1,202 research outputs found
The Dependence of Globular Cluster Number on Density for Abell Cluster Central Galaxies
A study of the globular cluster systems of 23 brightest, or central, galaxies
in 19 Abell clusters has recently been completed. This Letter presents some of
the newly discovered correlations of the globular cluster specific frequency
in these galaxies with other galaxy and cluster properties and puts forth
an interpretation of these results. is found to correlate strongly with
measures of the cluster density, such as the velocity dispersion of the cluster
galaxies and the cluster X-ray temperature and luminosity (especially ``local''
X-ray luminosity). Within a cluster, galaxies at smaller projected distances
from the X-ray center are found to have higher values of . Taken together,
the scaling of with cluster density and the relative constancy of central
galaxy luminosity suggest a scenario in which globular clusters form in
proportionate numbers to the available mass, but galaxy luminosity
``saturates'' at a maximum threshold, resulting in higher \sn\ values for
central galaxies in denser clusters as well as the suitability of these
galaxies as ``standard candles.'' Thus, these galaxies do not have too many
globular clusters for their luminosity; rather, they are underluminous for
their number of globular clusters.Comment: 10 pages, with 4 included postscript figures; AASTeX (aaspp4.sty); to
appear in ApJ Letter
Stellar Populations and Surface Brightness Fluctuations: New Observations and Models
We examine the use of surface brightness fluctuations (SBF) for both stellar
population and distance studies. New V-band SBF data are reported for five
Fornax cluster galaxies and combined with literature data to define a new
V-band SBF distance indicator. We use new stellar population models, based on
the latest Padua isochrones transformed empirically to the observational plane,
to predict SBF magnitudes and integrated colours for a wide range of population
ages and metallicities. We examine the sensitivity of the predictions to
changes in the isochrones, transformations, and IMF. The new models reproduce
the SBF data for globular clusters fairly well, especially if higher
metallicity globulars are younger. The models also give a good match to the
"fluctuation colors" of elliptical galaxies. In order to obtain theoretical
calibrations of the SBF distance indicators, we combine our single-burst models
into composite population models. These models reproduce the observed behavior
of the SBF magnitudes as a function of stellar population parameters, including
the steep colour dependence found for HST/WFPC2 F814W SBF data. Because the
theoretical SBF calibrations are fairly sensitive to uncertain details of
stellar evolution, the empirical calibrations are more secure. However, the
sensitivity of SBF to these finer details potentially makes it a powerful
constraint for stellar evolution and population synthesis. [abbridged]Comment: 24 pages with 17 embedded figures. MNRAS, in pres
Simulating CCD images of elliptical galaxies
We introduce a procedure developed by the ``Teramo Stellar Populations
Tools'' group (Teramo-SPoT), specifically optimized to obtain realistic
simulations of CCD images of elliptical galaxies.
Particular attention is devoted to include the Surface Brightness Fluctuation
(SBF) signal observed in ellipticals and to simulate the Globular Cluster (GC)
system in the galaxy, and the distribution of background galaxies present in
real CCD frames. In addition to the physical properties of the simulated
objects - galaxy distance and brightness profile, luminosity function of GC and
background galaxies, etc. - the tool presented allows the user to set some of
the main instrumental properties - FoV, zero point magnitude, exposure time,
etc.Comment: Presented at From Stars to Galaxies: Building the Pieces to Build up
the Universe (StarGal 2006), Venice, Italy, 16-20 Oct 200
Structural Parameters of the M87 Globular Clusters
We derive structural parameters for ~2000 globular clusters in the giant
Virgo elliptical M87 using extremely deep Hubble Space Telescope images in
F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes
(half-light radii r_h) and ellipticities are determined from PSF-convolved
King-model profile fitting. We find that the r_h distribution closely resembles
the inner Milky Way clusters, peaking at r_h~2.5 pc and with virtually no
clusters more compact than r_h ~ 1 pc. The metal-poor clusters have on average
an r_h 24% larger than the metal-rich ones. The cluster scale size shows a
gradual and noticeable increase with galactocentric distance. Clusters are very
slightly larger in the bluer waveband V a possible hint that we may be
beginning to see the effects of mass segregation within the clusters. We also
derived a color magnitude diagram for the M87 globular cluster system which
show a striking bimodal distribution.Comment: ApJ accepte
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