217 research outputs found
Halo orbits in cosmological disk galaxies : tracers of information history
We analyze the orbits of stars and dark matter particles in the halo of a disk galaxy formed in a cosmological hydrodynamical simulation. The halo is oblate within the inner ∼20 kpc and triaxial beyond this radius. About 43% of orbits are short axis tubes—the rest belong to orbit families that characterize triaxial potentials (boxes, long-axis tubes and chaotic orbits), but their shapes are close to axisymmetric. We find no evidence that the self-consistent distribution function of the nearly oblate inner halo is comprised primarily of axisymmetric short-axis tube orbits. Orbits of all families and both types of particles are highly eccentric, with mean eccentricity �0.6. We find that randomly selected samples of halo stars show no substructure in “integrals of motion” space. However, individual accretion events can clearly be identified in plots of metallicity versus formation time. Dynamically young tidal debris is found primarily on a single type of orbit. However, stars associated with older satellites become chaotically mixed during the formation process (possibly due to scattering by the central bulge and disk, and baryonic processes), and appear on all four types of orbits. We find that the tidal debris in cosmological hydrodynamical simulations experiences significantly more chaotic evolution than in collisionless simulations, making it much harder to identify individual progenitors using phase space coordinates alone. However, by combining information on stellar ages and chemical abundances with the orbital properties of halo stars in the underlying self-consistent potential, the identification of progenitors is likely to be possible
The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History
Fully cosmological, high resolution N-Body + SPH simulations are used to
investigate the chemical abundance trends of stars in simulated stellar halos
as a function of their origin. These simulations employ a physically motivated
supernova feedback recipe, as well as metal enrichment, metal cooling and metal
diffusion. As presented in an earlier paper, the simulated galaxies in this
study are surrounded by stellar halos whose inner regions contain both stars
accreted from satellite galaxies and stars formed in situ in the central
regions of the main galaxies and later displaced by mergers into their inner
halos. The abundance patterns ([Fe/H] and [O/Fe]) of halo stars located within
10 kpc of a solar-like observer are analyzed. We find that for galaxies which
have not experienced a recent major merger, in situ stars at the high [Fe/H]
end of the metallicity distribution function are more [alpha/Fe]-rich than
accreted stars at similar [Fe/H]. This dichotomy in the [O/Fe] of halo stars at
a given [Fe/H] results from the different potential wells within which in situ
and accreted halo stars form. These results qualitatively match recent
observations of local Milky Way halo stars. It may thus be possible for
observers to uncover the relative contribution of different physical processes
to the formation of stellar halos by observing such trends in the halo
populations of the Milky Way, and other local L* galaxies.Comment: Version accepted for publication in ApJ Part 1. This version of the
paper has been extended to include a detailed discussion of numerical issue
Stellar distances from spectroscopic observations: a new technique
A Bayesian approach to the determination of stellar distances from
photometric and spectroscopic data is presented and tested both on pseudodata,
designed to mimic data for stars observed by the RAVE survey, and on the real
stars from the Geneva-Copenhagen survey. It is argued that this method is
optimal in the sense that it brings to bear all available information and that
its results are limited only by observational errors and the underlying physics
of stars. The method simultaneously returns the metallicities, ages and masses
of programme stars. Remarkably, the uncertainty in the output metallicity is
typically 44 per cent smaller than the uncertainty in the input metallicity.Comment: 16 pages, 17 figures, 2 tables; accepted by MNRAS. Reference updated
The stellar mass distribution in early-type disk galaxies: surface photometry and bulge-disk decompositions
We present deep B- and R-band surface photometry for a sample of 21 galaxies
with morphological types between S0 and Sab. We present radial profiles of
surface brightness, colour, ellipticity, position angle and deviations of
axisymmetry for all galaxies, as well as isophotal and effective radii and
total magnitudes. We have decomposed the images into contributions from a
spheroidal bulge and a flat disk, using an interactive, 2D decomposition
technique.
We study in detail the relations between various bulge and disk parameters.
