17,645 research outputs found
The nature and evolution of Ultraluminous Infrared Galaxies: A mid-infrared spectroscopic survey
We report the first results of a low resolution mid-infrared spectroscopic
survey of an unbiased, far-infrared selected sample of 60 ultraluminous
infrared galaxies, using ISOPHOT-S on board ISO. We use the ratio of the 7.7um
`PAH' emission feature to the local continuum as a discriminator between
starburst and AGN activity. About 80% of all the ULIRGs are found to be
predominantly powered by star formation but the fraction of AGN powered objects
increases with luminosity.
Observed ratios of the PAH features in ULIRGs differ slightly from those in
lower luminosity starbursts. This can be plausibly explained by the higher
extinction and/or different physical conditions in the interstellar medium of
ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of
feature-free 5.9um continuum to the IRAS 60um continuum, confirming suggestions
that strong mid-IR continuum is a prime AGN signature. The location of
starburst-dominated ULIRGs in such a diagram is consistent with previous
ISO-SWS spectroscopy which implies significant extinction even in the
mid-infrared.
We have searched for indications that ULIRGs which are advanced mergers might
be more AGN-like, as postulated by the classical evolutionary scenario. No such
trend has been found amongst those objects for which near infrared images are
available to assess their likely merger status.Comment: aastex, 4 eps figures. Revised version, accepted by ApJ (Letters
CTMC calculations of electron capture and ionization in collisions of multiply charged ions with elliptical Rydberg atoms
We have performed classical trajectory Monte Carlo (CTMC) studies of electron
capture and ionization in multiply charged (Q=8) ion-Rydberg atom collisions at
intermediate impact velocities. Impact parallel to the minor and to the major
axis, respectively, of the initial Kepler electron ellipse has been
investigated. The important role of the initial electron momentum distribution
found for singly charged ion impact is strongly disminished for higher
projectile charge, while the initial spatial distribution remains important for
all values of Q studied.Comment: 3 pages, 5 figure
Computing FO-Rewritings in EL in Practice: from Atomic to Conjunctive Queries
A prominent approach to implementing ontology-mediated queries (OMQs) is to
rewrite into a first-order query, which is then executed using a conventional
SQL database system. We consider the case where the ontology is formulated in
the description logic EL and the actual query is a conjunctive query and show
that rewritings of such OMQs can be efficiently computed in practice, in a
sound and complete way. Our approach combines a reduction with a decomposed
backwards chaining algorithm for OMQs that are based on the simpler atomic
queries, also illuminating the relationship between first-order rewritings of
OMQs based on conjunctive and on atomic queries. Experiments with real-world
ontologies show promising results
Mid-Infrared line diagnostics of Active Galaxies -- A spectroscopic AGN survey with ISO-SWS
We present medium resolution (R approx. 1500) ISO-SWS 2.4--45 micron spectra
of a sample of 29 galaxies with active nuclei. This data set is rich in fine
structure emission lines tracing the narrow line regions and (circum-)nuclear
star formation regions, and it provides a coherent spectroscopic reference for
future extragalactic studies in the mid-infrared. We use the data set to
briefly discuss the physical conditions in the narrow line regions (density,
temperature, excitation, line profiles) and to test for possible differences
between AGN sub-types. Our main focus is on new tools for determining the
propertibes of dusty galaxies and on the AGN-starburst connection. We present
mid-IR line ratio diagrams which can be used to identify composite (starburst +
AGN) sources and to distinguish between emission excited by active nuclei and
emission from (circum-nuclear) star forming regions. For instance, line ratios
of high to low excitation lines like [O IV]25.9um/[Ne II]12.8um, that have been
used to probe for AGNs in dusty objects, can be examined in more detail and
with better statistics now. In addition, we present two-dimensional diagnostic
diagrams that are fully analogous to classical optical diagnostic diagrams, but
better suited for objects with high extinction. Finally, we discuss
correlations of mid-infrared line fluxes to the mid- and far-infrared
continuum. We compare these relations to similar relations in starburst
galaxies in order to examine the contribution of AGNs to the bolometric
luminosities of their host galaxies. The spectra are available in electronic
form from the authors.Comment: 24 pages, 23 figures, 5 tables. Accepted for A&
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