Progress with SMIRFS
SMIRFS was built by the Astronomical Instrumentation Group at Durham University,
UK, to provide a fibre-based multi-object capability for the CGS4 spectrograph.
An Integral Field Unit (IFU) fibre bundle was commissioned in June 1997,
as a prototype for the units being constructed for the William Herschel
and Gemini telescopes. With the SMIRFS-IFU, it is possible to resolve a
6'' x 4'' field of view simultaneously into 72 hexagonally packed spatial
elements by 256 spectral elements (see UKIRT Newsletter, March 1998 and
http://star-www.dur.ac.uk/~jra/ukirt_ifu.html).
This article is mainly concerned with progress related to the unit's second
observing run, in February 1998.
Since the 1997 run, adjustments to the IFU slit block have resulted
in an increase of throughput to 50%. Improved electrical insulation also
seems to have eliminated the ripple effect seen on the CGS4 array which
appeared in the 1997 images. Perhaps more importantly, a dedicated package
of software tools for handling the IFU data has been in development for
a year, alongside extensive investigation of issues relating to reduction.
Observations can now be reduced to a stage suitable for analysis in a relatively
short time - work remaining in this respect will centre on analyzing reduced
spectra, improving their extraction, propagating more detailed error information
and automating the location of spectra in CGS4 integrations.
Excitation mechanisms in Seyfert galaxies
Background
The scientific aim of the February 1998 run (Morris et.al.) was to investigate
the mechanisms leading to near-infrared emission features in Seyfert galaxies;
the IFU data are complemented by very high resolution images obtained by
Morris and Chapman at the CFHT.
At infrared wavelengths it is possible to view the centres of active
galaxies from which visible light is obscured by dust in the torus. A fibre-lenslet
IFU allows imaging to be performed over a small field of view with the
full wavelength resolution and coverage of a conventional long slit. Thus
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spectral line strengths and corresponding velocities can be
mapped across an object from a single set of
data, revealing any association with structures seen in broad-band
images or radio observations. Such an ability to form connections between
spectral and physical features has proved crucial in identifying line excitation
mechanisms.
Understanding excitation is important for studying kinematics as well
as energy transfer. Previous work (Hutchings et.al. 1998, AJ, 116,634;
Gallimore et.al. 1997, AstroPh 9708062) suggests that the optical narrow
line regions of Seyfert galaxies are driven by photoionization by a cone
of radiation from the active nucleus. This scenario is compatible with
gravitationally dominated dynamics in those regions, indicated also by
measurements of velocity dispersion (Nelson & Whittle, 1996, ApJ, 465,
96). However, emission may also occur through shock excitation (Genzel
et.al. 1995, ApJ, 444, 129), in which case corresponding velocity measurements
cannot be used to constrain the overall galactic gas kinematics.
The strongest near-infrared emission lines in active galaxies are two
forbidden [FeII] transitions (1.26 and 1.64 microns), produced in regions
where hydrogen is partially ionized. Usually the transition between ionized
and neutral hydrogen is very sharply defined, so there has been recent
speculation as to how the partially ionized regions may arise (Veilleux
et.al., 1997, ApJ, 477, 631). Proposed mechanisms involve X-ray photoionization
of optically deep narrow line clouds, or shocks from either supernova remnants
in starbursts or the interaction of a radio jet with the interstellar medium.
The 1.257 micron [FeII] line in the J-band falls close to Pa-beta, at
1.28 microns; the ratio [FeII]/Pa-beta has been found to be greater where
emission is due to shock excitation than where photoionization dominates
(Simpson et.al., MNRAS, 283, 777), so may help to separate X-ray illumination
from radio jet induced shocks. The two emission lines can be observed simultaneously
with the 150 lines/mm grating in CGS4 (albeit without much bare continuum
for reference), and being so close together provide a combined indicator
which is relatively insensitive to reddening. |
