1,065 research outputs found
H2 formation and excitation in the Stephan's Quintet galaxy-wide collision
Context. The Spitzer Space Telescope has detected a powerful (L(H2)~10^41 erg
s-1) mid-infrared H2 emission towards the galaxy-wide collision in the
Stephan's Quintet (SQ) galaxy group. This discovery was followed by the
detection of more distant H2-luminous extragalactic sources, with almost no
spectroscopic signatures of star formation. These observations set molecular
gas in a new context where one has to describe its role as a cooling agent of
energetic phases of galaxy evolution. Aims. The SQ postshock medium is observed
to be multiphase, with H2 gas coexisting with a hot (~ 5 10^6 K), X-ray
emitting plasma. The surface brightness of H2 lines exceeds that of the X-rays
and the 0-0 S(1) H2 linewidth is ~ 900 km s-1, of the same order of the
collision velocity. These observations raise three questions we propose to
answer: (i) Why H2 is present in the postshock gas ? (ii) How can we account
for the H2 excitation ? (iii) Why H2 is a dominant coolant ? Methods. We
consider the collision of two flows of multiphase dusty gas. Our model
quantifies the gas cooling, dust destruction, H2 formation and excitation in
the postshock medium. Results. (i) The shock velocity, the post-shock
temperature and the gas cooling timescale depend on the preshock gas density.
The collision velocity is the shock velocity in the low density volume filling
intercloud gas. This produces a ~ 5 10^6 K, dust-free, X-ray emitting plasma.
The shock velocity is smaller in clouds. We show that gas heated to
temperatures less than 10^6 K cools, keeps its dust content and becomes H2
within the SQ collision age (~ 5 10^6 years). (ii) Since the bulk kinetic
energy of the H2 gas is the dominant energy reservoir, we consider that the H2
emission is powered by the dissipation of kinetic turbulent energy. (Abridged)Comment: 19 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysics Minor editing and typo
H_2 formation and excitation in the Stephan's Quintet galaxy-wide collision
Context. The Spitzer Space Telescope has detected a powerful (L_(H_2) ~ 10^(41) erg s^(-1)) mid-infrared H_2 emission towards the galaxy-wide collision in the Stephan's Quintet (henceforth SQ) galaxy group. This discovery was followed by the detection of more distant H_2-luminous extragalactic sources, with almost no spectroscopic signatures of star formation. These observations place molecular gas in a new context where one has to describe its role as a cooling agent of energetic phases of galaxy evolution.
Aims. The SQ postshock medium is observed to be multiphase, with H_2 gas coexisting with a hot (~5 × 10^6 K), X-ray emitting plasma. The surface brightness of H_2 lines exceeds that of the X-rays and the 0-0 S(1)H_2 linewidth is ~900 km s^(-1), of the order of the collision velocity. These observations raise three questions we propose to answer: (i) why is H_2 present in the postshock gas? (ii) How can we account for the H_2 excitation? (iii) Why is H_2 a dominant coolant?
Methods. We consider the collision of two flows of multiphase dusty gas. Our model quantifies the gas cooling, dust destruction, H_2 formation and excitation in the postshock medium.
Results. (i) The shock velocity, the post-shock temperature and the gas cooling timescale depend on the preshock gas density. The collision velocity is the shock velocity in the low density volume-filling intercloud gas. This produces a ~5 × 10^6 K, dust-free, X-ray emitting plasma. The shock velocity is lower in clouds. We show that gas heated to temperatures of less than 10^6 K cools, keeps its dust content and becomes H_2 within the SQ collision age (~5 × 10^6 years). (ii) Since the bulk kinetic energy of the H_2 gas is the dominant energy reservoir, we consider that the H_2 emission is powered by the dissipation of kinetic turbulent energy. We model this dissipation with non-dissociative MHD shocks and show that the H_2 excitation can be reproduced by a combination of low velocities shocks (5-20 km s^(-1)) within dense (n_H > 10^3 cm^(-3)) H_2 gas. (iii) An efficient transfer of the bulk kinetic energy to turbulent motion of much lower velocities within molecular gas is required to make H_2 a dominant coolant of the postshock gas. We argue that this transfer is mediated by the dynamic interaction between gas phases and the thermal instability of the cooling gas. We quantify the mass and energy cycling between gas phases required to balance the dissipation of energy through the H_2 emission lines.
