79,014 research outputs found
Short-Timescale monitoring of the X-ray, UV and broad double-peak emission line of the nucleus of NGC 1097
Recent studies have suggested that the short-timescale ( days)
variability of the broad (10,000 km s) double-peaked H
profile of the LINER nucleus of NGC1097 could be driven by a variable X-ray
emission from a central radiatively inefficient accretion flow (RIAF). To test
this scenario, we have monitored the NGC1097 nucleus in X-ray and UV continuum
with Swift and the H flux and profile in the optical spectrum using
SOAR and Gemini-South from 2012 August to 2013 February. During the monitoring
campaign, the H flux remained at a very low level --- 3 times lower
than the maximum flux observed in previous campaigns and showing only limited
() variability. The X-ray variations were small, only
throughout the campaign, while the UV did not show significant variations. We
concluded that the timescale of the H profile variation is close to the
sampling interval of the optical observations, which results in only marginal
correlation between the X-ray and H fluxes. We have caught the AGN in
NGC1097 in a very low activity state, in which the ionizing source was very
weak and capable of ionizing just the innermost part of the gas in the disk.
Nonetheless, the data presented here still support the picture in which the gas
that emits the broad double-peaked Balmer lines is illuminated/ionized by a
source of high-energy photons which is located interior to the inner radius of
the line-emitting part of the disk.Comment: The paper contains 14 pages, 7 figures and is accepted for
publication at the Astrophysical Journa
The star cluster - field star connection in nearby spiral galaxies I. Data analysis techniques and application to NGC 4395
It is generally assumed that a large fraction of stars are initially born in
clusters. However, a large fraction of these disrupt on short timescales and
the stars end up belonging to the field. Understanding this process is of
paramount importance if we wish to constrain the star formation histories of
external galaxies using star clusters. We attempt to understand the relation
between field stars and star clusters by simultaneously studying both in a
number of nearby galaxies. As a pilot study, we present results for the
late-type spiral NGC 4395 using HST/ACS and HST/WFPC2 images. Different
detection criteria were used to distinguish point sources (star candidates) and
extended objects (star cluster candidates). Using a synthetic CMD method, we
estimated the star formation history. Using simple stellar population model
fitting, we calculated the mass and age of the cluster candidates. The field
star formation rate appears to have been roughly constant, or to have possibly
increased by up to about a factor of two, for ages younger than 300 Myr
within the fields covered by our data. Our data do not allow us to constrain
the star formation histories at older ages. We identify a small number of
clusters in both fields. Neither massive ( M) clusters nor
clusters with ages Gyr were found in the galaxy and we found few
clusters older than 100 Myr. Based on our direct comparison of field stars and
clusters in NGC 4395, we estimate the ratio of star formation rate in clusters
that survive for to years to the total star formation to be
. We suggest that this relatively low value is caused
by the low star formation rate of NGC 4395.Comment: 16 pages, 20 figures, accepted for publication in A&
An Optimal Strategy for Accurate Bulge-to-disk Decomposition of Disk Galaxies
The development of two-dimensional (2D) bulge-to-disk decomposition
techniques has shown their advantages over traditional one-dimensional (1D)
techniques, especially for galaxies with non-axisymmetric features. However,
the full potential of 2D techniques has yet to be fully exploited. Secondary
morphological features in nearby disk galaxies, such as bars, lenses, rings,
disk breaks, and spiral arms, are seldom accounted for in 2D image
decompositions, even though some image-fitting codes, such as GALFIT, are
capable of handling them. We present detailed, 2D multi-model and
multi-component decomposition of high-quality -band images of a
representative sample of nearby disk galaxies selected from the Carnegie-Irvine
Galaxy Survey, using the latest version of GALFIT. The sample consists of five
barred and five unbarred galaxies, spanning Hubble types from S0 to Sc.
