747 research outputs found
On the origin of the helium-rich population in the peculiar globular cluster Omega Centauri
In this contribution we discuss the origin of the extreme helium-rich stars
which inhabit the blue main sequence (bMS) of the Galactic globular cluster
Omega Centauri. In a scenario where the cluster is the surviving remnant of a
dwarf galaxy ingested by the Milky Way many Gyr ago, the peculiar chemical
composition of the bMS stars can be naturally explained by considering the
effects of strong differential galactic winds, which develop owing to multiple
supernova explosions in a shallow potential well.Comment: 2 pages, 1 figure, to appear in the Proceedings of IAU Symposium No.
268, Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
Dust and gas in the central region of NGC 1316 (Fornax A) -- Its origin and nature
The early-type galaxy NGC 1316 hosts about 10^7 solar masses of dust within a
central radius of 5 kpc. These prominent dust structures are believed to have
an external origin, which is also a popular interpretation for other dusty
early-type galaxies. We use archival Hubble Space Telescope/ACS data to
construct colour maps that delineate the dust pattern in detail, and we compare
these data with maps constructed with data from MUSE of the VLT at the European
Southern Observatory. Twelve MUSE pointings in wide field mode form a mosaic of
the central 3.3'x2.4'. We use the tool PyParadise to fit the stellar
population. We use the residual emission lines and the residual interstellar
absorption NaI D-lines, and we measure line strengths, the velocity field, and
the velocity dispersion field. The emission lines resemble LINER lines, with
[NII] being the strongest line everywhere. Ionising sources are plausibly the
post-asymptotic giant branch stars of the old or intermediate-age stellar
population. There is a striking match between the dust structures, ionised gas,
and atomic gas distributions, the last of which is manifested by interstellar
absorption residuals of the stellar NaI D-lines. In the dust-free regions, the
interstellar NaI D-lines appear in emission, which is indicative of a galactic
wind. The velocity field of the ionised gas (and thus of the dust) is
characterised by small-scale turbulent movements that indicate short lifetimes.
At the very centre, a bipolar velocity field of the ionised gas is observed,
which we interpret as an outflow. We identify a strongly inclined gaseous dusty
disc along the major axis of NGC1316. A straight beam of ionised gas with a
length of about 4 kpc emanates from the centre. Our findings are strongly
suggestive of a dusty outflow. Nuclear outflows may be important dust-producing
machines in galaxies. (Abridged)Comment: 9 pages, 8 figures, accepted by Astron.Astrophy
inducibility, severity and success of egg-killing leaf necrosis depends on plant genotype and egg clustering
Plants employ various defences killing the insect attacker in an early stage.
Oviposition by cabbage white butterflies (Pieris spp.) on brassicaceous
plants, including Brassica nigra, induces a hypersensitive response (HR) -
like leaf necrosis promoting desiccation of eggs. To gain a deeper insight
into the arms race between butterflies and plants, we conducted field and
greenhouse experiments using different B. nigra genotypes. We investigated
variation in HR and consequent survival of P. brassicae egg clusters. Impact
of egg density, distribution type and humidity on HR formation and egg
survival was tested. HR differed among plant genotypes as well as plant
individuals. Egg density per plant did not affect HR formation. Remarkably,
egg survival did not depend on the formation of HR, unless butterflies were
forced to lay single eggs. Larval hatching success from single eggs was lower
on plants expressing HR. This may be due to increased vulnerability of single
eggs to low humidity conditions at necrotic leaf sites. We conclude that
effectiveness of HR-like necrosis in B. nigra varies with plant genotype,
plant individual and the type of egg laying behaviour (singly or clustered).
