4,952 research outputs found
Group theory in cryptography
This paper is a guide for the pure mathematician who would like to know more
about cryptography based on group theory. The paper gives a brief overview of
the subject, and provides pointers to good textbooks, key research papers and
recent survey papers in the area.Comment: 25 pages References updated, and a few extra references added. Minor
typographical changes. To appear in Proceedings of Groups St Andrews 2009 in
Bath, U
Cryptanalysis of three matrix-based key establishment protocols
We cryptanalyse a matrix-based key transport protocol due to Baumslag, Camps,
Fine, Rosenberger and Xu from 2006. We also cryptanalyse two recently proposed
matrix-based key agreement protocols, due to Habeeb, Kahrobaei and Shpilrain,
and due to Romanczuk and Ustimenko.Comment: 9 page
Obscuring Material around Seyfert Nuclei with Starbursts
The structure of obscuring matter in the environment of active galactic
nuclei with associated nuclear starbursts is investigated using 3-D
hydrodynamical simulations. Simple analytical estimates suggest that the
obscuring matter with energy feedback from supernovae has a torus-like
structure with a radius of several tens of parsecs and a scale height of about
10 pc. These estimates are confirmed by the fully non-linear numerical
simulations, in which the multi-phase inhomogeneous interstellar matter and its
interaction with the supernovae are consistently followed. The globally stable,
torus-like structure is highly inhomogeneous and turbulent. To achieve the high
column densities (> 10^{24} cm^{-2}) as suggested by observations of some
Seyfert 2 galaxies with nuclear starbursts, the viewing angle should be larger
than about 70 degree from the pole-on for a 10^8 solar mass massive black hole.
Due to the inhomogeneous internal structure of the torus, the observed column
density is sensitive to the line-of-sight, and it fluctuates by a factor of
order 100. The covering fraction for N > 10^{23} cm^{-2} is about 0.4. The
average accretion rate toward R < 1 pc is 0.4 solar mass/yr, which is boosted
to twice that in the model without the energy feedback.Comment: ApJL in press (4 pages, 3 figures) A gziped ps file with high
resolution figures is available at http://th.nao.ac.jp/~wada/AGN
Beware of fake AGNs
In the BPT diagram, the distribution of the emission-line galaxies from the
Sloan Digital Sky Survey (SDSS) evokes the wings of a seagull. Traditionally,
galaxies in the right wing are considered to host AGNs. Our study of the
stellar populations of SDSS galaxies showed that about1/4 of galaxies thought
to host LINERS are in fact "retired galaxies", i.e. galaxies that stopped
forming stars and are ionized by hot post-AGB stars and white dwarfs (Stasinska
et al. 2008). When including the galaxies that lack some of the lines needed to
place them in the BPT diagram the fraction of retired galaxies is even larger
(Cid Fernandes et al., 2009, arXiv:0912.1376)Comment: to be published in "Co-evolution of central black holes and galaxies:
feeding and feed-back" Proceedings IAU Symposium No. 267, Peterson, Rachel
Somerville, & Thaisa Storchi-Bergmann ed
BOND: Bayesian Oxygen and Nitrogen abundance Determinations in giant H II regions using strong and semi-strong lines
We present BOND, a Bayesian code to simultaneously derive oxygen and nitrogen
abundances in giant H II regions. It compares observed emission lines to a grid
of photoionization models without assuming any relation between O/H and N/O.
Our grid spans a wide range in O/H, N/O and ionization parameter U, and covers
different starburst ages and nebular geometries. Varying starburst ages
accounts for variations in the ionizing radiation field hardness, which arise
due to the ageing of H II regions or the stochastic sampling of the initial
mass function. All previous approaches assume a strict relation between the
ionizing field and metallicity. The other novelty is extracting information on
the nebular physics from semi-strong emission lines. While strong lines ratios
alone ([O III]/Hbeta, [O II]/Hbeta and [N II]/Hbeta) lead to multiple O/H
solutions, the simultaneous use of [Ar III]/[Ne III] allows one to decide
whether an H II region is of high or low metallicity. Adding He I/Hbeta pins
down the hardness of the radiation field. We apply our method to H II regions
and blue compact dwarf galaxies, and find that the resulting N/O vs O/H
relation is as scattered as the one obtained from the temperature-based method.
As in previous strong-line methods calibrated on photoionization models, the
BOND O/H values are generally higher than temperature-based ones, which might
indicate the presence of temperature fluctuations or kappa distributions in
real nebulae, or a too soft ionizing radiation field in the models.Comment: MNRAS in press; 21 pages, 22 figures, 2 tables; code, data and
results available at http://bond.ufsc.b
A comprehensive classification of galaxies in the SDSS: How to tell true from fake AGN?
