1,166 research outputs found
2MASS photometry and age estimate of globular clusters in the outer halo of M31
We present the first photometric results in J, H, and K_s from 2MASS imaging
of 10 classical globular clusters in the far outer regions of M31. Combined
with the V and I photometric data from previous literature, we constructed the
color-color diagram between J-K_s and V-I. By comparing the integrated
photometric measurements with evolutionary models, we estimate the ages of
these clusters. The results showed that, all of these clusters are older than
yrs, of which 4 are older than 10 Gyrs and the other 6 are in
intermediate ages between 3-8 Gyrs. The masses for these outer halo GCs are
from to . We argued that, GC2 and
GC3, the ages, metallicities and the distance moduli of which are nearly the
same, were accreted from the same satellite galaxy, if they did not form {\it
in situ}. The statistical results show that, ages and metallicities for these
10 M31 outer halo GCs do not vary with projected radial position, and the
relationship between age and metallicity doest not exit.Comment: Accepted for Publication in RAA, 14 pages, 8 figures and 3 table
Age and structure parameters of a remote M31 globular cluster B514 based on HST, 2MASS, GALEX and BATC observations
B514 is a remote M31 globular cluster which locating at a projected distance
of R_p~55 kpc. Deep observations with the Advanced Camera for Surveys (ACS) on
the Hubble Space Telescope (HST) are used to provide the accurate integrated
light and star counts of B514. By coupling analysis of the distribution of the
integrated light with star counts, we are able to reliably follow the profile
of the cluster out to ~40". Based on the combined profile, we study in detail
its surface brightness distribution in F606W and F814W filters, and determine
its structural parameters by fitting a single-mass isotropic King model. The
results showed that, the surface brightness distribution departs from the
best-fit King model for r>10". B514 is quite flatted in the inner region, and
has a larger half-light radius than majority of normal globular clusters of the
same luminosity. It is interesting that, in the M_V versus log R_h plane, B514
lies nearly on the threshold for ordinary globular clusters as defined by
Mackey & van den Bergh. In addition, B514 was observed as part of the
Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey, using 13
intermediate-band filters covering a wavelength range of 3000--8500 \AA. Based
on aperture photometry, we obtain its SEDs as defined by the 13 BATC filters.
We determine the cluster's age and mass by comparing its SEDs (from 2267 to
20000{\AA}, comprising photometric data in the near-ultraviolet of GALEX, 5
SDSS bands, 13 BATC intermediate-band, and 2MASS near-infrared JHKs} filters)
with theoretical stellar population synthesis models, resulting in age of
Gyr. This age confirms the previous suggestion that B514 is an old
GC in M31. B514 has a mass of , and is a
medium-mass globular cluster in M31.Comment: Accepted for Publication in AJ, 18 pages, 6 figures and 9 table
Spectroscopic Study of Globular Clusters in the Halo of M31 with Xinglong 2.16m Telescope II: Dynamics, Metallicity and Age
(Abridged) We performed the spectroscopic observations of 11 confirmed GCs in
M31 with the Xinglong 2.16m telescope and we mainly focus on the fits method
and the metallicity gradient for the M31 GC sample. We analyzed and discussed
more about the dynamics, metallicity and age, and their distributions as well
as the relationships between these parameters. Eight more confirmed GCs in the
halo of M31 were observed, most of which lack the spectroscopic information
before. These star clusters are located far from the galactic center at a
projected radius of ~14 to ~117 kpc. The Lick absorption-line indices and the
radial velocities have been measured and ages, metallicities [Fe/H] and
alpha-element [alpha/Fe] have also been fitted by comparing the observed
spectral feature indices and Thomas et al.SSP model. Our results show that most
of the star clusters of our sample are older than 10 Gyr except B290 ~5.5 Gyr,
and most of them are metal-poor with the metallicity [Fe/H]<-1, suggesting that
these clusters were born at the early stage of the galaxy's formation. We find
that the metallicity gradient for the outer halo clusters with r_p>25 kpc may
not exist with a slope of -0.005+-0.005 dex kpc^-1. We also find that the
metallicity is not a function of age for the GCs with age < 7 Gyr while for the
old GCs with age >7 Gyr there seems to be a trend that the older ones have
lower metallicity. Besides, We plot metallicity distributions with the largest
sample of M31 GCs so far and it shows the bimodality is not significant and the
number of the metal-poor and metal-rich groups becomes comparable. The spatial
distributions shows that the metal-rich group is more centrally concentrated
while the metal-poor group is occupy a more extended halo and the young
population is centrally concentrated while the old population is more extended
spatially to the outer halo.Comment: 17 pages, 10 figures and 6 tables. Accepted for publication in RA
The M33 Globular Cluster System with PAndAS Data: The Last Outer Halo Cluster?
