605 research outputs found
A Broad Search for Counterrotating Gas and Stars: Evidence for Mergers and Accretion
We measure the frequency of bulk gas-stellar counterrotation in a sample of
67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative
survey of the local galaxy population down to M_B-15. We detect 4
counterrotators among 17 E/S0's with extended gas emission (24% +8 -6). In
contrast, we find no clear examples of bulk counterrotation among 38 Sa-Sbc
spirals, although one Sa does show peculiar gas kinematics. This result implies
that, at 95% confidence, no more than 8% of Sa-Sbc spirals are bulk
counterrotators. Among types Sc and later, we identify only one possible
counterrotator, a Magellanic irregular. We use these results together with the
physical properties of the counterrotators to constrain possible origins for
this phenomenon.Comment: 19 pages, 4 figures, AJ, accepte
Vacuum energy in the presence of a magnetic string with delta function profile
We present a calculation of the ground state energy of massive spinor fields
and massive scalar fields in the background of an inhomogeneous magnetic string
with potential given by a delta function. The zeta functional regularization is
used and the lowest heat kernel coefficients are calculated. The rest of the
analytical calculation adopts the Jost function formalism. In the numerical
part of the work the renormalized vacuum energy as a function of the radius
of the string is calculated and plotted for various values of the strength of
the potential. The sign of the energy is found to change with the radius. For
both scalar and spinor fields the renormalized energy shows no logarithmic
behaviour in the limit , as was expected from the vanishing of the heat
kernel coefficient , which is not zero for other types of profiles.Comment: 30 pages, 10 figure
Mogelijkheden voor ondersteunende en kleinschalige horeca op agrarische bedrijven binnen bestaande regelgeving
Het platteland trekt steeds meer recreanten en agrarische ondernemers en hun gezinsleden spelen hierop in met onder andere campings, rondleidingen op het bedrijf, verkoop van streekproducten en kleinschalige horeca. Gemeenten kiezen ervoor om ondersteunde horeca te stimuleren omdat deze bijdraagt aan de kwaliteit van het gebied. Diverse gemeenten hebben beleid opgesteld voor ondersteunende en kleinschalige horeca met afgeleide regelgeving, bijvoorbeeld ten behoeve van detailhandel. Dit biedt veel overeenkomsten en aanknopingspunten voor horeca op agrarische bedrijven
Starbursts versus Truncated Star Formation in Nearby Clusters of Galaxies
We present long-slit spectroscopy, B and R bandpass imaging, and 21 cm
observations of a sample of early-type galaxies in nearby clusters which are
known to be either in a star-forming phase or to have had star formation which
recently terminated. From the long-slit spectra, obtained with the Blanco 4-m
telescope, we find that emission lines in the star-forming cluster galaxies are
significantly more centrally concentrated than in a sample of field galaxies.
The broadband imaging reveals that two currently star-forming early-type
galaxies in the Pegasus I cluster have blue nuclei, again indicating that
recent star formation has been concentrated. In contrast, the two galaxies for
which star formation has already ended show no central color gradient. The
Pegasus I galaxy with the most evident signs of ongoing star formation
(NGC7648), exhibits signatures of a tidal encounter. Neutral hydrogen
observations of that galaxy with the Arecibo radiotelescope reveal the presence
of ~4 x 10^8 solar masses of HI. Arecibo observations of other current or
recent star-forming early-type galaxies in Pegasus I indicate smaller amounts
of gas in one of them, and only upper limits in others.Comment: to be published in Astronomical Journa
Radial Color Gradients in K+A Galaxies in Distant Clusters of Galaxies
Galaxies in rich clusters with z 0.3 are observed to have a higher
fraction of photometrically blue galaxies than their nearby counterparts. This
raises the important question of what environmental effects can cause the
termination of star formation between z 0.3 and the present. The star
formation may be truncated due to ram-pressure stripping, or the gas in the
disk may be depleted by an episode of star formation caused by some external
perturbation. To help resolve this issue, surface photometry was carried out
for a total of 70 early-type galaxies in the cluster Cl1358+62, at z
0.33, using two-color images from the Hubble Archive. The galaxies were divided
into two categories based on spectroscopic criteria: 24 are type K+A (e.g.,
strong Balmer lines, with no visible emission lines), while the remaining 46
are in the control sample with normal spectra. Radial color profiles were
produced to see if the K+A galaxies show bluer nuclei in relation to their
surrounding disks. Specifically, a linear gradient was fit to the radial color
profile of each galaxy. We find that the K+A galaxies on average tend to have
slightly bluer gradients towards the center than the normals. A
Kolmogorov-Smirnov two-sample test has been applied to the two sets of color
gradients. The result of the test indicates that there is only a 2%
probability that the K+A and normal samples are drawn from the same parent
distribution. There is a possible complication from a trend in the apparent
magnitude vs. color gradient relation, but overall our results favor the
centralized star formation scenario as an important process in the evolution of
galaxies in dense clusters.Comment: 16 pages, 12 figures, accepted for publication in A
Low concentrations of maternal thyroxin during early gestation:a risk factor of breech presentation?
