49 research outputs found
What are black holes
In the very hearts of galaxies, like our Milky Way, lurk giant black holes that sometimes evolve into monstrous powerhouses of light. How do we know that they exist? How are they born? How do they grow? Are they important in the ‘big picture’
Sun wonder! non-trivial concepts through day-time astronomy experiments with self-constructed equipment
Enchanting is the sight of the Milky Way on a
moonless night. Mesmerising are the photographs
of the distant universe, brought home to us via
the Internet by powerful telescopes like Hubble,
Spitzer and Chandra. The sky is accessible to
everyone and is a ‘universal laboratory’. However,
school hours are nearly always during day-time.
Combine this reality with the scourge of light
pollution, and practical star gazing within regular
school curricula is virtually ruled out - with one
exception. Our nearest star, the Sun, can play
‘laboratory’ during school hours! Thus, learning
science by doing and discovering can indeed
happen with day-time astronomy experiments. The Sun should not be stared at directly - it could
harm our eyes. The projection of the Sun’s image
described in the activities below is one of the safe
methods of viewing the Sun
Low-frequency radio observations of Seyfert galaxies: A test to the unification scheme
Aims. We present low-frequency radio imaging and spectral properties of a
well defined sample of Seyfert galaxies using GMRT 240/610 MHz dual frequency
observations. Radio spectra of Seyfert galaxies over 240 MHz to 5.0 GHz are
investigated using 240 MHz, 610 MHz flux densities derived from GMRT, and 1.4
GHz and 5.0 GHz flux densities mainly from published VLA data. We test the
predictions of Seyfert unification scheme by comparing the radio properties of
Seyfert type 1s and type 2s. Methods. We choose a sample such that the two
Seyfert subtypes have matched distributions in parameters that are independent
to the orientation of AGN, obscuring torus and the host galaxy. Our sample
selection criteria allow us to assume that the two Seyfert subtypes are
intrinsically similar within the framework of the unification scheme. Results.
The new observations at 240/610 MHz, together with archival observations at 1.4
GHz, 5.0 GHz show that type 1s and type 2s have statistically similar radio
luminosity distributions at 240 MHz, 610 MHz, 1.4 GHz and 5.0 GHz. The spectral
indices at selected frequency intervals as well as index measured over 240 MHz
to 5.0 GHz for the two Seyfert subtypes have similar distributions with median
spectral index -0.7, consistent with the synchrotron emission from
optically thin plasma. In our snap-shot 240/610 MHz GMRT observations, most of
the Seyfert galaxies show primarily an unresolved central radio component,
except a few sources in which faint kpc-scale extended emission is apparent at
610 MHz. Our results on the statistical comparison of the multifrequency radio
properties of our sample Seyfert galaxies are in agreement with the predictions
of the Seyfert unification scheme.Comment: 21 pages, 5 figures, Accepted for publication in Astronomy &
Astrophysic
Probing the physics of narrow-line regions of Seyfert galaxies I: The case of NGC 5427
We have used the Wide Field Spectrograph (WiFeS) on the ANU 2.3m telescope at
Siding Spring to observe the nearby, nearly face-on, Seyfert 2 galaxy, NGC
5427. We have obtained integral field spectroscopy of both the nuclear regions
and the HII regions in the spiral arms. We have constrained the chemical
abundance in the interstellar medium of the extended narrow line region (ENLR)
by measuring the abundance gradient in the circum-nuclear \ion{H}{ii} regions
to determine the nuclear chemical abundances, and to use these to in turn
determine the EUV spectral energy distribution for comparison with theoretical
models. We find a very high nuclear abundance, times solar, with
clear evidence of a nuclear enhancement of N and He, possibly caused by massive
star formation in the extended (pc) central disk structure. The
circum-nuclear narrow-line region spectrum is fit by a radiation pressure
dominated photoionisation model model with an input EUV spectrum from a Black
Hole with mass radiating at of its Eddington
luminosity. The bolometric luminosity is closely constrained to be erg s. The EUV spectrum characterised by
a soft accretion disk and a harder component extending to above 15keV. The ENLR
region is extended in the NW-SE direction. The line ratio variation in
circum-nuclear spaxels can be understood as the result of mixing \ion{H}{ii}
regions with an ENLR having a radius-invariant spectrum.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 13
figure
S7 : Probing the physics of Seyfert Galaxies through their ENLR & HII Regions
Here we present the first results from the Siding Spring Southern Seyfert
Spectroscopic Snapshot Survey (S7) which aims to investigate the physics of
~140 radio-detected southern active Galaxies with z<0.02 through Integral Field
Spectroscopy using the Wide Field Spectrograph (WiFeS). This instrument
provides data cubes of the central 38 x 25 arc sec. of the target galaxies in
the waveband 340-710nm with the unusually high resolution of R=7000 in the red
(530-710nm), and R=3000 in the blue (340-560nm). These data provide the
