517,617 research outputs found
Morphologically-Identified Merging Galaxies in the SWIRE Fields
We investigate the evolutional and environmental effects on star formation
efficiency for more than 400 merging galaxies. The ~400 merging systems, with
photometric redshifts smaller than 0.7, are obtained from a catalog of ~15000
morphologically identified merging galaxies derived from observations of the
Canada-France-Hawaii Telescope. We also obtained the IR data of the merging
galaxies from the Spitzer Wide-area InfraRed Extragalactic Survey (SWIRE). The
redshift differences \Delta z between the member galaxies of these merging
pairs show a large distribution with 0 < \Delta z < 0.4. We divide our merging
pairs into two sub-samples with \Delta z 0.05 for further
analyses. We find a statistically significant anti-correlation between the
specific star formation rate (SSFR) and the separation of the merging galaxies
for both sub-samples. Our analyses also show that although most of the merging
systems do have enhanced star formation activity, only very rare ones display
extremely high SFRs. Additionally, the SSFR of the merging galaxies also
decreases when the magnitude difference between two member galaxies becomes
large. However, we find that for the merging pairs with large luminosity
contrast, the fainter components show higher SSFR than the brighter ones.
Finally, there is a higher fraction of gas-poor mergers in galaxy clusters, and
the SSFR of gas-rich mergers is reduced in cluster environments.Comment: 32 pages, 12 figures and 7 tables; accepted for publication in Ap
Automatic Image Segmentation by Dynamic Region Merging
This paper addresses the automatic image segmentation problem in a region
merging style. With an initially over-segmented image, in which the many
regions (or super-pixels) with homogeneous color are detected, image
segmentation is performed by iteratively merging the regions according to a
statistical test. There are two essential issues in a region merging algorithm:
order of merging and the stopping criterion. In the proposed algorithm, these
two issues are solved by a novel predicate, which is defined by the sequential
probability ratio test (SPRT) and the maximum likelihood criterion. Starting
from an over-segmented image, neighboring regions are progressively merged if
there is an evidence for merging according to this predicate. We show that the
merging order follows the principle of dynamic programming. This formulates
image segmentation as an inference problem, where the final segmentation is
established based on the observed image. We also prove that the produced
segmentation satisfies certain global properties. In addition, a faster
algorithm is developed to accelerate the region merging process, which
maintains a nearest neighbor graph in each iteration. Experiments on real
natural images are conducted to demonstrate the performance of the proposed
dynamic region merging algorithm.Comment: 28 pages. This paper is under review in IEEE TI
Auto-dual connected operators based on iterative merging algorithms
This paper proposes a new set of connected operators that are autodual. Classical connected operators are analyzed within the framework of merging algorithms. The discussion highlights three basic notions: merging order , merging criterion and region model. As a result a general merging algorithm is proposed. It can be used to create new connected operators and in particular autodual operators. Implementation issues are also discussed.Peer ReviewedPostprint (published version
The collaborative effect of ram pressure and merging on star formation and stripping fraction
Aims: We investigate the effect of ram pressure stripping (RPS) on
simulations of merging pairs of gas-rich spiral galaxies. Our goal is to
provide an estimate of the combined effect of merging and RPS on stripping
efficiency and star formation rate.
Methods: We make use of the combined N-body/hydrodynamic code GADGET-2. In
our simulations, we vary mass ratios between 1:4 and 1:8 in a binary merger. We
sample different geometric configurations of the merging systems (edge-on and
face-on mergers, different impact parameters). Furthermore, we vary the
properties of the intracluster medium (ICM) in rough steps: The speed of the
merging system relative to the ICM between 500 and 1000 km/s, the ICM density
between and g/cm, and the ICM direction relative to
the mergers' orbital plane. Ram pressure is kept constant within a simulation
time period, as is the ICM temperature of K. Each simulation in the ICM
is compared to simulations of the merger in vacuum and the non-merging galaxies
with acting ram pressure.
Results: Averaged over the simulation time (1 Gyr) the merging pairs show a
negligible 5% enhancement in SFR, when compared to single galaxies under the
same environmental conditions. The SFRs peak at the time of the galaxies first
fly-through. There, our simulations show SFRs of up to 20 M/yr
(compared to 3 M/yr of the non-merging galaxies in vacuum). In the
most extreme case, this constitutes a short-term ( Myr) SFR increase of
50% over the non-merging galaxies experiencing ram pressure. The wake of
merging galaxies in the ICM typically has a third to half the star mass seen in
the non-merging galaxies and 5% to 10 % less gas mass. The joint effect of RPS
and merging, according to our simulations, is not significantly different from
pure ram pressure effects.Comment: 12 pages, 13 figures, 3 tables, A&
Galaxy merging in MOND
We present the results of N-body simulations of dissipationless galaxy
merging in Modified Newtonian Dynamics (MOND). For comparison, we also studied
Newtonian merging between galaxies embedded in dark matter halos, with internal
dynamics equivalent to the MOND systems. We found that the merging timescales
are significantly longer in MOND than in Newtonian gravity with dark matter,
suggesting that observational evidence of rapid merging could be difficult to
explain in MOND. However, when two galaxies eventually merge, the MOND merging
end-product is hardly distinguishable from the final stellar distribution of an
equivalent Newtonian merger with dark matter.Comment: 5 pages, 2 color figures. To appear in MNRAS Letters. Added
references and discussion, conclusions unchange
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