Contents of: VI/111/./abstract/HSMITH_IRBGALS.abs

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 SCIENTIFIC ABSTRACT
One the most exciting discoveries of the IRAS mission was that a
significant number of galaxies emit a large fraction (up to 90%) of
their energy in the far-IR.  With luminosities exceeding 10^12
L(solar) these IR bright (IRB) galaxies may represent the initial dust
enshrouded stages of quasar formation.  IRBs are an important
component of the extragalactic sky at all luminosities, and are the
single most numerous class of objects in the local universe with
luminosities in excess of 10^11 L(solar).  What is the source of this
prodigious far-IR energy?

For some galaxies, it is clear the energy source is a tremendous burst
of star formation.  For others, the far-IR luminosity may be the
outward ramification of an obscured active galactic nucleus (AGN).
Many of these galaxies appear to be in collision or merger, suggesting
that the far-IR emission is the cooling radiation of galaxy-galaxy
collisions.

We propose LWS spectroscopic measurements of a representative set of
these IRBs to shed light on the nature of these objects, with special
attention to the origins of the far-IR luminosity.  Detailed studies of
the line intensity ratios will (1) determine the physical conditions of
the interstellar medium (ISM) in IRBs including abundances, hence the
degree of processing of the ISM (2) reveal the spatial extent and age
of any starburst (3) determine the hardness of the UV radiation field
in these galaxies and (together with SWS GT Proposal observations)
distinguish the origins of the field, be it AGN or starburst, and (4)
determine the contribution from galaxy-galaxy collisions to the far-IR
luminosity.  By striving for the highest signal-to-noise thought
possible, we will also be sensitive to the presence of low level
emission of molecular species such as H20, CO, and OH.  We will also
use the SWS to observe [FeII] 26.0 um and [FeIII] 22.9 um line emission
in a selected set of interacting galaxies with prominent circumnuclear
starbursts.  These observations are designed to constrain the
ionization equilibrium of gas phase iron in the starbursts, and to
explore the effect of supernova activity and star formation on [Fe II]
and [Fe III] line emission in interaction induced starbursts.


OBSERVATIONAL SUMMARY.
Our scientific program requires detection of more than half a dozen
bright fine-structure lines.  Therefore, for observational efficiency,
we will take full LWS01 scans on most of our objects.  Our LWS01
program is separated into two primary sections: detailed mapping of a
few nearby representative IRB galaxies, and LWS01 scans of the nuclei
of a sample of ultraluminous, IR bright galaxies.  Our first priority
is full LWS grating scans of several representative IRB galaxies
including objects known to be powered by starbursts (e.g. M82), objects
thought to contain embedded AGNs (e.g. Arp 220) and colliding galaxy
pairs (e.g.  IC694/NGC 3690).  Our Priority 1 also includes the
ultraluminous "protogalaxy" IRAS1021+4724.  These full scans and the
full scan maps outlined below will provide a template by which we may
compare the more distant or enigmatic sources.  Our primary diagnostic
lines include [CII] 158 um, [OI] 63 and 146 um, [NII] 122 um, [OIII] 52
and 88 um, and [NIII] 57 um (Table 1).  The first three lines trace the
physical conditions of the atomic and photodissociated molecular gas
and together with photodissociation region models constrain the
strength of the local UV radiation field.  The [NII] line traces the
diffuse ionized medium, and is used to discern the fraction of [CII]
emission which arises from this low density ionized gas.  The [OIII]
pair are a somewhat higher density tracer for the ionized gas, and
together with the [NIII] and [NII] lines, they trace the O/N abundance
ratio, hence, age of the interstellar medium.  The [NIII]/[NII] line
intensity ratio is a sensitive indicator of the hardness of the UV
radiation field when T(eff)   33,000 K.  This pair thus indicates the
upper mass limit (or, age) of any purported starburst.  Our [OIII]
data, when combined with SWS GT observations of the [OIV] 26 um line
will be a sensitive indicator of the very hard UV radiation fields
associated with AGNs.  Finally, clear signatures of the shocks
associated with galaxy-galaxy collisions include a high [OI]/[CII]
ratio, and detectable [SI] 56 um and/or CO (14-13) 186 um radiation.
Naturally, our LWS01 spectra may also reveal unexpectedly strong lines
from other important species.  Of special interest would be strong H20
line emission - an indicator of warm dense molecular gas (shocked or UV
exposed).  For the Priority 1 objects, we set the integration time so
that we would detect the weakest of our seven priority lines (likely
[NIII]) with SNR > 7 (except in the case of the "protogalaxy" IRAS
1021+4724, where the signal-to-noise ratio achieved on the continuum is
2-10).  Line flux estimates were based on typical observed KAO [CII]
line-to-continuum ratios and standard models of the ISM.

Our Priority 2 observations consist of LWS01 full scans of several
ultraluminous galaxies. 

Priority 3 includes

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