25,623 research outputs found
Globular Clusters in NGC 4365: New K-band Imaging and a Reassessment of the Case for Intermediate-age Clusters
We study the globular cluster (GC) system of the Virgo giant elliptical
galaxy NGC 4365, using new wide-field VIK imaging. The GC colour distribution
has (at least) two peaks, but the colours of the red GCs appear more strongly
weighted towards intermediate colours compared to most other large ellipticals
and the integrated galaxy light. The intermediate-color/red peak may itself be
composed of two sub-populations, with clusters of intermediate colours more
concentrated towards the centre of the galaxy than both the blue and red GCs.
Nearly all intermediate-colour and red GCs in our sample show an offset towards
red V-K and/or blue V-I colours compared to SSP models for old ages in a
(V-K,V-I) diagram. This has in the past been interpreted as evidence for
intermediate ages. We also combine our VIK data with previously published
spectroscopy. The differences between observed and model colour-metallicity
relations are consistent with the offsets observed in the two-colour diagram,
with the metal-rich GCs being too red (by about 0.2 mag) in V-K and too blue
(by about 0.05 mag) in V-I compared to the models at a given metallicity. These
offsets cannot easily be explained as an effect of younger ages. We conclude
that, while intermediate GC ages cannot be definitively ruled out, an
alternative scenario is more likely whereby all the GCs are old but the
relative number of intermediate-metallicity GCs is greater than typical for
giant ellipticals. The main obstacle to reaching a definitive conclusion is the
lack of robust calibrations of integrated spectral and photometric properties
for stellar populations with near-solar metallicity. In any case, it is
puzzling that the intermediate-colour GCs in NGC 4365 are not accompanied by a
corresponding shift of the integrated galaxy light towards bluer colours.Comment: 23 pages, including 20 figures and 5 tables. Accepted for publication
in A&
Detailed abundances from integrated-light spectroscopy: Milky Way globular clusters
We test the performance of our analysis technique for integrated-light
spectra by applying it to seven well-studied Galactic GCs that span a wide
range of metallicities. Integrated-light spectra were obtained by scanning the
slit of the UVES spectrograph on the ESO Very Large Telescope across the
half-light diameters of the clusters. We modelled the spectra using resolved
HST colour-magnitude diagrams (CMDs), as well as theoretical isochrones, in
combination with standard stellar atmosphere and spectral synthesis codes. The
abundances of Fe, Na, Mg, Ca, Ti, Cr, and Ba were compared with literature data
for individual stars in the clusters. The typical differences between iron
abundances derived from our integrated-light spectra and those compiled from
the literature are less than 0.1 dex. A larger difference is found for one
cluster (NGC 6752), and is most likely caused primarily by stochastic
fluctuations in the numbers of bright red giants within the scanned area. As
expected, the alpha-elements (Ca, Ti) are enhanced by about 0.3 dex compared to
the Solar-scaled composition, while the [Cr/Fe] ratios are close to Solar. When
using up-to-date line lists, our [Mg/Fe] ratios also agree well with literature
data. Our [Na/Fe] ratios are, on average, 0.08-0.14 dex lower than average
values quoted in the literature, and our [Ba/Fe] ratios may be overestimated by
0.20-0.35 dex at the lowest metallicities. We find that analyses based on
theoretical isochrones give very similar results to those based on resolved
CMDs. Overall, the agreement between our integrated-light abundance
measurements and the literature data is satisfactory. Refinements of the
modelling procedure, such as corrections for stellar evolutionary and non-LTE
effects, might further reduce some of the remaining offsets.Comment: 35 pages, 16 figures, accepted for A&
Near-IR Spectroscopy of a Young Super-Star Cluster in NGC 6946: Chemical Abundances and Abundance Patterns
Using the NIRSPEC spectrograph at Keck II, we have obtained H and K-band
echelle spectra for a young (10-15 Myr), luminous (MV=-13.2) super-star cluster
in the nearby spiral galaxy NGC 6946. From spectral synthesis and equivalent
width measurements we obtain for the first time accurate abundances and
abundance patterns in an extragalactic super-star cluster. We find
[Fe/H]=-0.45+/-0.08 dex, an average alpha-enhancement of +0.22+/-0.1 dex, and a
relatively low 12C/13C~ 8+/-2 isotopic ratio. We also measure a velocity
dispersion of ~9.1 km/s, in agreement with previous estimates. We conclude that
integrated high-dispersion spectroscopy of massive star clusters is a promising
alternative to other methods for abundance analysis in extragalactic young
stellar populations.Comment: 5 pages, incl. 2 figures. Accepted for publication in MNRAS Letters.
