263 research outputs found
Low-mass X-ray binaries and globular clusters streamers and ARCS in NGC 4278
We report significant inhomogeneities in the projected two-dimensional spatial distributions of low-mass X-ray binaries (LMXBs) and globular clusters (GCs) of the intermediate mass elliptical galaxy NGC 4278. In the inner region of NGC 4278, a significant arc-like excess of LMXBs extending south of the center at ∼50″ in the western side of the galaxy can be associated with a similar overdensity of the spatial distribution of red GCs from Brassington et al. Using a recent catalog of GCs produced by Usher et al. and covering the whole field of the NGC 4278 galaxy, we have discovered two other significant density structures outside the D 25 isophote to the W and E of the center of NGC 4278, associated with an overdensity and an underdensity, respectively. We discuss the nature of these structures in the context of the similar spatial inhomogeneities discovered in the LMXBs and GCs populations of NGC 4649 and NGC 4261, respectively. These features suggest streamers from disrupted and accreted dwarf companions.Peer reviewe
AGN activity and the misaligned hot ISM in the compact radio elliptical NGC4278
The analysis of a deep (579 ks) Chandra ACIS pointing of the elliptical
galaxy NGC4278, which hosts a low luminosity AGN and compact radio emission,
allowed us to detect extended emission from hot gas out to a radius of \sim 5
kpc, with a 0.5--8 keV luminosity of 2.4x10^{39} erg/s. The emission is
elongated in the NE-SW direction, misaligned with respect to the stellar body,
and aligned with the ionized gas, and with the Spitzer IRAC 8\mum non-stellar
emission. The nuclear X-ray luminosity decreased by a factor of \sim 18 since
the first Chandra observation in 2005, a dimming that enabled the detection of
hot gas even at the position of the nucleus. Both in the projected and
deprojected profiles, the gas shows a significantly larger temperature (kT=0.75
keV) in the inner \sim 300 pc than in the surrounding region, where it stays at
\sim 0.3 keV, a value lower than expected from standard gas heating
assumptions. The nuclear X-ray emission is consistent with that of a low
radiative efficiency accretion flow, accreting mass at a rate close to the
Bondi one; estimates of the power of the nuclear jets require that the
accretion rate is not largely reduced with respect to the Bondi rate. Among
possibile origins for the central large hot gas temperature, such as
gravitational heating from the central massive black hole and a recent AGN
outburst, the interaction with the nuclear jets seems more likely, especially
if the latter remain confined, and heat the nuclear region frequently. The
unusual hot gas distribution on the galactic scale could be due to the
accreting cold gas triggering the cooling of the hot phase, a process also
contributing to the observed line emission from ionize gas, and to the hot gas
temperature being lower than expected; alternatively, the latter could be due
to an efficiency of the type Ia supernova energy mixing lower than usually
adopted.Comment: 48 pages, submitted to Ap
The Spectral and Temporal Properties of Transient Sources in Early-type Galaxies
Copyright 2012 Elsevier B.V., All rights reserved.We report the spectral and temporal variability properties of 18 candidate transient (TC) and potential transient (PTC) sources detected in deep multi-epoch Chandra observation of the nearby elliptical galaxies, NGC3379, NGC4278, and NGC4697. Only one source can be identified with a background counterpart, leaving 17 TCs + PTCs in the galaxies. Of these, 14 are in the galaxy field, supporting the theoretical picture that the majority of field X-ray binaries (XRBs) will exhibit transient accretion for >75% of their lifetime. Three sources are coincident with globular clusters, including two high-luminosity candidate black hole (BH) XRBs, with Lx= 5.4 × 10 erg/s and Lx= 2.8 × 10 erg/s, respectively. The spectra, luminosities, and temporal behavior of these 17 sources suggest that the transient population is heterogeneous, including neutron star (NS) and BH XRBs in both normal and high-rate accretion modes, and super soft sources containing white dwarf binaries. Our TC and PTC detections are noticeably fewer than the number expected from the population synthesis (PS) models of Fragos et al., tailored to our new Chandra pointings of NGC4278. We attribute this discrepancy to the PS assumption that the transient population is composed of NS XRBs, as well as differences between the statistical analysis and error estimates used in the model and our observationsPeer reviewe
Simulated Galaxy Interactions as Probes of Merger Spectral Energy Distributions
We present the first systematic comparison of ultraviolet-millimeter spectral
energy distributions (SEDs) of observed and simulated interacting galaxies. Our
sample is drawn from the Spitzer Interacting Galaxy Survey, and probes a range
of galaxy interaction parameters. We use 31 galaxies in 14 systems which have
been observed with Herschel, Spitzer, GALEX, and 2MASS. We create a suite of
GADGET-3 hydrodynamic simulations of isolated and interacting galaxies with
stellar masses comparable to those in our sample of interacting galaxies.