We use CFHT/MegaCam data to search for outer halo star clusters in M33 as
part of the Pan-Andromeda Archaeological Survey (PAndAS). This work extends
previous studies out to a projected radius of 50 kpc and covers over 40 square
degrees. We find only one new unambiguous star cluster in addition to the five
previously known in the M33 outer halo (10 kpc <= r <= 50 kpc). Although we
identify 2440 cluster candidates of various degrees of confidence from our
objective image search procedure, almost all of these are likely background
contaminants, mostly faint unresolved galaxies. We measure the luminosity,
color and structural parameters of the new cluster in addition to the five
previously-known outer halo clusters. At a projected radius of 22 kpc, the new
cluster is slightly smaller, fainter and redder than all but one of the other
outer halo clusters, and has g' ~ 19.9, (g'-i') ~ 0.6, concentration parameter
c ~ 1.0, a core radius r_c ~ 3.5 pc, and a half-light radius r_h ~ 5.5 pc. For
M33 to have so few outer halo clusters compared to M31 suggests either tidal
stripping of M33's outer halo clusters by M31, or a very different, much calmer
accretion history of M33.Comment: 37 pages, 9 figures. Accepted by the Astrophysical Journa
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Catastrophic photometric redshift errors: weak lensing survey requirements
We study the sensitivity of weak lensing surveys to the effects of
catastrophic redshift errors - cases where the true redshift is misestimated by
a significant amount. To compute the biases in cosmological parameters, we
adopt an efficient linearized analysis where the redshift errors are directly
related to shifts in the weak lensing convergence power spectra. We estimate
the number Nspec of unbiased spectroscopic redshifts needed to determine the
catastrophic error rate well enough that biases in cosmological parameters are
below statistical errors of weak lensing tomography. While the straightforward
estimate of Nspec is ~10^6 we find that using only the photometric redshifts
with z<=2.5 leads to a drastic reduction in Nspec to ~30,000 while negligibly
increasing statistical errors in dark energy parameters. Therefore, the size of
spectroscopic survey needed to control catastrophic errors is similar to that
previously deemed necessary to constrain the core of the z_s-z_p distribution.
We also study the efficacy of the recent proposal to measure redshift errors by
cross-correlation between the photo-z and spectroscopic samples. We find that
this method requires ~10% a priori knowledge of the bias and stochasticity of
the outlier population, and is also easily confounded by lensing magnification
bias. The cross-correlation method is therefore unlikely to supplant the need
for a complete spectroscopic redshift survey of the source population.Comment: 14 pages, 3 figure
Multicolour Photometric Study of M31 Globular Clusters
We present the photometry of 30 globular clusters (GCs) and GC candidates in
15 intermediate-band filters covering from ~3000 to ~10000 \AA using the
archival CCD images of M31 observed as part of the Beijing - Arizona - Taiwan -
Connecticut (BATC) Multicolour Sky Survey. We transform these intermediate-band
photometric data to the photometry in the standard UBVRI broad-bands. These M31
GC candidates are selected from the Revised Bologna Catalogue (RBC V.3.5), and
most of these candidates do not have any photometric data. Therefore the
present photometric data are supplement to RBC V.3.5. We find that 4 out of 61
GCs and GC candidates in RBC V.3.5 do not show any signal on the BATC images at
their locations. By linear fit of the distribution in colour-magnitude diagram
of blue GCs and GC candidates using the data from RBC V.3.5, and in this study
we find the ``blue-tilt'' of blue M31 GCs with a high confidence at 99.95% or
3.47 sigma for the confirmed GCs, and >99.99% or 4.87 sigma for GCs and GC
candidates.Comment: Accepted for publication in RAA. 22 pages including 9 figures and 6
table
HST/ACS color-magnitude diagrams of candidate intermediate-age M 31 globular clusters. The role of blue horizontal branches
We present deep (V ~ 28.0) BV photometry obtained with the wide field channel
of the Advanced Camera for Surveys on board HST for four M31 globular clusters
that were identified as candidate intermediate-age (age ~ 1-9 Gyr) by various
authors, based on their integrated spectra and/or broad/intermediate-band
colors. Two of them (B292 and B350) display an obvious blue horizontal branch,
indicating that they are as old as the oldest Galactic globulars. On the other
hand, for the other two (B058 and B337), which display red horizontal branches,
it was not possible either to confirm or disconfirm the age estimate from
integrated spectra. The analysis of the distribution in the spectral indices
Mg2 and H_beta of the M31 and Milky Way clusters whose horizontal branch can be
classified as red or blue based on existing CMDs, strongly suggests that
classical age diagnostics from integrated spectra may be significantly
