983 research outputs found
Young Star Clusters in the Dwarf Irregular Galaxy, UGC 7636, Interacting with the Giant Elliptical Galaxy NGC 4472
We present integrated Washington CT1 photometry of 18 bright blue objects
discovered in the dwarf galaxy UGC 7636 which is located 5'.5 southeast of the
giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster.
Several lines of evidence indicate that UGC 7636 is interacting violently with
NGC 4472. These objects are very blue with colors of -0.4 < (C-T1) < 0.6, and
their magnitudes are in the range of 20.6 < T1 < 22.9 mag which corresponds to
absolute magnitudes of -10.6 < M(T1) < -8.3 mag for a distance modulus of
(m-M)o = 31.2. These objects are grouped spatially in three regions: the
central region of UGC 7636, the tidal tail region, and the HI cloud region. No
such objects were found in the counter tail region. It is concluded that these
objects are probably young star clusters which formed < 0.1Gyr ago during the
interaction between UGC 7636 and NGC 4472. Surface photometry of UGC 7636 (r <
83") shows that there is a significant excess of blue light along the tidal
tail region compared with other regions. The star clusters are bluer than the
stellar light in the tidal tail region, indicating that these clusters might
have formed later than most stars in the tidal tail region which were formed
later than most stars in the main body of the galaxy.Comment: 18 pages (AASLaTeX), 6 Postscript figures, Accepted for publication
in the Astronomical Journal, Nov. 30th, 199
Wide Field CCD Surface Photometry of the Giant Elliptical Galaxy NGC 4472 in the Virgo Cluster
We present deep wide field (16'.4 x 16'.4) Washington CT1 CCD surface
photometry of the giant elliptical galaxy NGC 4472, the brightest member in the
Virgo cluster. Our data cover a wider field than any previous CCD photometry as
well as going deeper. Surface brightness profiles of NGC 4472 are not well fit
by a single King model, but they can be fit approximately by two King models:
with separate models for the inner and outer regions. Surface brightness
profiles for the outer region can also be fit approximately by a deVaucouleurs
law. There is clearly a negative color gradient within 3' of NGC 4472, in the
sense that the color gets bluer with increasing radius. The slope of the color
gradient for this region is derived to be = -0.08 mag
arcsec for , which corresponds to a metallicity
gradient of [Fe/H] dex. However, the surface color gets
redder slowly with increasing radius beyond 3'. A comparison of the structural
parameters of NGC 4472 in C and T1 images has shown that there is little
difference in the ellipse shapes between isochromes and isophotes. In addition,
photometric and structural parameters of NGC 4472 have been determined.Comment: 8 pages(mnrasLaTeX), 8 Postscript figures, Accepted for publication
in MNRAS, 2000
Disturbed Fossil Group Galaxy NGC 1132
We have analyzed the Chandra archival data of NGC 1132, a well-known fossil
group, i.e. a system expected to be old and relaxed long after the giant
elliptical galaxy assembly. Instead, the Chandra data reveal that the hot gas
morphology is disturbed and asymmetrical, with a cold front following a
possible bow shock. We discuss possible origins of the disturbed hot halo,
including sloshing by a nearby object, merger, ram pressure by external hotter
gas and nuclear outburst. We consider that the first two mechanisms are likely
explanations for the disturbed hot halo, with a slight preference for a minor
merger with a low impact parameter because of the match with simulations and
previous optical observations. In this case, NGC 1132 may be a rare example of
unusual late mergers seen in recent simulations. Regardless of the origin of
the disturbed hot halo, the paradigm of the fossil system needs to be
reconsidered.Comment: 16 pages, 6 figures, accepted for publication in Ap
A Review of the Proposed KIsi Offset-Secant Method for Size-Insensitive Linear-Elastic Fracture Toughness Evaluation
Recently proposed modifications to ASTM E399 would provide a new size-insensitive approach to analyzing the force-displacement test record. The proposed size-insensitive linear-elastic fracture toughness, KIsi, targets a consistent 0.5mm crack extension for all specimen sizes by using an offset secant that is a function of the specimen ligament length. The KIsi evaluation also removes the Pmax/PQ criterion and increases the allowable specimen deformation. These latter two changes allow more plasticity at the crack tip, prompting the review undertaken in this work to ensure the validity of this new interpretation of the force-displacement curve. This paper provides a brief review of the proposed KIsi methodology and summarizes a finite element study into the effects of increased crack tip plasticity on the method given the allowance for additional specimen deformation. The study has two primary points of investigation: the effect of crack tip plasticity on compliance change in the force-displacement record and the continued validity of linear-elastic fracture mechanics to describe the crack front conditions. The analytical study illustrates that linear-elastic fracture mechanics assumptions remain valid at the increased deformation limit; however, the influence of plasticity on the compliance change in the test record is problematic. A proposed revision to the validity criteria for the KIsi test method is briefly discussed
Changes in brain activity in response to problem solving during the abstinence from online game play
Background and aims: Several studies have suggested that addictive disorders including substance abuse and pathologic gambling might be associated with dysfunction on working memory and prefrontal activity. We hypothesized that excessive online game playing is associated with deficits in prefrontal cortex function and that recovery from excessive online game playing might improve prefrontal cortical activation in response to working memory stimulation. Methods: Thirteen adolescents with excessive online game playing (AEOP) and ten healthy adolescents (HC) agreed to participate in this study. The severity of online game play and playing time were evaluated for a baseline measurement and again following four weeks of treatment. Brain activation in response to working memory tasks (simple and complex calculations) at baseline and subsequent measurements was assessed using BOLD functional magnetic resonance imaging (fMRI). Results: Compared to the HC subjects, the AEOP participants exhibited significantly greater activity in the right middle occipital gyrus, left cerebellum posterior lobe, left premotor cortex and left middle temporal gyrus in response to working memory tasks during baseline measurements. After four weeks of treatment, the AEOP subjects showed increased activity within the right dorsolateral prefrontal cortex and left occipital fusiform gyrus. After four weeks of treatment, changes in the severity of online game playing were negatively correlated with changes in the mean β value of the right dorsolateral prefrontal cortex in response to complex stimulation. Conclusions: We suggest that the effects of online game addiction on working memory may be similar to those observed in patients with substance dependence
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