In particular, we find that the bulges of our galaxies have surface brightness
profiles ranging from exponential to De Vaucouleurs, with the average value of
the Sersic shape parameter n being 2.5. In agreement with previous studies, we
find that the shape of the bulge intensity distribution depends on luminosity,
with the more luminous bulges having more centrally peaked light profiles. By
comparing the ellipticity of the isophotes in the bulges to those in the outer,
disk dominated regions, we are able to derive the intrinsic axis ratio q_b of
the bulges. The average axis ratio is 0.55, with an rms spread of 0.12. None of
the bulges in our sample is spherical, whereas in some cases, the bulges can be
as flat as q_b = 0.3 - 0.4. The bulge flattening seems to be weakly coupled to
luminosity, more luminous bulges being on average slightly more flattened than
their lower-luminosity counterparts. Our finding that most bulges are
significantly flattened and have an intensity profile shallower than R^{1/4}
suggests that `pseudobulges', formed from disk material by secular processes,
do not only occur in late-type spiral galaxies, but are a common feature in
early-type disk galaxies as well. (abridged)Comment: 26 pages, 11 figures. Accepted for publication in MNRAS. A
pdf-version with full resolution figures and the full atlas can be found at
http://www.nottingham.ac.uk/~ppzen/surfphot.accepted.pd
White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy
The nature of the several microlensing events observed by the MACHO team
towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass
substellar objects and stars with masses larger than ~M_{sun} have been ruled
out as major components of a Massive Astrophysical Compact Halo Object (MACHO)
Galactic halo, while stars of half a solar mass seem to be viable candidates.
Main sequence stars have been already discarded, and there are tight
restrictions on the role played by white dwarfs with hydrogen-dominated
atmospheres. In this paper we evaluate the contribution to the dark matter
content of the Galaxy of white dwarfs with hydrogen-deficient atmospheres. For
this purpose we use a Monte Carlo simulator which incorporates up-to-date
evolutionary sequences of white dwarfs with hydrogen-rich and
hydrogen-deficient atmospheres. We also take into account detailed descriptions
of the thick disk and the halo of our Galaxy as well as of a reliable model of
the LMC. We find that the contribution of white dwarfs with hydrogen-deficient
atmospheres moderately increases the theoretical estimate of the optical depth
with respect to the value obtained when only hydrogen-rich white dwarfs are
considered. We also find that the contribuiton of the thick disk population of
white dwarfs is comparable to the halo contribution. However, the contributions
of both the halo and the thick disk white-dwarf populations are still
insufficient to explain the number of events observed by the MACHO team.
Finally, we find that the contribution to the halo dark matter of the entire
population under study is less than 10% at the 95% conficence level.Comment: 12 pages, 5 figures, accepted for publication in A&
Star Formation at From the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH)
Using the first 50% of data collected for the Spitzer Large Area Survey with
Hyper-Suprime-Cam (SPLASH) observations on the 1.8 deg Cosmological
Evolution Survey (COSMOS) we estimate the masses and star formation rates of
3398 star-forming galaxies at with a
substantial population up to . We find that the
strong correlation between stellar mass and star formation rate seen at lower
redshift (the "main sequence" of star-forming galaxies) extends to .
The observed relation and scatter is consistent with a continued increase in
star formation rate at fixed mass in line with extrapolations from
lower-redshift observations. It is difficult to explain this continued
correlation, especially for the most massive systems, unless the most massive
galaxies are forming stars near their Eddington-limited rate from their first
collapse. Furthermore, we find no evidence for moderate quenching at higher
masses, indicating quenching either has not occurred prior to or
else occurs rapidly, so that few galaxies are visible in transition between
star-forming and quenched.Comment: ApJL, accepte
Abundance analysis of the outer halo globular cluster Palomar 14
We determine the elemental abundances of nine red giant stars belonging to
Palomar 14 (Pal 14). Pal 14 is an outer halo globular cluster (GC) at a
distance of \sim 70 kpc. Our abundance analysis is based on high-resolution
spectra and one-dimensional stellar model atmospheres.We derived the abundances
for the iron peak elements Sc, V, Cr, Mn, Co, Ni, the {\alpha}-elements O, Mg,
Si, Ca, Ti, the light odd element Na, and the neutron-capture elements Y, Zr,
Ba, La, Ce, Nd, Eu, Dy, and Cu. Our data do not permit us to investigate light
element (i.e., O to Mg) abundance variations. The neutron-capture elements show
an r-process signature. We compare our measurements with the abundance ratios
of inner and other outer halo GCs, halo field stars, GCs of recognized
extragalactic origin, and stars in dwarf spheroidal galaxies (dSphs). The
abundance pattern of Pal 14 is almost identical to those of Pal 3 and Pal 4,
the next distant members of the outer halo GC population after Pal 14. The
abundance pattern of Pal 14 is also similar to those of the inner halo GCs,
halo field stars, and GCs of recognized extragalactic origin, but differs from
what is customarily found in dSphs field stars. The abundance properties of Pal
14 as well as those of the other outer halo GCs are thus compatible with an
