969 research outputs found
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
A First Comparison of the SBF Survey Distances with the Galaxy Density Field: Implications for H_0 and Omega
We compare the peculiar velocities measured in the SBF Survey of Galaxy
Distances with the predictions from the density fields of the IRAS 1.2 Jy
flux-limited redshift survey and the Optical Redshift Survey (ORS) to derive
simultaneous constraints on the Hubble constant and the density parameter
, where is the linear bias. We find
and for the IRAS and ORS
comparisons, respectively, and \kmsMpc (with an additional 9%
uncertainty due to the Cepheids themselves). The match between predicted and
observed peculiar velocities is good for these values of and , and
although there is covariance between the two parameters, our results clearly
point toward low-density cosmologies. Thus, the unresolved discrepancy between
the ``velocity-velocity'' and ``density-density'' measurements of
continues.Comment: 4 pages with 3 embedded ps figures; uses emulateapj.sty (included).
Accepted for publication in ApJ Letter
Maxwell currents beneath thunderstorms
Analyses of single station measurements of the Maxwell current density (Jm) made under Florida thunderstorms during the summer of 1981 have been completed. The results of these analyses indicate that: (1) Jm is usually dominated by the displacement current component when the electric field is close to zero; (2) Jm is steady with time in the intervals between lightning flashes; (3) Jm is not altered significantly by lightning; and (4) the average value of Jm changes slowly and over time scales that are comparable to those required for storm development. Maps have also been derived of the surface Maxwell current density for a number of the Florida TRIP (76-78) storms using field mill data to estimate Jm from the displacement current density. Studies show that these maps provide a good indication of the location and relative intensity of the storm current generators, and area-integrations of the current contours provide estimates of the total storm currents
On the Metallicity-Color Relations and Bimodal Color Distributions in Extragalactic Globular Cluster Systems
We perform a series of numerical experiments to study how the nonlinear
metallicity--color relations predicted by different stellar population models
affect the color distributions observed in extragalactic globular cluster
systems. % We present simulations in the bandpasses based on five
different sets of simple stellar population (SSP) models. The presence of
photometric scatter in the colors is included as well. % We find that unimodal
metallicity distributions frequently ``project'' into bimodal color
distributions. The likelihood of this effect depends on both the mean and
dispersion of the metallicity distribution, as well as of course on the SSP
model used for the transformation. % Adopting the Teramo-SPoT SSP models for
reference, we find that optical--to--near-IR colors should be favored with
respect to other colors to avoid the bias effect in globular cluster color
distributions discussed by \citet{yoon06}. In particular, colors such as \vh\
or \vk are more robust against nonlinearity of the metallicity--color relation,
and an observed bimodal distribution in such colors is more likely to indicate
a true underlying bimodal metallicity distribution. Similar conclusions come
from the simulations based on different SSP models, although we also identify
exceptions to this result.Comment: ApJ accepte
The Next Generation Virgo Cluster Survey (NGVS). XVIII. Measurement and Calibration of Surface Brightness Fluctuation Distances for Bright Galaxies in Virgo (and Beyond)
We describe a program to measure surface brightness fluctuation (SBF)
distances to galaxies observed in the Next Generation Virgo Cluster Survey
(NGVS), a photometric imaging survey covering of the Virgo cluster
in the bandpasses with the Canada-France Hawaii Telescope. We
describe the selection of the sample galaxies, the procedures for measuring the
apparent -band SBF magnitude , and the calibration of the absolute
as a function of observed stellar population properties. The
multi-band NGVS data set provides multiple options for calibrating the SBF
distances, and we explore various calibrations involving individual color
indices as well as combinations of two different colors. Within the color range
of the present sample, the two-color calibrations do not significantly improve
the scatter with respect to wide-baseline, single-color calibrations involving
. We adopt the calibration as reference for the present
galaxy sample, with an observed scatter of 0.11 mag. For a few cases that lack
good photometry, we use an alternative relation based on a combination
of and colors, with only a slightly larger observed scatter of
0.12 mag. The agreement of our measurements with the best existing distance
estimates provides confidence that our measurements are accurate. We present a
preliminary catalog of distances for 89 galaxies brighter than
mag within the survey footprint, including members of the background M and W
Clouds at roughly twice the distance of the main body of the Virgo cluster. The
extension of the present work to fainter and bluer galaxies is in progress.Comment: ApJ accepte
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