Traditional 1D decomposition is also presented for comparison. In detailed case
studies of the 10 galaxies, we successfully model the secondary morphological
features. Through a comparison of best-fit parameters obtained from different
input surface brightness models, we identify morphological features that
significantly impact bulge measurements. We show that nuclear and inner
lenses/rings and disk breaks must be properly taken into account to obtain
accurate bulge parameters, whereas outer lenses/rings and spiral arms have a
negligible effect. We provide an optimal strategy to measure bulge parameters
of typical disk galaxies, as well as prescriptions to estimate realistic
uncertainties of them, which will benefit subsequent decomposition of a larger
galaxy sample.Comment: 30 pages, 14 figures, published in ApJ; minor typos correcte
Fuzzy ARTMAP: A Neural Network Architecture for Incremental Supervised Learning of Analog Multidimensional Maps
A new neural network architecture is introduced for incremental supervised learning of recognition categories and multidimensional maps in response to arbitrary sequences of analog or binary input vectors. The architecture, called Fuzzy ARTMAP, achieves a synthesis of fuzzy logic and Adaptive Resonance Theory (ART) neural networks by exploiting a close formal similarity between the computations of fuzzy subsethood and ART category choice, resonance, and learning. Fuzzy ARTMAP also realizes a new Minimax Learning Rule that conjointly minimizes predictive error and maximizes code compression, or generalization. This is achieved by a match tracking process that increases the ART vigilance parameter by the minimum amount needed to correct a predictive error. As a result, the system automatically learns a minimal number of recognition categories, or "hidden units", to met accuracy criteria. Category proliferation is prevented by normalizing input vectors at a preprocessing stage. A normalization procedure called complement coding leads to a symmetric theory in which the MIN operator (Λ) and the MAX operator (v) of fuzzy logic play complementary roles. Complement coding uses on-cells and off-cells to represent the input pattern, and preserves individual feature amplitudes while normalizing the total on-cell/off-cell vector. Learning is stable because all adaptive weights can only decrease in time. Decreasing weights correspond to increasing sizes of category "boxes". Smaller vigilance values lead to larger category boxes. Improved prediction is achieved by training the system several times using different orderings of the input set. This voting strategy can also be used to assign probability estimates to competing predictions given small, noisy, or incomplete training sets. Four classes of simulations illustrate Fuzzy ARTMAP performance as compared to benchmark back propagation and genetic algorithm systems. These simulations include (i) finding points inside vs. outside a circle; (ii) learning to tell two spirals apart; (iii) incremental approximation of a piecewise continuous function; and (iv) a letter recognition database. The Fuzzy ARTMAP system is also compared to Salzberg's NGE system and to Simpson's FMMC system.British Petroleum (89-A-1204); Defense Advanced Research Projects Agency (90-0083); National Science Foundation (IRI 90-00530); Office of Naval Research (N00014-91-J-4100); Air Force Office of Scientific Research (90-0175
The Local Radio-IR Relation in M51
We observed M51 at three frequencies, 1.4 GHz (20 cm), 4.9 GHz (6 cm), and 8.4 GHz (3.6 cm), with the Very Large Array and the Effelsberg 100 m telescope to obtain the highest quality radio continuum images of a nearby spiral galaxy. These radio data were combined with deconvolved Spitzer IRAC 8 μm and MIPS 24 μm images to search for and investigate local changes in the radio-IR correlation. Utilizing wavelet decomposition, we compare the distribution of the radio and IR emission on spatial scales between 200 pc and 30 kpc. We show that the radio-IR correlation is not uniform across the galactic disk. It presents a complex behavior with local extrema corresponding to various galactic structures, such as complexes of H II regions, spiral arms, and interarm filaments, indicating that the contribution of the thermal and non-thermal radio emission is a strong function of environment. In particular, the relation of the 24 μm and 20 cm emission presents a linear relation within the spiral arms and globally over the galaxy, while it deviates from linearity in the interarm and outer regions as well in the inner region, with two different behaviors: it is sublinear in the interarm and outer region and overlinear in the central 3.5 kpc. Our analysis suggests that the changes in the radio/IR correlation reflect variations of interstellar medium properties between spiral arms and interarm region. The good correlation in the spiral arms implies that 24 μm and 20 cm are tracing recent star formation, while a change in the dust opacity, "Cirrus" contribution to the IR emission and/or the relation between the magnetic field strength and the gas density can explain the different relations found in the interarm, outer, and inner regions
IP Pegasi in outburst: Echelle spectroscopy & Modulation Doppler Tomography
We analyse a unique set of time-resolved echelle spectra of the dwarf nova IP
Peg, obtained at ESO's NTT with EMMI. The dataset covers the wavelength range
of 4000-7500A and shows Balmer, HeI, HeII and heavier elements in emission. IP
Peg was observed one day after the peak of an outburst. The trailed spectra,
spectrograms and Doppler maps show characteristics typical of IP Pegasi during
the early stages of its outburst. The high-ionisation line of HeII 4686A is the
most centrally located line and has the greatest radial extension compared to
the HeI lines. The Balmer lines extend from close to the white dwarf up to