accretion origin from dSphs. Whether or not GC accretion played a role, it
seems that the formation conditions of outer halo GCs and GCs in dSphs were
similar.Comment: 19 pages, 15 figures. Accepted by A&
The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid
The combination of large size, high stellar density, high metallicity, and
Sersic surface brightness profile of the spheroidal component of the Andromeda
galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent
of the Milky Way. In this work we capitalize on our proximity to and external
view of M31 to probe the kinematical properties of this "inner spheroid." We
employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar
kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to
disentangle M31's inner spheroid from its stellar disk. We measure the mean
velocity and dispersion of the spheroid in each of five spatial bins after
accounting for a locally cold stellar disk as well as the Giant Southern Stream
and associated tidal debris. For the first time, we detect significant spheroid
rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion
decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc,
consistent to 2 sigma with existing measurements and models. We calculate the
probability that a given star is a member of the spheroid and find that the
spheroid has a significant presence throughout the spatial extent of our
sample. Lastly, we show that the flattening of the spheroid is due to velocity
anisotropy in addition to rotation. Though this suggests that the inner
spheroid of M31 more closely resembles an elliptical galaxy than a typical
spiral galaxy bulge, it should be cautioned that our measurements are much
farther out (2 - 14 r_eff) than for the comparison samples.Comment: Accepted for publication in Ap
Neuroimaging alterations of the suicidal brain and its relevance to practice: an updated review of MRI studies
Suicide is a leading cause of death in the United States. Historically, scientific inquiry has focused on psychological theory. However, more recent studies have started to shed light on complex biosignatures using MRI techniques, including task-based and resting-state functional MRI, brain morphometry, and diffusion tensor imaging. Here, we review recent research across these modalities, with a focus on participants with depression and Suicidal Thoughts and Behavior (STB). A PubMed search identified 149 articles specific to our population of study, and this was further refined to rule out more diffuse pathologies such as psychotic disorders and organic brain injury and illness. This left 69 articles which are reviewed in the current study. The collated articles reviewed point to a complex impairment showing atypical functional activation in areas associated with perception of reward, social/affective stimuli, top-down control, and reward-based learning. This is broadly supported by the atypical morphometric and diffusion-weighted alterations and, most significantly, in the network-based resting-state functional connectivity data that extrapolates network functions from well validated psychological paradigms using functional MRI analysis. We see an emerging picture of cognitive dysfunction evident in task-based and resting state fMRI and network neuroscience studies, likely preceded by structural changes best demonstrated in morphometric and diffusion-weighted studies. We propose a clinically-oriented chronology of the diathesis-stress model of suicide and link other areas of research that may be useful to the practicing clinician, while helping to advance the translational study of the neurobiology of suicide
Light Curve Templates and Galactic Distribution of RR Lyrae Stars from Sloan Digital Sky Survey Stripe 82
We present an improved analysis of halo substructure traced by RR Lyrae stars
in the SDSS stripe 82 region. With the addition of SDSS-II data, a revised
selection method based on new ugriz light curve templates results in a sample
of 483 RR Lyrae stars that is essentially free of contamination. The main
result from our first study persists: the spatial distribution of halo stars at
galactocentric distances 5--100 kpc is highly inhomogeneous. At least 20% of
halo stars within 30 kpc from the Galactic center can be statistically
associated with substructure. We present strong direct evidence, based on both
RR Lyrae stars and main sequence stars, that the halo stellar number density
profile significantly steepens beyond a Galactocentric distance of ~30 kpc, and
a larger fraction of the stars are associated with substructure. By using a
novel method that simultaneously combines data for RR Lyrae and main sequence
stars, and using photometric metallicity estimates for main sequence stars
derived from deep co-added u-band data, we measure the metallicity of the
Sagittarius dSph tidal stream (trailing arm) towards R.A.2h-3h and Dec~0 deg to
be 0.3 dex higher ([Fe/H]=-1.2) than that of surrounding halo field stars.
Together with a similar result for another major halo substructure, the
Monoceros stream, these results support theoretical predictions that an early
forming, smooth inner halo, is metal poor compared to high surface brightness
material that have been accreted onto a later-forming outer halo. The mean
metallicity of stars in the outer halo that are not associated with detectable
clumps may still be more metal-poor than the bulk of inner-halo stars, as has
been argued from other data sets.Comment: Submitted to ApJ, 68 pages, 26 figures, supplemental material (light
curves, templates, animation) can be downloaded from
http://www.astro.washington.edu/bsesar/S82_RRLyr.htm
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