449 research outputs found
High resolution kinematics of galactic globular clusters. II. On the significance of velocity dispersion measurements
Small number statistics may heavily affect the structure of the broadening
function in integrated spectra of galactic globular cluster centers. As a
consequence, it is a priori unknown how closely line broadening measure- ments
gauge the intrinsic velocity dispersions at the cores of these stel- lar
systems. We have tackled this general problem by means of Monte Carlo
simulations. An examination of the mode and the frequency distribution of the
measured values of the simulations indicates that the low value measured for
the velocity dispersion of M30 (Zaggia etal 1992) is likely a reliable estimate
of the velocity dispersion at the center of this cluster. The same methodology
applied to the case of M15 suggests that the steep inward rise of the velocity
dispersion found by Peterson, Seitzer and Cudworth (1989) is real, although
less pronounced. Large-aperture observa- tions are less sensitive to
statistical fluctuations, but are unable to detect strong variations in the
dispersion wich occur within the aperture itself.Comment: 6 pages, 8 figures upon request, Latex A&A style version 3.0,
DAPD-20
On the Cepheid variables of nearby galaxies III. NGC 3109
We extended to the R and I bands the light curve coverage for 8 Cepheids
already studied in B and V by Capaccioli et al [AJ, 103, 1151 (1992)]. Sixteen
additional Cepheid candidates have been identified and preliminary periods are
proposed. The new Cepheids allow the period-luminosity relation to be extended
one magnitude fainter. Apparent B, V, R, and I distance moduli have been
calculated. Combining the data at different wavelengths, and assuming a true
distance modulus of 18.50 mag for the LMC, we obtain for NGC 3109 a true
distance modulus , corresponding to Mpc.
Adopting for the LMC, the interstellar reddening for the Cepheids
in NGC 3109 is consistent with 0. A discussion on the possible implications of
this result is presented. A comparison of the period-color, period-amplitude,
and period-luminosity relations suggests similar properties for the Cepheids in
the LMC, NGC 3109, Sextans A, Sextans B, and IC 1613, though the uncertainties
in the main parameter determination are still unsatisfactorily high for a firm
conclusion on the universality of the period-luminosity relation.Comment: 35 pages, aaspp4.sty, accepted for pubblication on Astron.
Detection of Surface Brightness Fluctuations in Elliptical Galaxies imaged with the Advanced Camera for Surveys. B- and I-band measurements
Taking advantage of the exceptional capabilities of ACS on board of HST, we
derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands
from images of six elliptical galaxies with . Given the
low S/N ratio of the SBF signal in the blue band images, the reliability of the
measurements is verified both with numerical simulations and experimental data
tests.
This paper presents the first published B- and I-band SBF measurements for
distant ( 20 Mpc) galaxies, essential for the comparisons of the models
to observations of normal ellipticals. By comparing I-band data with our new
Simple Stellar Population (SSP) models we find an excellent agreement and we
confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of
the dominant stellar component in the galaxy, and are not strongly affected by
the contribution of possible secondary stellar components. As a consequence
I-band fluctuations magnitudes are ideal for distance studies. On the other
hand, we show that standard SSP models do not reproduce the B-band SBF
magnitudes of red ((B-I)_0 \gsim 2.1) galaxies in our sample. We explore the
capability of two non--canonical models in properly reproducing the high
sensitivity of B SBF to the presence of even small fractions of bright, hot
stars (metal poor stars, hot evolved stars, etc.). The disagreement is solved
both by taking into account hot (Post--AGB) stars in SSP models and/or by
adopting Composite Stellar Population models. Finally, we suggest a limit value
of the S/N for the B-band SBF signal required to carry out a detailed study of
stellar population properties based on this technique.Comment: ApJ accepte
A VST and VISTA study of globular clusters in NGC253
Aims. We analyze the properties of the sources in the NGC253 to define an up
to date catalog of GC candidates in the galaxy. Methods. Our analysis is based
on the science verification data of two ESO survey telescopes, VST and VISTA.
Using ugri photometry from VST and JKs from VISTA, GC candidates were selected
using the morpho-photometric and color properties of spectroscopically
confirmed GCs available in the literature. The strength of the results was
verified against available archival HST/ACS data from the GHOSTS survey.
Results. The adopted GC selection leads to the definition of a sample of ~350
GC candidates. At visual inspection, we find that 82 objects match all the
requirements for selecting GC candidates and 155 are flagged as uncertain GC
candidate; 110 are unlikely GCs, most likely background galaxies. Furthermore,
our analysis shows that four of the previously spectroscopically confirmed GCs,
i.e., ~20% of the total spectroscopic sample, are more likely either background
galaxies or high-velocity Milky Way stars. The radial density profile of the
selected best candidates shows the typically observed r1/4-law radial profile.
