472 research outputs found
Galaxy Formation by Galactic Magnetic Fields
Galaxies exhibit a sequence of various morphological types, i.e., the Hubble
sequence, and they are basically composed of spheroidal components (elliptical
galaxies and bulges in spiral galaxies) and disks. It is known that spheroidal
components are found only in relatively massive galaxies with M=10^{10-12}
M_sun, and all stellar populations in them are very old, but there is no clear
explanation for these facts. Here we present a speculative scenario for the
origin of the Hubble sequence, in which magnetic fields ubiquitously seen in
galaxies have played a crucial role. We first start from a strange
observational fact that magnetic field strengths observed in spiral galaxies
sharply concentrate at a few microgauss, for a wide range of galaxy luminosity
and types. We then argue that this fact and the observed correlation between
star formation activity and magnetic field strength in spiral galaxies suggest
that spheroidal galaxies have formed by starbursts induced by strong magnetic
fields. Then we show that this idea naturally leads to the formation of
spheroidal systems only in massive and high-redshift objects in hierarchically
clustering universe, giving a simple explanation for various observations.Comment: 7 pages including 2 figures. Accepted by ApJ Letter
Forming Young Bulges within Existing Disks: Statistical Evidence for External Drivers
Contrary to traditional models of galaxy formation, recent observations
suggest that some bulges form within preexisting disk galaxies. Such late-epoch
bulge formation within disks seems to be linked to disk gas inflow and central
star formation, caused by either internal secular processes or galaxy mergers
and interactions. We identify a population of galaxies likely to be
experiencing active bulge growth within disks, using the criterion that the
color within the half-light radius is bluer than the outer disk color. Such
blue-centered galaxies make up >10% of star-forming disk galaxies within the
Nearby Field Galaxy Survey, a broad survey designed to represent the natural
diversity of the low-z galaxy population over a wide range of luminosities and
environments. Blue-centered galaxies correlate at 99% confidence with
morphological peculiarities suggestive of minor mergers and interactions. From
this and other evidence, we argue that external drivers rather than internal
secular processes probably account for the majority of blue-centered galaxies.
We go on to discuss quantitative plausibility arguments indicating that
blue-centered evolutionary phases may represent an important mode of bulge
growth for most disk galaxies, leading to significant changes in bulge-to-disk
ratio without destroying disks. If this view is correct, bulge growth within
disks may be a natural consequence of the repeated galaxy mergers and
interactions inherent in hierarchical galaxy formation.Comment: 18 pages including 12 figures, AJ, accepte
Additional modifications to the Blumgart pancreaticojejunostomy: Results of a propensity score-matched analysis versus Cattel-Warren pancreaticojejunostomy
Background: Postoperative pancreatic fistula continues to occur frequently after pancreatoduodenectomy. Methods: We have described a modification of the Blumgart pancreaticojejunostomy. The modification of the Blumgart pancreaticojejunostomy was compared to the Cattel-Warren pancreaticojejunostomy in cohorts of patients matched by propensity scores based on factors predictive of clinically relevant postoperative pancreatic fistula, which was the primary endpoint of this study. Based on a noninferiority study design, 95 open pancreatoduodenectomies per group were needed. Feasibility of the modification of the Blumgart pancreaticojejunostomy in robotic pancreatoduodenectomy was also shown. All pancreaticojejunostomies were performed by a single surgeon. Results: Between October 2011 and May 2019, there were 415 pancreatoduodenectomies with either a Cattel-Warren pancreaticojejunostomy (n = 225) or a modification of the Blumgart pancreaticojejunostomy (n = 190). There was 1 grade C postoperative pancreatic fistula in 190 consecutive modification of the Blumgart pancreaticojejunostomies (0.5%). Logistic regression analysis showed that the rate of clinically relevant postoperative pancreatic fistula was not affected by consecutive case number. After exclusion of robotic pancreatoduodenectomies (the Cattel-Warren pancreaticojejunostomy: 82; modification of the Blumgart pancreaticojejunostomy: 66), 267 open pancreatoduodenectomies were left, among which the matching process identified 109 pairs. The modification of the Blumgart pancreaticojejunostomy was shown to be noninferior to the Cattel-Warren pancreaticojejunostomy with respect to clinically relevant postoperative pancreatic fistula (11.9% vs 22.9%; odds ratio: 0.46 [0.21–0.93]; P = .03), grade B postoperative pancreatic fistula (11.9% vs 18.3%; P = .18), and grade C postoperative pancreatic fistula (0 vs 4.6%; P = .05) as well as to all secondary study endpoints. The modification of the Blumgart pancreaticojejunostomy was feasible in 66 robotic pancreatoduodenectomies. In this subgroup with 1 conversion to open surgery (1.5%), a clinically relevant postoperative pancreatic fistula occurred after 9 procedures (13.6%) with no case of grade C postoperative pancreatic fistula and a 90-day mortality of 3%. Conclusion: The modification of the Blumgart pancreaticojejunostomy described herein is noninferior to the Cattel-Warren pancreaticojejunostomy in open pancreatoduodenectomy. This technique is also feasible in robotic pancreatoduodenectomy
An Imaging and Spectroscopic Study of the z=3.38639 Damped Lyman Alpha System in Q0201+1120: Clues to Star Formation Rate at High Redshift
We present the results of a series of imaging and spectroscopic observations
aimed at identifying and studying the galaxy responsible for the z = 3.38639
damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part
of a concentration of matter which includes at least four galaxies (probably
many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber
may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up
spectroscopy is still required for positive identification. The gas is
