities for thorough formation and dynamical follow-up
studies.Comment: 12 pages, 9 figures, 5 tables. Accepted for publication as a Letter
in Astronomy and Astrophysics, section 1. Letters to the Edito
Stellar populations of bulges at low redshift
This chapter summarizes our current understanding of the stellar population
properties of bulges and outlines important future research directions.Comment: Review article to appear in "Galactic Bulges", Editors: Laurikainen
E., Peletier R., Gadotti D., Springer Publishing. 34 pages, 12 figure
Post conjunction detection of Pictoris b with VLT/SPHERE
With an orbital distance comparable to that of Saturn in the solar system,
\bpic b is the closest (semi-major axis \,9\,au) exoplanet that has
been imaged to orbit a star. Thus it offers unique opportunities for detailed
studies of its orbital, physical, and atmospheric properties, and of
disk-planet interactions. With the exception of the discovery observations in
2003 with NaCo at the Very Large Telescope (VLT), all following astrometric
measurements relative to \bpic have been obtained in the southwestern part of
the orbit, which severely limits the determination of the planet's orbital
parameters. We aimed at further constraining \bpic b orbital properties using
more data, and, in particular, data taken in the northeastern part of the
orbit.
We used SPHERE at the VLT to precisely monitor the orbital motion of beta
\bpic b since first light of the instrument in 2014. We were able to monitor
the planet until November 2016, when its angular separation became too small
(125 mas, i.e., 1.6\,au) and prevented further detection. We redetected \bpic b
on the northeast side of the disk at a separation of 139\,mas and a PA of
30 in September 2018. The planetary orbit is now well constrained.
With a semi-major axis (sma) of au (1 ), it
definitely excludes previously reported possible long orbital periods, and
excludes \bpic b as the origin of photometric variations that took place in
1981. We also refine the eccentricity and inclination of the planet. From an
instrumental point of view, these data demonstrate that it is possible to
detect, if they exist, young massive Jupiters that orbit at less than 2 au from
a star that is 20 pc away.Comment: accepted by A&
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