276 research outputs found
On the Location and Composition of the Dust in the MCG-6-30-15 Warm Absorber
Hubble Space Telescope images of MCG-6-30-15 show a dust lane crossing the
galaxy just below the nucleus. In this paper, we argue that this dust lane is
responsible for the observed reddening of the nuclear emission and the Fe I
edge hinted at in the Chandra spectrum of MCG-6-30-15. We further suggest that
the gas within the dust lane can comprise much of the low ionization component
(i.e., the one contributing the O VII edge) of the observed warm absorber.
Moreover, placing the warm absorbing material at such distances (hundreds of
pc) can account for the small outflow velocities of the low ionization
absorption lines as well as the constancy of the O VIII edge. Photoionization
models of a dusty interstellar gas cloud (with a column appropriate for the
reddening toward MCG-6-30-15) using a toy Seyfert 1 spectral energy
distribution show that it is possible to obtain a significant O VII edge
(\tau~0.2) if the material is ~150 pc from the ionizing source. For
MCG-6-30-15, such a distance is consistent with the observed dust lane. The
current data on MCG-6-30-15 is unable to constrain the dust composition within
the warm absorber. Astronomical silicate is a viable candidate, but there are
indications of a very low O abundance in the dust, which is inconsistent with a
silicate origin. If true, this may indicate that there were repeated cycles of
grain destruction and growth from shocks in the interstellar medium of
MCG-6-30-15. Pure iron grains are an unlikely dust constituent due to the limit
on their abundance in the Galaxy, yet they cannot be ruled out. The high column
densities inferred from the highly ionized zone of the warm absorber implies
that this gas is dust-free.Comment: 7 pages, 3 Figures, A&A accepte
Study on the financial sector in Greece during the economic adjustment programmes: 2010-2018
study on Greece\u27s various economic adjustment programs, their results, efficacy, and related program
Influence of doping on charge carrier collection in normal and inverted geometry polymer: fullerene solar cells
While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 1016 cm−3, resulting in space-charge regions ~100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent
The Radio Continuum-Star Formation Rate Relation in WSRT SINGS Galaxies
This is an author-created, un-copyedited version of an article accepted for published in The Astronomical Journal. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/0004-6256/147/5/103.We present a study of the spatially resolved radio continuum–star formation rate (RC–SFR) relation using stateof-the-art star formation tracers in a sample of 17 THINGS galaxies. We use SFR surface density (ΣSFR) maps created by a linear combination of GALEX far-UV (FUV) and Spitzer 24μm maps. We use RC maps at λλ22 and 18 cm from the WSRT SINGS survey and Hα emission maps to correct for thermal RC emission. We compare azimuthally averaged radial profiles of the RC and FUV/mid-IR (MIR) based ΣSFR maps and study pixel-by-pixel correlations at fixed linear scales of 1.2 and 0.7 kpc. The ratio of the integrated SFRs from the RC emission to that of the FUV/MIR-based SF tracers is Rint = 0.78 ± 0.38, consistent with the relation by Condon. We find a tight correlation between the radial profiles of the radio and FUV/MIR-based ΣSFR for the entire extent of the disk. The ratio R of the azimuthally averaged radio to FUV/MIR-based ΣSFR agrees with the integrated ratio and has only quasi-random fluctuations with galactocentric radius that are relatively small (25%). Pixel-by-pixel plots show a tight correlation in log-log diagrams of radio to FUV/MIR-based ΣSFR, with a typical standard deviation of a factor of two. Averaged over our sample we find (ΣSFR)RC ∝ (ΣSFR) 0.63±0.25 hyb , implying that data points with high ΣSFR are relatively radio dim, whereas the reverse is true for low ΣSFR. We interpret this as a result of spectral aging of cosmic-ray electrons (CREs), which are diffusing away from the star formation sites where they are injected into the interstellar medium. This is supported by our finding that the radio spectral index is a second parameter in pixel-by-pixel plots: those data points dominated by young CREs are relatively radio dim, while those dominated by old CREs are slightly more RC bright than what would be expected from a linear extrapolation. We studied the ratio R of radio to FUV/MIR-based integrated SFR as a function of global galaxy parameters and found no clear correlation. This suggests that we can use RC emission as a universal star formation tracer for galaxies with a similar degree of accuracy as other tracers, if we restrict ourselves to global or azimuthally averaged measurements. We can reconcile our finding of an almost linear RC–SFR relation and sub-linear resolved (on 1 kpc scale) RC–ΣSFR relation by proposing a non-linear magnetic field–SFR relation, B ∝ SFR0.30±0.02 hyb , which holds both globally and locally.Peer reviewe
The Toroidal Obscuration of Active Galactic Nuclei
