80 research outputs found
NGC 5011C: an overlooked dwarf galaxy in the Centaurus A group
(abridged) We report the discovery of a previously unnoticed member of the
Centaurus A Group, NGC 5011C. While the galaxy is a well known stellar system
listed with a NGC number its true identity remained hidden because of
coordinate confusion and wrong redshifts in the literature. NGC 5011C attracted
our attention since, at a putative distance of 45.3 Mpc, it would be a peculiar
object having a very low surface brightness typical of a dwarf galaxy, and at
the same time having the size of an early-type spiral or S0 galaxy. To confirm
or reject this peculiarity, our immediate objective was to have the first
reliable measurement of its recession velocity. The observations were carried
out with EFOSC2 at the 3.6m ESO telescope. We found that NGC 5011C has indeed a
low redshift of v_sun=647+/-96 km/sec and thus is a nearby dwarf galaxy rather
than a member of the distant Centaurus cluster as believed for the past 23
years. Rough distance estimates based on photometric parameters also favor this
scenario. As a byproduct of our study we update the redshift for NGC 5011B at
v_sun=3227+/-50 km/sec. Applying population synthesis techniques, we find that
NGC 5011B has a luminosity-weighted age of 4+/-1 Gyr and a solar metallicity,
and that the luminosity-weighted age and metallicity of NGC 5011C are 0.9+/-0.1
Gyr and 1/5 solar. Finally we estimate a stellar mass of NGC 5011C comparable
to that of dwarf spheroidal galaxies in the Local Group.Comment: 13 pages, 6 figures, accepted by the Astronomical Journa
Morphological transformation of NGC 205?
NGC 205 is a dwarf elliptical galaxy which shows many features that are more
typical of disk galaxies, and our recent study of the central stellar
population has added another peculiarity. In the central regions, star
formation has been on-going continuously for a few hundred Myr, until ca. 20
Myr ago, perhaps fed by gas funneled to the center in the course of
morphological transformation. In this contribution we use a deep, wide-field
image obtained at a scale of 2"/px to show that subtle structures can be
detected in and near the body of the dwarf galaxy. The southern tidal tail can
be mapped out to unprecedented distances from the center, and we suggest that
the northern tail is partially hidden behind a very extended dust lane, or
ring, belonging to M31. A spiral pattern emerges across the body of the galaxy,
but it might be explained by another M31 dust filament.Comment: 2 pages, 1 figure, poster contributed to IAU Symposium 262, Stellar
Populations -- Planning for the Next Decade, G. Bruzual & S. Charlot, ed
The relative ages of LMC old clusters, and the case of NGC 1841
Using archival HST/WFPC2 imaging of 7 LMC globular clusters, and following
the methods outlined in our previous study, we have reached the tightest
constrain so far on their age dispersion, which cannot be greater than ~ 0.5
Gyr. We also confirm earlier results that their average age is comparable to
that of the metal-poor Galactic globulars. Evidence is also provided that NGC
1841 is younger than the rest of LMC globulars.Comment: 2 pages, 1 figure, requires newpasp.sty. To appear in "New Horizons
in Globular Cluster Astronomy", Giampaolo Piotto, George Meylan, George
Djorgovski and Marco Riello, ed
The red giant branches of Galactic globular clusters in the [(V-I)o,Mv] plane: metallicity indices and morphology
The purpose of this study is to carry out a thorough investigation of the
changes in morphology of the red giant branch (RGB) of Galactic globular
clusters (GGC) as a function of metallicity, in the V,I bands. To this aim, two
key points are developed in the course of the analysis.
(a) Using our photometric V,I database for Galactic globular clusters (the
largest homogeneous data sample to date; Rosenberg et al. 1999) we measure a
complete set of metallicity indices, based on the morphology and position of
the red-giant branch. In particular, we provide here the first calibration of
the S, DV_(1.1) and DV_(1.4) indices in the (V-I,V) plane. We show that our
indices are internally consistent, and we calibrate each index in terms of
metallicity, both on the Zinn & West (1984) and the Carretta & Gratton (1997)
scales. Our new calibrations of the (V-I)o,g, DV_(1.2), (V-I)_(-3.0) and
(V-I)_(-3.5) indices are consistent with existing relations.
(b) Using a grid of selected RGB fiducial points, we define a function in the
{(V-I)o,Mi,[Fe/H]} space which is able to reproduce the whole set of GGC giant
branches in terms of a single parameter (the metallicity). As a first test, we
show that the function is able to predict the correct trend of our observed
indices with metallicity.
The usage of this function will improve the current determinations of
metallicity and distances within the Local Group, since it allows to easily map
[(V-I)o,Mi] coordinates into {[Fe/H],Mi} ones. To this aim the ``synthetic''
RGB distribution is generated both for the currently used Lee et al. (1990)
distance scale, and for the most recent results on the RR Lyr distance scale.Comment: 14 pages, 11 PS figures, accepted for publication in Astronomy &
Astrophysics, main journa
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