181 research outputs found
The Globular Cluster System of NGC 1399: III. VLT Spectroscopy and Database
Radial velocities of 468 globular clusters around NGC 1399, the central
galaxy in the Fornax cluster, have been obtained with FORS2 and the Mask
Exchange Unit (MXU) at the ESO Very Large Telescope. This is the largest sample
of globular cluster velocities around any galaxy obtained so far. The mean
velocity uncertainty is 50 km/sec. This data sample is accurate and large
enough to be used in studies of the mass distribution of NGC 1399 and the
properties of its globular cluster system. Here we describe the observations,
the reduction procedure, and discuss the uncertainties of the resulting
velocities. The complete sample of cluster velocities which is used in a
dynamical study of NGC 1399 is tabulated. A subsample is compared with
previously published values.Comment: 34 pages, 9 figures, 2 tables, accepted by A
Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations
Theoretical integrated broad-band colors ranging from far-UV to near-IR have
been computed for old stellar systems from our evolutionary population
synthesis code. These models take into account, for the first time, the
detailed systematic variation of horizontal-branch (HB) morphology with age and
metallicity. Our models show that some temperature-sensitive color indices are
significantly affected by the presence of `blue HB stars'. Our models are
calibrated in the B-V, V-I, C-T1, and M-T1 vs. [Fe/H] planes, using
low-reddened Galactic globular clusters (GCs) [E(B-V) < 0.2] and the relative
age difference between the older inner halo Galactic GCs and younger outer halo
counterparts is well reproduced. Several empirical linear color-metallicity
transformation relations are assessed with our models and it is noted that they
may not be safely used to estimate metallicity if there are sizable age
differences amongst GCs within and between galaxies. It is anticipated that the
detailed population models presented here coupled with further precise
spectrophotometric observations of globular cluster systems in external
galaxies from the large ground-based telescopes and space UV facilities will
enable us to accurately estimate their ages and metallicities. (abridged)Comment: 40 pages, 14 figures, Tentatively scheduled for the November 2002
issue of The Astronomical Journa
M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation
New VRI photometry is presented for the globular clusters in the innermost
140'' of the M87 halo. The results are used to discuss several issues
concerning the formation and evolution of globular cluster systems in
supergiant ellipticals like M87. (1) we find no significant change in the
globular cluster luminosity function (GCLF) with galactocentric radius, for
cluster masses M < 10^5 solar masses, indicating that the main effects of
dynamical evolution may be only on lower-mass clusters. (2) Within the core
radius (1') of the globular cluster system, the metallicity distribution is
uniform, but at larger radii the mean metallicity declines steadily as Z ~
r^-0.9. (3) The various options for explaining the existence of high specific
frequency galaxies like M87 are evaluated, and scaling laws for the GCSs in
these galaxies are given. Interpretations involving secondary evolution
(formation of many globular clusters during mergers, intergalactic globular
clusters, etc.) are unlikely to be the primary explanation for high-S_N
galaxies. (4) We suggest that central-supergiant E galaxies may have formed in
an exceptionally turbulent or high-density environment in which an early,
powerful galactic wind drove out a high fraction of the protogalactic gas, thus
artificially boosting the specificComment: 67 pages, 17 figures. To appear in Astronomical Journal, in press for
May 1998. Preprints also available from W.Harris; send e-mail request to
[email protected]
The Soluble CTLA-4 Splice Variant Protects From Type 1 Diabetes and Potentiates Regulatory T-Cell Function
Normal Globular Cluster Systems in Massive Low Surface Brightness Galaxies
We present the results of a study of the globular cluster systems of 6
massive spiral galaxies, originally cataloged as low surface brightness
galaxies but here shown to span a wide range of central surface brightness
values, including two intermediate to low surface brightness galaxies. We used
the Advanced Camera for Surveys on board HST to obtain photometry in the F475W
and F775W bands and select sources with photometric and morphological
properties consistent with those of globular clusters. A total of 206
candidates were identified in our target galaxies. From a direct comparison
with the Galactic globular cluster system we derive specific frequency values
for each galaxy that are in the expected range for late-type galaxies. We show
that the globular cluster candidates in all galaxies have properties consistent
with globular cluster systems of previously studied galaxies in terms of
luminosity, sizes and color. We establish the presence of globular clusters in
the two intermediate to low surface brightness galaxies in our sample and show
that their properties do not have any significant deviation from the behavior
observed in the other sample galaxies. Our results are broadly consistent with
a scenario in which low surface brightness galaxies follow roughly the same
evolutionary history as normal (i.e. high surface) brightness galaxies except
at a much lower rate, but require the presence of an initial period of star
formation intense enough to allow the formation of massive star clusters.Comment: 14 pages, 6 figures. AJ accepte
The Globular Cluster System of NGC 1399. II. Kinematics of a Large Sample of Globular Clusters
We study the kinematics and dynamics of the globular cluster system of NGC
1399, the central galaxy of the Fornax cluster. The observational data consists
of medium resolution spectra, obtained at the Very Large Telescope. Our sample
comprises 468 radial velocities in the magnitude range 20 < m_R < 23. This is
the largest sample of globular cluster velocities around any galaxy obtained so
far. The radial range is 2 arcmin < r < 9 arcmin, corresponding to 11 kpc to 50
kpc of galactocentric distance. There is the possibility that unbound clusters
and/or objects in the foreground contaminate the NGC 1399 cluster sample. Under
strong error selection, practically no objects are found with velocities lower
than 800 km/s or higher than 2000 km/s. Since the extreme velocities influence
the velocity dispersion considerably, uncertainty regarding the exact value of
the dispersion remains. Within the above velocity limits, we derive a projected
velocity dispersion for the total sample of 274+-9 km/s which within the
uncertainties remains constant over the entire radial range. Without any
velocity restriction, it increases to 325 km/s. Blue and red clusters show
different dispersions corresponding to their different surface density
profiles. Spherical models point to a circular velocity of 415+-30$ km/s,
assuming isotropy for the red clusters. This value is constant out to 40 kpc.
The inferred dark halo potential can be well represented by a logarithmic
potential. Also a halo of the NFW type fits well to the observations. Some mass
profiles derived from X-ray analyses do not agree with a constant circular
velocity within our radial range, irrespective of its exact value.Comment: 38 pages, 20 figures, accepted by A
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