246 research outputs found
High-mass star formation in southern disk galaxies
As part of a major study of the physical processes of star formation and the evolution of galactic discs, the detailed distribution of high-mass star formation within southern late-type spirals and Magellanic-type galaxies is being measured by means of narrow-band imaging in Ha and the continuum, spectroscopic studies of prominent HII regions identified in the Ha images, and by radio mapping in neutral hydrogen and the continuum. The radio mapping will be undertaken with the Southern Hemisphere's first large, multi-frequency synthesis array, the Australia Telescope. Some optical imaging and spectroscopic data has already been acquired; the optical data and some preliminary results are described
Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons
The measurement of electron temperatures and metallicities in H ii regions
and Planetary Nebulae (PNe) has-for several decades-presented a problem:
results obtained using different techniques disagree. What it worse, they
disagree consistently. There have been numerous attempts to explain these
discrepancies, but none has provided a satisfactory solution to the problem. In
this paper, we explore the possibility that electrons in H ii regions and PNe
depart from a Maxwell-Boltzmann equilibrium energy distribution. We adopt a
"kappa-distribution" for the electron energies. Such distributions are widely
found in Solar System plasmas, where they can be directly measured. This simple
assumption is able to explain the temperature and metallicity discrepancies in
H ii regions and PNe arising from the different measurement techniques. We find
that the energy distribution does not need to depart dramatically from an
equilibrium distribution. From an examination of data from Hii regions and PNe
it appears that kappa ~ 10 is sufficient to encompass nearly all objects. We
argue that the kappa-distribution offers an important new insight into the
physics of gaseous nebulae, both in the Milky Way and elsewhere, and one that
promises significantly more accurate estimates of temperature and metallicity
in these regions.Comment: 16 pages, 11 figures, 2 tables, published in Ap
Radio-Excess IRAS Galaxies: IV. Optical Spectroscopy
This is the fourth in our series of papers investigating radio-excess
galaxies, which have radio emission associated with an active nucleus but which
do not fit into the traditional categories of either radio-loud or radio-quiet
active galaxies. In this paper, we present optical spectra of our sample of
FIR-luminous radio-excess galaxies. Optical emission line diagnostics are used
to determine the dominant source of the ionizing radiation. We find that radio
excess is an excellent indicator of the presence of an active nucleus: the
radio-excess sample contains a much higher fraction of AGN than samples
selected on FIR luminosity alone, or using other criteria such as warm FIR
colors. Several objects have ambiguous classifications and are likely to be
composite objects with mixed excitation. The type of optical spectrum appears
to be associated with the radio-loudness: radio-loud objects may be more `pure'
AGN than radio-intermediate objects. We find strong evidence for interaction
between the radio plasma and the surrounding gas. The jet energy fluxes of the
radio-excess objects, inferred from the [O III] luminosities, are lower than in
powerful radio sources, consistent with our previous results. We conclude that
the jets of radio-intermediate sources are intrinsically weaker than those in
sources with more powerful radio emission. A significant fraction of the sample
spectra show post-starburst stellar continuum, with A-star absorption lines,
consistent with the large fraction of merging or disturbed host galaxies in the
sample. The ages of the radio sources are significantly less than those of A
stars indicating that, if the radio sources are associated with merging
activity, there is a delay between the interaction and the initiation of the
radio activity. (Abridged.)Comment: Accepted for publication in AJ; version with high resolution figures
available from http://www.cis.rit.edu/~clbsps/papers/paper4.pd
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