1,129 research outputs found
LINERs as Low-Luminosity Active Galactic Nuclei
Many nearby galaxies contain optical signatures of nuclear activity in the
form of LINER nuclei. LINERs may be the weakest and most common manifestation
of the quasar phenomenon. The physical origin of this class of objects,
however, has been ambiguous. I draw upon a number of recent observations to
argue that a significant fraction of LINERs are low-luminosity active galactic
nuclei.Comment: Invited review to appear in The 32nd COSPAR Meeting, The AGN-Galaxy
Connection (Advances in Space Research). LaTex, 10 pages including embedded
figure
Phase Statistics of Soliton
The characteristic function of soliton phase jitter is found analytically
when the soliton is perturbed by amplifier noise. In additional to that from
amplitude jitter, the nonlinear phase noise due to frequency and timing jitter
is also analyzed. Because the nonlinear phase noise is not Gaussian
distributed, the overall phase jitter is also non-Gaussian. For a fixed mean
nonlinear phase shift, the contribution of nonlinear phase noise from frequency
and timing jitter decreases with distance and signal-to-noise ratio.Comment: 8 pages, submitted to JOSA
The Narrow-Line Regions of LINERs as Resolved with the Hubble Space Telescope
LINERs exist in the nuclei of a large fraction of luminous galaxies, but
their connection with the AGN phenomenon has remained elusive. We present
Hubble Space Telescope narrowband [O III]5007 and H-alpha+[N II] emission-line
images of the central regions of 14 galaxies with LINER nuclei. The compact, ~1
arcsec-scale, unresolved emission that dominates the line flux in ground-based
observations is mostly resolved by HST. The bulk of this emission comes from
regions with sizes of tens to hundreds of parsecs that are resolved into knots,
filaments, and diffuse gas whose morphology differs from galaxy to galaxy. Most
of the galaxies do not show clear linear structures or ionization cones
analogous to those often seen in Seyfert galaxies. An exception is NGC 1052,
the prototypical LINER, in which we find a 3 arcsec-long (~ 250 pc) biconical
structure that is oriented on the sky along the galaxy's radio jet axis. Seven
of the galaxies have been shown in previously published HST images to have a
bright compact ultraviolet nuclear source, while the other seven do not have a
central UV source. Our images find evidence of dust in the nuclear regions of
all 14 galaxies, with clear indications of nuclear obscuration in most of the
"UV-dark" cases. The data suggest that the line-emitting gas in most LINERs is
photoionized by a central source (which may be stellar, nonstellar, or a
combination thereof) but that this source is often hidden from direct view. We
find no obvious morphological differences between LINERs with detected weak
broad H-alpha wings in their spectra and those with only narrow lines.
Likewise, there is no clear morphological distinction between objects whose UV
spectra are dominated by hot stars (e.g., NGC 4569) and those that are AGN-like
(e.g., NGC 4579).Comment: Accepted for publication in the ApJ. 25 pages, 3 tables, 9 JPEG
Figure
X-ray Properties of the Weak Seyfert 1 Nucleus in NGC 4639
We obtained observations of NGC 4639 with ASCA in order to investigate its
mildly active Seyfert 1 nucleus at hard X-ray energies. Koratkar et al. (1995)
have previously shown that the nucleus is a pointlike source in the ROSAT soft
X-ray band. We detected in the 2-10 keV band a compact central source with a
luminosity of 8.3E+40 erg/s. Comparison of the ASCA data with archival data
taken with the Einstein and ROSAT satellites shows that the nucleus varies on
timescales of months to years. The variability could be intrinsic, or it could
be caused by variable absorption. More rapid variability, on a timescale of
\~10^4 s, may be present in the ASCA data. The spectrum from 0.5 to 10 keV is
well described by a model consisting of a lightly absorbed (N_H = 7.3E+20
cm^-2) power law with a photon index of 1.68. We find no evidence for
significant emission from a thermal plasma; if present, it can account for no
more than 25% of the flux in the 0.5-2.0 keV band. The limited photon
statistics of our data do not allow us to place significant limits on the
presence of iron K emission. (abridged)Comment: To appear in The Astrophysical Journal. LaTex, 18 pages including
embedded figures and table
ASCA Observations of "Type 2" LINERs: Evidence for a Stellar Source of Ionization
We present ASCA observations of LINERs without broad H emission in
their optical spectra. The sample of "type 2" LINERs consists of NGC 404, 4111,
4192, 4457, and 4569. We have detected X-ray emission from all the objects
except for NGC 404; among the detected objects are two so-called transition
objects (NGC 4192 and NGC 4569), which have been postulated to be composite
nuclei having both an HII region and a LINER component. The images of NGC 4111
and NGC 4569 in the soft (0.5-2 keV) and hard (2-7 keV) X-ray bands are
extended on scales of several kpc. The X-ray spectra of NGC 4111, NGC 4457 and
NGC 4569 are well fitted by a two-component model that consists of soft thermal
emission with keV and a hard component represented by a power law
(photon index 2) or by thermal bremsstrahlung emission ( several
keV). The extended hard X-rays probably come from discrete sources, while the
soft emission most likely originates from hot gas produced by active star
formation in the host galaxy. We have found no clear evidence for the presence
of active galactic nuclei (AGNs) in the sample. If an AGN component is the
primary ionization source of the optical emission lines, then it must be
heavily obscured with a column density significantly larger than
cm. Alternatively, the optical emission could be ionized by a population
of exceptionally hot stars.Comment: 13 pages, 5 figures, emulateapj.sty, Accepted for publication in the
Astrophysical Journa
Bistability in Apoptosis by Receptor Clustering
Apoptosis is a highly regulated cell death mechanism involved in many
physiological processes. A key component of extrinsically activated apoptosis
is the death receptor Fas, which, on binding to its cognate ligand FasL,
oligomerize to form the death-inducing signaling complex. Motivated by recent
experimental data, we propose a mathematical model of death ligand-receptor
dynamics where FasL acts as a clustering agent for Fas, which form locally
stable signaling platforms through proximity-induced receptor interactions.
