Present-day stress orientations and tectonic provinces of the NW Borneo collisional margin

Extent: 15p.Borehole failure observed on image and dipmeter logs from 55 petroleum wells across the NW Borneo collisional margin were used to determine maximum horizontal stress (σH) orientations; combined with seismic and outcrop data, they define seven tectonic provinces. The Baram Delta–Deepwater Fold-Thrust Belt exhibits three tectonic provinces: its inner shelf inverted province (σH is NW-SE, margin-normal), its outer shelf extension province (σH is NE-SW, margin-parallel), and its slope to basin floor compression province (σH is NW-SE, margin-normal). In the inverted province, σH reflects inversion of deltaic normal faults. The σH orientations in the extension and compression provinces reflect deltaic gravitational tectonics. The shale and minibasin provinces have been recognized in offshore Sabah. In the shale province, σH is N010°E, which aligns around the boundary of a massif of mobile shale. Currently, no data are available to determine σH in the minibasin province. In the Balingian province, σH is ESE-WNW, reflecting ESE absolute Sunda plate motions due to the absence of a thick detachment seen elsewhere in NW Borneo. The Central Luconia province demonstrates poorly constrained and variable σH orientations. These seven provinces result from the heterogeneous structural and stratigraphic development of the NW Borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages.Rosalind C. King, Mark R. P. Tingay, Richard R. Hillis, Christopher K. Morley, and James Clar

A VLBA search for binary black holes in active galactic nuclei with double-peaked optical emission line spectra

We have examined a subset of 11 active galactic nuclei (AGN) drawn from a sample of 87 objects that possess double-peaked optical emission line spectra, as put forward by Wang et al. (2009a) and are detectable in the FIRST survey at radio wavelengths. The double-peaked nature of the optical emission line spectra has been suggested as evidence for the existence of binary black holes in these AGN, although this interpretation is controversial. We make a simple suggestion, that direct evidence of binary black holes in these objects could be searched for in the form of dual sources of compact radio emission associated with the AGN. To explore this idea, we have used the Very Long Baseline Array to observe these 11 objects from the Wang et al. (2009a) sample. Of the 11 objects, we detect compact radio emission from two, SDSS J151709+335324 and SDSS J160024+264035. Both objects show single components of compact radio emission. The morphology of SDSS J151709+335324 is consistent with a recent comprehensive multi-wavelength study of this object by Rosario et al. (2010). Assuming that the entire sample consists of binary black holes, we would expect of order one double radio core to be detected, based on radio wavelength detection rates from FIRST and VLBI surveys. We have not detected any double cores, thus this work does not substantially support the idea that AGN with double-peaked optical emission lines contain binary black holes. However, the study of larger samples should be undertaken to provide a more secure statistical result, given the estimated detection rates.Comment: 14 pages, 3 figures. To appear in A

Parsec-scale shocks in the kiloparsec-scale jet of Centaurus A

High angular resolution VLBI observations of Centaurus A have been undertaken that allow access to a wide field-of-view, encompassing both the well-studied pc-scale jet and the inner part of the kpc-scale jet. The VLBI observations have detected compact regions of synchrotron emission in the kpc-scale jet that coincide with three stationary features identified from previous VLA monitoring observations. Each of these stationary features is associated with strong localised X-ray emission. The VLBI results strengthen arguments made by previous authors suggesting that the stationary features may be the result of stellar objects or gas clouds traversing the jet flow, intercepting the jet and causing strong shocks. The VLBI data show that the most strongly shocked regions in these features are resolved but have extents no larger than a few pc, reducing the required mass of the typical intercepting object by a factor of ~10 relative to previous estimates, making explanations based on high mass loss stars or low density gas clouds more plausible.Comment: Submitted to the Astronomical Journal. 18 pages, 3 figure

Absorption variability as a probe of the multiphase interstellar media surrounding active galaxies

© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. We examine a model for the variable free-free and neutral hydrogen absorption inferred towards the cores of some compact radio galaxies in which a spatially fluctuating medium drifts in front of the source. We relate the absorption-induced intensity fluctuations to the statistics of the underlying opacity fluctuations. We investigate models in which the absorbing medium consists of either discrete clouds or a power-law spectrum of opacity fluctuations. We examine the variability characteristics of a medium comprised of Gaussian-shaped clouds in which the neutral and ionized matter are co-located, and in which the clouds comprise spherical constant-density neutral cores enveloped by ionized sheaths. The cross-power spectrum indicates the spatial relationship between neutral and ionized matter, and distinguishes the two models, with power in the Gaussian model declining as a featureless power-law, but that in the ionized sheath model oscillating between positive and negative values. We show how comparison of the H I and free-free power spectra reveals information on the ionization and neutral fractions of the medium. The background source acts as a low-pass filter of the underlying opacity power spectrum, which limits temporal fluctuations to frequencies ? ? ???/?src, where ??? is the angular drift speed of the matter in front of the source, and it quenches the observability of opacity structures on scales smaller than the source size ?src. For drift speeds of ~103 km s-1 and source brightness temperatures ~1012 K, this limitation confines temporal opacity fluctuations to time-scales of order several months to decades

HCO+ and HCN J=3-2 absorption toward the center of Centaurus A

We have investigated the presence of dense gas toward the radio source Cen A by looking at the absorption of the HCO+ and HCN (3-2) lines in front of the bright continuum source with the Submillimeter Array. We detect narrow HCO+ (3-2) absorption, and tentatively HCN (3-2), close to the systemic velocity. For both molecules, the J=3-2 absorption is much weaker than for the J=1-0 line. From simple excitation analysis, we conclude that the gas density is on the order of a few 10^4 cm^-3 for a column density N(HCO+)/dV of 3x10^12 cm^-2 km^-1 s and a kinetic temperature of 10 K. In particular, we find no evidence for molecular gas density higher than a few 10^4 cm^-3 on the line of sight to the continuum source. We discuss the implications of our finding on the nature of the molecular gas responsible for the absorption toward Cen A.Comment: Accepted for publication in Ap

Indigenous Australian artists and astrophysicists come together to communicate science and culture via art

During the International Year of Astronomy in 2009, we initiated a collaboration between astrophysicists in Western Australia working toward building the largest telescope on Earth, the Square Kilometre Array (SKA), and Indigenous artists living in the region where the SKA is to be built. We came together to explore deep traditions in Indigenous culture, including perspectives of the night sky, and the modern astrophysical understanding of the Universe. Over the course of the year, we travelled as a group and camped at the SKA site, we sat under the stars and shared stories about the constellations, and we talked about the telescopes we wanted to build and how they could sit on the Indigenous traditional country. We found lots of interesting points of connection in our discussions and both artists and astronomers found inspiration. The artists then produced > 150 original works of art, curated as an exhibition called "Ilgarijiri - Things belonging to the Sky" in the language of the Wadjarri Yamatji people. This was exhibited in Geraldton, Perth, Canberra, South Africa, Brussels, the U.S.A., and Germany over the course of the next few years. In 2015, the concept went further, connecting with Indigenous artists from South Africa, resulting in the "Shared Sky" exhibition, which now tours the ten SKA member countries. The exhibitions communicate astrophysics and traditional Indigenous stories, as well as carry to the world Indigenous culture and art forms. The process behind the collaboration is an example of the Reconciliation process in Australia, successful through thoughtful and respectful engagements, built around common human experiences and points of contact (the night sky). This Commentary briefly describes the collaboration, its outcomes, and future work