By clustering eggs, cabbage white butterflies can escape the egg-killing,
direct plant defence trait
The Rotation of Sub-Populations in omega Centauri
We present the first result of the Ital-FLAMES survey of red giant branch
(RGB) stars in omega Cen. Radial velocities with a precision of ~0.5 km/s are
presented for 650 members of omega Cen observed with FLAMES-Giraffe at the Very
Large Telescope. We found that stars belonging to the metal -poor (RGB-MP),
metal-intemediate (RGB-MInt) and metal-rich (RGB-a) sub -populations of Omega
Cen are all compatible with having the same rotational pattern. Our results
appear to contradict past findings by Norris et al., who could not detect any
rotational signature for metal -rich stars. The slightly higher precision of
the present measurements and the much larger sample size, especially for the
metal-richer stars, appear as the most likely explanation for this discrepancy.
The result presented here weakens the body of evidence in favour of a merger
event in the past history of omega Cen.Comment: 5 pages, 3 fiures, electronic table can be obtained from E. Pancino.
Accepted for publication in ApJ Letter
Dust and star formation in the centre of NGC 3311
NGC 3311 is the central galaxy of the Hydra I galaxy cluster. It has a hot
interstellar medium and hosts a central dust lane with emission lines. These
dust lanes are frequent in elliptical galaxies, but the case of NGC 3311 might
be particularly interesting for problems of dust lifetime and the role of cool
gas in the central parts. We aim to use archival HST images and MUSE data to
investigate the central dust structure of NGC 3311. We used the tool PyParadise
to model the stellar population and extract the emission lines. The HST/ACS
colour map reveals the known dust structures, but also blue spots, which are
places of strong line emission. A dusty 'mini-jet' emanates from the centre.
The distribution of the emission line gas matches the dust silhouette almost
exactly. Close to the brightest Halpha emission, the ratio [NII]/Halpha
resembles that of HII-regions; in the outer parts, [NII] gets stronger and is
similar to LINER-like spectra. The gas kinematics is consistent with that of a
rotating disc. The Doppler shifts of the strongest line emissions, which
indicate the areas of highest star formation activity, smoothly fit into the
disc symmetry. The metallicity is supersolar. The presence of neutral gas is
indicated by the fit residuals of the stellar NaI D absorption line, which we
interpret as interstellar absorption. We estimate the mass of the neutral gas
to be of the order of the X-ray mass. The dynamical mass infers a stellar
population of intermediate age, whose globular clusters have already been
identified. Our findings can be harmonised in a scenario in which the star
formation is triggered by the accretion of cold gas onto a pre-existing
gas/dust disc or ring. Newly produced dust then contributes to the longevity of
the dust.Comment: 9 pages, 6 figures, accepted by Astron.Astrophy
Extragalactic Globular Clusters in the Near-Infrared II. The Globular Clusters Systems of NGC 3115 and NGC 4365
We combine near-infrared photometry obtained with the VLT/ISAAC instrument
and archival HST/WFPC2 optical images to determine VIK magnitudes and colours
of globular clusters in two early-type galaxies, NGC 3115 and NGC 4365. The
combination of near-IR and optical photometry provides a way to lift the
age-metallicity degeneracy. For NGC 3115, the globular cluster colours reveal
two major sub-populations, consistent with previous studies. By comparing the
V-I, V-K colours of the NGC 3115 globular clusters with SSP models, we find
that the colour difference between the two >10 Gyr old major sub-populations is
primarily due to a difference in metallicity. We find \Delta[Fe/H] = 1.0 +/-
0.3 dex and the blue and red globular cluster sub-populations being coeval
within 3 Gyr. In contrast to the NGC 3115 globular clusters, the globular
cluster system in NGC 4365 exhibits a more complex age and metallicity
structure. We find a significant population of intermediate-age very metal-rich
globular clusters along with an old population of both metal-rich and
metal-poor clusters. Specifically, we observe a large population of globular
clusters with V-K and V-I colours, for which all current SSP models give ages
and metallicities in the range ~2-8 Gyr and ~0.5-3 Z_solar, respectively. After
10 Gyr of passive evolution, the intermediate-age globular clusters in NGC 4365
