761 research outputs found
Limits on [OIII] 5007 emission from NGC4472's globular clusters: constraints on planetary nebulae and ultraluminous black hole X-ray binaries in globular clusters
We have searched for [OIII] 5007 emission in high resolution spectroscopic
data from Flames/Giraffe VLT observations of 174 massive globular clusters
(GCs) in NGC4472. No planetary nebulae (PNe) are observed in these clusters,
constraining the number of PNe per bolometric luminosity,
\alpha<0.8*10^{-7}PN/L_{\odot}. This is significantly lower than the rate
predicted from stellar evolution, if all stars produce PNe. Comparing our
results to populations of PNe in galaxies, we find most galaxies have a higher
\alpha than these GCs (more PNe per bolometric luminosity - though some massive
early-type galaxies do have similarly low \alpha). The low \alpha required in
these GCs suggests that the number of PNe per bolometric luminosity does not
increase strongly with decreasing mass or metallicity of the stellar
population. We find no evidence for correlations between the presence of known
GC PNe and either the presence of low mass X-ray binaries (LMXBs) or the
stellar interaction rates in the GCs. This, and the low \alpha observed,
suggests that the formation of PNe may not be enhanced in tight binary systems.
These data do identify one [OIII] emission feature, this is the (previously
published) broad [OIII] emission from the cluster RZ 2109. This emission is
thought to originate from the LMXB in this cluster, which is accreting at
super-Eddington rates. The absence of any similar [OIII] emission from the
other clusters favors the hypothesis that this source is a black hole LMXB,
rather than a neutron star LMXB with significant geometric beaming of its X-ray
emission.Comment: 10 pages, 3 figures, accepted for publication in Ap
Keck Spectroscopy of Globular Clusters in the Virgo Cluster Dwarf Elliptical VCC 1386
We present the results of a Keck spectroscopic study of globular clusters
associated with the Virgo Cluster dwarf elliptical VCC 1386. We analyze blue
spectroscopic absorption lines from 3500-5500 A for 13 globular cluster
candidates and confirm that five are associated with VCC 1386. By comparing
metal and Balmer line indices of these globular clusters with alpha-enhanced
single stellar population models we find that these systems are metal poor with
[Fe/H] 5 Gyr at 3 sigma confidence, placing their
formation at z >1. This is one of the first spectroscopic studies of globular
clusters surrounding dwarfs in a cluster, revealing that some low mass galaxies
in rich environments form at least part of their stellar mass early in the
history of the universe. We further find that the luminosity weighted stellar
population of VCC 1386 itself is younger, and more metal rich than its globular
clusters, consistent with (V-I)_0 colors from Hubble Space Telescope imaging.
This implies that VCC 1386, like Local Group dEs, has had multiple episodes of
star formation. Globular clusters associated with low luminosity systems
however appear to be roughly as old as those associated with giant galaxies,
contrary to the `downsizing' formation of their bulk stellar populations.Comment: ApJ, in pres
A new globular cluster black hole in NGC 4472
We discuss CXOU~1229410+075744, a new black hole candidate in a globular
cluster in the elliptical galaxy NGC~4472. By comparing two Chandra
observations of the galaxy, we find a source that varies by at least a factor
of 4, and has a peak luminosity of at least ergs/sec. As such,
the source varies by significantly more than the Eddington luminosity for a
single neutron star, and is a strong candidate for being a globular cluster
black hole. The source's X-ray spectrum also evolves in a manner consistent
with what would be expected from a single accreting stellar mass black hole. We
consider the properties of the host cluster of this source and the six other
strong black hole X-ray binary candidates, and find that there is suggestive
evidence that black hole X-ray binary formation is favored in bright and metal
rich clusters, just as is the case for bright X-ray sources in general.Comment: 6 pages, one 2-panel figure, 2 tables; accepted to MNRA
Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate
We have studied the low mass X-ray binary (LMXB) populations within and
outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age
and metallicity constraints from optical and IR observations of their GCs, we
do not find any evidence for an increase in the LMXB formation rate in the
intermediate age cluster population of NGC 4365, as has been proposed in some
scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red
population of GCs in NGC 3115 on the other hand is {\it at least} three times
as efficient at creating LMXBs as the old, metal-poor, blue clusters. These
data suggest that the higher formation efficiency of LMXBs in the red GC
subsystems of many galaxies is largely a consequence of their higher
metallicity. A comparison of the densities of field LMXBs in different galaxies
does not reveal an obvious correlation with the age of the field stars as
predicted by models in which the LMXB formation rate in the field drops
monotonically with time after an initial burst. This suggests that either a
significant fraction of the field LMXBs are created in GCs and subsequently
injected into the field, or the LMXB formation rate has a more complex time
evolution pattern.Comment: Accepted for publication in ApJ Letters. 7 pages including 5 figure
Probing the Low Mass X-ray Binaries/Globular Cluster connection in NGC1399
We present a wide field study of the Globular Clusters/Low Mass X-ray
Binaries connection in the cD elliptical NGC1399, combining HST/ACS and Chandra
high resolution data. We find evidence that LMXB formation likelihood is
influenced by GCs structural parameters, in addition to the well known effects
of mass and metallicity, independently from galactocentric distance.Comment: in press in the Proceedings of the X-ray 2009 Conference, 7-11
September 2009, Bologna, Ital
PATENTS, R&D AND LAG EFFECTS: EVIDENCE FROM FLEXIBLE METHODS FOR COUNT PANEL DATA ON MANUFACTURING FIRMS
Hausman, Hall and Griliches (1984) and Hall, Griliches and Hausman (1986) investigated whether there was a lag in the patent-R&D relationship for the U.S. manufacturing sector using 1970¿s data. They found that there was little evidence of anything but contemporaneous movement of patents and R&D. We reexamine this important issue employing new longitudinal patent data at the firm level for the U.S. manufacturing sector from 1982 to 1992. To address unique features of the data, we estimate various distributed lag and dynamic multiplicative panel count data models. The paper also develops a new class of count panel data models based on series expansion of the distribution of individual effects. The empirical analyses show that, although results are somewhat sensitive to different estimation methods, the contemporaneous relationship between patenting and R&D expenditures continues to be rather strong, accounting for over 60% of the total R&D elasticity. Regarding the lag structure of the patents-R&D relationship, we do find a significant lag in all empirical specifications. Moreover, the estimated lag effects are higher than have previously been found, suggesting that the contribution of R&D history to current patenting has increased from the 1970¿s to the 1980¿s.Innovative activity, Patents and R&D, Individual effects, count panel data methods.
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