approximately 0.45 times R_L, with the outer radius gradually increasing when
moving from H delta to H alpha. The application, for the first time, of the
modulation Doppler tomography technique, maps any harmonically varying
components present in the system configuration. We find, as expected, that part
of the strong secondary star emission in Balmer and HeI lines is modulated
predominantly with the cosine term, consistent with the emission originating
from the irradiated front side of the mass-donor star, facing the accreting
white dwarf. For the Balmer lines the level of the modulation, compared to the
average emission, decreases when moving to higher series. Emission from the
extended accretion disk appears to be only weakly modulated, with amplitudes of
at most a few percent of the non-varying disk emission. We find no evidence of
modulated emission in the spiral arms, which if present, is relatively weak at
that our signal-to-noise ratio was good enough to put a lower detection limit
of any modulated emission at 5--6%. Only in one arm of the HeII 4686A line, is
there a possibility of modulated emission, but again, we cannot be sure this is
not caused by blending with the nearby Bowen complex of lines.Comment: 20 pages, 8 figures, submitted to Journal of Astronomical Dat
Molecular Gas Evolution across a Spiral Arm in M 51
We present sensitive and high angular resolution CO(1-0) data obtained by the
Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations
toward the nearby grand-design spiral galaxy M 51. The angular resolution of
0.7" corresponds to 30 pc, which is similar to the typical size of Giant
Molecular Clouds (GMCs), and the sensitivity is also high enough to detect
typical GMCs. Within the 1' field of view centered on a spiral arm, a number of
GMC-scale structures are detected as clumps. However, only a few clumps are
found to be associated with each Giant Molecular Association (GMA), and more
than 90% of the total flux is resolved out in our data. Considering the high
sensitivity and resolution of our data, these results indicate that GMAs are
not mere confusion of GMCs but plausibly smooth structures. In addition, we
have found that the most massive clumps are located downstream of the spiral
arm, which suggests that they are at a later stage of molecular cloud evolution
across the arm and plausibly are cores of GMAs. By comparing with H-alpha and
Pa-alpha images, most of these cores are found to have nearby star forming
regions. We thus propose an evolutionary scenario for the interstellar medium,
in which smaller molecular clouds collide to form smooth GMAs at spiral arm
regions and then star formation is triggered in the GMA cores. Our new CO data
have revealed the internal structure of GMAs at GMC scales, finding the most
massive substructures on the downstream side of the arm in close association
with the brightest H II regions.Comment: accepted for publication in Ap
Micro & strong lensing with the Square Kilometer Array: The mass--function of compact objects in high--redshift galaxies
We present the results from recent VLA 8.5-GHz and WSRT 1.4 and 4.9-GHz
monitoring campaigns of the CLASS gravitational lens B1600+434 and show how the
observed variations argue strongly in favor of microlensing by MACHOs in the
halo of a dark-matter dominated edge-on disk galaxy at z=0.4. The population of
flat-spectrum radio sources with micro-Jy flux-densities detected with the
Square-Kilometer-Array is expected to have dimensions of micro-arcsec. They
will therefore vary rapidly as a result of Galactic scintillation (diffractive
and refractive). However, when positioned behind distant galaxies they will
also show variations due to microlensing, even more strongly than in the case
of B1600+434. Relativistic or superluminal motion in these background sources
typically leads to temporal variations on time scales of days to weeks.
Scintillation and microlensing can be distinguished, and separated, by their
different characteristic time scales and the frequency dependence of their
modulations. Monitoring studies with Square-Kilometer-Array at GHz frequencies
will thus probe both microscopic and macroscopic properties of dark matter and
its mass-function as a function of redshift, information very hard to obtain by
any other method.Comment: 8 pages, 5 figures, to appear in Perspectives in Radio Astronomy:
Scientific Imperatives at cm and m Wavelengths (Dwingeloo: NFRA), Edited by:
M.P. van Haarlem & J.M. van der Huls
The Secular Evolution of the Primordial Kuiper Belt
A model that computes the secular evolution of a gravitating disk-planet
system is developed. The disk is treated as a set of gravitating rings, with
the rings'/planets' time-evolution governed by the classical Laplace-Lagrange
solution for secular evolution but modified to account for the disk's finite
thickness h. This system's Lagrange planetary equations yield a particular
class of spiral wave solutions, usually denoted as apsidal density waves and
nodal bending waves. There are two varieties of apsidal waves:long waves and
short waves. Planets typically launch long density waves at the disk's nearer
edge or else at a secular resonance in the disk, and these waves ultimately
reflect downstream at a more distant disk edge or else at a Q-barrier in the
disk, whereupon they return as short density waves. Planets also launch nodal
bending waves, and these have the property that they can stall in the disk,
that is, their group velocity plummets to zero upon approaching a disk region
too thick to support the continued propagation of bending waves. The rings
model is used to compute the secular evolution of a Kuiper Belt having a
variety of masses, and it is shown that the early massive Belt was very
susceptible to the propagation of low-amplitude apsidal and nodal waves
launched by the giant planets.Comment: 45 pages, 6 figure
- …