Breaking up is hard to do: RalA, mitochondrial fission and cancer
The small GTPases RalA and RalB are activated downstream of oncogenic Ras. While activation of RalA is critically important for tumor initiation and growth of Ras-driven cancers, the highly similar small GTPase RalB is implicated in cell survival and metastasis. This difference in function between these two related proteins maps to the C-terminus, a 30 amino acid region that regulates subcellular localization and contains several potential phosphorylation sites. Here we discuss our recent evidence that phosphorylation by the mitotic kinase Aurora A promotes RalA relocalization to mitochondrial membranes, where it recruits the effector RalBP1 and the large dynamin-related GTPase Drp1 to promote mitochondrial fission. As upregulation of both RalA and Aurora A have been observed in human tumors, and phosphorylation of RalA at the site targeted by Aurora A promotes tumorigenesis, it is possible that regulation of mitochondrial fission is one mechanism by which RalA promotes cancer
Color Gradients and Surface Brightness Profiles of Galaxies in the Hubble Deep Field-North
We fit elliptical isophotes to the Hubble Deep Field-North WFPC-2 and NICMOS
data to study the rest-frame UV_{218}-U_{300} color profiles and rest-frame B
surface brightness profiles of 33 intermediate redshift galaxies (0.5 <= z <=
1.2) with I_{814} < 25 and 50 high redshift galaxies (2.0 <= z <= 3.5) with
H_{160}< 27. From the weighted least-squares fit to the color profiles we find
that, at intermediate redshifts, the galaxies possess negative color gradients
indicating a reddening towards the center of the profile similar to local
samples whereas, at high redshifts, the galaxies possess positive color
gradients. This indicates that star formation is more centrally concentrated in
the distant galaxy sample which differs from the prevalent mode of extended
disk star formation that we observe in the local universe. Additionally, we
find that it is critical to correct for PSF effects when evaluating the surface
brightness profiles since at small scale lengths and faint magnitudes, an
r^{1/4} profile can be smoothed out substantially to become consistent with an
exponential profile. After correcting for PSF effects, we find that at higher
look-back time, the fraction of galaxies possessing exponential profiles have
slightly decreased while the fraction of galaxies possessing r^{1/4} profiles
have slightly increased. Our results also suggest a statistically insignificant
increase in the fraction of peculiar/irregular type galaxies. We compare our
results with recent semi-analytical models which treat galaxy formation and
evolution following the cold dark matter hierarchical framework.Comment: 31 pages, 10 JPEG figures. To be published in AJ Vol. 124, October
200
Galactic bulge formation as a maximum intensity starburst
Properties of normal galactic star formation, including the density
dependence, threshold density, turbulent scaling relations, and clustering
properties, are applied to the formation of galactic bulges. One important
difference is that the bulge potential well is too deep to have allowed
self-regulation or blow-out by the pressures from young stars, unlike galactic
disks or dwarf galaxies. As a result, bulge formation should have been at the
maximum rate, which is such that most of the gas would get converted into stars
in only a few dynamical time scales, or ~10^8 years. The gas accretion phase
can be longer than this, but once the critical density is reached, which
depends primarily on the total virial density from dark matter, the formation
of stars in the bulge should have been extremely rapid. Such three-dimensional
star formation should also have formed many clusters, like normal disk star
formation today. Some of these clusters may have survived as old globulars, but
most got dispersed, although they might still be observable as concentrated
streams in phase space.Comment: 10 pages, 1 figure, scheduled for ApJ, vol. 517, May 20, 199
Natuurorganisaties en hun achterban: doelgroep of partner?
Veel Nederlanders dragen bij aan natuur en landschap. Natuurorganisaties zoeken meer particuliere bijdragen. Het Planbureau voor de Leefomgeving liet onderzoek doen naar de relatie natuurorganisaties en burgers. Uit het rapport âEnergiek aan de slag: natuur- en landschapsorganisaties en hun achterbanâ blijkt dat de achterban op veel manieren bijdraagt, maar dat natuurorganisaties deze vaker als doelgroep ziet die bijdraagt aan hun beoogde activiteiten dan als partner die zelf wensen en oplossingen voorstelt
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