817 research outputs found
Stellar haloes in Milky-Way mass galaxies: From the inner to the outer haloes
We present a comprehensive study of the chemical properties of the stellar
haloes of Milky-Way mass galaxies, analysing the transition between the inner
to the outer haloes. We find the transition radius between the relative
dominance of the inner-halo and outer-halo stellar populations to be ~15-20 kpc
for most of our haloes, similar to that inferred for the Milky Way from recent
observations. While the number density of stars in the simulated inner-halo
populations decreases rapidly with distance, the outer-halo populations
contribute about 20-40 per cent in the fiducial solar neighborhood, in
particular at the lowest metallicities. We have determined [Fe/H] profiles for
our simulated haloes; they exhibit flat or mild gradients, in the range
[-0.002, -0.01 ] dex/kpc. The metallicity distribution functions exhibit
different features, reflecting the different assembly history of the individual
stellar haloes. We find that stellar haloes formed with larger contributions
from massive subgalactic systems have steeper metallicity gradients. Very
metal-poor stars are mainly contributed to the halo systems by lower-mass
satellites. There is a clear trend among the predicted metallicity distribution
functions that a higher fraction of low-metallicity stars are found with
increasing radius. These properties are consistent with the range of behaviours
observed for stellar haloes of nearby galaxies.Comment: 11 pages, 6 figures. Accepted MNRAS. Revised version after referee's
comment
Evolution of field early-type galaxies: The view from GOODS CDFS
We explore the evolution of field early-type galaxies in a sample extracted from the ACS images of the southern GOODS field. The galaxies are selected by means of a nonparametric analysis, followed by visual inspection of the candidates with a concentrated surface brightness distribution. We furthermore exclude from the final sample those galaxies that are not consistent with an evolution into the Kormendy relation between surface brightness and size that is observed for z = 0 ellipticals. The final set, which comprises 249 galaxies with a median redshift z(m) = 0.71, represents a sample of early-type systems not selected with respect to color, with similar scaling relations as those of bona fide elliptical galaxies. The distribution of number counts versus apparent magnitude rejects a constant number density with cosmic time and suggests a substantial decrease with redshift: n proportional to (1 + z)(-2.5). The majority of the galaxies (78%) feature passively evolving old stellar populations. One-third of those in the upper half of the redshift distribution have blue colors, in contrast to only 10% in the lower redshift subsample. An adaptive binning of the color maps using an optimal Voronoi tessellation is performed to explore the internal color distribution. We find that the red and blue early-type galaxies in our sample have distinct behavior with respect to the color gradients, so that most blue galaxies feature blue cores whereas most of the red early-types are passively evolving stellar populations with red cores, i.e., similar systems to local early-type galaxies. Furthermore, the color gradients and scatter do not evolve with redshift and are compatible with the observations at z 0, assuming a radial dependence of the metallicity within each galaxy. Significant gradients in the stellar age are readily ruled out. This work emphasizes the need for a careful sample selection, as we found that most of those galaxies that were visually classified as candidate early types-but then rejected based on the Kormendy relation-feature blue colors characteristic of recent star formation
Large stellar disks in small elliptical galaxies
We present the rotation velocities V and velocity dispersions sigma along the
principal axes of seven elliptical galaxies less luminous than M_B= -19.5.
These kinematics extend beyond the half-light radii for all systems in this
photometrically selected sample. At large radii the kinematics not only confirm
that rotation and "diskiness" are important in faint ellipticals, as was
previously known, but also demonstrate that in most sample galaxies the stars
at large galactocentric distances have (V/sigma)_max of about 2, similar to the
disks in bona-fide S0 galaxies. Comparing this high degree of ordered stellar
motion in all sample galaxies with numerical simulations of dissipationless
mergers argues against mergers with mass ratios <=3:1 as an important mechanism
in the final shaping of low-luminosity ellipticals, and favors instead the
dissipative formation of a disk.Comment: 11 pages LaTex with 4 Postscript figure
Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348
A new model atmosphere analysis for the peculiar DQ white dwarf discovered by
Carollo et al. (2002) is presented. The effective temperature and carbon
abundance have been estimated by fitting both the photometric data
(UBJ,VRF,IN,JHK) and a low resolution spectrum (3500<lambda<7500 A) with a new
model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We
estimate Teff ~ 5120 +/- 200 K and log[C/He] ~ -5.8 +/- 0.5, which make GSC2U
J131147.2+292348 the coolest DQ star ever observed. This result indicates that
the hypothetical transition from C2 to C2H molecules around Teff = 6000 K,
which was inferred to explain the absence of DQ stars at lower temperatures,
needs to be reconsidered.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics Letter
Dark matter in elliptical galaxies
We present measurements of the shape of the stellar line-of-sight velocity
distribution out to two effective radii along the major axes of the four
elliptical galaxies NGC 2434, 2663, 3706, and 5018. The velocity dispersion
profiles are flat or decline gently with radius. We compare the data to the
predictions of f=f(E,L_z) axisymmetric models with and without dark matter.
