908 research outputs found
Untangling the X-ray Emission From the Sa Galaxy NGC1291 With Chandra
We present a Chandra ACIS-S observation of the nearby bulge-dominated Sa
galaxy NGC1291. The X-ray emission from the bulge resembles the X-ray emission
from a sub-class of elliptical and S0 galaxies with low L_X/L_B luminosity
ratios. The X-ray emission is composed of a central point-like nucleus, ~50
point sources that are most likely low mass X-ray binaries (LMXBs), and diffuse
gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global
temperature of 0.32^{+0.04}_{-0.03} keV and metallicity of 0.06 +/- 0.02 solar,
and both quantities marginally decrease with increasing radius. The hot gas
fills the hole in the HI distribution, and the softening of the spectrum of the
X-ray gas with radius might indicate a thermal coupling of the hot and cold
phases of the interstellar medium as previously suggested. The integrated X-ray
luminosity of the LMXBs, once normalized by the optical luminosity, is a factor
of 1.4 less than in the elliptical galaxy NGC4697 or S0 galaxy NGC1553. The
difference in L_{X,stellar}/L_B between the galaxies appears to be because of a
lack of very bright sources in NGC1291. No sources above 3 x 10^38 ergs/s were
found in NGC1291 when ~7 were expected from scaling from NGC4697 and NGC1553.
The cumulative L_{X,stellar}/L_B value including only sources below 1.0 x 10^38
ergs/s is remarkably similar between NGC1291 and NGC4697, if a recent surface
brightness fluctuation-determined distance is assumed for NGC4697. If this is a
common feature of the LMXB population in early-type systems, it might be used
as a distance indicator. Finally, a bright, variable (1.6-3.1 x 10^39 ergs/s)
source was detected at the optical center of the galaxy. Its spectrum shows
excess soft emission superimposed on a highly absorbed power law component,
similar to what has been found in several other low luminosity AGN (ABRIDGED).Comment: 13 pages in emulateapj5 style with 11 embedded Postscript figures;
minor revisions since last version; accepted by Ap
Simultaneous multi-band detection of Low Surface Brightness galaxies with Markovian modelling
We present an algorithm for the detection of Low Surface Brightness (LSB)
galaxies in images, called MARSIAA (MARkovian Software for Image Analysis in
Astronomy), which is based on multi-scale Markovian modeling. MARSIAA can be
applied simultaneously to different bands. It segments an image into a
user-defined number of classes, according to their surface brightness and
surroundings - typically, one or two classes contain the LSB structures. We
have developed an algorithm, called DetectLSB, which allows the efficient
identification of LSB galaxies from among the candidate sources selected by
MARSIAA. To assess the robustness of our method, the method was applied to a
set of 18 B and I band images (covering 1.3 square degrees in total) of the
Virgo cluster. To further assess the completeness of the results of our method,
both MARSIAA, SExtractor, and DetectLSB were applied to search for (i) mock
Virgo LSB galaxies inserted into a set of deep Next Generation Virgo Survey
(NGVS) gri-band subimages and (ii) Virgo LSB galaxies identified by eye in a
full set of NGVS square degree gri images. MARSIAA/DetectLSB recovered ~20%
more mock LSB galaxies and ~40% more LSB galaxies identified by eye than
SExtractor/DetectLSB. With a 90% fraction of false positives from an entirely
unsupervised pipeline, a completeness of 90% is reached for sources with r_e >
3" at a mean surface brightness level of mu_g=27.7 mag/arcsec^2 and a central
surface brightness of mu^0 g=26.7 mag/arcsec^2. About 10% of the false
positives are artifacts, the rest being background galaxies. We have found our
method to be complementary to the application of matched filters and an
optimized use of SExtractor, and to have the following advantages: it is
scale-free, can be applied simultaneously to several bands, and is well adapted
for crowded regions on the sky.Comment: 39 pages, 18 figures, accepted for publication in A
Parallel Evolution of Quasi-separatrix Layers and Active Region Upflows
Persistent plasma upflows were observed with Hinode's EUV Imaging
Spectrometer (EIS) at the edges of active region (AR) 10978 as it crossed the
solar disk. We analyze the evolution of the photospheric magnetic and velocity
fields of the AR, model its coronal magnetic field, and compute the location of
magnetic null-points and quasi-sepratrix layers (QSLs) searching for the origin
of EIS upflows. Magnetic reconnection at the computed null points cannot
explain all of the observed EIS upflow regions. However, EIS upflows and QSLs
are found to evolve in parallel, both temporarily and spatially. Sections of
two sets of QSLs, called outer and inner, are found associated to EIS upflow
streams having different characteristics. The reconnection process in the outer
QSLs is forced by a large-scale photospheric flow pattern which is present in
the AR for several days. We propose a scenario in which upflows are observed
provided a large enough asymmetry in plasma pressure exists between the
pre-reconnection loops and for as long as a photospheric forcing is at work. A
similar mechanism operates in the inner QSLs, in this case, it is forced by the
emergence and evolution of the bipoles between the two main AR polarities. Our
findings provide strong support to the results from previous individual case
studies investigating the role of magnetic reconnection at QSLs as the origin
of the upflowing plasma. Furthermore, we propose that persistent reconnection
along QSLs does not only drive the EIS upflows, but it is also responsible for
a continuous metric radio noise-storm observed in AR 10978 along its disk
transit by the Nan\c{c}ay Radio Heliograph.Comment: 29 pages, 10 figure
How Can Active Region Plasma Escape into the Solar Wind from below a Closed Helmet Streamer?
