47 research outputs found
A new low mass for the Hercules dSph: the end of a common mass scale for the dwarfs?
We present a new mass estimate for the Hercules dwarf spheroidal galaxy
(dSph), based on the revised velocity dispersion obtained by Aden et al. (2009,
arXiv:0908.3489).
The removal of a significant foreground contamination using newly acquired
Stromgren photometry has resulted in a reduced velocity dispersion. Using this
new velocity dispersion of 3.72 +/- 0.91 km/s, we find a mass of
M_300=1.9^{+1.1}_{-0.8} 10^6 M_sun within the central 300 pc, which is also the
half-light radius, and a mass of M_433=3.7_{-1.6}^{+2.2} 10^6 M_sun within the
reach of our data to 433 pc, significantly lower than previous estimates. We
derive an overall mass-to-light ratio of M_433/L=103^{+83}_{-48} M_sun/L_sun.
Our mass estimate calls into question recent claims of a common mass scale for
dSph galaxies.
Additionally, we find tentative evidence for a velocity gradient in our
kinematic data of 16 +/- 3 km/s/kpc, and evidence of an asymmetric extension in
the light distribution at about 0.5 kpc. We explore the possibility that these
features are due to tidal interactions with the Milky Way. We show that there
is a self-consistent model in which Hercules has an assumed tidal radius of r_t
= 485 pc, an orbital pericentre of r_p = 18.5 +/- 5 kpc, and a mass within r_t
of M_{tid,r_t}=5.2 +/- 2.7 10^6 M_sun. Proper motions are required to test this
model. Although we cannot exclude models in which Hercules contains no dark
matter, we argue that Hercules is more likely to be a dark matter dominated
system which is currently experiencing some tidal disturbance of its outer
parts.Comment: 10 pages, 3 figures, Accepted for publication by ApJ
An abundance study of red-giant-branch stars in the Hercules dwarf spheroidal galaxy
Using high-resolution spectroscopy, we provide a determination of [Fe/H] and
[Ca/H] for confirmed red-giant branch member stars of the Hercules dwarf
spheroidal galaxy. Based on this we explore the ages of the prevailing stellar
populations in Hercules, and the enrichment history from supernovae.
Additionally, we provide a new simple metallicity calibration for Stromgren
photometry for metal-poor, red giant branch stars. We find that the red-giant
branch stars of the Hercules dSph galaxy are more metal-poor than estimated in
our previous study that was based on photometry alone. Additionally, we find an
abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and
lower for more metal-rich stars, with a spread of about 0.8 dex. The [Ca/Fe]
trend suggests an early rapid chemical enrichment through supernovae of type
II, followed by a phase of slow star formation dominated by enrichment through
supernovae of type Ia. A comparison with isochrones indicates that the red
giants in Hercules are older than 10 Gyr.Comment: 12 pages, 11 figures. Accepted for publication in A&
Signatures of an intermediate-age metal-rich bulge population
We have determined detailed elemental abundances and stellar ages for a
sample of now 38 microlensed dwarf and subgiant stars in the Galactic bulge.
Stars with sub-solar metallicities are all old and have enhanced alpha-element
abundances -- very similar to what is seen for local thick disk stars. The
metal-rich stars on the other hand show a wide variety of stellar ages, ranging
from 3-4 Gyr to 12 Gyr, and an average around 7-8 Gyr. The existence of young
and metal-rich stars are in conflict with recent photometric studies of the
bulge which claim that the bulge only contains old stars.Comment: Poster contribution at Galactic archeology, near-field cosmology and
the formation of the Milky Way, Shuzenji, Japan, 1-4 November 2011, to be
published in ASP Conference Serie
External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters
Accretion studies have been focused on the flow around bodies with point mass
gravitational potentials, but few general results are available for non-point
mass distributions. Here, we study the accretion flow onto non-divergent, core
potentials moving through a background medium. We use Plummer and Hernquist
potentials as examples to study gas accretion onto star clusters, dwarf and
large galaxy halos and galaxy clusters in a variety of astrophysical
environments. The general conditions required for a core potential to
collectively accrete large quantities of gas from the external medium are
derived using both simulations and analytic results. The consequences of large
mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are
twofold. First, if the gas cools effectively star formation can be triggered,
generating new stellar members in the system. Second, if the collective
potential of the system is able to alter the ambient gas properties before the
gas is accreted onto the individual core members, the augmented mass supply
rates could significantly alter the state of the various accreting stellar
populations and result in an enhanced central black hole accretion luminosity.Comment: 24 pages, 15 figures, accepted to Ap
Elemental abundances in the Galactic bulge from microlensed dwarf stars
We present elemental abundances of 13 microlensed dwarf and subgiant stars in
the Galactic bulge, which constitute the largest sample to date. We show that
these stars span the full range of metallicity from Fe/H=-0.8 to +0.4, and that
they follow well-defined abundance trends, coincident with those of the
Galactic thick disc.Comment: Poster contribution to Chemical abundances in the Universe,
connecting first stars to planets, Proceedings of the International
Astronomical Union, IAU Symposium, Volume 265, K. Cunha, M. Spite and B.
Barbuy, eds, Cambridge University Press, in pres
A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules
Our aim is to provide as clean and as complete a sample as possible of red
giant branch stars that are members of the Hercules dSph galaxy. With this
sample we explore the velocity dispersion and the metallicity of the system.
