468 research outputs found
Extending the baseline: Spitzer Mid-Infrared Photometry of Globular Cluster Systems in the Centaurus A and Sombrero Galaxies
Spitzer IRAC mid-infrared photometry is presented for the globular cluster
(GC) systems of the NGC 5128 ("Centaurus A") and NGC 4594 ("Sombrero")
galaxies. Existing optical photometric and spectroscopic are combined with this
new data in a comprehensive optical to mid-IR colour catalogue of 260 GCs.
Empirical colour-metallicity relationships are derived for all optical to
mid-IR colour combinations.
These colours prove to be very effective quantities to test the photometric
predictions of simple stellar population (SSP) models. In general, four SSP
models show larger discrepancies between each other and the data at bluer
wavelengths, especially at high metallicities. Such differences become very
important when attempting to use colour-colour model predictions to constrain
the ages of stellar populations. Furthermore, the age-substructure determined
from colour-colour diagrams and 91 NGC 5128 GCs with spectroscopic ages from
Beasley et al. (2008) are inconsistent, suggesting any apparent GC system
age-substructure implied by a colour-colour analysis must be verified
independently.
Unlike blue wavebands, certain optical to mid-IR colours are insensitive to
the flux from hot horizontal branch stars and thus provide an excellent
metallicity proxy. The NGC 5128 GC system shows strong bimodality in the
optical R-band to mid-IR colour distributions, hence proving it is bimodal in
metallicity. In this new colour space, a colour-magnitude trend, a "blue tilt",
is found in the NGC 5128 metal-poor GC data. The NGC 5128 young GCs do not
contribute to this trend. [abridged]Comment: 16 pages, 12 colour figures. To be published in MNRAS. Catalogue
available from the first author. Full resolution copy available here
http://lee.spitler.googlepages.com/spitzer_spitler.pd
Evidence for the disky origin of luminous Virgo dwarf ellipticals from the kinematics of their globular cluster systems
We report evidence for dynamically significant rotation in the globular
cluster systems of two luminous Virgo dwarf ellipticals, VCC1261 and VCC1528.
Including previous results for VCC1087, the globular cluster systems of all
three Virgo dwarf ellipticals studied in detail to date exhibit v_rot/sigma >
1. Taking the rotation seen in the globular clusters as maximal disk rotation,
we find all three dEs lie on the r-band Tully-Fisher relation. We argue that
these data support the hypothesis that luminous dEs are the remnants of
transformed disk galaxies. We also obtained deep, longslit data for the stars
in VCC1261 and VCC1528. Both these galaxies show rapid rotation in their inner
regions, with spatial scales of ~0.5 kpc. These rotation velocities are similar
to those seen in the GC systems. Since our longslit data for Virgo dEs extend
out to 1-2 effective radii (typical of deep observations), whereas the globular
clusters extend out to 4--7 effective radii, we conclude that non-detections of
rotation in many luminous dEs may simply be due to a lack of radial coverage in
the stellar data, and that globular clusters represent singularly sensitive
probes of the dynamics of dEs. Based on these data, we suggest that gas disks
are significant sites of globular cluster formation in the early universe.Comment: To appear in the AJ, corrected typographical errors in Table 1, added
a referenc
Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs
The optical integrated spectra of three LMC young stellar clusters (NGC 1984,
NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands
which deviate significantly from the predictions of single stellar population
(SSP) models. In order to understand the appearance of these spectra, we create
a set of young stellar population (MILES) models, which we make available to
the community. We use archival International Ultraviolet Explorer integrated UV
spectra to independently constrain the cluster masses and extinction, and rule
out strong stochastic effects in the optical spectra. In addition, we also
analyze deep colour-magnitude diagrams of the clusters to provide independent
age determinations based on isochrone fitting. We explore hypotheses including
age-spreads in the clusters, a top-heavy initial mass function, different SSP
models and the role of red supergiant stars (RSG). We find that the strong
molecular features in the optical spectra can only be reproduced by modeling an
increased fraction of about 20 per cent by luminosity of RSG above what is
predicted by canonical stellar evolution models. Given the uncertainties in
stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr
age-spreads in these clusters. Our work combines different wavelengths as well
as different approaches (resolved data as well as integrated spectra for the
same sample) in order to reveal the complete picture. We show that each
approach provides important information but in combination can we better
understand the cluster stellar populations.Comment: Accepted for publication in MNRA
UBRI Photometry of Globular Clusters in the Leo Group Galaxy NGC 3379
We present wide area UBRI photometry for globular clusters around the Leo
group galaxy NGC 3379. Globular cluster candidates are selected from their
B-band magnitudes and their (U-B)o vs (B-I)o colours. A colour-colour selection
region was defined from photometry of the Milky Way and M31 globular cluster
systems. We detect 133 globular cluster candidates which, supports previous
claims of a low specific frequency for NGC 3379. The Milky Way and M31 reveal
blue and red subpopulations, with (U-B)o and (B-I)o colours indicating mean
metallicities similar to those expected based on previous spectroscopic work.