Recent studies show that active-region (AR) upflowing plasma, observed by the
EUV-Imaging Spectrometer (EIS), onboard Hinode, can gain access to open
field-lines and be released into the solar wind (SW) via magnetic-interchange
reconnection at magnetic null-points in pseudo-streamer configurations. When
only one bipolar AR is present on the Sun and it is fully covered by the
separatrix of a streamer, such as AR 10978 in December 2007, it seems unlikely
that the upflowing AR plasma can find its way into the slow SW. However,
signatures of plasma with AR composition have been found at 1 AU by Culhane et
al. (2014) apparently originating from the West of AR 10978. We present a
detailed topology analysis of AR 10978 and the surrounding large-scale corona
based on a potential-field source-surface (PFSS) model. Our study shows that it
is possible for the AR plasma to get around the streamer separatrix and be
released into the SW via magnetic reconnection, occurring in at least two main
steps. We analyse data from the Nan\c{c}ay Radioheliograph (NRH) searching for
evidence of the chain of magnetic reconnections proposed. We find a noise storm
above the AR and several varying sources at 150.9 MHz. Their locations suggest
that they could be associated with particles accelerated during the first-step
reconnection process and at a null point well outside of the AR. However, we
find no evidence of the second-step reconnection in the radio data. Our results
demonstrate that even when it appears highly improbable for the AR plasma to
reach the SW, indirect channels involving a sequence of reconnections can make
it possible.Comment: 26 pages, 10 figures. appears in Solar Physics, 201
Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states
We studied the rate of spontaneous emission from colloidal CdSe and CdTe
nanocrystals at room temperature. The decay rate, obtained from luminescence
decay curves, increases with the emission frequency in a supra-linear way. This
dependence is explained by the thermal occupation of dark exciton states at
room temperature, giving rise to a strong attenuation of the rate of emission.
The supra-linear dependence is in agreement with the results of tight-binding
calculations.Comment: 11 page
The Arecibo L-band Feed Array Zone of Avoidance Survey I: Precursor Observations through the Inner and Outer Galaxy
The Arecibo L-band Feed Array (ALFA) is being used to conduct a low-Galactic
latitude survey, to map the distribution of galaxies and large-scale structures
behind the Milky Way through detection of galaxies' neutral hydrogen (HI) 21-cm
emission. This Zone of Avoidance (ZOA) survey finds new HI galaxies which lie
hidden behind the Milky Way, and also provides redshifts for partially-obscured
galaxies known at other wavelengths. Before the commencement of the full
survey, two low-latitude precursor regions were observed, totalling 138 square
degrees, with 72 HI galaxies detected. Detections through the inner Galaxy
generally have no cataloged counterparts in any other waveband, due to the
heavy extinction and stellar confusion. Detections through the outer Galaxy are
more likely to have 2MASS counterparts. We present the results of these
precursor observations, including a catalog of the detected galaxies, with
their HI parameters. The survey sensitivity is well described by a flux- and
linewidth-dependent signal-to-noise ratio of 6.5. ALFA ZOA galaxies which also
have HI measurements in the literature show good agreement between our
measurements and previous work. The inner Galaxy precursor region was chosen to
overlap the HI Parkes Zone of Avoidance Survey so ALFA performance could be
quickly assessed. The outer Galaxy precursor region lies north of the Parkes
sky. Low-latitude large-scale structure in this region is revealed, including
an overdensity of galaxies near l = 183 deg and between 5000 - 6000 km/s in the
ZOA. The full ALFA ZOA survey will be conducted in two phases: a shallow survey
using the observing techniques of the precursor observations, and also a deep
phase with much longer integration time, with thousands of galaxies predicted
for the final catalog.Comment: 26 pages, 7 figures, 2 tables, Astronomical Journal accepte
- …