2,093 research outputs found
The Chemical Enrichment History of the Large Magellanic Cloud
Ca II triplet spectroscopy has been used to derive stellar metallicities for
individual stars in four LMC fields situated at galactocentric distances of
3\arcdeg, 5\arcdeg, 6\arcdeg\@ and 8\arcdeg\@ to the north of the Bar. Observed
metallicity distributions show a well defined peak, with a tail toward low
metallicities. The mean metallicity remains constant until 6\arcdeg\@
([Fe/H]-0.5 dex), while for the outermost field, at 8\arcdeg, the mean
metallicity is substantially lower than in the rest of the disk
([Fe/H]-0.8 dex). The combination of spectroscopy with deep CCD
photometry has allowed us to break the RGB age--metallicity degeneracy and
compute the ages for the objects observed spectroscopically. The obtained
age--metallicity relationships for our four fields are statistically
indistinguishable. We conclude that the lower mean metallicity in the outermost
field is a consequence of it having a lower fraction of intermediate-age stars,
which are more metal-rich than the older stars. The disk age--metallicity
relationship is similar to that for clusters. However, the lack of objects with
ages between 3 and 10 Gyr is not observed in the field population. Finally, we
used data from the literature to derive consistently the age--metallicity
relationship of the bar. Simple chemical evolution models have been used to
reproduce the observed age--metallicity relationships with the purpose of
investigating which mechanism has participated in the evolution of the disk and
bar. We find that while the disk age--metallicity relationship is well
reproduced by close-box models or models with a small degree of outflow, that
of the bar is only reproduced by models with combination of infall and outflow.Comment: 45 pages, 10 figures, accepted for publication in Astronomical
Journa
The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?
We discuss how knowledge of the whole evolutionary history of dwarf galaxies,
including details on the early star formation events, can provide insight on
the origin of the different dwarf galaxy types. We suggest that these types may
be imprinted by the early conditions of formation rather than being only the
result of a recent morphological transformation driven by environmental
effects. We present precise star formation histories of a sample of Local Group
dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest
main-sequence turnoffs. We argue that these galaxies can be assigned to two
basic types: fast dwarfs that started their evolution with a dominant and short
star formation event, and slow dwarfs that formed a small fraction of their
stars early and have continued forming stars until the present time (or
almost). These two different evolutionary paths do not map directly onto the
present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast
dwarfs also differ in their inferred past location relative to the Milky Way
and/or M31, which hints that slow dwarfs were generally assembled in lower
density environments than fast dwarfs. We propose that the distinction between
a fast and slow dwarf galaxy reflects primarily the characteristic density of
the environment where they form. At a later stage, interaction with a large
host galaxy may play a role in the final gas removal and ultimate termination
of star formation.Comment: 7 pages, 3 figures, ApJ Letters, submitted. Comments welcom
The ACS LCID project. VI. The SFH of the Tucana dSph and the relative ages of the isolated dSph galaxies
We present a detailed study of the star formation history (SFH) of the Tucana
dwarf spheroidal galaxy. High quality, deep HST/ACS data, allowed us to obtain
the deepest color-magnitude diagram to date, reaching the old main sequence
turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on
three different SFH codes, shows that Tucana is an old and metal-poor stellar
system, which experienced a strong initial burst of star formation at a very
early epoch (~ 13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We
are not able to unambiguously answer the question of whether most star
formation in Tucana occurred before or after the end of the reionization era,
and we analyze alternative scenarios that may explain the transformation of
Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial
velocity do not preclude that Tucana may have crossed the inner regions of the
Local Group once, and so gas stripping by ram pressure and tides due to a close
interaction cannot be ruled out. On the other hand, the high star formation
rate measured at early times may have injected enough energy into the
interstellar medium to blow out a significant fraction of the initial gas
content. Gas that is heated but not blown out would also be more easily
stripped via ram pressure. We compare the SFH inferred for Tucana with that of
Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the
formation time of the bulk of star formation in Cetus is clearly delayed with
respect to that of Tucana. This reinforces the conclusion of Monelli et al.