Photometry for the simulated systems is then calculated with the SUNRISE
radiative transfer code for comparison with the observed systems. For most of
the observed systems, one or more of the simulated SEDs match reasonably well.
The best matches recover the infrared luminosity and the star formation rate of
the observed systems, and the more massive systems preferentially match SEDs
from simulations of more massive galaxies. The most morphologically distorted
systems in our sample are best matched to simulated SEDs close to coalescence,
while less evolved systems match well with SEDs over a wide range of
interaction stages, suggesting that an SED alone is insufficient to identify
interaction stage except during the most active phases in strongly interacting
systems. This result is supported by our finding that the SEDs calculated for
simulated systems vary little over the interaction sequence.Comment: 24 pages, 16 figures, 2 tables, accepted for publication in ApJ.
Animations of the evolution of the simulated SEDs can be found at
http://www.cfa.harvard.edu/~llanz/sigs_sim.htm
Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291
We present Chandra studies of the X-ray binary (XRB) populations in the bulge
and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources
in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available
Chandra observations totaling an effective exposure of 179 ks. We report
photometric properties of these sources in a point-source catalog. There are
~40% of the bulge sources and ~25% of the ring sources showing >3\sigma
long-term variability in their X-ray count rate. The X-ray colors suggest that
a significant fraction of the bulge (~75%) and ring (~65%) sources are likely
low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate
that it is a low-luminosity AGN with moderate obscuration; spectral variability
is observed between individual observations. We construct 0.3-8.0 keV X-ray
luminosity functions (XLFs) for the bulge and ring XRB populations, taking into
account the detection incompleteness and background AGN contamination. We reach
90% completeness limits of ~1.5\times10^{37} and ~2.2\times10^{37} erg/s for
the bulge and ring populations, respectively. Both XLFs can be fit with a
broken power-law model, and the shapes are consistent with those expected for
populations dominated by LMXBs. We perform detailed population synthesis
modeling of the XRB populations in NGC 1291, which suggests that the observed
combined XLF is dominated by an old LMXB population. We compare the bulge and
ring XRB populations, and argue that the ring XRBs are associated with a
younger stellar population than the bulge sources, based on the relative
overdensity of X-ray sources in the ring, the generally harder X-ray color of
the ring sources, the overabundance of luminous sources in the combined XLF,
and the flatter shape of the ring XLF.Comment: 15 pages, 11 figures. Accepted for publication in Ap
An evaluation of airborne laser scan data for coalmine subsidence mapping
The accurate mapping of coalmine subsidence is necessary for the continued management of potential subsidence impacts. The use of airborne laser scan (ALS) data for subsidence mapping provides an alternative method to traditional ground-based approaches that affords increased accessibility and complete spatial coverage. This paper evaluates the suitability and potential of ALS data for subsidence mapping, primarily through the examination of two pre-mining surveys in a rugged, densely vegetated study site. Data quality, in terms of mean point spacing and coverage, is evaluated, along with the impact of interpolation methods, resolution, and terrain. It was assumed that minimal surface height changes occurred between the two pre-mining surfaces. Therefore any height changes between digital elevation models of the two ALS surveys were interpreted as errors associated with the use of ALS data for subsidence mapping. A mean absolute error of 0.23 m was observed, though this error may be exaggerated by the presence of a systematic 0.15 m offset between the two surveys. Very large (several metres) errors occur in areas of steep or dynamic terrain, such as along cliff lines and watercourses. Despite these errors, preliminary subsidence mapping, performed using a third, post-mining dataset, clearly demonstrates the potential benefits of ALS data for subsidence mapping, as well as some potential limitations and the need for further careful assessment and validation concerning data errors
