Galactic Cirrus

High-density clouds hundreds of parsecs above the midplane should have an effect on the fraction of total Hαintensity due to scattered light. Observations of the cloud LDN1780 led Witt et al. (2010) to determine that dust-scattered Hαaccounts for∼19% of the Hαemission at high altitudes in the Galaxy. LDN 1780 is located approximate 110 pc above the midplane of the galaxy (Franco, 1989), is approximately 1.2 pc in diameter and has an average density of_∼ 103 cm−3. To determine the contribution of dust-scattered light from such clouds the thickness and Hαintensity of the ionised shell that results from photoionisation are calculated using the following analysis. A slab geometry for the cloud and plane parallel illumination are assumed.

The ionising luminosity needed to ionise a volumeδV is determined by

Q=n2α_BδV (4.3)

where Q is the number of ionising photons per second incident on volumeδV,nis the density of the gas (cm−3),α_B is the recombination coefficient assuming Case B recombination. The case B recombination condition is that a photon emitted in the n2P0 _→ 12S transition is absorbed immediately in the nebula, thereby populating then2P0 level in another hydrogen atom, therefore recombinations to this level are ignored. This can also be written in terms of the total ionising flux available from O stars in the galaxyFLyC:

FLyCfδA=n2αBδAδl (4.4)

whereδAis the area of the cloud exposed to the ionising radiation,f is the fraction of ionising photons that escape HIIregions to produce the DIG, andδl is the depth of the ionised volume in the cloud. Given an ionising flux, the depth to which a cloud can be ionised is:

δl = FLyCf n2_α

B

(4.5)

The intensity of Hαemission in Rayleighs is related to the emission measure by

E M= Z

(Haffner, Reynolds & Tufte, 1999). The Hαintensity can therefore be found using

I_Hα(R) = R

n2d l

2.75T₄0.9 (4.7)

assuming the density is constant along the pathd l this can be simplified to

I_Hα(R) = n

2_δl

2.75T₄0.9 (4.8)

where T4 is the temperature of the gas in units ofT/104K andIHα is the intensity of Hα in Rayleighs (R).

Since the simulations shown in figure 4.3 are not able to resolve an ionised skin of thickness ∼1AU in the large-scale simulations presented in section 4.2, scattering on this smaller scale was explored with a model of a single cloud. The total Lyman continuum flux in the Galaxy is 3.74_×107 cm−2s−1 (Vacca, Garmany & Shull, 1996). It was assumed that half of this flux travels upwards from the midplane of the Galaxy, towards the cirrus cloud, while the other half travels downwards, away from the cloud. Approximately 5% of the Lyman continuum photons from each source escape the galaxy (e.g. Kim et al. 2013, Barger, Haffner & Bland- Hawthorn 2013), 15% produce the DIG (Reynolds, 1990) and the remaining 80% produce local HII regions around each source. Adopting f = 0.15, it is expected that a cloud with densityn=103cm−3andT=104K, ionised by the galactic ionising flux from O stars, FLyC=

3×107cm−2s−1(Reynolds, 1984), would be ionised to a depth of 6×10−6pc and produce an Hαintensity of 2.2R from in situ recombinations.

Assuming case B recombination, each Lyman continuum photon produces 0.46 Hαphotons (Martin, 1988). Therefore the Hαflux impinging on galactic cirrus clouds is 0.46_×0.8FL y C =

0.37F_{L y C}=6.9×106cm−2s−1.

To determine the contribution of dust scattering in this cloud, simulations of a spherical cloud with r =0.5pc andn=103cm−3 using a 2003 pixel grid were produced. The Hαflux incident on the cloud in this simulation is assumed to be directed upwards from HIIregions close to the midplane of the Galaxy. Scattered light simulations similar to those described above were then run to find that the intensity of Hαscattered by the cloud is 1.4R, which is about 40% of the total Hα intensity from the cloud, similar to that found by Planck Collab- oration et al. (2015). These results indicate that the presence of high density galactic cirrus

4.4. Conclusions can increase the contribution of dust scattered light to the total Hαintensity we observe in the Galaxy. However the intensity of Hαemission that results from ionisation is still larger than that from scattering.

4.4

Conclusions

Using MHD and analytic fractal models for the 3D density structure appropriate for the ISM in the outer disk of a spiral galaxy, the relative contributions to the Hαintensity from in situ recombinations of diffuse ionised gas and dust-scattered Hα originating in HII regions have

been investigated. The models presented here self consistently compute the Hαemissivity from diffuse ionised gas. The small scale HIIregions within the photoionisation simulations are not

resolved, so it is assumed that the Hα luminosity from HIIregions is equal to that computed for the DIG as observed in other galaxies. The main results of the combined photoionisation and Hαscattered light models are:

• The intensity of scattered Hαoriginating from HIIregions differs depending on the den- sity structure. In both fractal and MHD structures the intensity of scattered light peaks around the midplane of the simulation closest to the HIIregions and where the gas den- sity is highest. The intensity of scattered light then decreases away from the midplane to less than about∼10% in the MHD and∼5% in fractal models. The larger scattered light fraction in the MHD simulations is due to the very low densities and hence low intrinsic Hαemissivity at large heights in those models.

• In low density regions a large fraction of the Hαin the simulations is dust scattered light that originates in HIIregions, a result of the small Hαemissivity from the lowest density DIG.

• Different scattering phase functions and albedo affect the intensity of scattered light in the models. However this does not significantly change the results, increasing the largest fraction of scattered light by 10%.

• Scattering of Hαphotons from HIIregions from high density cirrus in the ISM can dom- inate over the Hα intensity from photoionisation, contributing 40% of the total Hα. However the covering fraction of such clouds is∼50% (Gillmon & Shull, 2006), so such clouds would not effect all sightlines through the Galaxy.