turbulent, with many absorption components distributed over approximately 270
km/s and a large spin temperature, T_s greater than 4000K. The metallicity is
relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From
consideration of the relative ratios of elements which have different
nucleosynthetic timescales, it would appear that the last major episode of star
formation in this DLA occurred at z greater than 4.3, more than approximately
500 Myr prior to the time when we observe it.Comment: Accepted for publication in Ap
Properties of Galaxies in and around Voids
Two surveys for intrinsically faint galaxies towards nearby voids have been
conducted at the MPI f\"ur Astronomie, Heidelberg. One selected targets from a
new diameter limited () catalog with morphological criteria while
the other used digitized objective prism Schmidt plates to select mainly HII
dwarf galaxies. For some 450 galaxies, redshifts and other optical data were
obtained. We studied the spatial distribution of the sample objects, their
luminosity function, and their intrinsic properties. Most of the galaxies
belong to already well known sheets and filaments. But we found about a dozen
highly isolated galaxies in each sample (nearest neighborhood distance ). These tend to populate additional structures and are not
distributed homogeneously throughout the voids. As our results on 'void
galaxies' still suffer from small sample statistics, I also tried to combine
similar existing surveys of nearby voids to get further hints on the larger
structure and on the luminosity function of the isolated galaxies. No
differences in the luminosity function of sheet and void galaxies could be
found. The optical and infrared properties of both samples are in the normal
range for samples dominated by late-type dwarfs. Follow-up HI studies show that
the isolated dwarfs in both samples have unusual high amount of neutral gas for
a given luminosity.Comment: 10 pages, 4 figures, latex, to appear in the proceedings of the
'Ringberg workshop on Large Scale Structure', hold Sep. 23-28, 199
Is the Butcher-Oemler effect a function of the cluster redshift ?
Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles,
size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies.
The cluster x-ray size, as measured by the Petrosian r_{\eta=2} radius, does
not change with redshift and is independent from x-ray luminosity. On the other
hand, the x-ray luminosity increases with redshift. Considering that fair
samples show no-evolution, or negative luminosity evolution, we conclude that
the BO sample is not formed from the same class of objects observed at
different look-back times. This is in conflict with the usual interpretation of
the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on
the assumption that we are comparing the same class of objects at different
redshifts. Other trends present in the BO sample reflect selection criteria
rather than differences in look-back time, as independently confirmed by the
fact that trends loose strength when we enlarge the sample with x-ray selected
sample of clusters. The variety of optical sizes and shapes of the clusters in
the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for
these selection effects that mimic the trends usually interpreted as changes
due to evolution.Comment: ApJ, in press, scheduled on May, 10 issue. 17 pages & 11 figure
AGN Jet-induced Feedback in Galaxies. II. Galaxy colours from a multicloud simulation
We study the feedback from an AGN on stellar formation within its host
galaxy, mainly using one high resolution numerical simulation of the jet
propagation within the interstellar medium of an early-type galaxy. In
particular, we show that in a realistic simulation where the jet propagates
into a two-phase ISM, star formation can initially be slightly enhanced and
then, on timescales of few million years, rapidly quenched, as a consequence
both of the high temperatures attained and of the reduction of cloud mass
(mainly due to Kelvin-Helmholtz instabilities). We then introduce a model of
(prevalently) {\em negative} AGN feedback, where an exponentially declining
star formation is quenched, on a very short time scale, at a time t_AGN, due to
AGN feedback. Using the Bruzual & Charlot (2003) population synthesis model and
our star formation history, we predict galaxy colours from this model and match
them to a sample of nearby early-type galaxies showing signs of recent episodes
of star formation (Kaviraj et al. 2007). We find that the quantity t_gal -
t_AGN, where t_gal is the galaxy age, is an excellent indicator of the presence
of feedback processes, and peaks significantly around t_gal - t_AGN \approx
0.85 Gyr for our sample, consistent with feedback from recent energy injection
by AGNs in relatively bright (M_{B} \lsim -19) and massive nearby early-type
galaxies. Galaxies that have experienced this recent feedback show an
enhancement of 3 magnitudes in NUV(GALEX)-g, with respect to the unperturbed,
no-feedback evolution. Hence they can be easily identified in large combined
near UV-optical surveys.Comment: 18 pages, 16 figures, accepted for publication on MNRAS. This version
includes revisions after the referee's repor
Nonthermal Emission from Accreting and Merging Clusters of Galaxies
We compare the nonthermal emission from clusters of galaxies undergoing minor
mergers (``accreting'' clusters) and major mergers (``merging'' clusters). For
accreting clusters, the radial distribution of the nonthermal emission in the
clusters is also calculated. The relativistic electrons, which are the origin
of the nonthermal radiation through inverse Compton (IC) and synchrotron
mission, are assumed to be accelerated at shocks produced by accretion or
mergers. We estimate the typical accretion rate and merger probability
according to a hierarchical clustering model. We show that the total luminosity
of IC emission from accreting and merging clusters are similar. On the other
hand, the luminosity of synchrotron radio emission of the former is much
smaller than that of the latter. We show that about 10% of clusters at z~0
should have hard X-ray and radio nonthermal emissions due to their last major
merger, which are comparable to or dominate those due to ongoing accretion.
