281 research outputs found
A Bit-String Model for Biological Aging
We present a simple model for biological aging. We studied it through
computer simulations and we have found this model to reflect some features of
real populations.Comment: LaTeX file, 4 PS figures include
Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438
Ram pressure stripping of the multiphase ISM is studied in the perturbed
Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction
~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA
radio continuum observations at 6 and 20 cm are presented. We detect prominent
extraplanar emission to the west of the galactic center, which extends twice as
far as the other tracers of extraplanar material. The spectral index of the
extraplanar emission does not steepen with increasing distance from the galaxy.
This implies in situ re-acceleration of relativistic electrons. The comparison
with multiwavelength observations shows that the magnetic field and the warm
ionized interstellar medium traced by Halpha emission are closely linked. The
kinematics of the northern extraplanar Halpha emission, which is ascribed to
star formation, follow those of the extraplanar CO emission. In the western and
southern extraplanar regions, the Halpha measured velocities are greater than
those of the CO lines. We suggest that the ionized gas of this region is
excited by ram pressure. The spatial and velocity offsets are consistent with a
scenario where the diffuse ionized gas is more efficiently pushed by ram
pressure stripping than the neutral gas. We suggest that the recently found
radio-deficient regions compared to 24 mum emission are due to this difference
in stripping efficiency.Comment: 8 pages, 6 figures, A&A, accepted for publicatio
The magnetic fields of large Virgo cluster spirals: Paper II
The Virgo cluster of galaxies provides excellent conditions for studying
interactions of galaxies with the cluster environment. Both the high-velocity
tidal interactions and effects of ram pressure stripping by the intracluster
gas can be investigated in detail. We extend our systematic search for possible
anomalies in the magnetic field structures of Virgo cluster spirals in order to
characterize a variety of effects and attribute them to different disturbing
agents. Six angularly large Virgo cluster spiral galaxies (NGC4192, NGC4302,
NGC4303, NGC4321, NGC4388, and NGC4535) were targets of a sensitive total power
and polarization study using the 100-m radio telescope in Effelsberg at 4.85GHz
and 8.35GHz (except for NGC4388 observed only at 4.85GHz, and NGC4535 observed
only at 8.35GHz). Magnetic field structures distorted to various extent are
found in all galaxies. Three galaxies (NGC4302, NGC4303, and NGC4321) show some
signs of possible tidal interactions, while NGC4388 and NGC4535 have very
likely experienced strong ram-pressure and shearing effects, respectively,
visible as distortions and asymmetries of polarized intensity distributions. As
in our previous study, even strongly perturbed galaxies closely follow the
radio-far-infrared correlation. In NGC4303 and NGC4321, we observe symmetric
spiral patterns of the magnetic field and in NGC4535 an asymmetric pattern.
Magnetic fields allow us to trace even weak interactions that are difficult to
detect with other observations. Our results show that the degree of distortions
of a galaxy is not a simple function of the distance to the cluster center but
reflects also the history of its interactions. The angle between the velocity
vector and the rotation vector of a galaxy may be a general parameter that
describes the level of distortions of galactic magnetic fields.Comment: 12 pages, 18 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Ram pressure stripping of the multiphase ISM and star formation in the Virgo spiral galaxy NGC 4330
It has been shown that the Virgo spiral galaxy NGC 4330 shows signs of
ongoing ram pressure stripping in multiple wavelengths: at the leading edge of
the interaction, the Halpha and dust extinction curve sharply out of the disk;
on the trailing side, a long Halpha/UV tail has been found which is located
upwind of a long HI tail. We complete the multiwavelength study with IRAM 30m
HERA CO(2-1) and VLA 6 cm radio continuum observations of NGC 4330. The data
are interpreted with the help of a dynamical model including ram pressure and,
for the first time, star formation. Our best-fit model reproduces qualitatively
the observed projected position, radial velocity of the galaxy, the molecular
and atomic gas distribution and velocity field, and the UV distribution in the
region where a gas tail is present. However, the observed red UV color on the
windward side is currently not reproduced by the model. Based on our model, the
galaxy moves to the north and still approaches the cluster center with the
closest approach occurring in ~100 Myr. In contrast to other Virgo spiral
galaxies affected by ram pressure stripping, NGC 4330 does not show an
asymmetric ridge of polarized radio continuum emission. We suggest that this is
due to the relatively slow compression of the ISM and the particular projection
of NGC 4330. The observed offset between the HI and UV tails is well reproduced
by our model. Since collapsing and starforming gas clouds decouple from the ram
pressure wind, the UV-emitting young stars have the angular momentum of the gas
at the time of their creation. On the other hand, the gas is constantly pushed
by ram pressure. The reaction (phase change, star formation) of the multiphase
ISM (molecular, atomic, ionized) to ram pressure is discussed in the framework
of our dynamical model.Comment: 18 pages, 17 figures, accepted by A&
The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals
The influence of the environment on the polarized and total power radio
continuum emission of cluster spiral galaxies is investigated. We present deep
scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo
spiral galaxies. These data are combined with existing optical, HI, and Halpha
data. Ram pressure compression leads to sharp edges of the total power
distribution at one side of the galactic disk. These edges coincide with HI
edges. In edge-on galaxies the extraplanar radio emission can extend further
than the HI emission. In the same galaxies asymmetric gradients in the degree
of polarization give additional information on the ram pressure wind direction.