Strong tangential anisotropy is ruled out at large radii. We conclude from our
measurements that massive dark halos must be present in three of the four
galaxies, while for the fourth galaxy (NGC 2663) the case is inconclusive.Comment: 15 pages, uuencoded compressed PostScript, includes 3 figure
Evolution of field early-type galaxies: The view from GOODS/CDFS
(Abridged) We explore the evolution of field early-type galaxies on a sample
extracted from GOODS/CDFS. The galaxies are selected by means of a
non-parametric analysis followed by visual inspection. We exclude those
galaxies which are not consistent with an evolution into the Kormendy relation.
The final set comprises 249 galaxies with a median redshift z=0.7. The
distribution of number counts versus apparent magnitude suggests a substantial
decrease of the comoving number density with redshift. The majority of the
galaxies feature passively evolving old stellar populations. One third of those
in the upper half of the redshift distribution have blue colors, in contrast to
only 10% in the lower redshift subsample. An adaptive binning of the color maps
is performed to explore the internal color distribution. We find that most blue
galaxies in our sample feature blue cores whereas most of the red early-types
are passively evolving stellar populations with red cores. The color gradients
and scatter do not evolve with redshift and are compatible with the
observations at z=0 assuming a radial dependence of the metallicity within each
galaxy. This work emphasizes the need for a careful sample selection, as we
found that most of those galaxies which were visually classified as early types
-- but then rejected based on the Kormendy relation -- feature blue colors
characteristic of recent star formation.Comment: 14 pages, 16 figures. Uses emulateapj. Accepted for publication in
ApJ. Some figures in low resolutio
Coherent evolution via reservoir driven holonomy
We show that in the limit of strongly interacting environment a system
initially prepared in a Decoherence Free Subspace (DFS) coherently evolves in
time, adiabatically following the changes of the DFS. If the reservoir cyclicly
evolves in time, the DFS states acquire an holonomy.Comment: 4 page
Stellar haloes of simulated Milky Way-like galaxies: Chemical and kinematic properties
We investigate the chemical and kinematic properties of the diffuse stellar
haloes of six simulated Milky Way-like galaxies from the Aquarius Project.
Binding energy criteria are adopted to defined two dynamically distinct stellar
populations: the diffuse inner and outer haloes, which comprise different
stellar sub-populations with particular chemical and kinematic characteristics.
Our simulated inner- and outer-halo stellar populations have received
contributions from debris stars (formed in sub-galactic systems while they were
outside the virial radius of the main progenitor galaxies) and endo-debris
stars (those formed in gas-rich sub-galactic systems inside the dark matter
haloes). The inner haloes possess an additional contribution from disc-heated
stars in the range , with a mean of . Disc-heated
stars might exhibit signatures of kinematical support, in particular among the
youngest ones. Endo-debris plus disc-heated stars define the so-called \insitu
stellar populations. In both the inner- and outer-halo stellar populations, we
detect contributions from stars with moderate to low [/Fe] ratios,
mainly associated with the endo-debris or disc-heated sub-populations. The
observed abundance gradients in the inner-halo regions are influenced by both
the level of chemical enrichment and the relative contributions from each
stellar sub-population. Steeper abundance gradients in the inner-halo regions
are related to contributions from the disc-heated and endo-debris stars, which
tend to be found at lower binding energies than debris stars. (Abridged).Comment: 12 pages, 6 figures. Accepted version. To appear in MNRA
The GSC-II-based survey of ancient cool white dwarfs I. The sample of spectroscopically confirmed WDs
The GSC-II white dwarf survey was designed to identify faint and high proper
motion objects, which we used to define a new and independent sample of cool
white dwarfs. With this survey we aim to derive new constraints on the halo
white dwarf space density. Also, these data can provide information on the age
of thick disk and halo through the analysis of the luminosity function. On the
basis of astrometric and photometric parameters, we selected candidates with mu
> 0.28 as/yr and R_F > 16 in an area of 1150 square degrees. Then, we separated
white dwarfs from late type dwarfs and subdwarfs by means of the reduced proper
motion diagram. Finally, spectroscopic follow-up observations were carried out
to confirm the white dwarf nature of the selected candidates. We found 41 white
dwarfs of which 24 are new discoveries. Here we present the full sample and for
each object provide positions, absolute proper motions, photometry, and
spectroscopy.Comment: 14 pages, 7 figures, submitted to A&
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