142 research outputs found
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
The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope V. Optimal algorithms for planning multi-object spectroscopic observations
We present an overview of the capabilities and key algorithms employed in the
so-called eMPT software suite developed for planning scientifically optimized,
multi-object spectroscopic (MOS) observations with the Micro-Shutter Array
(MSA) of the Near-Infrared Spectrograph (NIRSpec) instrument on board the James
Webb Space Telescope (JWST), the first multi-object spectrograph to operate in
space. NIRSpec MOS mode is enabled by a programmable MSA, a regular grid of
~250,000 individual apertures that projects to a static, semi-regular pattern
of available slits on the sky and makes the planning and optimization of an MSA
observation a rather complex task. As such, the eMPT package is offered to the
NIRSpec user community as a supplement to the MSA Planning Tool (MPT) included
in the STScI Astronomer's Proposal Tool (APT) to assist in the planning of
NIRSpec MOS proposals requiring advanced functionality to meet ambitious
science goals. The eMPT produces output that can readily be imported and
incorporated into the user's observing program within the APT to generate a
customized MPT MOS observation. Furthermore, its novel algorithms and modular
approach make it highly flexible and customizable, providing users the option
to finely control the workflow and even insert their own software modules to
tune their MSA slit masks to the particular scientific objectives at hand
Do Jet-Driven Shocks ionize the Narrow Line Regions of Seyfert Galaxies?
We consider a model in which the narrow line regions (NLRs) of Seyfert
galaxies are photoionized ``in situ'' by fast (300 -- 1,000 km/s), radiative
shock waves driven into the interstellar medium of the galaxy by radio jets
from the active nucleus. Such shocks are powerful sources of soft X-rays. We
compute the expected ratio of the count rates in the ROSAT PSPC and Einstein
IPC detectors to the [OIII] \lambda 5007 flux as a function of shock velocity,
and compare these ratios with observations of type 2 Seyferts. If most of the
observed soft X-ray emission from these galaxies originates in the NLR and the
absorbing hydrogen column is similar to that inferred from the reddening of the
NLR, a photoionizing shock model with shock velocity 400 -- 500 km/s
is compatible with the observed ratios. High angular resolution observations
with AXAF are needed to isolate the X-ray emission of the NLR and measure its
absorbing column, thus providing a more conclusive test. We also calculate the
expected coronal iron line emission from the shocks. For most Seyfert 2s, the
[Fe X] \lambda 6374/H \beta$ ratio is a factor of 2 -- 14 lower than the
predictions of 300 -- 500 km/s shock models, suggesting that less hot gas is
present than required by these models.Comment: Astrophys J. Letters 1999 March 10 issue, Vol. 51
OASIS High-Resolution Integral Field Spectroscopy of the SAURON Ellipticals and Lenticulars
We present a summary of high-spatial resolution follow-up observations of the
elliptical (E) and lenticular (S0) galaxies in the SAURON survey using the
OASIS integral field spectrograph. The OASIS observations explore the central
8x10" regions of these galaxies using a spatial sampling four times higher than
SAURON, often revealing previously undiscovered features. Around 75% (31/48) of
the SAURON E/S0s with central velocity dispersion >= 120 km/s were observed
with OASIS, covering well the original SAURON representative sample. We present
here an overview of this follow-up survey, and some preliminary results on
individual objects, including a previously unreported counter-rotating core in
NGC 4382; the decoupled stellar and gas velocity fields of NGC 2768; and the
strong age gradient towards the centre of NGC 3489.Comment: 4 pages, 5 figures. Accepted for publication in Astron. Nachr. as
refereed proceedings of Euro3D Science Workshop, IoA Cambridge, May 200
Nuclear gas dynamics in Arp 220 - sub-kiloparsec scale atomic hydrogen disks
We present new, high angular resolution (~0.22") MERLIN observations of
neutral hydrogen (HI) absorption and 21-cm radio continuum emission across the
central ~900 parsecs of the ultraluminous infrared galaxy, Arp220. Spatially
resolved HI absorption is detected against the morphologically complex and
extended 21-cm radio continuum emission, consistent with two counterrotating
disks of neutral hydrogen, with a small bridge of gas connecting the two.
