832 research outputs found
The impact of giving feedback in online discussions: effects of evaluative reply comments on the authors of evaluated user comments
Abstract. In online discussions, users often evaluate comments from other users. On the basis of face theory, the present study analyzed the effects of evaluative replies on the evaluated comment authors. The investigation complements existing research, which has mainly focused on effects of comments on uninvolved readers. In the experimental study presented here, disapproving evaluations provoked negative and less positive emotions, and the evaluated authors were less willing to participate in the online discussion further. The authors’ perception of face threat mediated these effects. The results contribute to face theory in computer-mediated interactions and to our understanding of online discussions with dissonant standpoints
The flattening and the orbital structure of early-type galaxies and collisionless N-body binary disk mergers
We use oblate axisymmetric dynamical models including dark halos to determine
the orbital structure of intermediate mass to massive Coma early-type galaxies.
We find a large variety of orbital compositions. Averaged over all sample
galaxies the unordered stellar kinetic energy in the azimuthal and the radial
direction are of the same order, but they can differ by up to 40 percent in
individual systems. In contrast, both for rotating and non-rotating galaxies
the vertical kinetic energy is on average smaller than in the other two
directions. This implies that even most of the rotating ellipticals are
flattened by an anisotropy in the stellar velocity dispersions. Using
three-integral axisymmetric toy models we show that flattening by stellar
anisotropy maximises the entropy for a given density distribution.
Collisionless disk merger remnants are radially anisotropic. The apparent lack
of strong radial anisotropy in observed early-type galaxies implies that they
may not have formed from mergers of disks unless the influence of dissipational
processes was significant.Comment: 14 pages, 8 figures; accepted for publication in MNRA
Monster black holes
A combination of ground-based and spacecraft observations has uncovered two
black holes of 10 billion solar masses in the nearby Universe. The finding
sheds light on how these cosmic monsters co-evolve with galaxies.Comment: 2 pages, 1 figure, LaTeX. Published in Nature "News & Views
Simulating magnetic fields in the Antennae galaxies
We present self-consistent high-resolution simulations of NGC4038/4039 (the
"Antennae galaxies") including star formation, supernova feedback and magnetic
fields performed with the N-body/SPH code Gadget, in which magnetohydrodynamics
are followed with the SPH method. We vary the initial magnetic field in the
progenitor disks from 1 nG to 100 muG. At the time of the best match with the
central region of the Antennae system the magnetic field has been amplified by
compression and shear flows to an equilibrium field of approximately 10 muG,
independent of the initial seed field. These simulations are a proof of the
principle that galaxy mergers are efficient drivers for the cosmic evolution of
magnetic fields. We present a detailed analysis of the magnetic field structure
in the central overlap region. Simulated radio and polarization maps are in
good morphological and quantitative agreement with the observations. In
particular, the two cores with the highest synchrotron intensity and ridges of
regular magnetic fields between the cores and at the root of the southern tidal
arm develop naturally in our simulations. This indicates that the simulations
are capable of realistically following the evolution of the magnetic fields in
a highly non-linear environment. We also discuss the relevance of the
amplification effect for present day magnetic fields in the context of
hierarchical structure formation.Comment: 18 pages, 14 figures, accepte
Probing for evolutionary links between local ULIRGs and QSOs from NIR spectroscopy
We present a study of the dynamical evolution of Ultraluminous Infrared
Galaxies (ULIRGs), merging galaxies of infrared luminosity >10^12 L_sun. During
our Very Large Telescope large program, we have obtained ISAAC near-infrared,
high-resolution spectra of 54 ULIRGs (at several merger phases) and 12 local
Palomar-Green QSOs to investigate whether ULIRGs go through a QSO phase during
their evolution. One possible evolutionary scenario is that after nuclear
coalescence, the black hole radiates close to Eddington to produce QSO
luminosities. The mean stellar velocity dispersion that we measure from our
spectra is similar (~160 km/s) for 30 post-coalescence ULIRGs and 7 IR-bright