The definitive version will be available at http://www.blackwell-synergy.co
Nitrogen abundances and multiple stellar populations in the globular clusters of the Fornax dSph
We use measurements of nitrogen abundances in red giants to search for
multiple stellar populations in the four most metal-poor globular clusters
(GCs) in the Fornax dwarf spheroidal galaxy (Fornax 1, 2, 3, and 5). New
imaging in the F343N filter, obtained with the Wide Field Camera 3 on the
Hubble Space Telescope, is combined with archival F555W and F814W observations
to determine the strength of the NH band near 3370 AA. After accounting for
observational errors, the spread in the F343N-F555W colors of red giants in the
Fornax GCs is similar to that in M15 and corresponds to an abundance range of
Delta([N/Fe])=2 dex, as observed also in several Galactic GCs. The spread in
F555W-F814W is, instead, fully accounted for by observational errors. The stars
with the reddest F343N-F555W colors (indicative of N-enhanced composition) have
more centrally concentrated radial distributions in all four clusters, although
the difference is not highly statistically significant within any individual
cluster. From double-Gaussian fits to the color distributions we find roughly
equal numbers of "N-normal" and "N-enhanced" stars (formally about 40% N-normal
stars in Fornax 1, 3, and 5 and 60% in Fornax 2). We conclude that GC
formation, in particular regarding the processes responsible for the origin of
multiple stellar populations, appears to have operated similarly in the Milky
Way and in the Fornax dSph. Combined with the high ratio of metal-poor GCs to
field stars in the Fornax dSph, this places an important constraint on
scenarios for the origin of multiple stellar populations in GCs.Comment: 16 pages, 7 figures, accepted for publication in Ap
A dynamical and kinematical model of the Galactic stellar halo and possible implications for galaxy formation scenarios
We re-analyse the kinematics of the system of blue horizontal branch field
(BHBF) stars in the Galactic halo (in particular the outer halo), fitting the
kinematics with the model of radial and tangential velocity dispersions in the
halo as a function of galactocentric distance r proposed by Sommer-Larsen,
Flynn & Christensen (1994), using a much larger sample (almost 700) of BHBF
stars. The basic result is that the character of the stellar halo velocity
ellipsoid changes markedly from radial anisotropy at the sun to tangential
anisotropy in the outer parts of the Galactic halo (r greater than approx 20
kpc). Specifically, the radial component of the stellar halo's velocity
ellipsoid decreases fairly rapidly beyond the solar circle, from approx 140 +/-
10 km/s at the sun, to an asymptotic value of 89 +/- 19 km/s at large r. The
rapid decrease in the radial velocity dispersion is matched by an increase in
the tangential velocity dispersion, with increasing r.
Our results may indicate that the Galaxy formed hierarchically (partly or
fully) through merging of smaller subsystems - the 'bottom-up' galaxy formation
scenario, which for quite a while has been favoured by most theorists and
recently also has been given some observational credibility by HST observations
of a potential group of small galaxies, at high redshift, possibly in the
process of merging to a larger galaxy (Pascarelle et al 1996).Comment: Latex, 16 pages. 2 postscript figures. Submitted to the Astrophysical
Journal. also available at http://astro.utu.fi/~cflynn/outerhalo.htm
Modal Interface Automata
De Alfaro and Henzinger's Interface Automata (IA) and Nyman et al.'s recent
combination IOMTS of IA and Larsen's Modal Transition Systems (MTS) are
established frameworks for specifying interfaces of system components. However,
neither IA nor IOMTS consider conjunction that is needed in practice when a
component shall satisfy multiple interfaces, while Larsen's MTS-conjunction is
not closed and Bene\v{s} et al.'s conjunction on disjunctive MTS does not treat
internal transitions. In addition, IOMTS-parallel composition exhibits a
compositionality defect. This article defines conjunction (and also
disjunction) on IA and disjunctive MTS and proves the operators to be
'correct', i.e., the greatest lower bounds (least upper bounds) wrt. IA- and
resp. MTS-refinement. As its main contribution, a novel interface theory called
Modal Interface Automata (MIA) is introduced: MIA is a rich subset of IOMTS
featuring explicit output-must-transitions while input-transitions are always
allowed implicitly, is equipped with compositional parallel, conjunction and
disjunction operators, and allows a simpler embedding of IA than Nyman's. Thus,
it fixes the shortcomings of related work, without restricting designers to
deterministic interfaces as Raclet et al.'s modal interface theory does.Comment: 28 page
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