453 research outputs found
Nuclear embedded star clusters in NGC 7582
We report on the discovery of several compact regions of mid-infrared
emission in the starforming circum nuclear disk of the starburst/Seyfert2
galaxy NGC7582. The compact sources do not have counterparts in the optical and
near-infrared, suggesting that they are deeply embedded in dust. We use the
[NeII]12.8 micron line emission to estimate the emission measure of the ionized
gas, which in turn is used to assess the number of ionizing photons. Two of the
brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the
fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst,
we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number
of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that
the compact mid-infrared sources are likely to be young, embedded star
clusters, of which only a few are known so far. Our observation highlights the
need for high resolution mid-infrared imaging to discover and study embedded
star clusters in the proximity of active galactic nuclei.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
Revisiting the location and environment of the central engine in NGC1068
We revisit in this paper the location of the various components observed in
the AGN of NGC1068. Discrepancies between previously published studies are
explained, and a new measurement for the absolute location of the K-band
emission peak is provided. It is found to be consistent with the position of
the central engine as derived by Gallimore (1997), Capetti (1997) and Kishimoto
(1999). A series of map overlays is then presented and discussed. Model
predictions of dusty tori show that the nuclear unresolved NIR-MIR emission is
compatible with a broad range of models: the nuclear SED alone does not
strongly constrain the torus geometry, while placing reasonable constraints on
its size and thickness. The extended MIR emission observed within the ionizing
cone is shown to be well explained by the presence of optically thick dust
clouds exposed to the central engine radiation and having a small covering
factor. Conversely, a distribution of diffuse dust particles within the
ionizing cone is discarded. A simple model for the H2 and CO emission observed
perpendicularly to the axis of the ionizing cone is proposed. We show that a
slight tilt between the molecular disc and the Compton thick central absorber
naturally reproduces the observed distribution of H2 of CO emission.Comment: 17 pages, 11 figures, revised version for A&
Embedded clusters in NGC1808 central starburst - Near-infrared imaging and spectroscopy
In the course of a mid-infrared imaging campaign of close-by active galaxies,
we discovered the mid-infrared counterparts of bright compact radio sources in
the central star-forming region of NGC1808. We aim at confirming that these
sources are deeply embedded, young star clusters and at deriving some of their
intrinsic properties. To complement the mid-infrared data, we have collected a
set of near-infrared data with ISAAC at the VLT: J, Ks, and L' images, as well
as low-resolution, long-slit spectra for three of the sources. Surprisingly,
the new images unveil a near-infrared counterpart for only one of the
mid-infrared/radio sources, namely M8 in the L' band. All the other sources are
so deeply embedded that their emission does not pop out above an extended
diffuse near-infrared emission. The near-infrared spectra of the sources look
alike, with intense, ionised hydrogen lines. This supports the interpretation
of these sources in terms of embedded young clusters. We derive extinctions and
ionising photon production rates for two of the clusters.Comment: accepted A&A research note, 7 pages, 4 figures and 1 tabl
Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
We present the results of a systematic study of mid-IR spectra of Galactic
regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral,
starburst), observed by the satellites ISO and Spitzer. We study the relative
variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially
resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well
as among 90 integrated spectra of 50 objects. Our main results are that the
6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios
between these bands and the 11.3 micron band varies by one order of magnitude.
This implies that the properties of the PAHs are remarkably universal
throughout our sample, and that the relative variations of the band ratios are
mainly controled by the fraction of ionized PAHs. In particular, we show that
we can rule out both the modification of the PAH size distribution, and the
mid-infrared extinction, as an explanation of these variations. Using a few
well-studied Galactic regions (including the spectral image of the Orion Bar),
we give an empirical relation between the I(6.2)/I(11.3) ratio and the
ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful
quantitative diagnostic tool of the physical conditions in the regions where
the PAH emission originates. Finally, we discuss the physical interpretation of
the I(6.2)/I(11.3) ratio, on galactic size scales.Comment: Accepted by the ApJ, 67 pages, 70 figure
Two-dimensional crystals far from equilibrium
When driven by nonequilibrium fluctuations, particle systems may display
phase transitions and physical behaviour with no equilibrium counterpart. We
study a two-dimensional particle model initially proposed to describe driven
non-Brownian suspensions undergoing nonequilibrium absorbing phase transitions.
We show that when the transition occurs at large density, the dynamics produces
long-range crystalline order. In the ordered phase, long-range translational
order is observed because equipartition of energy is lacking, phonons are
suppressed, and density fluctuations are hyperuniform. Our study offers an
explicit microscopic model where nonequilibrium violations of the Mermin-Wagner
theorem stabilize crystalline order in two dimensions.Comment: 6 pages, 4 figures. V2 accepted for publication at Phys. Rev. Let
Mid-infrared imaging of NGC1068 with VISIR at the VLT
High resolution mid-infrared (MIR) images of the central region of NGC1068
have been obtained with VISIR, the multi-mode MIR instrument recently installed
at the ESO/VLT on Paranal. A map of the emission at 12.8mic with increased
sensitivity over the central 8"x 8" area is discussed. It shows a central core
(unresolved along the E-W direction) and an extended emission which draws a
spiral pattern similar to that observed on near-infrared images. Patches of MIR
emission can be detected up to a distance of 4" from the core. The deconvolved
12.8mic map is fully consistent with previous high-resolution MIR observations.