QSOs. The black holes in both populations have masses of order 10^7-10^8 M_sun
(calculated from the relation to the host dispersion) and accrete at rates >0.5
Eddington. Placing ULIRGs and IR-bright QSOs on the fundamental plane of
early-type galaxies shows that they are located on a similar region (that of
moderate-mass ellipticals), in contrast to giant ellipticals and radio-loud
QSOs. While this preliminary comparison of the ULIRG and QSO host kinematical
properties indicates that (some) ULIRGs may undergo a QSO phase in their
evolutionary history before they settle down as ellipticals, further data on
non-IR excess QSOs are necessary to test this scenario.Comment: To appear in the "QSO Host Galaxies: Evolution and Environment"
conference proceedings; meeting held in Leiden, August 200
Early-type galaxies at large galactocentric radii - I. Stellar kinematics and photometric properties
We present the results of a combined analysis of the kinematic and
photometric properties at large galactocentric radii of a sample of 14
low-luminosity early-type galaxies in the Fornax and Virgo clusters. From
Gemini South GMOS long-slit spectroscopic data we measure radial profiles of
the kinematic parameters v_{rot}, sigma, h_{3}, and h_{4} out to ~ 1 - 3
effective radii. Multi-band imaging data from the HST/ACS are employed to
evaluate surface brightness profiles and isophotal shape parameters of
ellipticity, position angle and discyness/boxiness. The galaxies are found to
host a cold and old stellar component which extend to the largest observed
radii and that is the dominant source of their dynamical support. The
prevalence of discy-shaped isophotes and the radial variation of their
ellipticity are signatures of a gradual gas dissipation. An early star-forming
collapse appears to be the main mechanism acting in the formation of these
objects. Major mergers are unlikely to have occurred in these galaxies. We can
not rule out a minor merging origin for these galaxies, but a comparison of our
results with model predictions of different merger categories places some
constraints on the possible merger progenitors. These merger events are
required to happen at high-redshift (i.e., z > 1), between progenitors of
different mass ratio (at least 3:1) and containing a significant amount of gas
(i.e., > 10 percent). A further scenario is that the low-luminosity galaxies
were originally late-type galaxies, whose star formation has been truncated by
removal of gas and subsequently the disc has been dynamically heated by high
speed encounters in the cluster environment.Comment: 19 pages, 16 figures (Contact author for high resolution version of
figures 1,2,3), MNRAS, accepted. The second paper of the series "Early-type
galaxies at large galactocentric radii - II. Metallicity gradients and the
[Z/H]-mass, [alpha/Fe] mass relations" can be found at arXiv:1006.169
Electron-capture branch of 100Tc and tests of nuclear wave functions for double-beta decays
We present a measurement of the electron-capture branch of Tc. Our
value, , implies that the
Mo neutrino absorption cross section to the ground state of Tc
is roughly one third larger than previously thought. Compared to previous
measurements, our value of prevents a smaller disagreement with
QRPA calculations relevant to double- decay matrix elements
Magnetic field structure due to the global velocity field in spiral galaxies
We present a set of global, self-consistent N-body/SPH simulations of the
dynamic evolution of galactic discs with gas and including magnetic fields. We
have implemented a description to follow the evolution of magnetic fields with
the ideal induction equation in the SPH part of the Vine code. Results from a
direct implementation of the field equations are compared to a representation
by Euler potentials, which pose a div(B)-free description, an constraint not
fulfilled for the direct implementation. All simulations are compared to an
implementation of magnetic fields in the Gadget code which includes also
cleaning methods for div(B).
Starting with a homogeneous seed field we find that by differential rotation
and spiral structure formation of the disc the field is amplified by one order
of magnitude within five rotation periods of the disc. The amplification is
stronger for higher numerical resolution. Moreover, we find a tight connection
of the magnetic field structure to the density pattern of the galaxy in our
simulations, with the magnetic field lines being aligned with the developing
spiral pattern of the gas. Our simulations clearly show the importance of
non-axisymmetry for the evolution of the magnetic field.Comment: 17 pages, 18 figure
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