Issue |
A&A
Volume 472, Number 2, September III 2007
|
|
---|---|---|
Page(s) | 509 - 517 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20077685 | |
Published online | 06 June 2007 |
SPEAR far UV spectral imaging of highly ionized emission from the North Galactic Pole region
1
Space Sciences Laboratory, University of California, 7 Gauss Way, Berkeley, CA 94720, USA e-mail: bwelsh@ssl.berkeley.edu
2
Korea Advanced Institute of Science & Technology, 305-70 Daejeon, Korea
3
Korea Astronomy & Space Science Institute, 305-348 Daejeon, Korea
Received:
20
April
2007
Accepted:
25
May
2007
Aims.We present far ultraviolet (FUV: 912–1750 Å) spectral imaging
observations recorded with the satellite
of the interstellar OVI (1032 Å), CIV (1550 Å), SiIV (1394 Å), SiII* (1533 Å) and AlII (1671 Å)
emission lines originating
in a 60
rectangular region lying close to the North Galactic Pole.
These data represent the first large area, moderate spatial resolution maps of the
distribution of UV spectral-line emission originating the
both the highly ionized medium (HIM) and the warm ionized medium (WIM)
recorded at high galactic latitudes.
Methods.By assessing and removing a local continuum level that underlies these
emission line spectra, we have obtained interstellar emission intensity maps for the aforementioned
lines constructed
in 8 spatial bins on the sky.
Results.Our maps of OVI, CIV, SiIV and SiII* line emission show the highest intensity levels
being spatially coincident with similarly high levels of soft X-ray emission originating
in the edge of the Northern Polar Spur feature. However, the distribution of the low
ionization AlII emission does not show this spatial correlation, and suggests that
warm-neutral and/or partially ionized gas with a temperature <20 000 K may be quite pervasive at
high galactic latitudes.
The variation of the emission line intensity ratios
as a function of
sky position is contrasted with theoretical predictions
concerning the physical state of interstellar gas
in the galactic halo. The observed line ratios alone unfortunately do not provide
us with a clear diagnostic tool to distinguish between the various physical production
mechanisms responsible for both high and low ion states. However, our results do
favor the hybrid model of Shull & Slavin (1994, ApJ, 427, 784) which incorporates
the combined effects of turbulent mixing layers and isobarically cooling supernova
remnant gas.
For this highly ionized gas, our results
are best explained assuming
that the observed OVI halo emission is somewhat clumpy in nature, consistent with its
production at interfaces between warm ( K) and hotter
(
K) soft X-ray emitting gas. CIV emission at these
interfaces occurs in the intermediate temperature (
K) gas, which
seems always present whenever OVI is strongly detected. Alternately, the data
are also consistent with
CIV emission being ubiquitous throughout the halo with
a fairly constant level of emission line intensity (of ~4000 LU), and our
observations mostly reflect the superposition of spatially
separate OVI emission originating
at the cloud interfaces of random clumps of high latitude gas.
Key words: ISM: atoms / ultraviolet: ISM
© ESO, 2007
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