Dissimilatory nitrate reductionpathways in an oligotrophic aquatic ecosystem: spatial and temporal trends

Elevated nitrate (NO3−) concentrations can cause eutrophication, which may lead to harmful algal blooms, loss of habitat and reduction in biodiversity. Denitrification, a dissimilatory process that removes NO3− mainly as dinitrogen gas (N2), is believed to be the dominant NO3− removal pathway in aquatic ecosystems. Evidence suggests that a less well-studied process, dissimilatory nitrate reduction to ammonium (DNRA), which retains nitrogen (N) in the system, may also be important under favorable conditions. Using stable isotope tracers in sealed microcosms, we measured the potential for NO3− losses due to DNRA and denitrification in an oligotrophic freshwater ecosystem. We took sediment and water samples at runoff and baseflow, across several ecotypes. Our objective was to quantify the relative importance of DNRA compared to denitrification with changes in ecotype and season. Potential denitrification rates ranged from 0 to 0.14 ± 0.03 µgN gAFDM−1 d−1. ­Potential DNRA rates ranged from 0 to 0.0051 ± 0.0008 µgN gAFDM−1 d−1. Denitrification losses peaked at the inflow stream ecotype at 96.2% of total dissimilatory NO3− removal, whereas losses due to DNRA peaked in the lake ecotype at 34.4%. When averaged over the entire system, denitrification peaked at baseflow (31.2%), while DNRA peaked at runoff (2.9%). Although NO3− transformations due to denitrification were higher than DNRA in all ecotype and temporal comparisons, our results suggest that DNRA is also important under favorable conditions

Interpretation of UV Absorption Lines in SN1006

We present a theoretical interpretation of the broad silicon and iron UV absorption features observed with the Hubble Space Telescope in the spectrum of the Schweizer-Middleditch star behind the remnant of Supernova 1006. These features are caused by supernova ejecta in SN1006. We propose that the redshifted SiII2 1260 A feature consists of both unshocked and shocked SiII. The sharp red edge of the line at 7070 km/s indicates the position of the reverse shock, while its Gaussian blue edge reveals shocked Si with a mean velocity of 5050 km/s and a dispersion of 1240 km/s, implying a reverse shock velocity of 2860 km/s. The measured velocities satisfy the energy jump condition for a strong shock, provided that all the shock energy goes into ions, with little or no collisionless heating of electrons. The line profiles of the SiIII and SiIV absorption features indicate that they arise mostly from shocked Si. The total mass of shocked and unshocked Si inferred from the SiII, SiIII and SiIV profiles is M_Si = 0.25 \pm 0.01 Msun on the assumption of spherical symmetry. Unshocked Si extends upwards from 5600 km/s. Although there appears to be some Fe mixed with the Si at lower velocities < 7070 km/s, the absence of FeII absorption with the same profile as the shocked SiII suggests little Fe mixed with Si at higher (before being shocked) velocities. The column density of shocked SiII is close to that expected for SiII undergoing steady state collisional ionization behind the reverse shock, provided that the electron to SiII ratio is low, from which we infer that most of the shocked Si is likely to be of a fairly high degree of purity, unmixed with other elements. We propose that the ambient interstellar density on the far side of SN1006 is anomalously low compared to the density around the rest of the remnant. ThisComment: 24 pages, with 8 figures included. Accepted for publication in the Astrophysical Journa

Simultaneous X-ray and Ultraviolet spectroscopy of the Seyfert galaxy NGC 5548. III. X-ray time variability

The Seyfert 1 galaxy NGC 5548 was observed for a week by Chandra using both the HETGS and LETGS spectrometers. In this paper we study the time variability of the continuum radiation. During our observation, the source showed a gradual increase in flux over four days, followed by a rapid decrease and flattening of the light curve afterwards. Superimposed upon these relatively slow variations several short duration bursts or quasi-periodic oscillations occured with a typical duration of several hours and separation between 0.6-0.9 days. The bursts show a delay of the hard X-rays with respect to the soft X-rays of a few hours. We interprete these bursts as due to a rotating, fluctuating hot spot at approximately 10 gravitational radii; the time delay of the hard X-rays from the bursts agree with the canonical picture of Inverse Compton scattering of the soft accretion disk photons on a hot medium that is relatively close to the central black hole.Comment: 6 pages, 7 figures, accepted for publication in Astronomy and Astrophysic

Observations of Outflowing UV Absorbers in NGC 4051 with the Cosmic Origins Spectrograph

