256 research outputs found
Assessing distances and consistency of kinematics in Gaia/TGAS
We apply the statistical methods by Schoenrich, Binney & Asplund to assess
the quality of distances and kinematics in the RAVE-TGAS and LAMOST-TGAS
samples of Solar neighbourhood stars. These methods yield a nominal distance
accuracy of 1-2%. Other than common tests on parallax accuracy, they directly
test distance estimations including the effects of distance priors. We show how
to construct these priors including the survey selection functions (SSFs)
directly from the data. We demonstrate that neglecting the SSFs causes severe
distance biases. Due to the decline of the SSFs in distance, the simple
1/parallax estimate only mildly underestimates distances. We test the accuracy
of measured line-of-sight velocities (v_los) by binning the samples in the
nominal v_los uncertainties. We find: a) the LAMOST v_los have a ~ -5 km/s
offset; b) the average LAMOST measurement error for v_los is ~7 km/s,
significantly smaller than, and nearly uncorrelated with the nominal LAMOST
estimates. The RAVE sample shows either a moderate distance underestimate, or
an unaccounted source of v_los dispersion (e_v) from measurement errors and
binary stars. For a subsample of suspected binary stars in RAVE, our methods
indicate significant distance underestimates. Separating a sample in
metallicity or kinematics to select thick-disc/halo stars, discriminates
between distance bias and e_v. For LAMOST, this separation yields consistency
with pure v_los measurement errors. We find an anomaly near longitude
l~(300+/-60)deg and distance s~(0.32+/-0.03)kpc on both sides of the galactic
plane, which could be explained by either a localised distance error or a
breathing mode.Comment: 21 pages, 14 figures accepted by MNRAS; now also includes comparison
to Astraatmadja & Bailer-Jones distance
The structural evolution of galaxies with both thin and thick discs
We perform controlled N-body simulations of disc galaxies growing within live
dark matter (DM) haloes to present-day galaxies that contain both thin and
thick discs. We consider two types of models: a) thick-disc initial conditions
to which stars on near-circular orbits are continuously added over ~10 Gyr and
b) models in which the birth velocity dispersion of stars decreases
continuously over the same time-scale. We show that both schemes produce
double-exponential vertical profiles similar to that of the Milky Way (MW). We
indicate how the spatial age structure of galaxies can be used to discriminate
between scenarios. We show that the presence of a thick disc significantly
alters and delays bar formation and thus makes possible models with a realistic
bar and a high baryon-to-DM mass ratio in the central regions, as required by
microlensing constraints. We examine how the radial mass distribution in stars
and DM is affected by disc growth and non-axisymmetries. We discuss how bar
buckling shapes the vertical age distribution of thin- and thick-disc stars in
the bar region. The extent to which the combination of observationally
motivated inside-out growth histories and cosmologically motivated dark halo
properties leads to the spontaneous formation of non-axisymmetries that steer
the models towards present-day MW-like galaxies is noteworthy.Comment: Accepted for publication in MNRAS, 22 pages, 13 figures, 2 table
Migration and kinematics in growing disc galaxies with thin and thick discs
We analyse disc heating and radial migration in N-body models of growing disc
galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies
with appropriate non-axisymmetric structures reproduce observational
constraints on radial disc heating in and migration to the Solar Neighbourhood
(Snhd). The presence of thick discs can suppress non-axisymmetries and thus
higher baryonic-to-dark matter fractions are required than in models that only
have a thin disc. Models that are baryon-dominated to roughly the Solar radius
R_0 are favoured, in agreement with data for the Milky Way. For inside-out
growing discs, today's thick-disc stars at R_0 are dominated by outwards
migrators. Whether outwards migrators are vertically hotter than non-migrators
depends on the radial gradient of the thick disc vertical velocity dispersion.
There is an effective upper boundary in angular momentum that thick disc stars
born in the centre of a galaxy can reach by migration, which explains the
fading of the high-alpha sequence outside R_0. Our models compare well to Snhd
kinematics from RAVE-TGAS. For such comparisons it is important to take into
account the azimuthal variation of kinematics at R ~ R_0 and biases from survey
selection functions. The vertical heating of thin disc stars by giant molecular
clouds is only mildly affected by the presence of thick discs. Our models
predict higher vertical velocity dispersions for the oldest stars than found in
the Snhd age-velocity dispersion relation, possibly because of measurement
uncertainties or an underestimation of the number of old cold stars in our
models.Comment: accepted for publication in MNRAS, 22 pages, 11 figures, 1 Table,
Appendi
Age velocity dispersion relations and heating histories in disc galaxies
We analyse the heating of stellar discs by non axisymmetric structures and
giant molecular clouds (GMCs) in N-body simulations of growing disc galaxies.
The analysis resolves long-standing discrepancies between models and data by
demonstrating the importance of distinguishing between measured age-velocity
dispersion relations (AVRs) and the heating histories of the stars that make up
the AVR. We fit both AVRs and heating histories with formulae proportional to
t^beta and determine the exponents beta_R and beta_z derived from in-plane and
vertical AVRs and ~beta_R and ~beta_z from heating histories. Values of beta_z
are in almost all simulations larger than values of ~beta_z, whereas values of
beta_R are similar to or mildly larger than values of ~beta_R. Moreover, values
of beta_z (~beta_z) are generally larger than values of beta_R (~beta_R). The
dominant cause of these relations is the decline over the life of the disc in
importance of GMCs as heating agents relative to spiral structure and the bar.
