Both FIR continuum emission from dust and CO spectral line emission can be used to estimate the masses of interstellar clouds, and the FIR data can be compared for consistency and/or calibration with the CO data. For isolated clouds, spatial coincidence and close morphological similarity is found between the CO emission and dust emission, especially after subtraction of the extended background Galactic FIR emission (Heyer et al. 1987; Langer et al. 1989; and Mooney 1992). In general, an accurate separation of the background Galactic FIR emission from the cloud emission is necessary before using the IRAS data to estimate the FIR luminosity of a cloud. However, for the region in this study the identification of the FIR emission is fairly straightforward as the cloud is well isolated. The only other contribution to the FIR emission would be atomic hydrogen. An HI map was available from the Weaver & Williams (1973) data base, although the resolution of the original data is very large, 36'. We have resampled it onto a 3' grid, assuming there is no specific structure. In Fig. 9 we present the HI integrated intensity map fitted to the CO mapped region. The distribution of HI emission is found to have a very small dynamic range of a factor of 1.3, whereas the CO emission without internal heating sources has a much larger dynamic range. Thus the atomic hydrogen does not affect significantly the slope of the relationship between the FIR emission and the CO emission; we have not included the HI contribution in this work.
Figure 10 represents the pixel to pixel correlation between the CO
integrated intensity and dust FIR intensity at 100 m. It shows
two distinctly different features; a very scattered distribution of
FIR intensity above 35 MJy sr-1, and the bulk of emission
confined to between 15 and 30 MJy sr-1. However, we note that
the pixels with FIR intensity greater than 35 MJy sr-1 are all
associated with the HII region Sh 241 and IRAS point sources.
Except for those points, a good correlation is found between the FIR
emission and the 12CO integrated intensity. To
determine the
relation between 12CO integrated intensity and FIR intensity,
pixels with the CO integrated intensity > 5 K km s-1
(
) have been fit by using the least-squares method. The
slope of the correlation is 0.38 (MJy sr-1) (K km
s-1)-1 and the intercept is 21.0 MJy sr-1, which is
represented as a thick solid line in Fig. 10. The non-zero
intercept most likely represents emission from dust in a region which
lacks 12CO. The amount of dust that lies in front of or behind
the target region may not be small, as the intercept of above equation
is nonnegligible. HI contribution to the FIR emission is also
involved as mentioned previously. However, considering that it is the
general dust emission from the Galaxy, thus not associated directly
with the CO integrated intensity, only the slope represents the true
relation between the 12CO integrated intensity and the FIR
intensity. The good correlation implies that there is little stray
dust emission in the line of sight.
Numerous studies have found a linear relationship between 100 m
intensity and either the HI column density in regions of atomic
gas or the CO column
density in molecular clouds (Boulanger & Pérault 1988; Snell
et al. 1989; Laureijs et al. 1995; Boulanger et al.
1998). Boulanger & Pérault (1988) found ratios between the
100
m intensity and CO integrated intensity in the range 0.6 to 2.5 (MJy
sr-1) (K km s-1)-1 with an average value of 1.4 (MJy sr-1) (K km
s-1)-1 for regions outside of star forming sites. This value can be compared
with the average slope found for the target region of 0.38 (MJy sr-1) (K km
s-1)-1, substantially less than this value. Based on the
slope of the least-squares fit to the data, we have computed the ratio
of 100
m intensity to total hydrogen column density. We can use
the CO conversion factor computed from the
-ray analysis
(Bloemen 1989), of
H2
to convert the CO intensities to hydrogen column
densities. Expressing this ratio in terms of the hydrogen, the values
found for the target region are
(H) = 0.07 (MJy
sr-1) (1020
H cm-2)-1. These values can be
compared to those for dark clouds;
(H) is 0.07 (MJy
sr-1)(1020 H cm-2)-1) for B18, and 0.10 (MJy
sr-1)(1020 H cm-2)-1 for HCL2 (Snell et al. 1989).
The values for warmer GMCs are substantially higher; for example, in
Orion Molecular Cloud (OMC):
(H) = 1.3 (MJy
sr-1)/(1020
H cm-2)-1 (Boulanger & Pérault
1988). Thus, the ratios found for the target region are comparable to
the dark cloud ratios, and a factor of
20 times smaller than
the ratios in OMC. Snell et al. (1989) attributed the low ratios in
the dark clouds to dust heated exclusively by the solar neighborhood
interstellar radiation field. A similar conclusion was reached by
Mooney (1992) for the clouds classified as IR-quiet. Thus, the low
ratios found for the target region probably results from the absence
of internal or significant nearby external heating sources. If this
is the case, the dust temperature should also be relatively low as is
seen in the dark clouds.
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