399 research outputs found
Diffuse stellar component in galaxy clusters and the evolution of the most massive galaxies at z<~1
The high end of the stellar mass function of galaxies is observed to have
little evolution since z~1. This represents a stringent constraint for
merger--based models, aimed at explaining the evolution of the most massive
galaxies in the concordance LambdaCDM cosmology. In this Letter we show that it
is possible to remove the tension between the above observations and model
predictions by allowing a fraction of stars to be scattered to the Diffuse
Stellar Component (DSC) of galaxy clusters at each galaxy merger, as recently
suggested by the analysis of N-body hydrodynamical simulations. To this
purpose, we use the MORGANA model of galaxy formation in a minimal version, in
which gas cooling and star formation are switched off after z=1. In this way,
any predicted evolution of the galaxy stellar mass function is purely driven by
mergers. We show that, even in this extreme case, the predicted degree of
evolution of the high end of the stellar mass function is larger than that
suggested by data. Assuming instead that a significant fraction, ~30 per cent,
of stars are scattered in the DSC at each merger event, leads to a significant
suppression of the predicted evolution, in better agreement with observational
constraints, while providing a total amount of DSC in clusters which is
consistent with recent observational determinations.Comment: 5 pages, figures included; ApJ Letters, in press. Revision: reference
adde
Unveiling Palomar 2: The Most Obscure Globular Cluster in the Outer Halo
We present the first color-magnitude study for Palomar 2, a distant and
heavily obscured globular cluster near the Galactic anticenter. Our (V,V-I)
color-magnitude diagram (CMD), obtained with the UH8K camera at the CFHT,
reaches V(lim) = 24 and clearly shows the principal sequences of the cluster,
though with substantial overall foreground absorption and differential
reddening. The CMD morphology shows a well populated red horizontal branch with
a sparser extension to the blue, similar to clusters such as NGC 1261, 1851, or
6229 with metallicities near [Fe/H] = -1.3, placing it about 34 kpc
from the Galactic center. We use starcounts of the bright stars to measure the
core radius, half-mass radius, and central concentration of the cluster. Its
integrated luminosity is M_V = -7.9, making it clearly brighter and more
massive than most other clusters in the outer halo.Comment: 25 pages, aastex, with 8 postscript figures; accepted for publication
in AJ, September 1997. Also available by e-mail from
[email protected]. Please consult Harris directly for (big)
postscript files of Figures 1a,b (the images of the cluster
Vortex deformation and breaking in superconductors: A microscopic description
Vortex breaking has been traditionally studied for nonuniform critical
current densities, although it may also appear due to nonuniform pinning force
distributions. In this article we study the case of a
high-pinning/low-pinning/high-pinning layered structure. We have developed an
elastic model for describing the deformation of a vortex in these systems in
the presence of a uniform transport current density for any arbitrary
orientation of the transport current and the magnetic field. If is above a
certain critical value, , the vortex breaks and a finite effective
resistance appears. Our model can be applied to some experimental
configurations where vortex breaking naturally exists. This is the case for
YBaCuO (YBCO) low angle grain boundaries and films on vicinal
substrates, where the breaking is experienced by Abrikosov-Josephson vortices
(AJV) and Josephson string vortices (SV), respectively. With our model, we have
experimentally extracted some intrinsic parameters of the AJV and SV, such as
the line tension and compared it to existing predictions based on
the vortex structure.Comment: 11 figures in 13 files; minor changes after printing proof
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
Investigating the Andromeda Stream: II. Orbital Fits and Properties of the Progenitor
We construct test-particle orbits and simple N-body models that match the
properties of the giant stellar stream observed to the south of M31, using the
model of M31's potential derived in the companion paper by Geehan et al.
(2006). We introduce a simple approximation to account for the difference in
position between the stream and the orbit of the progenitor; this significantly
affects the best-fitting orbits. The progenitor orbits we derive have orbital
apocenter \sim 60 \kpc and pericenter \sim 3 \kpc, though these quantities
vary somewhat with the current orbital phase of the progenitor which is as yet
unknown. Our best combined fit to the stream and galaxy properties implies a
mass within 125 kpc of M31 of (7.4 \pm 1.2) \times 10^{11} \Msun. Based on
its length, width, luminosity, and velocity dispersion, we conclude that the
stream originates from a progenitor satellite with mass M_s \sim 10^9 \Msun,
and at most modest amounts of dark matter; the estimate of is again
correlated with the phase of the progenitor. M31 displays a large number of
faint features in its inner halo which may be progenitors or continuations of
the stream. While the orbital fits are not constrained enough for us to
conclusively identify the progenitor, we can identify several plausible
candidates, of which a feature in the planetary nebula distribution found by
Merrett et al. is the most plausible, and rule out several others. We make
predictions for the kinematic properties of the successful candidates. These
may aid in observational identification of the progenitor object, which would
greatly constrain the allowed models of the stream.Comment: 17 pages, 10 color figures, 4 tables. Accepted by Monthly Notices;
some minor revisions and corrected typo
Photometry and the Metallicity Distribution of the Outer Halo of M31
We have conducted a wide-field CCD-mosaic study of the resolved red-giant
branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the
SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top
three magnitudes of the M31 halo are strongly present. Photometry of a more
distant control field to subtract field contamination is used to derive the
`cleaned' luminosity function and metallicity distribution function (MDF) of
the M31 halo field. From the color distribution of the foreground Milky Way
halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from
the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47
+/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an
extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated
by a moderately high-metallicity population ([m/H]~ -0.5) found previously in
more interior M31 halo/bulge fields, and is much more metal-rich than the
[m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending
on AGB star contribution) metal-poor population is also present. To first
order, the shape of the MDF resembles that predicted by a simple,
single-component model of chemical evolution starting from primordial gas with
an effective yield y=0.0055. It strongly resembles the MDF recently found for
the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though
NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in
both NGC 5128 and M31, the metallicity distribution of the globular clusters in
M31 does not match the halo stars; the clusters are far more heavily weighted
to metal-poor objects. We suggest similarities in the formation and early
evolution of massive, spheroidal stellar systems.Comment: to appear in the Astronomical Journal; 43 pages, including 15 figure
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