1,269 research outputs found
Gigahertz-Peaked Spectrum Radio Sources in Nearby Galaxies
There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain
accreting massive black holes and that the nuclear radio emission is dominated
by parsec-scale jets launched by these black holes. Here, we present
preliminary results on the 1.4 GHz to 667 GHz spectral shape of a well-defined
sample of 16 LLAGNs. The LLAGNs have a falling spectrum at high GHz
frequencies. Several also show a low-frequency turnover with a peak in the 1-20
GHz range. The results provide further support for jet dominance of the core
radio emission. The LLAGNs show intriguing similarities with gigahertz-peaked
spectrum (GPS) sources.Comment: 6 pages, to appear in ASP Conference series, 2002, Vol. 25
Radio Variability in Seyfert Nuclei
Comparison of 8.4-GHz radio images of a sample of 11 early-type Seyfert
galaxies with previous observations reveals possible variation in the nuclear
radio flux density in 5 of them over a 7-yr period. We find no correlation
between radio variability and nuclear radio luminosity or Seyfert nuclear type,
although the sample is small and dominated by type 2 Seyferts. Instead, a
possible correlation between the presence of nuclear radio variability and the
absence of ~100-pc-scale radio emission is seen. NGC2110 is the only source
with significant extended radio structure and strong nuclear variability (>38%
nuclear decline over seven years). Our results suggest that all Seyferts may
exhibit variation in their nuclear radio flux density at 8.4 GHz, but that
variability is more easily recognised in compact sources in which emission from
the variable nucleus is not diluted by unresolved, constant flux density
radio-jet emission within the central ~50 pc. If flares in radio light curves
correspond to ejection of new relativistic components or emergence of shocks in
the underlying flow, we suggest that radio jets may be intrinsically
non-relativistic during quiescence, but that Seyferts, as black-hole driven
AGN, have the capacity to accelerate relativistic jets during radio flares.
Taken together with the increased detection rate of flat spectrum radio nuclei
in Seyferts imaged at VLBI resolutions and the detection of variable water
megamaser emission, our results support the paradigm of intermittent periods of
quiescence and nuclear outburst across the Seyfert population. (Abridged).Comment: Accepted for publication in Astrophysical Journal; 15 pages, 7
figures and 3 table
Chandra Observations of Arp 220: The Nuclear Source
We present the first results from 60ks of observations of Arp 220 using the
ACIS-S instrument on Chandra. We report the detection of several sources near
the galaxy's nucleus, including a point source with a hard spectrum that is
coincident with the western radio nucleus B. This point source is mildly
absorbed (N_H ~ 3 x 10^22 cm^-2) and has an estimated luminosity of 4 x 10^40
erg/s. In addition, a fainter source may coincide with the eastern nucleus A.
Extended hard X-ray emission in the vicinity raises the total estimated nuclear
2-10 keV X-ray luminosity to 1.2 x 10^41 erg/s, but we cannot rule out a hidden
AGN behind columns exceeding 5 x 10^24 cm^-2. We also detect a peak of soft
X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from
the nucleus with a luminosity of 6 x 10^39 erg/s.Comment: Accepted for publication in Ap
Chandra Observations of Arp 220: The Nuclear Source
We present the first results from 60ks of observations of Arp 220 using the
ACIS-S instrument on Chandra. We report the detection of several sources near
the galaxy's nucleus, including a point source with a hard spectrum that is
coincident with the western radio nucleus B. This point source is mildly
absorbed (N_H ~ 3 x 10^22 cm^-2) and has an estimated luminosity of 4 x 10^40
erg/s. In addition, a fainter source may coincide with the eastern nucleus A.
