4,278 research outputs found
Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution
Using the Hubble Space Telescope, we resolve the red giant branch in the halo
of the S0 galaxy NGC 3115. We measure magnitudes and colours for stars
down to 1.5 magnitudes below the tip of the red giant branch. From the
brightest stars we estimate a distance modulus ,
corresponding to a distance of Mpc. This is in excellent
agreement with the value determined from the planetary
nebula luminosity function. Our results rule out the shorter distance modulus
determined from surface brightness fluctuations. A histogram of
colours shows a clear bimodality, indicating the presence of two
distinct halo populations of roughly equal size. One has [Fe/H] and
one has [Fe/H]. This is the most distant galaxy in which a
Population II halo has been resolved, and it is the first time a colour
bimodality has been observed among the halo stars of any early-type galaxy.Comment: 15 pages, 4 figures. MNRAS, in pres
Fish tracking technology development. Phases 1 and 2, project definition desk study and equipment
The document reports on the major findings from a definition study to appraise the options to develop fish tracking equipment, in particular tags and data logging systems, in order to improve the effeciency of the Agency tracking studies and to obtain a greater understanding of fish biology. The definition study was in two parts. The first, Phase 1, collated and evaluated all the known tracking systems that may be suitable for studies of fish which are either produced commercially or have been constructed for specific in-house studies. Phase 2 was an evaluation of all the tracking equipment considered to merit further investigation in Phase 1. The deficiencies between existing and required technologies to improve the efficiency of Agency's tracking studies and to obtain a greater understanding of fish biology are also identified
The Ages and Abundances of the M87 Globular Clusters
A subset of 150 globular clusters in M87 has been selected on the basis of
S/N ratio for abundance and age determinations from the sample of Paper I.
Indices measuring the strength of the strongest spectral features were
determined for the M87 GCs and from new data for twelve galactic GCs. Combining
the new and existing data for the galactic GCs and comparing the colors
and the line indices gives qualitative indications for the ages and abundances
of the GCs. Quantitative results are obtained by applying the Worthey (1994)
models for the integrated light of stellar systems of a single age, calibrated
by observations of galactic GCs, to deduce abundances and ages for the objects
in our sample.
We find that the M87 GCs span a wide range in metallicity, from very metal
poor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex is
higher than that of the galactic GC system, and there is a metal rich tail that
reaches to higher [Fe/H] than one finds among the galactic GCs. The mean
metallicity of the M87 GC system is about a factor of four lower than that of
the M87 stellar halo at a fixed projected radius . The metallicity inferred
from the X-ray studies is similar to that of the M87 stellar halo, not to that
of GCs. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs from
the strength of their spectral features. The behavior of these elements between
the metal rich and metal poor M87 GCs is similar to that shown by the galactic
GCs and by halo stars in the Galaxy. The pattern of chemical evolution in these
disparate old stellar systems is indistinguishable. We obtain a median age for
the M87 GC system of 13 Gyr, similar to that of the galactic GCs, with a small
dispersion about this value.Comment: 56 pages with included postscript figures; added derived M87 GC
metallicities to Table 2, a statistical analysis of possible bimodality, an
appendix on the metallicity calibration of U-R and the Washington system, and
other smaller changes. Accepted for publication in ApJ. (See paper for
complete version of the Abstract.
HST Star Counts at High Galactic Latitudes
We use star counts from 13 deep HST fields imaged with the {\it Wide Field
Camera - 2} in order to constrain the amount of dark matter in the Galaxy that
can be in the form of low-mass main sequence stars or white-dwarfs. Based on
the number of red stars found in our fields, we exclude the possibility that
more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs;
fairly massive () and yet extremely faint ( \gtsima
) stellar candidates would have to be invoked in order to make the
observed number of stars compatible with that predicted by a stellar dark halo.
White dwarfs must also be intrinsically very faint ( \gtsima ) in
order to be consistent with the observed number of faint stars in the HST
fields. We also rule out an increasing or flat stellar luminosity function
beyond . The inferred slope of the disk luminosity function is
intermediary between local, volume-limited surveys and ground-based photometric
ones. Finally, the magnitude counts are well fitted by existing models for the
structure of the Galaxy, with only small changes in the fiducial values of the
model parameters. The colour distribution, however, is not well reproduced by
the models. It is unclear at present if this reflects inadequacies of the
available models or uncertainties in the colour-magnitude diagrams for low
metallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure
HST Observations of the Field Star Population in the Large Magellanic Cloud
We present and photometry, obtained with the Hubble Space Telescope,
for stars in a field in the inner disk of the Large Magellanic
Cloud. We confirm previous results indicating that an intense star formation
event, probably corresponding to the formation of the LMC disk, occurred a few
times years ago. We find a small but real difference between our field
and one further out in the disk observed by Gallagher et al (1996): either star
formation in the inner disk commenced slightly earlier, or the stars are
slightly more metal rich. We also find evidence for a later burst, around 1 Gyr
ago, which may correspond to the formation of the LMC bar. About 5% of the
stars in our field are substantially older than either burst, and are probably
members of an old disk or halo population with age Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete
copies by request from [email protected]
Efficient and optimized identification of generalized Maxwell viscoelastic relaxation spectra
Viscoelastic relaxation spectra are essential for predicting and interpreting the mechanical responses of materials and structures. For biological tissues, these spectra must usually be estimated from viscoelastic relaxation tests. Interpreting viscoelastic relaxation tests is challenging because the inverse problem is expensive computationally. We present here an efficient algorithm that enables rapid identification of viscoelastic relaxation spectra. The algorithm was tested against trial data to characterize its robustness and identify its limitations and strengths. The algorithm was then applied to identify the viscoelastic response of reconstituted collagen, revealing an extensive distribution of viscoelastic time constants. © 2015 Elsevier Ltd
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Using the surface brightness fluctuations in HST WFPC-2 images, we determine
that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC
4476 lies in the background at ~21 Mpc. We also examine the globular clusters
of M87 using archived HST fields. We detect the bimodal color distribution, and
find that the amplitude of the red peak relative to the blue peak is greatest
near the center. This feature is in good agreement with the merger model of
elliptical galaxy formation, where some of the clusters originated in
progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure
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