Conclusions. This study provides a physical framework to interpret H_2 emission from H_2-luminous galaxies. It highlights the role that H_2 formation and cooling play in dissipating mechanical energy released in galaxy collisions. This physical framework is of general relevance for the interpretation of observational signatures, in particular H_2 emission, of mechanical energy dissipation in multiphase gas
Collisional excitation of water by hydrogen atoms
We present quantum dynamical calculations that describe the rotational
excitation of HO due to collisions with H atoms. We used a recent, high
accuracy potential energy surface, and solved the collisional dynamics with the
close-coupling formalism, for total energies up to 12 000 cm. From these
calculations, we obtained collisional rate coefficients for the first 45 energy
levels of both ortho- and para-HO and for temperatures in the range T =
5-1500 K. These rate coefficients are subsequently compared to the values
previously published for the HO / He and HO / H collisional
systems. It is shown that no simple relation exists between the three systems
and that specific calculations are thus mandatory
ISOCAM spectro-imaging of the H2 rotational lines in the supernova remnant IC443
We report spectro-imaging observations of the bright western ridge of the
supernova remnant IC 443 obtained with the ISOCAM circular variable filter
(CVF) on board the Infrared Space Observatory (ISO). This ridge corresponds to
a location where the interaction between the blast wave of the supernova and
ambient molecular gas is amongst the strongest. The CVF data show that the 5 to
14 micron spectrum is dominated by the pure rotational lines of molecular
hydrogen (v = 0--0, S(2) to S(8) transitions). At all positions along the
ridge, the H2 rotational lines are very strong with typical line fluxes of
10^{-4} to 10^{-3} erg/sec/cm2/sr. We compare the data to a new time-dependent
shock model; the rotational line fluxes in IC 443 are reproduced within factors
of 2 for evolutionary times between 1,000 and 2,000 years with a shock velocity
of 30 km/sec and a pre-shock density of 10^4 /cm3.Comment: To appear in Astronomy and Astrophysic
Observations and modeling of the dust emission from the H_2-bright galaxy-wide shock in Stephan's Quintet
Context. Spitzer Space Telescope observations have detected powerful mid-infrared (mid-IR) H_2 rotational line emission from the X-ray emitting large-scale shock (~15 × 35 kpc^2) associated with a galaxy collision in Stephan's Quintet (SQ). Because H_2 forms on dust grains, the presence of H_2 is physically linked to the survival of dust, and we expect some dust emission to originate in the molecular gas.
Aims. To test this interpretation, IR observations and dust modeling are used to identify and characterize the thermal dust emission from the shocked molecular gas.
Methods. The spatial distribution of the IR emission allows us to isolate the faint PAH and dust continuum emission associated with the molecular gas in the SQ shock. We model the spectral energy distribution (SED) of this emission, and fit it to Spitzer observations. The radiation field is determined with GALEX UV, HST V-band, and ground-based near-IR observations. We consider two limiting cases for the structure of the H_2 gas: it is either diffuse and penetrated by UV radiation, or fragmented into clouds that are optically thick to UV.
Results. Faint PAH and dust continuum emission are detected in the SQ shock, outside star-forming regions. The 12/24 μm flux ratio in the shock is remarkably close to that of the diffuse Galactic interstellar medium, leading to a Galactic PAH/VSG abundance ratio. However, the properties of the shock inferred from the PAH emission spectrum differ from those of the Galaxy, which may be indicative of an enhanced fraction of large and neutrals PAHs. In both models (diffuse or clumpy H_2 gas), the IR SED is consistent with the expected emission from dust associated with the warm (> 150 K) H_2 gas, heated by a UV radiation field of intensity comparable to that of the solar neighborhood. This is in agreement with GALEX UV observations that show that the intensity of the radiation field in the shock is GUV = 1.4±0.2 [Habing units].