(2010) that Cetus formed the vast majority of its stars after the end of the
reionization era implying, therefore, that small dwarf galaxies are not
necessarily strongly affected by reionization, in agreement with many
state-of-the-art cosmological models. [abridged]Comment: Accepted for publication on ApJ, 19 pages, 10 figures, 2 tables. A
version with full resolution figures is available at
http://www.iac.es/project/LCID/?p=publication
The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana
We present the first investigation of the Blue Straggler star (BSS)
population in two isolated dwarf spheroidal galaxies of the Local Group, Cetus
and Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and
1214 candidates, respectively. The analysis of the star formation histories of
the two galaxies suggests that both host a population of BSSs. Specifically, if
the BSS candidates are interpreted as young main sequence stars, they do not
conform to their galaxy's age-metallicity relationship. The analysis of the
luminosity function and the radial distributions support this conclusion, and
suggest a non-collisional mechanism for the BSS formation, from the evolution
of primordial binaries. This scenario is also supported by the results of new
dynamical simulations presented here. Both galaxies coincide with the
relationship between the BSS frequency and the absolute visual magnitude Mv
found by Momany et al (2007). If this relationship is confirmed by larger
sample, then it could be a valuable tool to discriminate between the presence
of BSSs and galaxies hosting truly young populations.Comment: Accepted for publication on ApJ. 15 pages, 3 tables, 13 figures. A
version with high resolution figure can be downloaded from
http://rialto.ll.iac.es/proyecto/LCID/?p=publication
On the kinematic separation of field and cluster stars across the Bulge globular NGC 6528
We present deep and precise multi-band photometry of the Galactic Bulge
globular cluster NGC6528. The current dataset includes optical and
near-infrared images collected with ACS/WFC, WFC3/UVIS, and WFC3/IR on board
the Hubble Space Telescope. The images cover a time interval of almost ten
years and we have been able to carry out a proper-motion separation between
cluster and field stars. We performed a detailed comparison in the m_F814W,
m_F606W - m_F814W Color-Magnitude Diagram with two empirical calibrators
observed in the same bands. We found that NGC6528 is coeval with and more
metal-rich than 47Tuc. Moreover, it appears older and more metal-poor than the
super-metal-rich open cluster NGC6791. The current evidence is supported by
several diagnostics (red horizontal branch, red giant branch bump, shape of the
sub-giant branch, slope of the main sequence) that are minimally affected by
uncertainties in reddening and distance. We fit the optical observations with
theoretical isochrones based on a scaled-solar chemical mixture and found an
age of 11 +- 1 Gyr and an iron abundance slightly above solar ([Fe/H = +0.20).
The iron abundance and the old cluster age further support the recent
spectroscopic findings suggesting a rapid chemical enrichment of the Galactic
Bulge.Comment: 14 pages, 12 figures (2 at low resolution); added references;
corrected figure 3, 4, 5, 8 and 9; results unchanged. Erratum to be published
in Ap
The Star Formation History of the Local Group dwarf galaxy Leo I
We present a quantitative analysis of the star formation history (SFH) of the
Local Group dSph galaxy Leo I, from the information in its HST [(V-I),I]
color-magnitude diagram (CMD). The method we use is based in comparing, via
synthetic CMDs, the expected distribution of stars in the CMD for different
evolutionary scenarios, with the observed distribution. We consider the SFH to
be composed by the SFR(t), the Z(t), the IMF, and a function ,
controlling the fraction and mass ratio distribution of binary stars.
The comparison between the observed CMD and the model CMDs is done through
chi-square minimization of the differences in the number of stars in a set of
regions of the CMD.