We use the W_Ha versus [NII]/Ha (WHAN) diagram to provide a comprehensive
emission-line classification of SDSS galaxies. This classification is able to
cope with the large population of weak line galaxies that do not appear in
traditional diagrams due to a lack of some of the diagnostic lines. A further
advantage of the WHAN diagram is to allow the differentiation between two very
distinct classes that overlap in the LINER region of traditional diagnostic
diagrams. These are galaxies hosting a weakly active nucleus (wAGN) and
"retired galaxies" (RGs), i.e. galaxies that have stopped forming stars and are
ionized by their hot evolved low-mass stars. A useful criterion to distinguish
true from fake AGN (i.e. the RGs) is the ratio (\xi) of the
extinction-corrected L_Ha with respect to the Ha luminosity expected from
photoionization by stellar populations older than 100 Myr. This ratio follows a
markedly bimodal distribution, with a \xi >> 1 population composed by systems
undergoing star-formation and/or nuclear activity, and a peak at \xi ~ 1
corresponding to the prediction of the RG model. We base our classification
scheme on the equivalent width of Ha, an excellent observational proxy for \xi.
Based on the bimodal distribution of W_Ha, we set the division between wAGN and
RGs at W_Ha = 3 A. Five classes of galaxies are identified within the WHAN
diagram: (a) Pure star forming galaxies: log [NII]/Ha 3 A.
(b) Strong AGN (i.e., Seyferts): log [NII]/Ha > -0.4 and W_Ha > 6 A. (c) Weak
AGN: log [NII]/Ha > -0.4 and W_Ha between 3 and 6 A. (d) RGs: W_Ha < 3 A. (e)
Passive galaxies (actually, line-less galaxies): W_Ha and W_[NII] < 0.5 A. A
comparative analysis of star formation histories and of other properties in
these different classes of galaxies corroborates our proposed differentiation
between RGs and weak AGN in the LINER-like family. (Abridged)Comment: Accepted for publication in MNRA
The many faces of LINER-like galaxies: a WISE view
We use the SDSS and WISE surveys to investigate the real nature of galaxies
defined as LINERs in the BPT diagram. After establishing a mid-infrared colour
W2-W3 = 2.5 as the optimal separator between galaxies with and without star
formation, we investigate the loci of different galaxy classes in the W_{Ha}
versus W2-W3 space. We find that: (1) A large fraction of LINER-like galaxies
are emission-line retired galaxies, i.e galaxies which have stopped forming
stars and are powered by hot low-mass evolved stars (HOLMES). Their W2-W3
colours show no sign of star formation and their Ha equivalent widths, W_{Ha},
are consistent with ionization by their old stellar populations. (2) Another
important fraction have W2-W3 indicative of star formation. This includes
objects located in the supposedly `pure AGN' zone of the BPT diagram. (3) A
smaller fraction of LINER-like galaxies have no trace of star formation from
W2-W3 and a high W_{Ha}, pointing to the presence of an AGN. (4) Finally, a few
LINERs tagged as retired by their W_{Ha} but with W2-W3 values indicative of
star formation are late-type galaxies whose SDSS spectra cover only the old
`retired' bulge. This reinforces the view that LINER-like galaxies are a mixed
bag of objects involving different physical phenomena and observational effects
thrusted into the same locus of the BPT diagram.Comment: Accepted for publication in MNRAS; 9 pages, 6 figure
Transition (LINER/HII) nuclei as evolved Composite (Seyfert 2/Starburst) nuclei
We compare the circumnuclear stellar population and environmental properies
of Seyfert and Composite (Seyfert + Starburst) nuclei with those of LINERs and
LINER/HII transition galaxies (TOs), and discuss evidences for evolution from
Seyfert/Composite to LINER/TO nuclei.Comment: 2 pages, 1 figure; to appear in the Proceedings of IAU Symp. No. 222:
The Interplay among Black Holes, Stars and ISM in Galactic Nuclei, CUP, eds.
T. Storchi-Bergmann, L. Ho and H. R. Schmit
Surface-induced near-field scaling in the Knudsen layer of a rarefied gas
We report on experiments performed within the Knudsen boundary layer of a
low-pressure gas. The non-invasive probe we use is a suspended
nano-electro-mechanical string (NEMS), which interacts with He gas at
cryogenic temperatures. When the pressure is decreased, a reduction of the
damping force below molecular friction had been first reported in
Phys. Rev. Lett. Vol 113, 136101 (2014) and never reproduced since. We
demonstrate that this effect is independent of geometry, but dependent on
temperature. Within the framework of kinetic theory, this reduction is
interpreted as a rarefaction phenomenon, carried through the boundary layer by
a deviation from the usual Maxwell-Boltzmann equilibrium distribution induced
by surface scattering. Adsorbed atoms are shown to play a key role in the
process, which explains why room temperature data fail to reproduce it.Comment: Article plus supplementary materia
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