Conclusions. The presence of PAHs and dust grains in the high-speed (~1000 km s^(-1)) galaxy collision suggests that dust survives. We propose that the dust that survived destruction was in pre-shock gas at densites higher than a few 0.1 cm^(-3), which was not shocked at velocities larger than ~200 km s^(-1). Our model assumes a Galactic dust-to-gas mass ratio and size distribution, and current data do not allow us to identify any significant deviations of the abundances and size distribution of dust grains from those of the Galaxy. Our model calculations show that far-IR Herschel observations will help in constraining the structure of the molecular gas, and the dust size distribution, and thereby to look for signatures of dust processing in the SQ shock
Real-Time Rough Extraction of Foreground Objects in MPEG1,2 Compressed Video
This paper describes a new approach to extract foreground objects in MPEG1,2 video streams, in the framework of “rough indexing paradigm”, that is starting from rough data obtained by only partially decoding the compressed stream. In this approach we use both P-frame motion information and I-frame colour information to identify and extract foreground objects. The particularity of our approach with regards to the state of the art methods consists in a robust estimation of camera motion and its use for localisation of real objects and filtering of parasite zones.
Secondly, a spatio-temporal filtering of roughly segmented objects at DC resolution is fulfilled using motion trajectory and gaussian-like shape characteristic function. This paradigm results in content description in real time, maintaining a good level of details
Hierarchical progressive surveys. Multi-resolution HEALPix data structures for astronomical images, catalogues, and 3-dimensional data cubes
Scientific exploitation of the ever increasing volumes of astronomical data
requires efficient and practical methods for data access, visualisation, and
analysis. Hierarchical sky tessellation techniques enable a multi-resolution
approach to organising data on angular scales from the full sky down to the
individual image pixels. Aims. We aim to show that the Hierarchical progressive
survey (HiPS) scheme for describing astronomical images, source catalogues, and
three-dimensional data cubes is a practical solution to managing large volumes
of heterogeneous data and that it enables a new level of scientific
interoperability across large collections of data of these different data
types. Methods. HiPS uses the HEALPix tessellation of the sphere to define a
hierarchical tile and pixel structure to describe and organise astronomical
data. HiPS is designed to conserve the scientific properties of the data
alongside both visualisation considerations and emphasis on the ease of
implementation. We describe the development of HiPS to manage a large number of
diverse image surveys, as well as the extension of hierarchical image systems
to cube and catalogue data. We demonstrate the interoperability of HiPS and
Multi-Order Coverage (MOC) maps and highlight the HiPS mechanism to provide
links to the original data. Results. Hierarchical progressive surveys have been
generated by various data centres and groups for ~200 data collections
including many wide area sky surveys, and archives of pointed observations.
These can be accessed and visualised in Aladin, Aladin Lite, and other
applications. HiPS provides a basis for further innovations in the use of
hierarchical data structures to facilitate the description and statistical
analysis of large astronomical data sets.Comment: 21 pages, 6 figures. Accepted for publication in Astronomy &
Astrophysic
Mumford dendrograms and discrete p-adic symmetries
In this article, we present an effective encoding of dendrograms by embedding
them into the Bruhat-Tits trees associated to -adic number fields. As an
application, we show how strings over a finite alphabet can be encoded in
cyclotomic extensions of and discuss -adic DNA encoding. The
application leads to fast -adic agglomerative hierarchic algorithms similar
to the ones recently used e.g. by A. Khrennikov and others. From the viewpoint
of -adic geometry, to encode a dendrogram in a -adic field means
to fix a set of -rational punctures on the -adic projective line
. To is associated in a natural way a
subtree inside the Bruhat-Tits tree which recovers , a method first used by
F. Kato in 1999 in the classification of discrete subgroups of
.
Next, we show how the -adic moduli space of
with punctures can be applied to the study of time series of
dendrograms and those symmetries arising from hyperbolic actions on
. In this way, we can associate to certain classes of dynamical
systems a Mumford curve, i.e. a -adic algebraic curve with totally
degenerate reduction modulo .
Finally, we indicate some of our results in the study of general discrete
actions on , and their relation to -adic Hurwitz spaces.Comment: 14 pages, 6 figure
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