90 research outputs found
Why stellar feedback promotes disc formation in simulated galaxies
We study how feedback influences baryon infall onto galaxies using
cosmological, zoom-in simulations of haloes with present mass
to . Starting
at z=4 from identical initial conditions, implementations of weak and strong
stellar feedback produce bulge- and disc-dominated galaxies, respectively.
Strong feedback favours disc formation: (1) because conversion of gas into
stars is suppressed at early times, as required by abundance matching
arguments, resulting in flat star formation histories and higher gas fractions;
(2) because 50% of the stars form in situ from recycled disc gas with angular
momentum only weakly related to that of the z=0 dark halo; (3) because
late-time gas accretion is typically an order of magnitude stronger and has
higher specific angular momentum, with recycled gas dominating over primordial
infall; (4) because 25-30% of the total accreted gas is ejected entirely before
z~1, removing primarily low angular momentum material which enriches the nearby
inter-galactic medium. Most recycled gas roughly conserves its angular
momentum, but material ejected for long times and to large radii can gain
significant angular momentum before re-accretion. These processes lower galaxy
formation efficiency in addition to promoting disc formation.Comment: 23 pages, 29 figures, accepted for publication in MNRA
Stellar population constraints on the ages of galactic bars
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. We present a study of the stellar populations within the central regions of four nearby barred galaxies, and use a novel technique to constrain the duration of bar activity. We focus on the star formation 'desert', a region within each of these galaxies where star formation appears to have been suppressed by the bar. New Hà spectroscopic data are presented, and used to produce spectroscopic line indices which are compared with theoretical predictions from population synthesis models for simple stellar populations and temporally truncated star formation histories. This analysis shows that the dearth of star formation activity in these regions appears to have been continuing for at least 1 Gyr, with time-scales of several Gyr indicated for two of the galaxies. This favours models in which strong bars can be long-lived features of galaxies, but our results also indicate a significant diversity in stellar population ages, and hence in the implied histories of bar activity in these four galaxies
Vibrational spectroscopic observation of ice dewetting on MgO(001)
The properties of the interfacial water monolayer on MgO(001) during growth of multilayer ice and, in particular, the dewetting of crystalline ice on MgO(001) are revealed by vibrational sum frequency generation and infrared reflection absorption spectroscopy
Cosmogenic Nuclide Systematics and the CRONUScalc Program
As cosmogenic nuclide applications continue to expand, the need for a common basis for calculation becomes increasingly important. In order to accurately compare between results from different nuclides, a single method of calculation is necessary. Calculators exist in numerous forms with none matching the needs of the CRONUS-Earth project to provide a simple and consistent method to interpret data from most commonly used cosmogenic nuclides. A new program written for this purpose, CRONUScalc, is presented here. This unified code presents a method applicable to 10Be, 26Al, 36Cl, 3He, and 14C, with 21Ne in testing. The base code predicts the concentration of a sample at a particular depth for a particular time in the past, which can be used for many applications. The multi-purpose code already includes functions for performing production rate calibrations as well as calculating erosion rates and surface exposure ages for single samples and depth profiles. The code is available under the GNU General Public License agreement and can be downloaded and modified to deal with specific atypical scenarios
The asymmetric drift, the local standard of rest, and implications from RAVE data
Context. The determination of the local standard of rest (LSR), which corresponds to the measurement of the peculiar motion of the Sun based on the derivation of the asymmetric drift of stellar populations, is still a matter of debate. The classical value of the tangential peculiar motion of the Sun with respect to the LSR was challenged in recent years, claiming a significantly larger value.
Aims. We present an improved Jeans analysis, which allows a better interpretation of the measured kinematics of stellar populations in the Milky Way disc. We show that the Radial Velocity Experiment (RAVE) sample of dwarf stars is an excellent data set to derive tighter boundary conditions to chemodynamical evolution models of the extended solar neighbourhood.
Methods. We propose an improved version of the Stromberg relation with the radial scalelengths as the only unknown. We redetermine the asymmetric drift and the LSR for dwarf stars based on RAVE data. Additionally, we discuss the impact of adopting a different LSR value on the individual scalelengths of the subpopulations.
Results. Binning RAVE stars in metallicity reveals a bigger asymmetric drift (corresponding to a smaller radial scalelength) for more metal-rich populations. With the standard assumption of velocity-dispersion independent radial scalelengths in each metallicity bin, we redetermine the LSR. The new Stromberg equation yields a joint LSR value of V-circle dot = 3.06 +/- 0.68 km s(-1), which is even smaller than the classical value based on Hipparcos data. The corresponding radial scalelength increases from 1.6 kpc for the metal-rich bin to 2.9 kpc for the metal-poor bin, with a trend of an even larger scalelength for young metal-poor stars. When adopting the recent Schonrich value of V-circle dot = 12.24 km s(-1) for the LSR, the new Stromberg equation yields much larger individual radial scalelengths of the RAVE subpopulations, which seem unphysical in part.
Conclusions. The new Stromberg equation allows a cleaner interpretation of the kinematic data of disc stars in terms of radial scalelengths. Lifting the LSR value by a few km s(-1) compared to the classical value results in strongly increased radial scalelengths with a trend of smaller values for larger velocity dispersions
An Evolutionary Perspective on Humor: Sexual Selection or Interest Indication?