We present new Hubble Space Telescope (HST)/Cosmic Origins Spectrograph observations of the Narrow-Line Seyfert 1 galaxy NGC 4051. These data were obtained as part of a coordinated observing program including X-ray observations with the Chandra/High Energy Transmission Grating (HETG) Spectrometer and Suzaku. We detected nine kinematic components of UV absorption, which were previously identified using the HST/Space Telescope Imaging Spectrograph. None of the absorption components showed evidence for changes in column density or profile within the \sim 10 yr between the STIS and COS observations, which we interpret as evidence of 1) saturation, for the stronger components, or 2) very low densities, i.e., n_H < 1 cm^-3, for the weaker components. After applying a +200 km s^-1 offset to the HETG spectrum, we found that the radial velocities of the UV absorbers lay within the O VII profile. Based on photoionization models, we suggest that, while UV components 2, 5 and 7 produce significant O VII absorption, the bulk of the X-ray absorption detected in the HETG analysis occurs in more highly ionized gas. Moreover, the mass loss rate is dominated by high ionization gas which lacks a significant UV footprint.Comment: 41 pages, 10 Figures; accepted for publication in the Astrophysical Journa

High Velocity Line Emission in the NLR of NGC 4151

Narrow-band imaging of the nuclear region of NGC 4151 with the Hubble Space Telescope is presented. The filter bandpasses isolate line emission in various high velocity ranges in several ions. Slitless and long-slit spectra of the region with the Space Telescope Imaging Spectrograph also indicate the locations of high velocity gas. These emission regions are faint and are interspersed among the bright emission clouds seen in direct images. They have radial velocities up to 1400 km/s relative to the nucleus, and are found in both approach and recession on both sides of the nucleus. This contrasts strongly with the bright emission line clouds which have been discussed previously as showing bidirectional outflow with velocities within 400 km/s of the nucleus. We discuss the possible connections of the high velocity material with the radio jet and the nuclear radiation.Comment: 12 pages plus 6 figures, to be published in A

New Indicators for AGN Power: The Correlation Between [O IV] lambda 25.89 micron and Hard X-ray Luminosity for Nearby Seyfert Galaxies

We have studied the relationship between the [O IV] lambda 25.89 micron emission line luminosities, obtained from Spitzer spectra, the X-ray continua in the 2-10 keV band, primarily from ASCA, and the 14-195 keV band obtained with the SWIFT/Burst Alert Telescope (BAT), for a sample of nearby (z < 0.08) Seyfert galaxies. For comparison, we have examined the relationship between the [O III] 5007, the 2-10 keV and the 14-195 keV luminosities for the same set of objects. We find that both the [O IV] and [O III] luminosities are well-correlated with the BAT luminosities. On the other hand, the [O III] luminosities are better-correlated with 2-10 keV luminosities than are those of [O IV]. When comparing [O IV] and [O III] luminosities for the different types of galaxies, we find that the Seyfert 2's have significantly lower [O III] to [O IV] ratios than the Seyfert 1's. We suggest that this is due to more reddening of the narrow line region (NLR) of the Seyfert 2's. Assuming Galactic dust to gas ratios, the average amount of extra reddening corresponds to a hydrogen column density of ~ few times 10^21 cm^-2, which is a small fraction of the X-ray absorbing columns in the Seyfert 2's. The combined effects of reddening and the X-ray absorption are the probable reason why the [O III] versus 2-10 keV correlation is better than the [O IV] versus 2-10 keV, since the [O IV] emission line is much less affected by extinction. Overall, we find the [O IV] to be an accurate and truly isotropic indicator of the power of the AGN. This suggests that it can be useful in deconvolving the contribution of the AGN and starburst to the spectrum of Compton-thick and/or X-ray weak sources.Comment: Accepted for publication in the Astrophysical Journal. 31 pages, 6 figures, 4 table

Transnationalism and Social Work Education

Transnational movements, networks, and relationships are everywhere in this “world on the move” (Williams & Graham, 2014, p. i1). Transnational peoples maintain relationships of interdependence and support with families and communities in their places of origin, often returning regularly, while starting new lives and making new connections. Transnationalism is characterized by mobilities and networks, by social integration, and by extended and extensive relationship ties of family, neighborhood, religious faith, or combinations thereof (Valtonen, 2008). While disciplines across the world including sociology, human geography, and cultural anthropology engage with the implications of transnationalism (Bauböck & Faist, 2010), social work in England and mainland Europe has not achieved similar levels of engagement. As Cox and Geisen state: “the social world is being transformed by migration and social work is playing catch-up” (2014, p. i162)

