272 research outputs found
The low mass end of the neutral gas mass and velocity width functions of galaxies in CDM
We use the high-resolution Aquarius cosmological dark matter simulations
coupled to the semi-analytic model by Starkenburg et al. (2013) to study the HI
content and velocity width properties of field galaxies at the low mass end in
the context of CDM. We compare our predictions to the observed ALFALFA
survey HI mass and velocity width functions, and find very good agreement
without fine-tuning, when considering central galaxies. Furthermore, the
properties of the dark matter halos hosting galaxies, characterised by their
peak velocity and circular velocity at 2 radial disk scalelengths overlap
perfectly with the inferred values from observations. This suggests that our
galaxies are placed in the right dark matter halos, and consequently at face
value, we do not find any discrepancy with the predictions from the
CDM model. Our analysis indicates that previous tensions, apparent
when using abundance matching models, arise because this technique cannot be
straightforwardly applied for objects with masses .Comment: Accepted for publication in MNRAS, 9 pages, 8 figure
Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy
We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and
S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the
Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving
power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are
derived for a large number of stars in a dwarf spheroidal. We found a trend for
the [C/Fe] abundance to decrease with increasing luminosity on the RGB across
the whole metallicity range, a phenomenon observed in both field and globular
cluster giants, which can be interpreted in the framework of evolutionary
mixing of partially processed CNO material. Both our measurements of [C/Fe] and
[N/Fe] are in good agreement with the theoretical predictions for stars at
similar luminosity and metallicity. We detected a dispersion in the carbon
abundance at a given [Fe/H], which cannot be ascribed to measurement
uncertainties alone. We interpret this observational evidence as the result of
the contribution of different nucleosynthesis sources over time to a not
well-mixed interstellar medium. We report the discovery of two new
carbon-enhanced, metal-poor stars. These are likely the result of pollution
from material enriched by asymptotic giant branch stars, as indicated by our
estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular
clusters in the field of the galaxy by looking for the distinctive C-N pattern
of second population globular clusters stars in a previously detected, very
metal-poor, chemodynamical substructure. We do not detect chemical anomalies
among this group of stars. However, small number statistics and limited spatial
coverage do not allow us to exclude the hypotheses that this substructure forms
part of a tidally shredded globular cluster.Comment: 18 pages, 14 figures, 3 tables. Accepted to A&
The Pristine survey II: a sample of bright stars observed with FEROS
Extremely metal-poor (EMP) stars are old objects formed in the first Gyr of
the Universe. They are rare and, to select them, the most successful strategy
has been to build on large and low-resolution spectroscopic surveys. The
combination of narrow- and broad band photometry provides a powerful and
cheaper alternative to select metal-poor stars. The on-going Pristine Survey is
adopting this strategy, conducting photometry with the CFHT MegaCam wide field
imager and a narrow-band filter centred at 395.2 nm on the CaII-H and -K lines.
In this paper we present the results of the spectroscopic follow-up conducted
on a sample of 26 stars at the bright end of the magnitude range of the Survey
(g<=15), using FEROS at the MPG/ESO 2.2 m telescope. From our chemical
investigation on the sample, we conclude that this magnitude range is too
bright to use the SDSS gri bands, which are typically saturated. Instead the
Pristine photometry can be usefully combined with the APASS gri photometry to
provide reliable metallicity estimates.Comment: AN accepte
Galactic Archeology with 4MOST
4MOST is a new wide-field, high-multiplex spectroscopic survey facility for
the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy more than 2400
fibres in a 4.1 square degree field-of-view using a positioner based on the
tilting spine principle. In this ontribution we give an outline of the major
science goals we wish to achieve with 4MOST in the area of Galactic Archeology.
The 4MOST Galactic Archeology surveys have been designed to address
long-standing and far-reaching problems in Galactic science. They are focused
on our major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical
characterisation of the major Milky Way stellar components, 3) The Galactic
Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor
stars. In addition to a top-level description of the Galactic surveys we
provide information about how the community will be able to join 4MOST via a
call for Public Spectroscopic Surveys that ESO will launch.Comment: To be published in "Rediscovering our Galaxy", IAU Symposium 334,
Eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentin
The early days of the Sculptor dwarf spheroidal galaxy
We present the high resolution spectroscopic study of five -3.9<=[Fe/H]<=-2.5
stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the
number of stars with comparable observations in this metallicity range. We
carry out a detailed analysis of the chemical abundances of alpha, iron peak,
light and heavy elements, and draw comparisons with the Milky Way halo and the
ultra faint dwarf stellar populations. We show that the bulk of the Sculptor
metal-poor stars follows the same trends in abundance ratios versus metallicity
as the Milky Way stars. This suggests similar early conditions of star
formation and a high degree of homogeneity of the interstellar medium. We find
an outlier to this main regime, which seems to miss the products of the most
massive of the TypeII supernovae. In addition to its value to help refining
galaxy formation models, this star provides clues to the production of cobalt
and zinc. Two of our sample stars have low odd-to-even barium isotope abundance
ratios, suggestive of a fair proportion of s-process; we discuss the
implication for the nucleosynthetic origin of the neutron capture elements.Comment: Replacement after language editio
VLT/FLAMES spectroscopy of red giant branch stars in the Fornax dwarf spheroidal galaxy
Fornax is one of the most massive dwarf spheroidal galaxies in the Local
Group. The Fornax field star population is dominated by intermediate age stars
but star formation was going on over almost its entire history. It has been
proposed that Fornax experienced a minor merger event. Despite recent progress,
only the high metallicity end of Fornax field stars ([Fe/H]>-1.2 dex) has been
sampled in larger number via high resolution spectroscopy. We want to better
understand the full chemical evolution of this galaxy by better sampling the
whole metallicity range, including more metal poor stars. We use the VLT-FLAMES
multi-fibre spectrograph in high-resolution mode to determine the abundances of
several alpha, iron-peak and neutron-capture elements in a sample of 47
individual Red Giant Branch stars in the Fornax dwarf spheroidal galaxy. We
combine these abundances with accurate age estimates derived from the age
probability distribution from the colour-magnitude diagram of Fornax. Similar
to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are
typically alpha-rich while the young metal-rich stars are alpha-poor. In the
classical scenario of the time delay between SNe II and SNe Ia, we confirm that
SNe Ia started to contribute to the chemical enrichment at [Fe/H] between -2.0
and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyrs
ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and
AGB stars in the abundance pattern of the younger stellar population of Fornax.
Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an
initial mass function that lacks the most massive stars and with star formation
that kept going on throughout the whole history of Fornax. We find that massive
stars kept enriching the interstellar medium in alpha-elements, although they
were not the main contributor to the iron enrichment.Comment: Resubmitted to A&A (18/09/2014) after Referee's comment
An artificial neural network to discover hypervelocity stars: candidates in Gaia DR1/TGAS
The paucity of hypervelocity stars (HVSs) known to date has severely hampered
their potential to investigate the stellar population of the Galactic Centre
and the Galactic Potential. The first Gaia data release gives an opportunity to
increase the current sample. The challenge is the disparity between the
expected number of hypervelocity stars and that of bound background stars. We
have applied a novel data mining algorithm based on machine learning
techniques, an artificial neural network, to the Tycho-Gaia astrometric
solution (TGAS) catalogue. With no pre-selection of data, we could exclude
immediately of the stars in the catalogue and find 80 candidates
with more than predicted probability to be HVSs, based only on their
position, proper motions, and parallax. We have cross-checked our findings with
other spectroscopic surveys, determining radial velocities for 30 and
spectroscopic distances for 5 candidates. In addition, follow-up observations
have been carried out at the Isaac Newton Telescope for 22 stars, for which we
obtained radial velocities and distance estimates. We discover 14 stars with a
total velocity in the Galactic rest frame > 400 km/s, and 5 of these have a
probability of being unbound from the Milky Way. Tracing back their
orbits in different Galactic potential models we find one possible unbound HVS
with velocity 520 km/s, 5 bound HVSs, and, notably, 5 runaway stars with
median velocity between 400 and 780 km/s. At the moment, uncertainties in the
distance estimates and ages are too large to confirm the nature of our
candidates by narrowing down their ejection location, and we wait for future
Gaia releases to validate the quality of our sample. This test successfully
demonstrates the feasibility of our new data mining routine.Comment: Published in MNRAS, 17 pages, 10 figure
On the genealogy of the Orphan Stream
We use N-body simulations to explore the origin and a plausible orbit for the
Orphan Stream, one of the faintest substructures discovered so far in the outer
halo of our Galaxy. We are able to reproduce its position, velocity and
distance measurements by appealing to a single wrap of a double-component
satellite galaxy. We find that the progenitor of the Orphan Stream could have
been an object similar to today's Milky Way dwarfs, such as Carina, Draco, Leo
II or Sculptor; and unlikely to be connected to Complex A or Ursa Major II. Our
models suggest that such progenitors, if accreted on orbits with apocenters
smaller than ~35 kpc, are likely to give rise to very low surface brightness
streams, which may be hiding in the outer halo and remain largely undetected
with current techniques. The systematic discovery of these ghostly
substructures may well require wide field spectroscopic surveys of the Milky
Way's outer stellar halo.Comment: 9 pages, 5 figures, MNRAS in press. Accepted version with minor
change
Carbon and nitrogen abundances of individual stars in the Sculptor dwarf spheroidal galaxy
We present [C/Fe] and [N/Fe] abundance ratios and CH(_4300) and S(_3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 Å. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material.
Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of di_erent nucleosynthesis sources over time to a not well-mixed interstellar medium.We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure.We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster
The star formation and chemical evolution history of the sculptor dwarf spheroidal galaxy
We have combined deep photometry in the B,V and I bands from CTIO/MOSAIC of
the Sculptor dwarf spheroidal galaxy, going down to the oldest Main Sequence
Turn-Offs, with spectroscopic metallicity distributions of Red Giant Branch
stars. This allows us to obtain the most detailed and complete Star Formation
History to date, as well as an accurate timescale for chemical enrichment. The
Star Formation History shows that Sculptor is dominated by old (10 Gyr),
metal-poor stars, but that younger, more metal-rich populations are also
present. Using Star Formation Histories determined at different radii from the
centre we show that Sculptor formed stars with an increasing central
concentration with time. The old, metal-poor populations are present at all
radii, while more metal-rich, younger stars are more centrally concentrated. We
find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre,
a total mass in stars of 7.8 M was formed, between 14
and 7 Gyr ago, with a peak at 1314 Gyr ago. We use the detailed Star
Formation History to determine age estimates for individual Red Giant Branch
stars with high resolution spectroscopic abundances. Thus, for the first time,
we can directly determine detailed timescales for the evolution of individual
chemical elements. We find that the trends in alpha-elements match what is
expected from an extended, relatively uninterrupted period of star formation
continuing for 67 Gyr. The knee in the alpha-element distribution occurs at
an age of 10.91Gyr, suggesting that SNe Ia enrichment began
1Gyr after the start of star formation in Sculptor.Comment: 23 pages, 23 figure
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