influenced by the HB morphology of the clusters and can lead to erroneous
age-classifications. We also provide the CMD for another two clusters that fall
into the field of the main targets, B336, an old and metal-poor globular with a
significant population of RR-Lyrae variables, and the newly discovered B531, a
cluster with a very red red giant branch.Comment: Accepted for publication in Astronomy and Astrophysics; 13 pages, 13
figures and 7 tables. Some figures have been decreased in quality, an higher
resolution version is available at http://www.bo.astro.it/M31/hstcatalog
Identification of novel subgroup a variants with enhanced receptor binding and replicative capacity in primary isolates of anaemogenic strains of feline leukaemia virus
<b>BACKGROUND:</b>
The development of anaemia in feline leukaemia virus (FeLV)-infected cats is associated with the emergence of a novel viral subgroup, FeLV-C. FeLV-C arises from the subgroup that is transmitted, FeLV-A, through alterations in the amino acid sequence of the receptor binding domain (RBD) of the envelope glycoprotein that result in a shift in the receptor usage and the cell tropism of the virus. The factors that influence the transition from subgroup A to subgroup C remain unclear, one possibility is that a selective pressure in the host drives the acquisition of mutations in the RBD, creating A/C intermediates with enhanced abilities to interact with the FeLV-C receptor, FLVCR. In order to understand further the emergence of FeLV-C in the infected cat, we examined primary isolates of FeLV-C for evidence of FeLV-A variants that bore mutations consistent with a gradual evolution from FeLV-A to FeLV-C.<p></p>
<b>RESULTS:</b>
Within each isolate of FeLV-C, we identified variants that were ostensibly subgroup A by nucleic acid sequence comparisons, but which bore mutations in the RBD. One such mutation, N91D, was present in multiple isolates and when engineered into a molecular clone of the prototypic FeLV-A (Glasgow-1), enhanced replication was noted in feline cells. Expression of the N91D Env on murine leukaemia virus (MLV) pseudotypes enhanced viral entry mediated by the FeLV-A receptor THTR1 while soluble FeLV-A Env bearing the N91D mutation bound more efficiently to mouse or guinea pig cells bearing the FeLV-A and -C receptors. Long-term in vitro culture of variants bearing the N91D substitution in the presence of anti-FeLV gp70 antibodies did not result in the emergence of FeLV-C variants, suggesting that additional selective pressures in the infected cat may drive the subsequent evolution from subgroup A to subgroup C.<p></p>
<b>CONCLUSIONS:</b>
Our data support a model in which variants of FeLV-A, bearing subtle differences in the RBD of Env, may be predisposed towards enhanced replication in vivo and subsequent conversion to FeLV-C. The selection pressures in vivo that drive the emergence of FeLV-C in a proportion of infected cats remain to be established
The removal of shear-ellipticity correlations from the cosmic shear signal: Influence of photometric redshift errors on the nulling technique
Cosmic shear is regarded one of the most powerful probes to reveal the
properties of dark matter and dark energy. To fully utilize its potential, one
has to be able to control systematic effects down to below the level of the
statistical parameter errors. Particularly worrisome in this respect is
intrinsic alignment, causing considerable parameter biases via correlations
between the intrinsic ellipticities of galaxies and the gravitational shear,
which mimic lensing. In an earlier work we have proposed a nulling technique
that downweights this systematic, only making use of its well-known redshift
dependence. We assess the practicability of nulling, given realistic conditions
on photometric redshift information. For several simplified intrinsic alignment
models and a wide range of photometric redshift characteristics we calculate an
average bias before and after nulling. Modifications of the technique are
introduced to optimize the bias removal and minimize the information loss by
nulling. We demonstrate that one of the presented versions is close to optimal
in terms of bias removal, given high quality of photometric redshifts. For
excellent photometric redshift information, i.e. at least 10 bins with a small
dispersion, a negligible fraction of catastrophic outliers, and precise
knowledge about the redshift distributions, one version of nulling is capable
of reducing the shear-intrinsic ellipticity contamination by at least a factor
of 100. Alternatively, we describe a robust nulling variant which suppresses
the systematic signal by about 10 for a very broad range of photometric
redshift configurations. Irrespective of the photometric redshift quality, a
loss of statistical power is inherent to nulling, which amounts to a decrease
of the order 50% in terms of our figure of merit.Comment: 26 pages, including 16 figures; minor changes to match accepted
version; published in Astronomy and Astrophysic
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