It highlights the structure of the extended emission, already seen on the
un-deconvolved image, and allows to identify a set of mid-infrared sources: 7
in the NE quadrant and 5 in the SW quadrant. The MIR emission map is compared
with those obtained at comparable angular resolution in the near-infrared and
in the [OIII] line emission. The very good correlation between the VISIR map
and the HST optical map supports the idea that the MIR emission not associated
with the torus arises from dust associated with the narrow line region clouds.
The N-S extension of the MIR core (0.44") is then probably simply due to the
mixing of the MIR emission from the dusty torus and the MIR emission from NLR
cloud B, located only 0.1" to the North.Comment: 5 pages, 1 figure (reduced quality), accepted MNRAS Letter. The paper
with full resolution figure can be downloaded at
http://www.sc.eso.org/~egallian/VISIR/N1068_VISIR.ps.g
Birth and death processes with absorption
Spectral measures and transition probabilities of birth and death processes with λ0=μ0=0 are obtained as limite when λ0→0+ of the corresponding quantities. In particular the case of finite population is discussed in full detail. Pure birth and death processes are used to derive an inequality for Dirichlet polynomials
High angular resolution near-infrared integral field observations of young star cluster complexes in NGC1365
This paper presents and examines new near-infrared integral field
observations of the three so-called 'embedded star clusters' located in the
nuclear region of NGC1365. Adaptive-optics- corrected K-band data cubes were
obtained with the ESO/VLT instrument SINFONI. The continuum in the K-band and
emission lines such as HeI, Bracket-gamma, and several H2 lines were mapped at
an achieved angular resolution of 0.2arcsec over a field of 3x3arcsec^2 around
each source. We find that the continuum emission of the sources is spatially
resolved. This means that they are indeed cluster complexes confined to regions
of about 50pc extension. We performed robust measurements of the equivalent
width of the CO absorption band at 2.3micro and of Bracket-gamma. For the main
mid-infrared bright sources, the data only allow us to determine an upper limit
to the equivalent width of the CO bands. Under the assumption of an
instantaneously formed standard initial mass function Starburst99 model, the
new measurements are found to be incompatible with previously published
mid-infrared line ratios. We show that an upper mass limit of 25 to 30 solar
masses, lower than the typically assumed 100solar masses, allows one to simply
remove this inconsistency. For such a model, the measurements are consistent
with ages in the range of 5.5Myr to 6.5Myr, implying masses in the range from 3
to 10 x 10^6 solar masses. We detect extended gas emission both in HII and H2.
We argue that the central cluster complexes are the sources of excitation for
the whole nebulae, through ionisation and shock heating. We detect a blue wing
on the Bracket-gamma emission profile, suggesting the existence of gas outflows
centred on the cluster complexes. We do not find any evidence for the presence
of a lower mass cluster population, which would fill up a 'traditional' power
law cluster mass function.Comment: 15 pages, 10 figures, accepted for publication in A&
Editorial: The olivo-cerebellar system
Investigation on the olivo-cerebellum system has attained a high level of sophistication leading to define several structural and functional properties of neurons, synapses, connections and circuits. Research has expanded and deepened in so many directions, and so many theories and models have been proposed, that an ensemble review of the matter is now neede
A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor
More complete knowledge of galaxy evolution requires understanding the
process of star formation and interaction between the interstellar radiation
field and the interstellar medium in galactic environments traversing a wide
range of physical parameter space. Here we focus on the impact of massive star
formation on the surrounding low metallicity ISM in 30 Doradus in the Large
Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the
early universe, results in less ultra-violet shielding for the formation of the
molecular gas necessary for star formation to proceed. The half-solar
metallicity gas in this region is strongly irradiated by the super star cluster
R136, making it an ideal laboratory to study the structure of the ISM in an
extreme environment. Our spatially resolved study investigates the gas heating
and cooling mechanisms, particularly in the photo-dissociation regions where
the chemistry and thermal balance are regulated by far-ultraviolet photons (6
eV< h\nu <13.6 eV).
We present Herschel observations of far-infrared fine-structure lines
obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines
from Herschel and Spitzer observations with ground-based CO data to provide
diagnostics on the properties and the structure of the gas by modeling it with
the Meudon PDR code. We derive the spatial distribution of the radiation field,
the pressure, the size, and the filling factor of the photodissociated gas and
molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K
and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR
modeling. Assuming a plane-parallel geometry and a uniform medium, we find a
total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to
3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three
dimensional structure of the gas. (Abridged)Comment: 20 pages, 23 figures, accepted in A&
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