The analysis of the color distributions reveals only marginal evidence of the
presence of color bimodality, which is normally observed in galaxies of similar
luminosity. The GC luminosity function does not show the typical symmetry,
mainly because of the lack of bright GCs. Part of the bright GCs missing might
be at very large galactocentric distances or along the line of sight of the
galaxy dusty disk. Conclusions. Using ugriJKs photometry we purged the list of
GCs with spectroscopic membership and photometric GC candidates in NGC 253. Our
results show that the use of either spectroscopic or photometric data only does
not generally ensure a contaminant-free sample and a combination of both
spectroscopy and photometry is preferred.Comment: 24 pages, 15 figures, Accepted by Astronomy and Astrophysic
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
VEGAS: A VST Early-type GAlaxy Survey. III. Mapping the galaxy structure, interactions and intragroup light in the NGC 5018 group
Most of the galaxies in the Universe at present day are in groups, which are
key to understanding the galaxy evolution. In this work we present a new deep
mosaic of 1.2 x 1.0 square degrees of the group of galaxies centered on NGC
5018, acquired at the ESO VLT Survey Telescope. We use u, g, r images to
analyse the structure of the group members and to estimate the intra-group
light. Taking advantage of the deep and multiband photometry and of the large
field of view of the VST telescope, we studied the structure of the galaxy
members and the faint features into the intra-group space and we give an
estimate of the intragroup diffuse light in the NGC 5018 group of galaxies. We
found that ~ 41% of the total g-band luminosity of the group is in the form of
intragroup light (IGL). The IGL has a (g - r) color consistent with those of
other galaxies in the group, indicating that the stripping leading to the
formation of IGL is ongoing. From the study of this group we can infer that
there are at least two different interactions involving the group members: one
between NGC 5018 and NGC 5022, which generates the tails and ring-like
structures detected in the light, and another between NGC 5022 and
MCG-03-34-013 that have produced the HI tail. A minor merging event also
happened in the formation history of NGC 5018 that have perturbed the inner
structure of this galaxy.Comment: 21 pages, 15 figures. Accepted for publication in Ap
Shapley Supercluster Survey: Construction of the photometric catalogues and i-band data release
The Shapley Supercluster Survey is a multi-wavelength survey covering an area of ~23 deg2 (~260 Mpc2 at z = 0.048) around the supercluster core, including nine Abell and two poor clusters, having redshifts in the range 0.045-0.050. The survey aims to investigate the role of the cluster-scale mass assembly on the evolution of galaxies, mapping the effects of the environment from the cores of the clusters to their outskirts and along the filaments. The optical (ugri) imaging acquired with OmegaCAM on the VLT Survey Telescope is essential to achieve the project goals providing accurate multi-band photometry for the galaxy population down to m * + 6. We describe the methodology adopted to construct the optical catalogues and to separate extended and point-like sources. The catalogues reach average 5s limitingmagnitudes within a 3 arcsec diameter aperture of ugri=[24.4,24.6,24.1,23.3] and are 93 per cent complete down to ugri = [23.8,23.8,23.5,22.0] mag, corresponding to ~mr * + 8.5. The data are highly uniform in terms of observing conditions and all acquired with seeing less than 1.1 arcsec full width at half-maximum. The median seeing in r band is 0.6 arcsec, corresponding to 0.56 kpc h70 -1 at z = 0.048. While the observations in the u, g and r bands are still ongoing, the i-band observations have been completed, and we present the i-band catalogue over the whole survey area. The latter is released and it will be regularly updated, through the use of the Virtual Observatory tools. This includes 734 319 sources down to i = 22.0 mag and it is the first optical homogeneous catalogue at such a depth, covering the central region of the Shapley supercluster
Apparent counter-rotation in the torus of NGC 1068: influence of an asymmetric wind
The recent ALMA maps together with observations of HO maser emission seem
to suggest the presence of a counter-rotation in the obscuring torus of NGC
1068. We propose to explain this phenomenon as due to the influence of a wind,
considered as radiation pressure, and the effects of torus orientation. In
order to test this idea: 1. we make -body simulation of a clumpy torus
taking into account mutual forces between particles (clouds); 2. we apply
ray-tracing algorithm with the beams from the central engine to choose the
clouds in the torus throat that can be under direct influence of the accretion
disk emission; 3. we use semi-analytical model to simulate the influence of the
asymmetrical radiation pressure (wind) forced on the clouds in the torus
throat. An axis of such a wind is tilted with respect to the torus symmetry
axis; 4. we orient the torus relative to an observer and again apply
ray-tracing algorithm. In this step the beams go from an observer to the
optically thick clouds that allows us to take into account the mutual
obscuration of clouds; 5. after projecting on the picture plane, we impose a
grid on the resulting cloud distribution and find the mean velocity of clouds
in each cells to mimic the ALMA observational maps. By choosing the parameters
corresponding to NGC 1068 we obtain the model velocity maps that emulate the
effect of an apparent counter-rotation and can explain the discovery made by
ALMA.Comment: 11 pages, 11 figures, Accepted for publication in MNRA
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