Observations give strong support for the unification scheme of active
galactic nuclei. The scheme is premised on toroidal obscuration of the central
engine by dusty clouds that are individually very optically thick. These
lectures summarize the torus properties, describe the handling and implications
of its clumpy nature and present speculations about its dynamic origin.Comment: To appear in proceedings of "Active Galactic Nuclei at the highest
angular resolution: theory and observations", 2007 summer school, Torun,
Polan
Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in the Local
Group spiral galaxy M33. The regions observed range from 1 to 7 kpc in distance
from the nucleus. Of the 13 H II regions observed, the [O III] 4363 Angstrom
line was detected in six regions. Electron temperatures were thus able to be
determined directly from the spectra using the [O III] 4959,5007 A/4363 A line
ratio. Based on these temperature measurements, oxygen and neon abundances and
their radial gradients were calculated. For neon, a gradient of -0.016 +/-
0.017 dex/kpc was computed, which agrees with the Ne/H gradient derived
previously from ISO spectra. A gradient of -0.012 +/- 0.011 dex/kpc was
computed for O/H, much shallower than was derived in previous studies. The
newly calculated O/H and Ne/H gradients are in much better agreement with each
other, as expected from predictions of stellar nucleosynthesis. We examine the
correlation between the WC/WN ratio and metallicity, and find that the new M33
abundances do not impact the observed correlation significantly. We also
identify two new He II-emitting H II regions in M33, the first to be discovered
in a spiral galaxy other than the Milky Way. In both cases the nebular He II
emission is not associated with Wolf-Rayet stars. Therefore, caution is
warranted in interpreting the relationship between nebular He II emission and
Wolf-Rayet stars when both are observed in the integrated spectrum of an H II
region.Comment: 32 pages, 9 figures. Accepted for publication in Ap
XMM-Newton observation of the bright Seyfert 1 galaxy, MCG+8-11-11
We report on the XMM-Newton observation of the bright Seyfert 1 galaxy,
MCG+8-11-11. Data from the EPIC/p-n camera, the Reflection Gratings
Spectrometers (RGS) and the Optical Monitor (OM) have been analyzed. The p-n
spectrum is well fitted by a power law, a spectrally unresolved Fe Kalpha line,
a Compton reflection component (whose large value, when compared to the iron
line equivalent width, suggests iron underabundance), and absorption by warm
material. Absorption lines are apparent in the RGS spectra, but their
identification is uncertain and would require large matter velocities. The UV
fluxes measured by the OM are well above the extrapolation of the X-ray
spectrum, indicating the presence of a UV bump.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic
The state of the Martian climate
60°N was +2.0°C, relative to the 1981–2010 average value (Fig. 5.1). This marks a new high for the record. The average annual surface air temperature (SAT) anomaly for 2016 for land stations north of starting in 1900, and is a significant increase over the previous highest value of +1.2°C, which was observed in 2007, 2011, and 2015. Average global annual temperatures also showed record values in 2015 and 2016. Currently, the Arctic is warming at more than twice the rate of lower latitudes
Binary black holes and tori in AGN II. Can stellar winds constitute a dusty torus?
We determine the properties of the stellar torus that we showed in a previous
paper to result as a product of two merging black holes. If the surrounding
stellar cluster is as massive as the binary black hole, the torque acting on
the stars ejects a fraction which extracts all the binary's angular momentum on
scales of ~10^7 yr, and a geometrically thick torus remains. In the present
article we show that a certain fraction of the stars has winds, shaped into
elongated tails by the central radiation pressure, which are optically thick
for line of sights aligned with them. These stars are sufficiently numerous to
achieve a covering factor of 1, so that the complete torus is optically thick.
We find the parameters of such a patchy torus to be in the right range to
explain the observed large column densities in AGN and their temporal
variations on time scales of about a decade. Within this model the BAL quasars
can be interpreted as quasars seen at intermediate inclination angles, with the
line of sight grazing the edge of the torus. The opening angle of the torus is
wider for major mergers and thus correlates with the central luminosity. In
this picture the spin of the merged black hole is possibly dominated by the
orbital angular momentum of the binary. Thus the spin of the merged black hole
points into a new direction, and consequently the jet experiences a spin-flip
according to the spin-paradigm. This re-orientation could be an explanation for
the X-shaped radio galaxies, and the advancing of a new jet through the ambient
medium for Compact Symmetric Objects.Comment: 20 pages, 10 figures, final version, A&A accepte
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