240 research outputs found
Relation between source and temperature fluctuations in photoionized nebulae
The magnitude of the temperature fluctuations (t^2) required to explain the
observed inconsistencies between metallicities inferred from recombination
lines and from forbidden lines cannot be attained by steady-state equilibrium
photoionization models. If on the other hand the nebular ionizing source was
variable, the temperature fluctuations t^2 would be significantly larger. We
investigate the time-dependent response of the nebular ionization and
temperature structure when photoionized by a periodically varying source. We
study how the asymptotic mean value, , behaves as a function of the period
or amplitude of the source variability. We find that the temperature
fluctuations occur only in the outer section of the nebula, close to the
ionization front, within a zone corresponding to 8-20% of the ionized layer's
thickness. We conclude that the amplitude of the exciting star variations
required to achieve a = 0.025 (as in the Orion nebula) is unacceptably
large. Source variability is therefore not a viable mechanism to explain the
observed values of t^2. We reach a similar conclusion from studies of the
temporal variability resulting from intermittent shadows behind opaque
condensations. We find that photoionized nebulae are on average less massive
but somewhat hotter in the case of cyclicly variable ionizing sources.Comment: 15 pages, 6 figures, submitted to Revista Mexicana de Astronomia y
Astrofisica, revised versio
A two-arm gaseous spiral in the inner 200 pc of the early-type galaxy NGC 2974: signature of an inner bar
TIGER integral-field spectrography and HST/WFPC2 imaging of the E3 galaxy NGC
2974 are used to derive the kinematics of the stellar and ionized gas
components in its central 500 pc. We derive a numerical two-integral
distribution function from a MGE mass model using the HQ formalism. The TIGER
as well as published long-slit stellar kinematics are well fitted with this
self-consistent model, requiring neither the addition of a significant mass
contribution from a hidden disc structure, nor the presence of a central dark
mass. The data reveal the presence of a striking, highly contrasted, two-arm
gaseous spiral structure within a radius of ~200 pc, corresponding to a total
mass of 6.8x10^4 Msun of ionized gas. We use a deconvolved TIGER datacube to
probe its kinematics at a resolution of about 0.35 arcsec. Strong departures
from circular motions are observed, as well as high velocity dispersion values
on the inner side of the arms. We interpret the observed gas morphology and
kinematics as the signature of streaming gas flows driven by a ~540 pc diameter
bar with Ome=700+/-100 km/s/kpc. This hypothesis is strongly supported by the
predictions of a density wave model. This model predicts that the bar should
lie at about 35 degree from the line of nodes, and implies gas inflow towards
the central ~50 pc. The quadrupole pertubation due to this bar is estimated to
represent less than 2% of the underlying gravitational potential. Despite its
weakness, the inner bar of NGC 2974 may be able to drive some gas within a 10
pc radius. We suggest that the presence of such inner bars might be more common
among early-type disk galaxies than is generally thought, and that deep
high-resolution emission-line imagery may be the best way to detect such
structures.Comment: 17 pages, 21 figures. MNRAS in press. Full resolution paper at
http://www-obs.univ-lyon1.fr/eric.emsellem/papers/n2974.ps.g
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
Gas and stellar dynamics in NGC 1068. Probing the galactic gravitational potential
We present Sauron 2D spectrography of the central 1.5 kpc of the nearby Sey2
galaxy NGC1068, encompassing the well-known NIR inner bar. We have successively
disentangled the respective contributions of the ionized gas and stars, thus
deriving their 2D distribution and kinematics. The [OIII] and Hbeta emission
lines exhibit very different spatial distribution and kinematics, the latter
following inner spiral arms with clumps associated with star formation. Strong
inwards streaming motions are observed in both the Hbeta and [OIII] kinematics.
The stellar kinematics also exhibit clear signatures of a non-axisymmetric
tumbling potential, with a twist in both the velocity and h3 fields. We
re-examined the long-slit data of Shapiro et al (2003) using pPXF: a strong
decoupling of h3 is revealed, and the central decrease in h4 hinted in the
Sauron data is confirmed. These data also suggest that NGC1068 is a good
candidate for a so-called sigma-drop. We confirm the possible presence of two
pattern speeds. We also examine the stellar kinematics of bars formed in
N-body+SPH simulations built from axisymmetric initial conditions. These
successfully reproduce a number of properties observed in the 2D kinematics of
NGC1068, and the long-slit data, showing that the kinematic signature of the
NIR bar is imprinted in the stellar kinematics. The remaining differences
between the models and the observed properties are mostly due to the exclusion
of star formation and the lack of the primary large-scale oval/bar in the
simulations. These models suggest that the inner bar could drive a significant
amount of gas down to a scale of ~300 pc. This is consistent with the
interpretation of the sigma-drop in NGC1068 being the result of central gas
accretion followed by an episode of star formation.Comment: accepted for publication in MNRAS, 20 pages, 17 figures (high res
version available at
www-obs.univ-lyon1.fr/eric.emsellem/preprints/NGC1068_Emsellemetal_final.pdf
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
Linking the Supermassive Black Hole Growth with the Megamaser Emission
High-resolution observations of the central few 100 pc of the galactic
nuclear environments remain prohibitive for large statistical samples, which
are crucial for tracing the links between central black hole formation, galaxy
formation and AGN activity over cosmic time. With this contribution, we present
novel ways of connecting the physics of black hole accretion with its immediate
environs via a new quantitative evaluation of the degree to which the strength
and spatial configuration of the water maser emission is associated with the
nuclear nebular galactic activity. We discuss possible evolutionary/causal
connections between these two types of emission, together with criteria that
could dramatically enhance our search for mega-maser systems in nearby galactic
centers. These are timely results given the interest in combining
high-resolution observations with extremely large optical telescopes and large
arrays that start to conquer the sub-millimeter window.Comment: 7 pages, 5 figures, to appear in "The Central Kiloparsec in Galactic
Nuclei: Astronomy at High Angular Resolution 2011", open access Journal of
Physics: Conference Series (JPCS), published by IOP Publishin
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
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