morphology, kinematics and the excitation structure of the extended narrow-line
region, probe relationships with the black hole characteristics and the host
galaxy, measures host galaxy abundance gradients and the determination of
nuclear abundances from the HII regions. From photoionisation modelling, we may
determine the shape of the ionising spectrum of the AGN, discover whether AGN
metallicities differ from nuclear abundances determined from HII regions, and
probe grain destruction in the vicinity of the AGN. Here we present some
preliminary results and modelling of both Seyfert galaxies observed as part of
the survey.Comment: 6 pages, 2 figures, Invited Talk at the IAU symposium 30
Probing the Physics of Narrow Line Regions in Active Galaxies II: The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7)
Here we describe the \emph{Siding Spring Southern Seyfert Spectroscopic
Snapshot Survey} (S7) and present results on 64 galaxies drawn from the first
data release. The S7 uses the Wide Field Spectrograph (WiFeS) mounted on the
ANU 2.3m telescope located at the Siding Spring Observatory to deliver an
integral field of ~ arcsec at a spectral resolution of in
the red (nm), and in the blue (nm). {From these data
cubes we have extracted the Narrow Line Region (NLR) spectra from a 4 arc sec
aperture centred on the nucleus. We also determine the H and
[OIII]~5007 fluxes in the narrow lines, the nuclear reddening, the
reddening-corrected relative intensities of the observed emission lines, and
the H and \lOIII\ luminosities {determined from spectra for which the
stellar continuum has been removed.} We present a set of images of the galaxies
in [OIII]~5007, [NII]~6584 and H which serve to
delineate the spatial extent of the extended narrow line region (ENLR) and {\bf
also to} reveal the structure and morphology of the surrounding \HII\ regions.
Finally, we provide a preliminary discussion of those Seyfert~1 and Seyfert~2
galaxies which display coronal emission lines in order to explore the origin of
these lines.Comment: Accepted for publication 9 Jan 2015, Astrophysical Journal
Supplements. 49pages, 8 figure
Probing the Physics of Narrow Line Regions in Active Galaxies III: Accretion and Cocoon Shocks in the LINER NGC1052
We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and
HST FOS spectroscopy for the LINER galaxy NGC 1052. We infer the presence of a
turbulent accretion flow forming a small-scale accretion disk. We find a
large-scale outflow and ionisation cone along the minor axis of the galaxy.
Part of this outflow region is photoionised by the AGN, and shares properties
with the ENLR of Seyfert galaxies, but the inner (~arcsec)
accretion disk and the region around the radio jet appear shock excited. The
emission line properties can be modelled by a "double shock" model in which the
accretion flow first passes through an accretion shock in the presence of a
hard X-ray radiation, and the accretion disk is then processed through a cocoon
shock driven by the overpressure of the radio jets. This model explains the
observation of two distinct densities ( and cm),
and provides a good fit to the observed emission line spectrum. We derive
estimates for the velocities of the two shock components and their mixing
fractions, the black hole mass, the accretion rate needed to sustain the LINER
emission and derive an estimate for the jet power. Our emission line model is
remarkably robust against variation of input parameters, and so offers a
generic explanation for the excitation of LINER galaxies, including those of
spiral type such as NGC 3031 (M81).Comment: Accepted for publication in Astrophysical Journal. 16 pages, 12
figure
Dissecting Galaxies: Separating Star Formation, Shock Excitation and AGN Activity in the Central Region of NGC 613
The most rapidly evolving regions of galaxies often display complex optical
spectra with emission lines excited by massive stars, shocks and accretion onto
supermassive black holes. Standard calibrations (such as for the star formation
rate) cannot be applied to such mixed spectra. In this paper we isolate the
contributions of star formation, shock excitation and active galactic nucleus
(AGN) activity to the emission line luminosities of individual spatially
resolved regions across the central 3 3 kpc region of the active
barred spiral galaxy NGC613. The star formation rate and AGN luminosity
calculated from the decomposed emission line maps are in close agreement with
independent estimates from data at other wavelengths. The star formation
component traces the B-band stellar continuum emission, and the AGN component
forms an ionization cone which is aligned with the nuclear radio jet. The
optical line emission associated with shock excitation is cospatial with strong
and [Fe II] emission and with regions of high ionized gas velocity
dispersion ( km s). The shock component also traces the
outer boundary of the AGN ionization cone and may therefore be produced by
outflowing material interacting with the surrounding interstellar medium. Our
decomposition method makes it possible to determine the properties of star
formation, shock excitation and AGN activity from optical spectra, without
contamination from other ionization mechanisms.Comment: 16 pages, 12 figures. Accepted for publication in MNRA