We have seen in Sect. 4.2 that we find a significant difference in the morphology of the gas density map obtained using as input to the PDR modeling the datasets with the two [OI] lines (Fig. 15), and this difference is independent on the correction we apply to the observed [CII] emission due to the contribution from ionized gas. If this difference were caused by the fact that [OI] at 63 μm is thick in these regions, we would expect to observe less [OI] 63 than what is predicted by the solution obtained using the [OI] line at 145 μm, which is optically thin. We have checked this and we find the opposite answer, that is in order to make the two density map solutions similar, we should reduce the input flux at 63 μm. In other words, it looks like, in these regions we have too much [OI] 63 μm flux. Although this could be related to the uncertainties of the geometrical factor equal to two we have applied to the observed [OI] 63 line flux (see Sect. 4.1.2), we wanted to check whether this difference could arise from our processing steps.
Each set of input values to the PDR modeling were obtained from maps (either [CII], FIR and [OI] at 63 μm or [CII], FIR and [OI] at 145 μm), rebinned to the largest pixel size (6″, that in the red channel), smoothed to the worse resolution (that of the [CII] map), reduced to the same size and aligned to the [CII] peak. In this chain of processing, the map which suffers the heaviest manipulation is that at 63 μm. This is because the original pixel size is smaller, and its PSF differs from that at 158 μm much more than that at 145 μm. Moreover, the observations at 158 and 145 μm were carried out in the same AOR, therefore the pointing accuracy is likely to affect these two observations in a similar way. This may give rise to alignment problems between the final map at 63 μm and those at higher wavelengths.
For these reasons we decided to run the models again, this time using the [OI] 63 μm map aligned on the [CII] map on the steep gradient of the flux distribution rather than to the peak (shift of 1 pixel = 6″). Figure A.1 clearly shows that now both gas density maps show an enhancement toward the north part of the starburst, although the values are lower for the solutions obtained with the [OI] 145 μm line than those obtained with the [OI] 63 μm line. The maps in the other parameters, G0, Tgas and φ, show the same morphology as those shown in Figs. 14, 16 and 17.
The PDR solution maps obtained for the gas density nH using the [OI] map at 63 μm shifted to match the steep gradient of the starburst region to that in the [CII] map, instead of the [CII] peak emission.
|Open with DEXTER|
© ESO, 2013