We examine how age errors and biases in solar neighbourhood surveys influence
the measured AVR: they tend to decrease beta values by smearing out ages and
thus measured dispersions. We compare AVRs and velocity ellipsoid shapes
sigma_z/sigma_R from simulations to Solar neighbourhood data. We conclude that
for the expected disc mass and dark halo structure, combined GMC and spiral/bar
heating can explain the AVR of the Galactic thin disc. Strong departures of the
disc mass or the dark halo structure from expectation spoil fits to the data.Comment: Accepted for publication in MNRAS, 19 pages, 8 figures, 1 tabl
Kinematic Detection of the Galactic Nuclear Disc
We report the detection of the Galactic nuclear disc in line-of-sight
kinematics of stars, measured with infrared spectroscopy from APOGEE. This
stellar component of the nuclear disc has an extent and rotation velocity V ~
120kms comparable to the gas disc in the central molecular zone. The current
data suggest that this disc is kinematically cool and has a small vertical
extent of order 50pc. The stellar kinematics suggest a truncation radius/steep
decline of the stellar disc at a galactocentric radius R ~ 150pc, and provide
tentative evidence for an overdensity at the position of the ring found in the
molecular gas disc.Comment: 6 pages, 4 figures, accepted by ApJ Letter
The Design of Everyday Hate: A Qualitative and Quantitative Analysis
Throughout history artists, poets, and writers have been interested in the nature of hate. Scientists from a variety of disciplines have also attempted to unravel its mysteries. Yet in spite of abundant theorizing and research, most modern scholars still complain that little is known about this complex emotion. In this study, a new approach has been taken. Following Heider’s (1958) observation that scientists can often learn a great deal by exploring people’s “common-sense” or “naïve psychologies,” students at the University of Texas and participants from a number of Internet sites were interviewed regarding their perceptions of the nature of emotion. Using grounded theory and employing mixed-method analyses (qualitative and quantitative), four questions were explored: (1) What do people mean by hate? (2) Whom do they hate? (3) Why do people hate the people they do? (4) How do people attempt to deal with such feelings? From participants’ answers, a theory concerning everyday hate was generated
Towards a more realistic population of bright spiral galaxies in cosmological simulations
We present an update to the multiphase SPH galaxy formation code by
Scannapieco et al. We include a more elaborate treatment of the production of
metals, cooling rates based on individual element abundances, and a scheme for
the turbulent diffusion of metals. Our SN feedback model now transfers energy
to the ISM in kinetic and thermal form, and we include a prescription for the
effects of radiation pressure from massive young stars on the ISM. We calibrate
our new code on the well studied Aquarius haloes and then use it to simulate a
sample of 16 galaxies with halo masses between 1x10^11 and 3x10^12 M_sun. In
general, the stellar masses of the sample agree well with the stellar mass to
halo mass relation inferred from abundance matching techniques for redshifts
z=0-4. There is however a tendency to overproduce stars at z>4 and to
underproduce them at z<0.5 in the least massive haloes. Overly high SFRs at z<1
for the most massive haloes are likely connected to the lack of AGN feedback in
our model. The simulated sample also shows reasonable agreement with observed
star formation rates, sizes, gas fractions and gas-phase metallicities at
z=0-3. Remaining discrepancies can be connected to deviations from predictions
for star formation histories from abundance matching. At z=0, the model
galaxies show realistic morphologies, stellar surface density profiles,
circular velocity curves and stellar metallicities, but overly flat metallicity
gradients. 15 out of 16 of our galaxies contain disk components with kinematic
disk fraction ranging between 15 and 65 %. The disk fraction depends on the
time of the last destructive merger or misaligned infall event. Considering the
remaining shortcomings of our simulations we conclude that even higher
kinematic disk fractions may be possible for LambdaCDM haloes with quiet merger
histories, such as the Aquarius haloes.Comment: 26 pages, 20 figures, accepted for publication in MNRA
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Consequences of dual-group membership among children
textIncreasing numbers of individuals are simultaneously members of two or more social categories. To investigate the effects of single- versus dual-identity status on children's group views and intergroup attitudes, elementary-school-age children (N = 91) attending a summer school program were assigned to novel color groups that included single-identity ("blue" and "red") and dual-identity ("bicolored," or half red and half blue) members. The degree to which dual-identity status was verified by the authority members was also manipulated: teachers in some classrooms were instructed to label and make use of three social groups ("blues," "reds," "bicolors") to organize their classrooms, whereas teachers in other classrooms were instructed to label and make use of only the two "mono-colored" groups ("blues" and "reds"). After several weeks in their classrooms, children's (a) views of group membership (i.e., importance, satisfaction, perceived similarity, group preference), (b) intergroup attitudes (i.e., traits ratings, group evaluations, peer preferences), and (c) categorization complexity (i.e., tendency to sort individuals along multiple dimensions simultaneously) were assessed. Results varied across measures but, in general, indicated that dual-identity status affected children's views of their ingroup. Specifically, dual-identity children in classrooms in which their status was not verified were more likely to (a) perceive themselves as similar to other ingroup members (i.e., bicolored children), (b) want to keep their shirt color, and (c) assume that a new student would want their shirt color more than their single-identity peers. They also showed higher levels of ingroup bias in their competency ratings of groups than their single-identity peers, and demonstrated greater cognitive flexibility when thinking about social categories than their single-identity peers. Overall, these results suggest that dual-identity children experience identity issues differently than their single-identity peers and that additional theories are needed to address the complexities of social membership and bias among children with dual memberships.Psycholog
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 Hß 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
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