Deep Chandra Monitoring Observations of NGC 4278 : Catalog of Source Properties
We present the properties of the discrete X-ray sources detected in our monitoring program of the globular cluster (GC) rich elliptical galaxy, NGC 4278, observed with Chandra ACIS-S in six separate pointings, resulting in a co-added exposure of 458 ks. From this deep observation, 236 sources have been detected within the region overlapped by all observations, 180 of which lie within the D 25 ellipse of the galaxy. These 236 sources range in L X from 3.5 × 1036 erg s-1 (with 3σ upper limit 1 × 1038 erg s-1. From X-ray source photometry, it has been determined that the majority of the 236 point sources that have well-constrained colors have values that are consistent with typical low-mass X-ray binary spectra, with 29 of these sources expected to be background objects from the log N-log S relation. There are 103 sources in this population that exhibit long-term variability, indicating that they are accreting compact objects. Three of these sources have been identified as transient candidates, with a further three possible transients. Spectral variations have also been identified in the majority of the source population, where a diverse range of variability has been identified, indicating that there are many different source classes located within this galaxy.Peer reviewe
Verification of the mixed layer depth in the OceanMAPS operational forecast model for Austral autumn
The ocean mixed layer depth is an important parameter
describing the exchange of fluxes between the atmosphere and ocean. In ocean
modelling a key factor in the accurate representation of the mixed layer is
the parameterization of vertical mixing. An ideal opportunity to investigate
the impact of different mixing schemes was provided when the Australian
Bureau of Meteorology upgraded its operational ocean forecasting model,
OceanMAPS to version 3.0. In terms of the mixed layer, the main difference
between the old and new model versions was a change of vertical mixing
scheme from that of Chen et al. (1994) to the General Ocean Turbulence Model.The model estimates of the mixed layer depth were compared with those
derived from Argo observations. Both versions of the model exhibited a deep
bias in tropical latitudes and a shallow bias in the Southern Ocean,
consistent with previous studies. The bias, however, was greatly reduced in
version 3.0, and variance between model runs decreased. Additionally, model
skill against climatology also improved significantly. Further analysis
discounted changes to model resolution outside of the Australian region
having a significant impact on these results, leaving the change in vertical\ud
mixing scheme as the main factor in the assessed improvements to mixed layer
depth representation.</p
3D ensemble-based online oceanic flow field estimation for underwater glider path planning
Estimating ocean flow fields in 3D is a critical step in enabling the
reliable operation of underwater gliders and other small, low-powered
autonomous marine vehicles. Existing methods produce depth-averaged 2D layers
arranged at discrete vertical intervals, but this type of estimation can lead
to severe navigation errors. Based on the observation that real-world ocean
currents exhibit relatively low velocity vertical components, we propose an
accurate 3D estimator that extends our previous work in estimating 2D flow
fields as a linear combination of basis flows. The proposed algorithm uses data
from ensemble forecasting to build a set of 3D basis flows, and then
iteratively updates basis coefficients using point measurements of underwater
currents. We report results from experiments using actual ensemble forecasts
and synthetic measurements to compare the performance of our method to the
direct 3D extension of the previous work. These results show that our method
produces estimates with dramatically lower error metrics, with and without
measurement noise.Comment: Submitted to IROS 202
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