will have colours which are consistent with the very metal-rich population of
globular clusters in giant elliptical galaxies, such as M87. Our results for
both globular cluster systems are consistent with previous age and metallicity
studies of the diffuse galactic light. In addition to the major globular
cluster populations in NGC 3115 and NGC 4365 we report on the detection of
objects with extremely red colours.Comment: accepted for publication in A&A, 19 pages, incl. 9 figure
Spatially Resolved Kinematics of an Ultra-Compact Dwarf Galaxy
We present the internal kinematics of UCD3, the brightest known ultra-compact
dwarf galaxy (UCD) in the Fornax cluster, making this the first UCD with
spatially resolved spectroscopy. Our study is based on seeing-limited
observations obtained with the ARGUS Integral Field Unit of the VLT/FLAMES
spectrograph under excellent seeing conditions (0.5 - 0.67 arcsec FWHM). The
velocity field of UCD3 shows the signature of weak rotation, comparable to that
found in massive globular clusters. Its velocity dispersion profile is fully
consistent with an isotropic velocity distribution and the assumption that mass
follows the light distribution obtained from Hubble Space Telescope imaging. In
particular, there is no evidence for the presence of an extended dark matter
halo contributing a significant (>~33 per cent within R < 200 pc) mass
fraction, nor for a central black hole more massive than ~5 per cent of the
UCD's mass. While this result does not exclude a galaxian origin for UCD3, we
conclude that its internal kinematics are fully consistent with it being a
massive star cluster.Comment: 5 pages, 3 figures; accepted for publication in MNRAS Letter
The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database
We have obtained high-resolution spectra of 23 ultra-compact dwarf galaxies
(UCDs) in the Fornax cluster with -10.4>M_V>-13.5 mag (10^6<M/M_*<10^8), using
FLAMES/Giraffe at the VLT. This is the largest homogeneous data set of UCD
internal dynamics assembled to date. We derive dynamical M/L ratios for 15 UCDs
covered by HST imaging. In the M_V-sigma plane, UCDs with M_V<-12 mag are
consistent with the extrapolated Faber-Jackson relation for luminous
ellipticals, while fainter UCDs are closer to the extrapolated globular cluster
(GC) relation. At a given metallicity, Fornax UCDs have on average 30-40% lower
M/L ratios than Virgo UCDs, suggesting possible differences in age or dark
matter content between Fornax and Virgo UCDs. For our sample of Fornax UCDs we
find no significant correlation between M/L ratio and mass. We combine our data
with available M/L ratio measurements of compact stellar systems with
10^4<M/M_*<10^8, and normalise all M/L estimates to solar metallicity. We find
that UCDs (M > 2*10^6 M_*) have M/L ratios twice as large as GCs (M < 2*10^6
M_*). We show that stellar population models tend to under-predict dynamical
M/L ratios of UCDs and over-predict those of GCs. Considering the scaling
relations of stellar spheroids, UCDs align well along the 'Fundamental
Manifold', constituting the small-scale end of the galaxy sequence. The
alignment for UCDs is especially clear for r_e >~ 7 pc, which corresponds to
dynamical relaxation times that exceed a Hubble time. In contrast, GCs exhibit
a broader scatter and do not appear to align along the manifold. We argue that
UCDs are the smallest dynamically un-relaxed stellar systems, with M > 2*10^6
M_* and 7<r_e<100 pc. Future studies should aim at explaining the elevated M/L
ratios of UCDs and the environmental dependence of their properties.Comment: 17 pages, 14 figures, accepted for publication in A&A. V3 taking into
account proof corrections: Table 3 radial velocity entries corrected by
heliocentric correction, updated sigma entries in Table 5 for a few CenA
sources, updated references for G1 and omega Ce
Evidence for a Young Stellar Population in NGC 5018
Two absorption line indices, Ca II and Hdelta/FeI4045, measured from high
resolution spectra are used with evolutionary synthesis models to verify the
presence of a young stellar population in NGC 5018. The derived age of this
population is about ~2.8 Gyr with a metallicity roughly solar and it completely
dominates the integrated light of the galaxy near 4000 A.Comment: 13 pages, 7 figures (figs 3-7 are color figures), to be published in
the May 2000 issue of the Astrophysical Journa
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