153 research outputs found
Independent Analysis of the Distance to NGC1052-DF2
The conclusion that the diffuse galaxy NGC1052-DF2 is deficient in dark
matter depends critically on the distance, as noted in the original publication
and subsequent works. The results of our preliminary assessment of the SBF
distance have already been referenced on the web. Here we present the details
of a more careful analysis, supply some context, and comment on the remaining
uncertainty.Comment: 2 pages, accepted by Research Notes of the American Astronomical
Society; text is a bit laconic because of the RNAAS word limi
VEGAS-SSS: A VST Early-Type GAlaxy Survey: Analysis of Small Stellar System
VEGAS-SSS is a program devoted to study the properties of small stellar systems (SSSs) around bright galaxies, built on the VEGAS survey. At completion, the survey will have collected detailed photometric information of ∼ 100 bright early-type galaxies to study the properties of diffuse light (surface brightness, colours, SBF, etc.) and the clustered light (compact stellar systems) out to previously unreached projected galactocentric radii. VEGAS-SSS will define an accurate and homogeneous dataset that will have an important legacy value for studies of the evolution and transformation processes taking place in galaxies through the fossil information provided by SSSs
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
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
An Ultra diffuse Galaxy in the NGC 5846 group from the VEGAS survey
Many ultra diffuse galaxies (UDGs) have now been identified in clusters of
galaxies. However, the number of nearby UDGs suitable for detailed follow-up
remain rare. Our aim is to begin to identify UDGs in the environments of nearby
bright early-type galaxies from the VEGAS survey. Here we use a deep g band
image of the NGC 5846 group, taken as part of the VEGAS survey, to search for
UDGs. We found one object with properties of a UDG if it associated with the
NGC 5846 group, which seems likely. The galaxy, we name NGC 5846UDG1, has
an absolute magnitude of M = -14.2, corresponding to a stellar mass of
10 M. It also reveals a system of compact sources which are
likely globular clusters. Based on the number of globular clusters detected we
estimate a halo mass that is greater than 810 M for
UDG1.Comment: 5 pages, 4 figures, accepted for publication in A&
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
The distance to NGC1316 (Fornax A): yet another curious case
The distance of NGC1316, the brightest galaxy in Fornax, is an interesting
test for the cosmological distance scale. First, because Fornax is the 2nd
largest cluster of galaxies at <~25 Mpc after Virgo and, in contrast to Virgo,
has a small line-of-sight depth; and second, because NGC1316 is the galaxy with
the largest number of detected SNeIa, giving the opportunity to test the
consistency of SNeIa distances internally and against other indicators.
We measure SBF mags in NGC1316 from ground and space-based imaging data,
providing a homogeneous set of measurements over a wide wavelength interval.
The SBF, coupled with empirical and theoretical calibrations, are used to
estimate the distance to the galaxy. We present the first B-band SBF
measurements of NGC1316 and use them together with the optical and near-IR SBF
data to analyze the properties of field stars.
Our distance modulus m-M=31.59 +-0.05(stat) +-0.14(sys), when placed in a
consistent Cepheid distance scale, agrees with the results from other
indicators. However, our result is ~17% larger than the most recent estimate
based on SNeIa. Possible explanations for this disagreement are the
uncertainties on internal extinction, or calibration issues. Concerning the
stellar population analysis, we confirm earlier results from other indicators:
the field stars in NGC1316 are dominated by a solar metallicity, intermediate
age component. A substantial mismatch exists between B-band SBF models and
data, a behavior that can be accounted for by an enhanced percentage of hot
horizontal branch stars.
Our study of the SBF distance to NGC1316, and the comparison with distances
from other indicators, raises some concern about the homogeneity between the
calibrations of different indicators. If not properly placed in the same
reference scale, significant differences can occur, with dramatic impact on the
cosmological distance ladder.Comment: 21 pages, 13 figures; A&A accepte
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