We propose a merger model in which the two nuclei represent the galaxy cores
which have survived the initial encounter and are now in the final stages of
merging, similar to conclusions drawn from previous CO studies (Sakamoto,
Scoville & Yun 1999). However, we suggest that instead of being coplanar with
the main CO disk (in which the eastern nucleus is embedded), the western
nucleus lies above it and, as suggested by bridge of HI connecting the two
nuclei, will soon complete its final merger with the main disk. We suggest that
the collection of radio supernovae (RSN) detected in VLBA studies in the more
compact western nucleus represent the second burst of star formation associated
with this final merger stage and that free-free absorption due to ionised gas
in the bulge-like component can account for the observed RSN distribution.
(Abridged)Comment: 26 pages including 8 figures and 1 table; accepted for publication in
Ap
Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275
Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been
combined with HST WFPC2 photometry to explore the nature and origin of these
objects and discriminate between merger and cooling flow scenarios for globular
cluster formation. The objects we have studied are not HII regions, but rather
star clusters, yet their integrated spectral properties do not resemble young
or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The
clusters' Balmer absorption appears to be too strong to be consistent with any
of the standard Bruzual & Charlot evolutionary models at any metallicity. If
these models are adopted, an IMF which is skewed to high masses provides a
better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces
the observed Balmer equivalent widths and colors at about 450 Myr. Formation in
a continuous cooling flow appears to be ruled out since the age of the clusters
is much larger than the cooling time, the spatial scale of the clusters is much
smaller than the cooling flow radius, and the deduced star formation rate in
the cooling flow favors a steep rather than a flat IMF. A merger would have to
produce clusters only in the central few kpc, presumably from gas in the
merging galaxies which was channeled rapidly to the center. Widespread shocks
in merging galaxies cannot have produced these clusters. If these objects are
confirmed to have a relatively flat, or truncated, IMF it is unclear whether or
not they will evolve into objects we would regard as bona fide globular
clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to
appear in the AJ (August issue), also available at
http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution
Fig.1) Revised Version: previous posted version was an uncorrect ealier
iteration, parts of the text, tables and figures changed. The overall
conclusions remain unchange
Chandra X-ray Observation of NGC 4151
We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC
4151. The images show the extended soft X-ray emission on the several hundreds
of pc scale with better sensitivity than previously obtained. The spectrum of
the unresolved nuclear source may be described by a heavily absorbed (N_{H}
\simeq 3 \times 10^{22} cm^-2), hard power-law (\Gamma \simeq 0.3) plus soft
emission from either a power-law (\Gamma \simeq 2.6) or a thermal (kT \simeq
0.6 keV) component. The flux of the high energy component has decreased from
that observed by ASCA in 1993 and the spectrum is much harder.The large
difference between the soft and hard spectral shapes does not favor the partial
covering or scattering model of the ``soft excess''. Instead, it is likely that
the hard and soft nuclear components represent intrinsically different X-ray
sources. Spectra of the extended emission to almost 1 kpc NE and SW of the
nucleus have also been obtained. The spectra of these regions may be described
by either thermal bremsstrahlung (kT \simeq 0.4-0.7 keV) or power-law (\Gamma
\simeq 2.5-3.2) continua plus 3 emission lines. There is an excellent
correlation between the extended X-ray and [O III]\lambda 5007 line emissions.
We discuss the nature of the extended X-ray emission. Upper limit to the
electron scattering column was obtained. This upper limit is much too low for
the soft X-rays to be electron scattered nuclear radiation.Comment: 18 pages, 17 figures, Accepted for publication in The Astrophysical
Journa
A companion to a quasar at redshift 4.7
There is a growing consensus that the emergence of quasars at high redshifts
is related to the onset of galaxy formation, suggesting that the detection of
concentrations of gas accompanying such quasars should provide clues about the
early history of galaxies. Quasar companions have been recently identified at
redshifts up to . Here we report observations of Lyman-
emission (a tracer of ionised hydrogen) from the companion to a quasar at
=4.702, corresponding to a time when the Universe was less than ten per cent
of its present age. We argue that most of the emission arises in a gaseous
nebula that has been photoionised by the quasar, but an additional component of
continuum light -perhaps quasar light scattered from dust in the companion
body, or emission from young stars within the nebula- appears necessary to
explain the observations. These observations may be indicative of the first
stages in the assembly of galaxy-sized structures.Comment: 8 pages, 4 figures, plain LaTeX. Accepted for publication in Natur
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