50 research outputs found
Circumnuclear obscuration of the Circinus galaxy by a starburst-radiation supported wall
We consider the radiation-hydrodynamical formation of a dusty wall in the
circumnuclear regions of the Circinus Seyfert 2 galaxy. We focus on the
radiative flux force due to the circumnuclear starburst, the nuclear starburst
and the active galactic nucleus (AGN), and analyze the equilibrium
configuration and stability of the dusty gas in the circumnuclear regions. The
mass distributions used to model the gravitational fields are determined by the
observed rotation velocities of the stars and of the gas. Moreover, by using
the simple stellar evolution in the circumnuclear starburst, the bolometric
luminosity of the starburst is estimated as a function of time. As a result, it
is shown that the radiation force by the circumnuclear starburst does play an
important role to build up a radiatively-supported dusty wall of \la 100 pc,
which obscures the circumnuclear regions with of a few magnitudes. It is
also found that the age of a circumnuclear starburst when the circumnuclear
regions are enshrouded by the dusty wall is constrained to be \la 10^8 yr,
which is consistent with the age observationally inferred for the starburst, yr yr.Comment: 6 pages, 4 figures, accepted for publication in A&
High speed videometric monitoring of rock breakage
Estimation of rock breakage characteristics plays an important role in optimising various industrial and mining processes used for rock comminution. Although little research has been undertaken into 3D photogrammetric measurement of the progeny kinematics, there is promising potential to improve the efficacy of rock breakage characterisation. In this study, the observation of progeny kinematics was conducted using a high speed, stereo videometric system based on laboratory experiments with a drop weight impact testing system. By manually tracking individual progeny through the captured video sequences, observed progeny coordinates can be used to determine 3D trajectories and velocities, supporting the idea that high speed video can be used for rock breakage characterisation purposes. An analysis of the results showed that the high speed videometric system successfully observed progeny trajectories and showed clear projection of the progeny away from the impact location. Velocities of the progeny could also be determined based on the trajectories and the video frame rate. These results were obtained despite the limitations of the photogrammetric system and experiment processes observed in this study. Accordingly there is sufficient evidence to conclude that high speed videometric systems are capable of observing progeny kinematics from drop weight impact tests. With further optimisation of the systems and processes used, there is potential for improving the efficacy of rock breakage characterisation from measurements with high speed videometric systems
Discovery of a nuclear gas bar feeding the active nucleus in Circinus
We report the discovery of gas inflow motions towards the active nucleus of
the Circinus galaxy caused by the non-axisymmetric potential of a nuclear gas
bar. Evidence for dust associated with the bar comes from the HST/NICMOS H-K
color map, whereas the streaming motions along the gas bar are seen in the
velocity field of the H2 S(1)(1-0) emission line. The gas bar is about 100 pc
long with a visual extinction in excess of 10 mag. Indication for the gaseous
nature of this bar comes from the lack of a stellar counterpart even in the K
band where the extinction is greatly reduced.
We also use the NICMOS emission line images (Pa-alpha, [SiVI], and [FeII]) to
study the innermost region of the ionization cones and the nuclear star forming
activity. We discuss the possible relationship of these components with the
gaseous bar.Comment: 14 pages, 7 figures (3 color plates), accepted for publication in Ap
X-ray imaging of the Seyfert 2 galaxy Circinus with Chandra
We present results from the zeroth-order imaging of a Chandra HETGS
observation of the nearby Seyfert 2 galaxy Circinus. Twelve X-ray sources were
detected in the ACIS-S image of the galaxy, embedded in diffuse X-ray emission.