Moreover, 20-40% of clusters should have significant EUV emission due to their
last merger. We also investigate the case where the criterion of mergers is
relaxed. If we extend the definition of a merger to an increase in the mass of
the larger subcluster by at least 10% of its initial mass, about 20-30% of
clusters at z~0 should have hard X-ray and radio nonthermal emissions due to
the merger even in a low density universe. We compare the results with
observations. We find that the observed EUV emission from clusters is not
attributed to accretion. If the diffuse radio emission observed in clusters is
synchrotron emission from electrons accelerated via accretion or merging, the
magnetic fields of clusters are generally as small as ~0.1 mu G.Comment: 33 pages, 18 figures, accepted by Ap
On the Origins of Starburst and Post-Starburst Galaxies in Nearby Clusters
HST WFPC2 images in B (F450W) and I (F814W) have been obtained for three
starburst (SB) and two post-starburst (PSB) galaxies in the Coma cluster, and
for three such galaxies in the cluster DC2048-52. V (F555W) and I images for an
additional PSB galaxy in Coma have been extracted from the archive. Seven of
these galaxies were previously classified as E/S0 on the basis of ground-based
images, one as Sa, and the other as an irregular.
The HST images reveal these SB/PSB galaxies to be heterogeneous in
morphology. Nevertheless a common theme is that many of them, especially the SB
galaxies, tend to have centralized spiral structure that appears simply as a
bright ``bulge''on ground-based images. In addition, while some PSB galaxies
exhibit distinct spiral structure, on the whole they have smoother morphologies
than the SB galaxies. The morphologies and luminosity profiles are generally
consistent with substantial starbursts in the form of centralized spiral
structure (the SB galaxies) which fade into smoother morphologies (the PSB
galaxies), with lingering spectroscopic evidence for past central starbursts.
An important point is that the PSB galaxies retain disks, i.e, they have not
evolved into spheroidal systems.Comment: 32 pages, 10 figures including 3 jpg images. To appear in the January
1999 Astronomical Journa
Environmental Effects on Evolution of Cluster Galaxies in a LCDM Universe
We investigate environmental effects on evolution of bright cluster galaxies
in a dominated cold dark matter universe using a combination of
dissipationless N-body simulations and a semi-analytic galaxy formation model.
The N-body simulations enable us to calculate orbits of galaxies in simulated
clusters. Therefore we can incorporate stripping of cold gas from galactic
disks by ram pressure from intracluster medium into our model. In this paper we
study how ram pressure stripping (RPS) and small starburst induced by a minor
merger affect colors, star formation rates, and morphologies of cluster
galaxies. We find that the RPS is not important for colors and SFRs of galaxies
in the cluster core if star formation time-scale is properly chosen, because
the star formation is sufficiently suppressed by consumption of the cold gas in
the disks. Then observed color and SFR gradients can be reproduced without the
RPS. The small starburst triggered by a minor merger hardly affects the SFRs
and colors of the galaxies as well. We also examine whether these two processes
can resolve the known problem that the major merger-driven bulge formation
scenario predict too few galaxies of intermediate bulge-to-total luminosity
ratio (B/T) in clusters. When the minor burst is taken into account, the
intermediate B/T population is increased and the observed morphology gradients
in clusters are successfully reproduced. When the minor burst is considered,
the RPS also plays an important role in formation of the intermediate B/T
galaxies. We present redshift evolution of morphological fractions predicted by
our models. The predicted number ratios of the intermediate B/T galaxies to the
bulge-dominated galaxies show nearly flat or slightly increasing trends with
increasing redshift.Comment: 12 pages, 9 figures, accepted for publication in ApJ (v587 n2 April
20, 2003
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