9,286 research outputs found
Regression Depth and Center Points
We show that, for any set of n points in d dimensions, there exists a
hyperplane with regression depth at least ceiling(n/(d+1)). as had been
conjectured by Rousseeuw and Hubert. Dually, for any arrangement of n
hyperplanes in d dimensions there exists a point that cannot escape to infinity
without crossing at least ceiling(n/(d+1)) hyperplanes. We also apply our
approach to related questions on the existence of partitions of the data into
subsets such that a common plane has nonzero regression depth in each subset,
and to the computational complexity of regression depth problems.Comment: 14 pages, 3 figure
Half-Megasecond Chandra Spectral Imaging of the Hot Circumgalactic Nebula around Quasar Mrk 231
A deep 400-ksec ACIS-S observation of the nearest quasar known, Mrk 231, is
combined with archival 120-ksec data obtained with the same instrument and
setup to carry out the first ever spatially resolved spectral analysis of a hot
X-ray emitting circumgalactic nebula around a quasar. The 65 x 50 kpc X-ray
nebula shares no resemblance with the tidal debris seen at optical wavelengths.
One notable exception is the small tidal arc 3.5 kpc south of the nucleus where
excess soft X-ray continuum emission and Si XIII 1.8 keV line emission are
detected, consistent with star formation and its associated alpha-element
enhancement, respectively. An X-ray shadow is also detected at the location of
the 15-kpc northern tidal tail. The hard X-ray continuum emission within 6 kpc
of the center is consistent with being due entirely to the bright central AGN.
The soft X-ray spectrum of the outer (>6 kpc) portion of the nebula is best
described as the sum of two thermal components with T~3 and ~8 million K and
spatially uniform super-solar alpha element abundances, relative to iron. This
result implies enhanced star formation activity over ~10^8 yrs accompanied with
redistribution of the metals on large scale. The low-temperature thermal
component is not present within 6 kpc of the nucleus, suggesting extra heating
in this region from the circumnuclear starburst, the central quasar, or the
wide-angle quasar-driven outflow identified from optical IFU spectroscopy on a
scale of >3 kpc. Significant azimuthal variations in the soft X-ray intensity
are detected in the inner region where the outflow is present. The soft X-ray
emission is weaker in the western quadrant, coincident with a deficit of Halpha
and some of the largest columns of neutral gas outflowing from the nucleus.
Shocks created by the interaction of the wind with the ambient ISM may heat the
gas to high temperatures at this location. (abridged)Comment: 43 pages, 11 figures, accepted for publication in the Astrophysical
Journa
A Chandra X-Ray Survey of Ultraluminous Infrared Galaxies
We present results from Chandra observations of 14 ultraluminous infrared
galaxies (ULIRGs; log(L_IR/L_Sun) >= 12) with redshifts between 0.04 and 0.16.
The goals of the observations were to investigate any correlation between
infrared color or luminosity and the properties of the X-ray emission and to
attempt to determine whether these objects are powered by starbursts or active
galactic nuclei (AGNs). The sample contains approximately the same number of
high and low luminosity objects and ``warm'' and ``cool'' ULIRGs. All 14
galaxies were detected by Chandra. Our analysis shows that the X-ray emission
of the two Seyfert 1 galaxies in our sample are dominated by AGN. The remaining
12 sources are too faint for conventional spectral fitting to be applicable.