The latter shows a prominent (~18arcsec) soft ``plume'' in the N-W direction,
coincident with the [OIII] ionization cone. The radial profiles of the
brightest X-ray source at various energies are consistent with an unresolved
(FWHM ~0.8arcsec) component, which we identify as the active nucleus, plus two
extended components with FWHMs ~ 2.3arcsec and 18arcsec, respectively. In a
radius of 3arcsec, the nucleus contributes roughly the same flux as the
extended components at the softest energies (< 2 keV). However, at harder
energies (> 2 keV), the contribution of the nucleus is dominant. The
zeroth-order ACIS spectrum of the nucleus exhibits emission lines at both soft
and hard X-rays, including a prominent Fe Kalpha line at 6.4 keV, showing that
most of the X-ray lines previously detected with ASCA originate in a compact
region (<15 pc). Based on its X-ray spectrum, we argue that the 2.3arcsec
extended component is scattered nuclear radiation from nearby ionized gas. The
large-scale extended component includes the emission from the N-W plume and
possibly from the outer starburst ring.Comment: Figure 1 in color. ApJ Letters, in pres
Unveiling the central parsec region of an AGN: the Circinus nucleus in the near infrared with the VLT
VLT J- to M\p-band adaptive optics observations of the Circinus Galaxy on
parsec scales resolve a central bright Ks-band source with a FWHM size of 1.9
0.6 pc. This source is only visible at wavelengths longward of 1.6 m
and coincides in position with the peak of the [Si VII]~2.48 m coronal
line emission. With respect to the peak of the central optical emission, the
source is shifted by 0.15\arcsec (2.8 pc) to the south-east. Indeed, it
defines the vertex of a fairly collimated beam which extends for 10 pc,
and which is seen in both continuum light shortward of 1.6 m and in
H line emission. The source also lies at the center of a 19 pc
size [Si VII] ionization {\it bicone}.
Identifying this source as the nucleus of Circinus, its size is compatible
with a putative parsec-scale torus. Its spectral energy distribution,
characterized by a prominent narrow peak, is compatible with a dust temperature
of 300 K. Hotter dust within a 1 pc radius of the center is not detected. The
AGN luminosity required to heat this dust is in the range of X-ray luminosities
that have been measured toward the central source. This in turn supports the
existence of highly obscuring material, with column densities of
cm, that must be located within 1 pc of the core.Comment: 15 pages, 4 figures; To appear in The Astrophysical Journa
The Circumnuclear Molecular Gas in the Seyfert Galaxy NGC4945
We have mapped the central region of NGC 4945 in the transition of
CO, CO, and CO, as well as the continuum at 1.3 mm, at an
angular resolution of 5\farc \times 3\farc with the Submillimeter Array. The
relative proximity of NGC 4945 (distance of only 3.8 Mpc) permits a detailed
study of the circumnuclear molecular gas and dust in a galaxy exhibiting both
an AGN (classified as a Seyfert 2) and a circumnuclear starburst in an inclined
ring with radius 2\farcs5 (50 pc). We find that all three molecular
lines trace an inclined rotating disk with major axis aligned with that of the
starburst ring and large-scale galactic disk, and which exhibits solid-body
rotation within a radius of 5\farc (95 pc). We infer an inclination
for the nuclear disk of , somewhat smaller than the
inclination of the large-scale galactic disk of . The
continuum emission at 1.3 mm also extends beyond the starburst ring, and is
dominated by thermal emission from dust. If it traces the same dust emitting in
the far-infrared, then the bulk of this dust must be heated by star-formation
activity rather than the AGN. We discover a kinematically-decoupled component
at the center of the disk with a radius smaller than 1\farcs4 (27 pc), but
which spans approximately the same range of velocities as the surrounding disk.
This component has a higher density than its surroundings, and is a promising
candidate for the circumnuclear molecular torus invoked by AGN unification
models.Comment: 13 pages, 10 figures,accepted by Ap
Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
To test the dust torus model for active galactic nuclei directly, we study
the extent and morphology of the nuclear dust distribution in the Circinus
galaxy using high resolution interferometric observations in the mid-infrared
with the MIDI instrument at the Very Large Telescope Interferometer. We find
that the dust distribution in the nucleus of Circinus can be explained by two
components, a dense and warm disk-like component of 0.4 pc size and a slightly
cooler, geometrically thick torus component with a size of 2.0 pc. The disk
component is oriented perpendicular to the ionisation cone and outflow and
seems to show the silicate feature at 10 micron in emission. It coincides with
a nuclear maser disk in orientation and size. From the energy needed to heat
the dust, we infer a luminosity of the accretion disk corresponding to 20% of
the Eddington luminosity of the nuclear black hole. We find that the
interferometric data are inconsistent with a simple, smooth and axisymmetric
dust emission. The irregular behaviour of the visibilities and the shallow
decrease of the dust temperature with radius provide strong evidence for a
clumpy or filamentary dust structure. We see no evidence for dust reprocessing,
as the silicate absorption profile is consistent with that of standard galactic
dust. We argue that the collimation of the ionising radiation must originate in
the geometrically thick torus component. Our findings confirm the presence of a
geometrically thick, torus-like dust distribution in the nucleus of Circinus,
as required in unified schemes of Seyfert galaxies. Several aspects of our data
require that this torus is irregular, or "clumpy".Comment: 20 pages, 16 figures, accepted for publication by A&
Atomic and Molecular Gas in the Starburst Galaxy NGC4945
Spatial and kinematical correlations between HI and CO (2-1) emission of the
southern spiral galaxy NGC4945 are studied with a common angular (23 arcsec)
and velocity resolution (7 km/s). The 21cm continuum emission is also observed.