Significantly, the model exhibits hysteresis, providing an upstream mechanism
for bistability and robustness. At low receptor concentrations, the bistability
is contingent on the trimerism of FasL. Moreover, irreversible bistability,
representing a committed cell death decision, emerges at high concentrations,
which may be achieved through receptor pre-association or localization onto
membrane lipid rafts. Thus, our model provides a novel theory for these
observed biological phenomena within the unified context of bistability.
Importantly, as Fas interactions initiate the extrinsic apoptotic pathway, our
model also suggests a mechanism by which cells may function as bistable
life/death switches independently of any such dynamics in their downstream
components. Our results highlight the role of death receptors in deciding cell
fate and add to the signal processing capabilities attributed to receptor
clustering.Comment: Accepted by PLoS Comput Bio
Nuclear and global X-ray properties of LINER galaxies: Chandra and BeppoSAX results for Sombrero and NGC 4736
We report on the 0.1--100 keV BeppoSAX observations of two nearby LINER
galaxies, Sombrero and NGC 4736. Chandra ACIS-S observations supplement this
broad-beam spectral study with a high resolution look into the nuclear region,
and show a dominating central point source in Sombrero and a complex X-ray
binary dominated/starburst region in NGC 4736. A compact non-thermal radio
source, present in the nucleus of both galaxies, coincides with the central
source in Sombrero, while in NGC 4736 its X-ray counterpart is a much fainter
point source, not the brightest of the central region. On the basis of these
and other results, we conclude that the LINER activity is linked to the
presence of a low luminosity AGN in Sombrero and to a recent starburst in NGC
4736, and that Chandra's spectroscopic capabilities coupled to high resolution
imaging are essential to establish the origin of the nuclear activity.Comment: 13 pages, 12 figures (Fig. 4 and Fig. 5 are colour figures), to be
published in Astronomy & Astrophysic
Mutations of the BRAF gene in human cancer
Cancers arise owing to the accumulation of mutations in critical genes that alter normal programmes of cell proliferation, differentiation and death. As the first stage of a systematic genome-wide screen for these genes, we have prioritized for analysis signalling pathways in which at least one gene is mutated in human cancer. The RAS RAF MEK ERK MAP kinase pathway mediates cellular responses to growth signals. RAS is mutated to an oncogenic form in about 15% of human cancer. The three RAF genes code for cytoplasmic serine/threonine kinases that are regulated by binding RAS. Here we report BRAF somatic missense mutations in 66% of malignant melanomas and at lower frequency in a wide range of human cancers. All mutations are within the kinase domain, with a single substitution (V599E) accounting for 80%. Mutated BRAF proteins have elevated kinase activity and are transforming in NIH3T3 cells. Furthermore, RAS function is not required for the growth of cancer cell lines with the V599E mutation. As BRAF is a serine/threonine kinase that is commonly activated by somatic point mutation in human cancer, it may provide new therapeutic opportunities in malignant melanoma
Sixteen Cases of Sclerosing Hemangioma of the Lung Including Unusual Presentations
Sclerosing hemangiomas (SH) of the lung are uncommon tumors and are thought to be benign. However, the biologic behavior of this tumor has not yet been characterized adequately. The clinicopathologic features were reviewed and analyzed for 16 cases of SH. The age of the patients ranged from 37 to 73 yr (mean 50.6 yr). There were fifteen female and one male patient. The SH located at the intraparenchyme in 14 cases, the interlobar fissure in one case and the visceral pleura in one case. The size of SH ranged from 0.3 cm to 8 cm (mean 2.6 cm). There were five unusual presentations of SH including a case having two SH with multiple nodules of atypical adenomatous hyperplasia in the same lobe, a case showing adenocarcinoma-like area within the SH, a case showing one peribronchial lymph node metastasis (N1 nodal stage) with location of interlobar major fissure, a case showing alveolar adenoma-like area within the SH, and one case with a large visceral pleural-based pedunculated mass presenting as mediastinal mass. All patients were alive and well without recurrence at the last follow up. Here, we reviewed previously published literatures and discussed the histogenesis of SH
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