Extended hard X-ray emission in the vicinity raises the total estimated nuclear
2-10 keV X-ray luminosity to 1.2 x 10^41 erg/s, but we cannot rule out a hidden
AGN behind columns exceeding 5 x 10^24 cm^-2. We also detect a peak of soft
X-ray emission to the west of the nucleus, and a hard point source 2.5 kpc from
the nucleus with a luminosity of 6 x 10^39 erg/s.Comment: Accepted for publication in Ap
Atomic Hydrogen Properties of AGN Host Galaxies: HI in 16 NUclei of GAlaxies (NUGA) Sources
We present a comprehensive spectroscopic imaging survey of the distribution
and kinematics of atomic hydrogen (HI) in 16 nearby spiral galaxies hosting low
luminosity AGN, observed with high spectral and spatial resolution (resolution:
~20 arcsec, 5 km/s) using the NRAO Very Large Array (VLA). The sample contains
a range of nuclear types, ranging from Seyfert to star-forming nuclei and was
originally selected for the NUclei of GAlaxies project (NUGA) - a spectrally
and spatially resolved interferometric survey of gas dynamics in nearby
galaxies designed to identify the fueling mechanisms of AGN and the relation to
host galaxy evolution. Here we investigate the relationship between the HI
properties of these galaxies, their environment, their stellar distribution and
their AGN type. The large-scale HI morphology of each galaxy is classified as
ringed, spiral, or centrally concentrated; comparison of the resulting
morphological classification with AGN type reveals that ring structures are
significantly more common in LINER than in Seyfert host galaxies, suggesting a
time evolution of the AGN activity together with the redistribution of the
neutral gas. Dynamically disturbed HI disks are also more prevalent in LINER
host galaxies than in Seyfert host galaxies. While several galaxies are
surrounded by companions (some with associated HI emission), there is no
correlation between the presence of companions and the AGN type
(Seyfert/LINER).Comment: 54 pages, 7 figures, accepted for publication in AJ. The
full-resolution version is available at
http://www.mpia.de/homes/haan/research.htm
Explaining two circumnuclear star forming rings in NGC5248
The distribution of gas in the central kiloparsec of a galaxy has a
dynamically rapid evolution. Nonaxisymmetries in the gravitational potential of
the galactic disk, such as a large scale stellar bar or spiral, can lead to
significant radial motion of gaseous material from larger radii to the central
region. The large influx of gas and the subsequent star formation keep the
central region constantly changing. However, the ability of gas to reach the
nucleus proper to fuel an AGN phase is not guaranteed. Gas inflow can be halted
at a circumnuclear star forming ring several hundred parsec away. The nearby
galaxy NGC5248 is especially interesting in this sense since it is said to host
2 circumnuclear star forming rings at 100pc and 370pc from its quiescent
nucleus. Here we present new subarcsecond PdBI+30m CO(2-1) emission line
observations of the central region. For the first time the molecular gas
distribution at the smallest stellar ring is resolved into a gas ring,
consistent with the presence of a quiescent nucleus. However, the molecular gas
shows no ring structure at the larger ring. We combine analyses of the gaseous
and stellar content in the central kiloparsec of this galaxy to understand the
gas distribution and dynamics of this star forming central region. We discuss
the probability of two scenarios leading to the current observations, given our
full understanding of this system, and discuss whether there are really two
circumnuclear star forming rings in this galaxy.Comment: Accepted for publication in A&A, 14pages + long tabl
Early GRB Optical and Infrared Afterglow Observations with the 2-m Robotic Liverpool Telescope
We present the first optical observations of a Gamma Ray Burst (GRB)
afterglow using the 2-m robotic Liverpool Telescope (LT), which is owned and
operated by Liverpool John Moores University and situated on La Palma. We
briefly discuss the capabilities of LT and its suitability for rapid follow-up
observations of early optical and infrared GRB light curves. In particular, the
combination of aperture, site, instrumentation and rapid response (robotic
over-ride mode aided by telescope's rapid slew and fully-opening enclosure)
makes the LT ideal for investigating the nature of short bursts, optically-dark
bursts, and GRB blast-wave physics in general. We briefly describe the LT's key
position in the RoboNet-1.0 network of robotic telescopes. We present the LT
observations of GRB041006 and use its gamma-ray properties to predict the time
of the break in optical light curve, a prediction consistent with the
observations.Comment: 4 pages, 1 figure, accepted for publication in Il nuovo cimento (4th
Workshop Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004
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