The HI kinematics yield a galaxy mass of 1.4x10^{11} Mo within a radius of 380
arcsec, with molecular and neutral atomic gas each contributing 2%. Nuclear HI
absorption at velocities 80 km/s higher than systemic indicates gas flowing
towards the centre. HI features at each end of the major axis (R ~ 600 arcsec)
are interpreted as spiral arms that are viewed tangentially and that also cause
prominent emission features in the radio continuum, HI, and CO further inside
the galaxy. A central elongated region showing non-circular motions is
interpreted as a bar which fuels the nuclear starburst. HI and CO
position-velocity data have been analysed using linear resonance theory and
possible locations of resonances are identified.Comment: 16 pages, Latex file, 9 Postscript Figures, aa.cls and psfig.sty
included. Accepted by Astronomy and Astrophysic
Extremely Rapid Variations of Water Maser Emission from the Circinus Galaxy
The water maser lines in the nucleus of the Circinus galaxy vary on time
scales as short as a few minutes, at least two orders of magnitude less than
for other Galactic or extragalactic water masers. The amplitude of one line
more than doubled in about 10 minutes. The intensity changes cannot be
attributed easily to a mechanism of intrinsic fluctuations. The variability may
be the result of strong interstellar diffractive scintillation along the line
of sight within our Galaxy, which would be the first example of diffractive
scintillation for any source at 22 GHz and for any source other than a pulsar.
However, only the very shortest timescales for interstellar scintillation,
obtained from pulsar observations and scaled to 22 GHz, correspond to the
observed maser variability. Alternatively, the intensity changes may be a
reaction to fluctuations in compact background or radiative pump sources and
thereby may be related to variability of the central engine. The maser spectral
features symmetrically bracket the systemic velocity of the galaxy, offset by
about +/-(100-200) km/s. The spectrum of the Circinus maser is similar in some
respects to that of the maser in NGC 4258, which probably traces a molecular
disk rotating around a supermassive object. VLBI observations could reveal
whether the maser source in the heart of the Circinus galaxy is part of a
similar dynamical system.Comment: 10 pages, 2 encapsulated postscript figures included. To appear in
Astrophysical Journal (Letters), January 10, 199
Very Large Baseline Array observations of Mrk 6 : probing the jet-lobe connection
We present the results of high-resolution VLBI (very long baseline interferometry) observations at 1.6 and 4.9 GHz of the radio-loud Seyfert galaxy, Mrk 6. These observations are able to detect a compact radio core in this galaxy for the first time. The core has an inverted spectral index (α1.6 4.9 = +1.0 ± 0.2) and a brightness temperature of 1 Ă 108 K. Three distinct radio components, which resemble jet elements and/or hotspots, are also detected. The position angles of these elongated jet elements point not only to a curved jet in Mrk 6, but also towards a connection between the AGN and the kpc-scale radio lobes/bubbles in this galaxy. Firmer constraints on the star formation rate provided by new Herschel observations (SFR <0.8 Mâ yr-1) make the starburst-wind-powered bubble scenario implausible. From plasma speeds, obtained via prior Chandra X-ray observations, and ram pressure balance arguments for the interstellar medium and radio bubbles, the north-south bubbles are expected to take 7.5 Ă 106 yr to form, and the east-west bubbles 1.4 Ă 106 yr. We suggest that the jet axis has changed at least once in Mrk 6 within the last â107 yr. A comparison of the nuclear radio-loudness of Mrk 6 and a small sample of Seyfert galaxies with a subset of low-luminosity FR I radio galaxies reveals a continuum in radio properties.Peer reviewe