Stromgren photometry and multi-fibre spectroscopy are combined to provide
information about the evolutionary state of the stars (via the Stromgren c_1
index) and their radial velocities. Based on this information we have selected
a clean sample of red giant branch stars, and show that foreground
contamination by Milky Way dwarf stars can greatly distort the results. Our
final sample consists of 28 red giant branch stars in the Hercules dSph galaxy.
Based on these stars we find a mean photometric metallicity of -2.35 dex which
is consistent with previous studies. We find evidence for an abundance spread.
Using those stars for which we have determined radial velocities we find a
systemic velocity of 45.2 km/s with a dispersion of 3.72 km/s, this is lower
than values found in the literature. Furthermore we identify the horizontal
branch and estimate the mean magnitude of the horizontal branch of the Hercules
dSph galaxy to be V_0=21.17, which corresponds to a distance of 147 kpc. We
have shown that a proper cleaning of the sample results in a smaller value for
the velocity dispersion of the system. This has implications for galaxy
properties derived from such velocity dispersions.Comment: 24 pages, 28 figure
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc
The Bulge is the least understood major stellar population of the Milky Way.
Most of what we know about the formation and evolution of the Bulge comes from
bright giant stars. The underlying assumption that giants represent all the
stars, and accurately trace the chemical evolution of a stellar population, is
under debate. In particular, recent observations of a few microlensed dwarf
stars give a very different picture of the evolution of the Bulge from that
given by the giant stars. [ABRIDGED] We perform a detailed elemental abundance
analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra
that were obtained while the stars were optically magnified during
gravitational microlensing events. [ABRIDGED] We present detailed elemental
abundances and stellar ages for six new dwarf stars in the Galactic bulge.
Combining these with previous events, here re-analysed with the same methods,
we study a homogeneous sample of 15 stars, which constitute the largest sample
to date of microlensed dwarf stars in the Galactic bulge. We find that the
stars span the full range of metallicities from [Fe/H]=-0.72 to +0.54, and an
average metallicity of =-0.08+/-0.47, close to the average metallicity
based on giant stars in the Bulge. Furthermore, the stars follow well-defined
abundance trends, that for [Fe/H]<0 are very similar to those of the local
Galactic thick disc. This suggests that the Bulge and the thick disc have had,
at least partially, comparable chemical histories. At sub-solar metallicities
we find the Bulge dwarf stars to have consistently old ages, while at
super-solar metallicities we find a wide range of ages. Using the new age and
abundance results from the microlensed dwarf stars we investigate possible
formation scenarios for the Bulge.Comment: New version accepted for publication in Astronomy and Astrophysic
Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. Detailed abundance analysis of OGLE-2008-BLG-209S
AIMS. Our aims are twofold. First we aim to evaluate the robustness and
accuracy of stellar parameters and detailed elemental abundances that can be
derived from high-resolution spectroscopic observations of microlensed dwarf
and subgiant stars. We then aim to use microlensed dwarf and subgiant stars to
investigate the abundance structure and chemical evolution of the Milky Way
Bulge. [ABRIDGED] METHODS. We present a detailed elemental abundance analysis
of OGLE-2008-BLG-209S, the source star of a new microlensing event towards the
Bulge, for which we obtained a high-resolution spectrum with the MIKE
spectrograph on the Magellan Clay telescope. We have performed four different
analyses of OGLE-2008-BLG-209S. [ABRIDGED] We have also re-analysed three
previous microlensed dwarf stars OGLE-2006-BLG-265S, MOA-2006-BLG-099S, and
OGLE-2007-BLG-349S with the same method. This homogeneous data set, although
small, enables a direct comparison between the different stellar populations.
RESULTS. We find that OGLE-2008-BLG-209S is a subgiant star that has a
metallicity of [Fe/H] ~-0.33. It possesses [alpha/Fe] enhancements similar to
what is found for Bulge giant stars at the same metallicity, and what also is
found for nearby thick disc stars at the same metallicity. In contrast, the
previous three microlensing dwarf stars have very high metallicities,
[Fe/H]>+0.4, and more solar-like abundance ratios, i.e. [alpha/Fe]~0. The
decrease in the [alpha/Fe] ratio with [Fe/H] is the typical signature of
enrichment from low and intermediate mass stars. We furthermore find that the
results for the four Bulge stars, in combination with results from studies of
giant stars in the Bulge, seem to favour a secular formation scenario for the
Bulge.Comment: Accepted for publication in A&A, 17 pages, online table will be
available in published version, or by contacting the first autho
Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation
(abridged) Predictions of the Concordance Cosmological Model (CCM) of the
structures in the environment of large spiral galaxies are compared with
observed properties of Local Group galaxies. Five new most probably
irreconcilable problems are uncovered. However, the Local Group properties
provide hints that may lead to a solution of the above problems The DoS and
bulge--satellite correlation suggest that dissipational events forming bulges
are related to the processes forming phase-space correlated satellite
populations. Such events are well known to occur since in galaxy encounters
energy and angular momentum are expelled in the form of tidal tails, which can
fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star
clusters. If Local Group satellite galaxies are to be interpreted as TDGs then
the sub-structure predictions of CCM are internally in conflict. All findings
thus suggest that the CCM does not account for the Local Group observations and
that therefore existing as well as new viable alternatives have to be further
explored. These are discussed and natural solutions for the above problems
emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor
text adjustments for conformity with the published version and additional
minor changes resulting from reader's feedback. The speculation on a dark
force has been added. Also, the Fritz Zwicky Paradox is now included to agree
with the published versio