The stellar population models of Maraston (2003) and Brocato etal (2000) are
consistent with both subpopulations being old, and with metallicities of [Fe/H]
\~ -1.5 and -0.6 for the blue and red subpopulations respectively. The models
of Worthey (1994) do not reproduce the (U-B)o colours of the red (metal-rich)
subpopulation for any modelled age. For NGC 3379 we detect a blue subpopulation
with similar colours and presumably age/metallicity, to that of the Milky Way
and M31 globular cluster systems. The red subpopulation is less well defined,
perhaps due to increased photometric errors, but indicates a mean metallicity
of [Fe/H] ~ -0.6.Comment: 12 pages, Latex, 10 figures, 1 table, submitted for publication in
MNRAS, Fig. 11 available in source file or from [email protected]
Dynamical evolution of globular cluster systems in clusters of galaxies I. The case of NGC 1404 in the Fornax cluster
We investigate, via numerical simulations, the tidal stripping and accretion
of globular clusters (GCs). In particular, we focus on creating models that
simulate the situation for the GC systems of NGC 1404 and NGC 1399 in the
Fornax cluster, which have poor (specific frequency 2) and
rich ( 10) GC systems respectively. We initially assign NGC
1404 in our simulation a typical ( 5) for cluster
ellipticals, and find that its GC system can only be reduced through stripping
to the presently observed value, if its orbit is highly eccentric (with orbital
eccentricity of 0.5) and if the initial scale length of the GCs system is
about twice as large as the effective radius of NGC 1404 itself. These stripped
GCs can be said to have formed a `tidal stream' of intracluster globular
clusters (ICGCs) orbiting the centre of Fornax cluster (many of which would be
assigned to NGC 1399 in an imaging study). The physical properties of these GCs
(e.g., number, radial distribution) depend on the orbit and initial
distribution of GCs in NGC 1404. Our simulations also predict a trend for
to rise with increasing clustercentric distance - a trend for which
there is some observational support in the Fornax cluster.Comment: 12 pages 12 figures, MNRAS in pres
Keck Spectroscopy and Imaging of Globular Clusters in the Lenticular Galaxy NGC 524
We have obtained Keck LRIS imaging and spectra for 29 globular clusters
associated with the lenticular galaxy NGC 524. Using the empirical calibration
of Brodie & Huchra we find that our spectroscopic sample spans a metallicity
range of --2.0 < [Fe/H] < 0. We have compared the composite spectrum of the
metal-poor ([Fe/H] < --1) and metal-rich clusters with stellar population
models and conclude that the clusters are generally old and coeval at the 2
sigma confidence level. To determine the mean [alpha/Fe] ratios of the globular
clusters, we have employed the Milone et al. 'alpha-enhanced' stellar
population models. We verified the reliability of these models by comparing
them with high S/N Galactic globular cluster data. We observe a weak trend of
decreasing [alpha/Fe] with increasing metallicity in the NGC 524 clusters.
Analysis of the cluster system kinematics reveals that the full sample exhibits
a rotation of 114+/-60 km/s around a position angle of 22+/-27 deg, and a
velocity dispersion of 186+/-29 km/s at a mean radius of 89 arcsec from the
galaxy centre. Subdividing the clusters into metal-poor and metal-rich
subcomponents we find that the metal-poor (17) clusters and metal-rich (11)
clusters have similar velocity dispersions (197+/-40 km/s and 169+/-47 km/s
respectively). The metal-poor clusters dominate the rotation in our sample with
147+/-75 km/s, whilst the metal-rich clusters show no significant rotation
(68+/-84 km/s). We derive a virial and projected mass estimation for NGC 524 of
between 4 and 13 x 10^11 Msun (depending on the assumed orbital distribution)
interior to 2 effective radii of this galaxy.Comment: to appear in MNRA
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