Are people who are funny more attractive? Or does being attractive lead people to be seen as funnier? The answer may depend on the underlying evolutionary function of humor. While humor has been proposed to signal "good genes," the authors propose that humor also functions to indicate interest in social relationships-in initiating new relationships and in monitoring existing ones. Consistent with this interest indicator model, across three studies both sexes were more likely to initiate humor and to respond more positively and consider the other person to be funny when initially attracted to that person. The findings support that humor dynamics- and not just humor displays-influence romantic chemistry for both men and women, suggesting that humor can ultimately function as a strategy to initiate and monitor social relationships
Understanding Dwarf Galaxies in order to Understand Dark Matter
Much progress has been made in recent years by the galaxy simulation
community in making realistic galaxies, mostly by more accurately capturing the
effects of baryons on the structural evolution of dark matter halos at high
resolutions. This progress has altered theoretical expectations for galaxy
evolution within a Cold Dark Matter (CDM) model, reconciling many earlier
discrepancies between theory and observations. Despite this reconciliation, CDM
may not be an accurate model for our Universe. Much more work must be done to
understand the predictions for galaxy formation within alternative dark matter
models.Comment: Refereed contribution to the Proceedings of the Simons Symposium on
Illuminating Dark Matter, to be published by Springe
Short-lived star-forming giant clumps in cosmological simulations of z~2 disks
Many observed massive star-forming z\approx2 galaxies are large disks that
exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We
present the largest sample to date of high-resolution cosmological SPH
simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun
galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong
stellar feedback parameterized as momentum-driven galactic winds. This model
reproduces many characteristic features of this observed class of galaxies,
such as their clumpy morphologies, smooth and monotonic velocity gradients,
high gas fractions (f_g\sim50%) and high specific star-formation rates
(\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps
(Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities.
However, the galactic winds are critical for their subsequent evolution. The
giant clumps we obtain are short-lived and are disrupted by wind-driven mass
loss. They do not virialise or migrate to the galaxy centers as suggested in
recent work neglecting strong winds. By phenomenologically implementing the
winds that are observed from high-redshift galaxies and in particular from
individual clumps, our simulations reproduce well new observational constraints
on clump kinematics and clump ages. In particular, the observation that older
clumps appear closer to their galaxy centers is reproduced in our simulations,
as a result of inside-out formation of the disks rather than inward clump
migration.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the
Astrophysical Journa
Stellar Disks in Aquarius Dark Matter Haloes
We investigate the gravitational interactions between live stellar disks and
their dark matter halos, using LCDM haloes similar in mass to that of the Milky
Way taken from the Aquarius Project. We introduce the stellar disks by first
allowing the haloes to respond to the influence of a growing rigid disk
potential from z = 1.3 to z = 1.0. The rigid potential is then replaced with
star particles which evolve self-consistently with the dark matter particles
until z = 0.0. Regardless of the initial orientation of the disk, the inner
parts of the haloes contract and change from prolate to oblate as the disk
grows to its full size. When the disk normal is initially aligned with the
major axis of the halo at z=1.3, the length of the major axis contracts and
becomes the minor axis by z=1.0. Six out of the eight disks in our main set of
simulations form bars, and five of the six bars experience a buckling
instability that results in a sudden jump in the vertical stellar velocity
dispersion and an accompanying drop in the m=2 Fourier amplitude of the disk
surface density. The bars are not destroyed by the buckling but continue to
grow until the present day. Bars are largely absent when the disk mass is
reduced by a factor of two or more; the relative disk-to-halo mass is therefore
a primary factor in bar formation and evolution. A subset of the disks is
warped at the outskirts and contains prominent non-coplanar material with a
ring-like structure. Many disks reorient by large angles between z=1 and z=0,
following a coherent reorientation of their inner haloes. Larger reorientations
produce more strongly warped disks, suggesting a tight link between the two
phenomena. The origins of bars and warps appear independent: some disks with
strong bars show no disturbances at the outskirts, while the disks with the
weakest bars show severe warps.Comment: 19 pages, 13 figures, accepted MNRAS; fixed compatibility problem in
figures 8,
A catalogue of young runaway Hipparcos stars within 3kpc from the Sun
Traditionally runaway stars are O and B type stars with large peculiar
velocities.We want to extend this definition to young stars (up to ~50 Myr) of
any spectral type and identify those present in the Hipparcos catalogue
applying different selection criteria such as peculiar space velocities or
peculiar one-dimensional velocities. Runaway stars are important to study the
evolution of multiple star systems or star clusters as well as to identify
origins of neutron stars. We compile distances, proper motions, spectral types,
luminosity classes, V magnitudes and B-V colours and utilise evolutionary
models from different authors to obtain star ages and study a sample of 7663
young Hipparcos stars within 3 kpc from the Sun. Radial velocities are obtained
from the literature. We investigate the distributions of the peculiar spatial
velocity, the peculiar radial velocity as well as the peculiar tangential
velocity and its one-dimensional components and obtain runaway star
probabilities for each star in the sample. In addition, we look for stars that
are situated outside any OB association or OB cluster and the Galactic plane as
well as stars of which the velocity vector points away from the median velocity
vector of neighbouring stars or the surrounding local OB association/ cluster
although the absolute velocity might be small. We find a total of 2547 runaway
star candidates (with a contamination of normal Population I stars of 20 per
cent at most). Thus, after subtraction of those 20 per cent, the runaway
frequency among young stars is about 27 per cent. We compile a catalogue of
runaway stars which will be available via VizieR.Comment: 12 pages, 8 figures, 7 tables, accepted for publication in MNRAS old
version replaced due to change of the title after journal proof-readin
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