A Kinematic Model for the Narrow-Line Region in NGC 4151

We present a simple kinematic model for the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151, based on our previous observations of extended [O III] emission with the Space Telescope Imaging Spectrograph (STIS). The model is similar to a biconical radial outflow model developed for the Seyfert 2 galaxy NGC 1068, except that the bicone axis is tilted much more into our line of sight (40 degrees out of the plane of the sky instead of 5 degrees), and the maximum space velocities are lower (750 km/s instead of 1300 km/s. We find evidence for radial acceleration of the emission-line knots to a distance of 160 pc, followed by deceleration that approaches the systemic velocity at a distance of 290 pc (for a distance to NGC 4151 of 13.3 Mpc). Other similarities to the kinematics of NGC 1068 are: 1) there are a number of high-velocity clouds that are not decelerated, suggesting that the medium responsible for the deceleration is patchy, and 2) the bicone in NGC 4151 is at least partially evacuated along its axis. Together, these two Seyfert galaxies provide strong evidence for radial outflow (e.g., due to radiation and/or wind pressure) and against gravitational motion or expansion away from the radio jets as the principal kinematic component in the NLR.Comment: 31 pages, Latex, includes 11 figures in postscript, Figures 5a,5b,6a,6b in color, to appear in the Astronomical Journa

Simultaneous X-ray and UV spectroscopy of the Seyfert 1 galaxy NGC 5548.II. Physical conditions in the X-ray absorber

We present the results from a 500 ks Chandra observation of the Seyfert 1 galaxy NGC 5548. We detect broadened emission lines of O VII and C VI in the spectra, similar to those observed in the optical and UV bands. The source was continuously variable, with a 30 % increase in luminosity in the second half of the observation. No variability in the warm absorber was detected between the spectra from the first 170 ks and the second part of the observation. The velocity structure of the X-ray absorber is consistent with the velocity structure measured simultaneously in the ultraviolet spectra. We find that the highest velocity outflow component, at -1040 km/s, becomes increasingly important for higher ionization parameters. This velocity component spans at least three orders of magnitude in ionization parameter, producing both highly ionized X-ray absorption lines (Mg XII, Si XIV) as well as UV absorption lines. A similar conclusion is very probable for the other four velocity components. Based upon our observations, we argue that the warm absorber probably does not manifest itself in the form of photoionized clumps in pressure equilibrium with a surrounding wind. Instead, a model with a continuous distribution of column density versus ionization parameter gives an excellent fit to our data. From the shape of this distribution and the assumption that the mass loss through the wind should be smaller than the accretion rate onto the black hole, we derive upper limits to the solid angle as small as 10^{-4} sr. From this we argue that the outflow occurs in density-stratified streamers. The density stratification across the stream then produces the wide range of ionization parameter observed in this source. Abridged.Comment: 21 pages, 12 figures accepted for publication in A&

IC5063: AGN driven outflow of warm and cold gas

We present new ATCA 17- and 24-GHz radio images and ESO-NTT optical spectra of the radio-loud Seyfert galaxy IC5063, the first galaxy in which a fast (~ 600 km/s) outflow of neutral hydrogen was discovered. The new radio data confirm the triple radio structure with a central core and two resolved radio lobes. This implies that the previously detected fast outflow of neutral gas is occurring off-nucleus, near a radio lobe about 0.5 kpc from the core. The ionised gas shows complex kinematics in the region co-spatial with the radio emission. Broad and blueshifted (~ 500 km/s) emission is observed in the region of the radio lobe, at the same location as the blueshifted HI absorption. The velocity of the ionised outflow is similar to the one found in HI. The first order correspondence between the radio and optical properties suggests that the outflow is driven by the interaction between the radio jet and the ISM. Despite the high outflow velocities, no evidence is found for the ionisation of the gas being due to fast shocks in the region of the outflow, indicating that photoionisation from the AGN is likely to be the dominant ionisation mechanism. The outflow rate of the warm (ionised) gas is small compared to that of the cold gas. The mass outflow rate associated with the HI is in the same range as for ``mild'' starburst-driven superwinds in ULIRGs. However, in IC5063, the AGN-driven outflow appears to be limited to the inner kpc region of the galaxy. The kinetic power associated with the HI outflow is a small fraction (a few x 10^-4) of the Eddington luminosity of the galaxy but is a significant fraction (~ 0.1) of the nuclear bolometric luminosity. In IC5063, the outflows may have sufficient kinetic power to have a significant impact on the evolution of the ISM in the host galaxy.Comment: Accepted for publication in A&A, 11 pages, 8 figure