Assessing the effect of geographically correlated failures on interconnected power-communication networks

We study the reliability of power transmission networks under regional disasters. Initially, we quantify the effect of large-scale non-targeted disasters and their resulting cascade effects on power networks. We then model the dependence of data networks on the power systems and consider network reliability in this dependent network setting. Our novel approach provides a promising new direction for modeling and designing networks to lessen the effects of geographical disasters.National Science Foundation (U.S.). (grant CNS-1017800)National Science Foundation (U.S.). (grant CNS-0830961)United States. Defense Threat Reduction Agency (HDTRA-09-1-005 )United States. Defense Threat Reduction Agency (HDTRA-1-13-10021

Network Reliability under Random Circular Cuts

Optical fiber networks consist of fibers that are laid out along physical terrestrial paths. As such, they are vulnerable to geographical physical failures, such as earthquakes and Electromagnetic Pulse (EMP) attacks. Moreover, such disasters can lead to multiple, geographically correlated, failures on the fiber network. Thus, the geographical layout of the fiber infrastructure has a critical impact on the robustness of the network in the face of such geographical physical failures. In this paper, we develop tools to analyze network connectivity after a `random' geographic disaster. The random location of the disaster allows us to model situations where the physical failures are not targeted attacks. In particular, we consider disasters that take the form of a `randomly' located disk in a plane. Using results from geometric probability, we are able to approximate some network performance metrics to such a disaster in polynomial time. We present some numerical results that make clear geographically correlated failures are fundamentally different from independent failures and then discuss network design in the context of random disk-cuts.National Science Foundation (U.S.) (Grant CNS-0830961)National Science Foundation (U.S.) (Grant CNS-1017800)United States. Defense Threat Reduction Agency (Grant HDTRA1-07-1-0004)United States. Defense Threat Reduction Agency (Grant HDTRA-09-1-005

An Upper Limit on the Mass of a Central Black Hole in the Large Magellanic Cloud from the Stellar Rotation Field

We constrain the possible presence of a central black hole (BH) in the center of the Large Magellanic Cloud (LMC). This requires spectroscopic measurements over an area of order a square degree, due to the poorly known position of the kinematic center. Such measurements are now possible with the impressive field of view of the Multi Unit Spectroscopic Explorer (MUSE) on the ESO Very Large Telescope. We used the Calcium Triplet (~850nm) spectral lines in many short-exposure MUSE pointings to create a two-dimensional integrated-light line-of-sight velocity map from the ~$10^8$ individual spectra, taking care to identify and remove Galactic foreground populations. The data reveal a clear velocity gradient at an unprecedented spatial resolution of 1 arcmin$^{2}$. We fit kinematic models to arrive at a $3\sigma$ upper-mass-limit of $10^{7.1}$ M$_{Sun}$ for any central BH - consistent with the known scaling relations for supermassive black holes and their host systems. This adds to the growing body of knowledge on the presence of BHs in low-mass and dwarf galaxies, and their scaling relations with host-galaxy properties, which can shed light on theories of BH growth and host system interaction.Comment: 12 pages, 11 figures, 1 table, ApJ - in pres

KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars

Knowing the metallicity distribution of stars in the Galactic Centre has important implications for the formation history of the Milky Way nuclear star cluster. However, this distribution is not well known, and is currently based on a small sample of fewer than 100 stars. We obtained near-infrared K-band spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way nuclear star cluster with the integral-field spectrograph KMOS (VLT). We analyse the medium-resolution spectra using a full-spectral fitting method employing the G\"ottingen Spectral library of synthetic PHOENIX spectra. The derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over the entire observed field. The origin of metal-poor stars remains unclear. They could originate from infalling globular clusters. For the metal-rich stellar population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there is only a very low fraction of metal-poor stars in the central 4 pc^2 of the Galactic Centre, we believe that our data can discard a scenario in which the Milky Way nuclear star cluster is purely formed from infalling globular clusters.Comment: 18 pages, 9 Figures, accepted for publication in MNRA

VLT Diffraction Limited Imaging and Spectroscopy in the NIR: Weighing the black hole in Centaurus A with NACO

We present high spatial resolution near-infrared spectra and images of the nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. The adaptive optics corrected data have a spatial resolution of 0.06" (FWHM) in K- and 0.11" in H-band, four times higher than previous studies. The observed gas motions suggest a kinematically hot disk which is orbiting a central object and is oriented nearly perpendicular to the nuclear jet. We model the central rotation and velocity dispersion curves of the [FeII] gas orbiting in the combined potential of the stellar mass and the (dominant) black hole. Our physically most plausible model, a dynamically hot and geometrically thin gas disk, yields a black hole mass of M_bh = (6.1 +0.6/-0.8) 10^7 M_sun. As the physical state of the gas is not well understood, we also consider two limiting cases: first a cold disk model, which completely neglects the velocity dispersion; it yields an M_bh estimate that is almost two times lower. The other extreme case is to model a spherical gas distribution in hydrostatic equilibrium through Jeans equation. Compared to the hot disk model the best-fit black hole mass increases by a factor of 1.5. This wide mass range spanned by the limiting cases shows how important the gas physics is even for high resolution data. Our overall best-fitting black hole mass is a factor of 2-4 lower than previous measurements. With our revised M_bh estimate, Cen A's offset from the M_bh-sigma relation is significantly reduced; it falls above this relation by a factor of ~2, which is close to the intrinsic scatter of this relation. (Abridged)Comment: 12 pages, 14 figures, including minor changes following the referee report; accepted for publication in The Astrophysical Journa

KMOS view of the Galactic Centre I. Young stars are centrally concentrated

The Galactic centre hosts a crowded, dense nuclear star cluster with a half-light radius of 4 pc. Most of the stars in the Galactic centre are cool late-type stars, but there are also >100 hot early-type stars in the central parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the number and distribution of early-type stars in the Galactic centre is incomplete. Only a few spectroscopic observations have been made beyond a projected distance of 0.5 pc of the Galactic centre. The distribution and kinematics of early-type stars are essential to understand the formation and growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic centre using the integral-field spectrograph KMOS. We extracted more than 1,000 spectra from individual stars and identified early-type stars based on their spectra. Our data set contains 114 bright early-type stars: 6 have narrow emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and 76 are O/B type stars. Our wide-field spectroscopic data confirm that the distribution of young stars is compact, with 90% of the young stars identified within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large radii. We estimate photometric masses of the O/B stars and show that the total mass in the young population is >12,000M_sun. The O/B stars all appear to be bound to the Milky Way nuclear star cluster, while less than 30% belong to the clockwise rotating disk. The central concentration of the early-type stars is a strong argument that they have formed in situ. A large part of the young O/B stars is not on the disk, which either means that the early-type stars did not all form on the same disk or that the disk is dissolving rapidly. [abridged]Comment: 27 pages, 17 figures, matches journal version: Corrected typos, corrected Notes in Table B.