The local total power emission is not sensitive to the effects of ram pressure.
The radio continuum spectrum might flatten in the compressed region only for
very strong ram pressure. This implies that neither the local star formation
rate nor the turbulent small-scale magnetic field are significantly affected by
ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc)
and is primarily detectable in polarized radio continuum emission.Comment: 16 pages, 10 figures, accepted for publication in A&
The characteristic polarized radio continuum distribution of cluster spiral galaxies
Deep observations of 6cm polarized radio continuum emission of 8 Virgo spiral
galaxies are presented. All galaxies show strongly asymmetric distributions of
polarized intensity with elongated ridges located in the outer galactic disk.
Such features are not found in existing observations of polarized radio
continuum emission of field spiral galaxies, where the distribution of 6cm
polarized intensity is generally relatively symmetric and strongest in the
interarm regions. We therefore conclude that most Virgo spiral galaxies and
most probably the majority of cluster spiral galaxies show asymmetric
distributions of polarized radio continuum emission due to their interaction
with the cluster environment. The polarized continuum emission is sensitive to
compression and shear motions in the plane of the sky and thus contains
important information about velocity distortions caused by these interactions.Comment: 5 pages, 1 figure, A&A accepted as a lette
AGB stars in the Magellanic Clouds. III. The rate of star formation across the SMC
This article compares the Ks magnitude distribution of Small Magellanic Cloud
asymptotic giant branch stars obtained from the DENIS and 2MASS data with
theoretical distributions. Theoretical Ks magnitude distributions have been
constructed using up-to-date stellar evolution calculations for low and
intermediate-mass stars, and in particular for thermally pulsing asymptotic
giant branch stars. Separate fits of the magnitude distributions of carbon- and
oxygen-rich stars allowed us to constrain the metallicity distribution across
the galaxy and its star formation rate. The Small Magellanic Cloud stellar
population is found to be on average 7-9 Gyr old but older stars are present at
its periphery and younger stars are present in the direction of the companion
galaxy the Large Magellanic Cloud. The metallicity distribution traces a
ring-like structure that is more metal rich than the inner region of the
galaxy. The C/M ratio discussed in Paper I is a tracer of the metallicity
distribution only if the underlying stellar population is of intermediate-age.Comment: 8 pages, 7 figures, accepted by A&
AGB stars in the Magellanic Clouds II. The rate of star formation across the LMC
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)This article compares the distribution of Ks magnitudes of Large Magellanic Cloud (LMC) asymptotic giant branch (AGB) stars obtained from the DENIS and 2MASS data with theoretical distributions. These have been constructed using up-to-date stellar evolution calculations for low and intermediate-mass stars, and in particular for thermally pulsing AGB stars. A fit of the magnitude distribution of both carbon- and oxygen-rich AGB stars allowed us to constrain the metallicity distribution across the LMC and its star formation rate (SFR). The LMC stellar population is found to be on average 5 − 6 Gyr old and is consistent with a mean metallicity corresponding to Z = 0.006. These values may however be affected by systematic errors in the underlying stellar models, and by the limited exploration of the possible SFR histories. Instead our method should be particularly useful for detecting variations in the mean metallicity and SFR across the LMC disk. There are well defined regions where both the metallicity and the mean-age of the underlying stellar population span the whole range of grid parameters. The C/M ratio discussed in paper I is a tracer of the metallicity distribution if the underlying stellar population is older than about a few Gyr. A similar study across the Small Magellanic Cloud is given in paper III of this series.Peer reviewe
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