Hardness ratios were used to estimate the spectral properties of these faint
sources. The photon indices for our sample plus the Chandra-observed sample
from Ptak et al.(2003) peak in the range of 1.0-1.5, consistent with
expectations for X-ray binaries in a starburst, an absorbed AGN, or hot
bremsstrahlung from a starburst or AGN. The values of photon index for the
objects in our sample classified as Seyferts (type 1 or 2) are larger than 2,
while those classified as HII regions or LINERs tend to be less than 2. The
hard X-ray to far-infrared ratios for the 12 weak sources are similar to those
of starbursts, but we cannot rule out the possibility of absorbed, possibly
Compton-thick, AGNs in some of these objects. Two of these faint sources were
found to have X-ray counterparts to their double optical and infrared nuclei.Comment: 40 pages, 5 tables, 14 figures, accepted by Ap
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Mechanism of Action of p-Hydroxybenzoate Hydrozylase from Pseudomonas putida. III. The Enzyme-Subtrate Complex
Detecting and Characterizing Small Dense Bipartite-like Subgraphs by the Bipartiteness Ratio Measure
We study the problem of finding and characterizing subgraphs with small
\textit{bipartiteness ratio}. We give a bicriteria approximation algorithm
\verb|SwpDB| such that if there exists a subset of volume at most and
bipartiteness ratio , then for any , it finds a set
of volume at most and bipartiteness ratio at most
. By combining a truncation operation, we give a local
algorithm \verb|LocDB|, which has asymptotically the same approximation
guarantee as the algorithm \verb|SwpDB| on both the volume and bipartiteness
ratio of the output set, and runs in time
, independent of the size of the
graph. Finally, we give a spectral characterization of the small dense
bipartite-like subgraphs by using the th \textit{largest} eigenvalue of the
Laplacian of the graph.Comment: 17 pages; ISAAC 201
Chandra Observations of Galaxy Zoo Mergers: Frequency of Binary Active Nuclei in Massive Mergers
We present the results from a Chandra pilot study of 12 massive galaxy
mergers selected from Galaxy Zoo. The sample includes major mergers down to a
host galaxy mass of 10 that already have optical AGN
signatures in at least one of the progenitors. We find that the coincidences of
optically selected active nuclei with mildly obscured ( cm) X-ray nuclei are relatively common (8/12), but the
detections are too faint ( counts per nucleus; erg s cm) to reliably separate starburst and
nuclear activity as the origin of the X-ray emission. Only one merger is found
to have confirmed binary X-ray nuclei, though the X-ray emission from its
southern nucleus could be due solely to star formation. Thus, the occurrences
of binary AGN in these mergers are rare (0-8%), unless most merger-induced
active nuclei are very heavily obscured or Compton thick.Comment: 8 pages, including 5 figures and 1 table. Accepted by Ap
Search for Intrinsic Excitations in 152Sm
The 685 keV excitation energy of the first excited 0+ state in 152Sm makes it
an attractive candidate to explore expected two-phonon excitations at low
energy. Multiple-step Coulomb excitation and inelastic neutron scattering
studies of 152Sm are used to probe the E2 collectivity of excited 0+ states in
this "soft" nucleus and the results are compared with model predictions. No
candidates for two-phonon K=0+ quadrupole vibrational states are found. A 2+,
K=2 state with strong E2 decay to the first excited K=0+ band and a probable 3+
band member are established.Comment: 4 pages, 6 figures, accepted for publication as a Rapid Communication
in Physical Review
Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)
Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101ฬ
4) cleavage surface (from natural โIceland Sparโ) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved โmirrorโ surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 ฮผm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (ยฑ 1.3) ร 10โ4 m sโ1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites
XMM-Newton Detection of a Compton-thick AGN in the 1-Jy ULIRG/LINER F04103-2838
We report on the detection of Fe Kalpha emission in F041032838, an
ultraluminous infrared galaxy (ULIRG; log[L/L] 12)
that is optically classified as a LINER. Previous {\it Chandra} observations
suggested the presence of both a starburst and an AGN in this source. A deeper
(20 ksec) {\it XMM-Newton} spectrum reveals an Fe Kalpha line at rest
frame energy 6.4 keV, consistent with cold neutral iron. The best-fit
spectral model indicates the Fe Kalpha line has an equivalent width of
1.6 keV. The hard X-ray emission is dominated by a Compton-thick AGN with
intrinsic 0.2--10 keV luminosity ergs s, while the soft
X-ray emission is from 0.1 keV gas attributed to the starburst. The X-ray
spectrum of this source bears a striking resemblance to that of the archetypal
luminous infrared galaxy NGC 6240 despite differences in merger state and
infrared properties.Comment: 23 pre-print pages including 5 figures and 1 table. Accepted by Ap
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