Our solution for the SFH of Leo I defines a minimum of chi-square in a well
defined position of the parameter space, and the derived SFR(t) is robust, in
the sense that its main characteristics are unchanged for different
combinations of the remaining parameters. However, only a narrow range of
assumptions for Z(t), IMF and result in a good agreement between
the data and the models, namely: Z=0.0004, a Kroupa et al. (1993) IMF or
slightly steeper, and a relatively large fraction of binary stars. Most star
formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago,
it abruptly dropped to a negligible value, but seems to have been active until
at least ~ 300 Myr ago. Our results don't unambiguously answer the question of
whether Leo I began forming stars around 15 Gyr ago, but it appears that the
amount of this star formation, if existing at all, would be small.Comment: 25 pages + 14 figures. Accepted by The Astronomical Journa
Neutral Hydrogen and Star Formation in the Irregular Galaxy NGC 2366
We present UBVJHKHalpha and HI data of the irregular galaxy NGC 2366. It is a
normal boxy-shaped disk seen at high inclination angle. We do not see any
unambiguous observational signature of a bar. There is an asymmetrical
extension of stars along one end of the major axis of the galaxy, and this is
where the furthest star-forming regions are found, at 1.3R_Holmberg. The HI is
normal in many respects but shows some anomalies: 1) The integrated HI shows
two ridges running parallel to the major axis that deproject to a large ring.
2) The velocity field exhibits several large-scale anomalies superposed on a
rotating disk. 3) The inclination and position angles derived from the
kinematics differ from those dervied from the optical and HI mor- phology. 4)
There are regions in the HI of unusually high velocity dispersion that
correlate with deficits of HI emission in a manner suggestive of long-range,
turbulent pressure equilibrium. Star-forming regions are found where the gas
densities locally exceed 6 Msolar/pc^2. NGC 2366, like other irregulars, has
low gas densities relative to the critical gas densities of gravitational
instability models. Because of the lack of shear in the optical galaxy, there
is little competition to the slow gravitational contraction that follows energy
dissipation. However, the peak gas densities in the star-forming regions are
equal to the local tidal densities for gravitational self-binding of a rotating
cloud. Evidently the large scale gas concentrations are marginally bound
against background galactic tidal forces. This condition for self-binding may
be more fundamental than the instability condition because it is local,
three-dimensional, and does not involve spiral arm generation as an
intermediate step toward star formation.Comment: To be published in ApJ; better figures available ftp.lowell.edu, cd
pub/dah/n2366pape
The ISLAndS project II: The Lifetime Star Formation Histories of Six Andromeda dSphs
The Initial Star formation and Lifetimes of Andromeda Satellites (ISLAndS)
project uses Hubble Space Telescope imaging to study a representative sample of
six Andromeda dSph satellite companion galaxies. The main goal of the program
is to determine whether the star formation histories (SFHs) of the Andromeda
dSph satellites demonstrate significant statistical differences from those of
the Milky Way, which may be attributable to the different properties of their
local environments. Our observations reach the oldest main sequence turn-offs,
allowing a time resolution at the oldest ages of ~ 1 Gyr, which is comparable
to the best achievable resolution in the MW satellites. We find that the six
dSphs present a variety of SFHs that are not strictly correlated with
luminosity or present distance from M31. Specifically, we find a significant
range in quenching times (lookback times from 9 to 6 Gyr), but with all
quenching times more than ~ 6 Gyr ago. In agreement with observations of Milky
Way companions of similar mass, there is no evidence of complete quenching of
star formation by the cosmic UV background responsible for reionization, but
the possibility of a degree of quenching at reionization cannot be ruled out.
We do not find significant differences between the SFHs of the three members of
the vast, thin plane of satellites and the three off-plane dSphs. The primary
difference between the SFHs of the ISLAndS dSphs and Milky Way dSph companions
of similar luminosities and host distances is the absence of very late
quenching (< 5 Gyr ago) dSphs in the ISLAndS sample. Thus, models that can
reproduce satellite populations with and without late quenching satellites will
be of extreme interest.Comment: 24 pages, 11 figures, 3 tables, submitted to the Ap
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