Complexes and notable regions

As mentioned in Section5.1.1we excluded the 6 dominant bright regions located in the survey area from the automated source extraction. As can be seen from Table5.2, these complexes are all located in a rather narrow band between Galactocentric distances from Rgal= 9.4 kpc at

`∼ 226<sup>◦</sup>to R<sub>gal</sub>= 11.5 kpc at `∼ 226^◦. This places most of these complexes slightly inward

2The broad peak between 1.4 and 3.2 kpc in Fig.5.12is not associated with the shell, but the superposition of different clusters of clumps in the observed region.

5.2. STRUCTURE OF THE OUTER GALAXY 95

0 2

4 6

8 10

x [kpc]

9 10 11 12 13 14

y[kpc]

clusters: 13

R=10 kpc R=15kpc

Figure 5.13: Identified clusters of CO clouds, in different colours, grey dots are not assigned to any cluster. The dashed lines indicate the loci of the Perseus and Outer arm, the dotted grey lines indicate Galactocentric radii of 10 and 15 kpc. The solid grey ellipse indicates the edge of the supershell. Be aware as projection effects sometimes make two close-by clusters appear to be one, although separated along the z-axis.

of the locus of the Perseus spiral arm. As these are located in the outer Galaxy, a density wave manifested in the Perseus spiral arm would travel faster than the rotation of the interstellar medium around the Galaxy. Being located slightly inward of the locus of the Perseus arm therefore means that these complexes would just have passed through the arm or are in the process of leaving it. With these complexes being the brightest in the survey area, and them just leaving the density wave of the spiral arm, it is possible that the activity of the regions is triggered by the passage through the arm. Such an effect is observed in M51, where the clouds after passing through the spiral arms are the brightest and most active observed (Colombo et al. 2014).

Unfortunately a detailed clustering analysis of clumps on the cloud size scale of∼30 pc (Miville-Deschênes et al. 2017) is not possible with our dataset, as the incompleteness of the survey would render the number of clusters as well as the clumps per cluster and all derived properties would be lower limits.

But as much larger structures are clearly visible above the cloud-scale, we first identified those by eye, and consecutively tried to identify them in a robust way using the DBSCAN algorithm (Density-Based Spatial Clustering of Applications with Noise; Ester et al. 1996) with a minimum density of 1, effectively implementing a friends-of-friends analysis. We find that these large structures are best represented with a distance threshold of 250 pc and a minimum cluster size of 10 clouds per cluster. In total we are able to identify 13 large clusters.

The results can be seen in Fig.5.13and a summary is given in Table5.7. The local emission is identified as a single cluster made out of 693 CO clouds, whereas the remaining clusters are comprised of 11 to 119 CO clouds. Excluding the local emission, we find these clusters to range in size from a few hundred parsec to more than a kiloparsec, constituting the largest structures in the Galaxy after the spiral arms.

Table 5.7: Summary of the clusters identified.

Label ` b v<sub>lsr</sub> R<sub>gal</sub> # clouds ∆` ∆b ∆v_lsr

[deg] [deg] [km s⁻¹] [kpc] [deg] [deg] [km s⁻¹]

1 251.05 −1.11 15.3 9.2 636 34.98 3.34 30.7

2 259.15 −1.14 57.5 11.3 115 4.31 2.75 15.9

3 232.76 −0.73 44.5 11.2 112 8.82 2.63 15.2

4 242.19 −0.61 66.4 12.5 72 6.46 1.99 11.4

5 235.48 −1.53 78.7 14.3 39 6.28 1.56 8.0

6 257.96 −1.63 43.2 10.5 32 2.59 2.61 5.6

7 257.63 −0.72 74.7 12.6 21 2.04 1.59 4.8

8 232.06 −0.08 56.8 12.3 18 4.75 2.07 5.4

9 227.92 −0.08 53.0 12.2 15 2.90 0.59 3.2

10 246.97 −0.21 57.3 11.6 13 4.44 2.06 2.5

11 251.90 −1.08 45.4 10.7 11 2.91 1.24 3.3

12 253.87 −1.21 37.7 10.2 11 2.89 1.70 3.4

5.2.4 Summary

In order to extend our previous analysis to the outer Galaxy between 225^◦≤ ` ≤ 260^◦we used Herschel/Hi-GAL 250 µm SPIRE continuum emission maps to select a representative sample of more than 800 sources from a rudimentary source catalogue of more than 25,000 extracted clumps using SExtracor (Bertin & Arnouts 1996), giving positions and source sizes for these clumps. To derive physical properties such as clump masses and bolometric luminosities, which require a known distance, we observed these sources in¹²CO(2–1) in order to obtain velocities and eventually determine distances to these dust clumps.

In the CO(2–1) emission spectra we identified 1757 clouds that consist of a total of 2034 individual velocity components, for a total of 1164 positions, including the recovered off-positions. 714 (61.3%) lines-of-sight were found with a single velocity component, whereas two or more clouds were found towards 450 (38.7%) lines-of sight, yielding on average 1.5 clouds per line of sight. Consecutively, distances were calculated using a rotation model of the Galaxy, applying the rotation curve fromBrand & Blitz(1993) for all clouds and velocity components. For every line-of-sight, we finally associated the cloud with the highest integrated intensity to the according dust clump.

Combining our velocity measurements with HI emission maps from the GASS survey (McClure-Griffiths et al. 2004) and CO(1–0) maps from Dame et al. (2001), we were able investigate the structure of the southern outer Galaxy. In order to be able to investigate star formation with respect to the spiral arms, the position of the latter was discussed. We focus on the model where both, the Perseus and the Outer arm, are continued from the 2nd Galactic quadrant. The Sagittarius arm is not located in the observed region but is located further inwards within the Solar circle. In general, we find the positions of the identified CO clouds to be strongly correlated with the dense parts of the HI emission. On the other hand, we were also able to identify a web of bridges, spurs and blobs of star forming regions spanning

5.2. STRUCTURE OF THE OUTER GALAXY 97

between the larger star forming regions, unveiling the complex three-dimensional structure of the outer Galaxy in unprecedented detail. Although the latter might be an indication of the outer Galaxy to be of a flucculent nature, a definite answer is difficult due to the influence of a large, expanding supershell in the survey area.

The Galactic supershell G242-03+37 as first identified byHeiles(1979) adds to the com-plexity of the structure in the outer Galaxy. In case there are indeed two arms present in the 3rd Galactic quadrant, the first one (Perseus) would have a large hole in between (∼ 234^◦ ≤ ` ≤∼ 253). Reviewing the literature, we hypothesize that this supershell was caused by a high-velocity cloud passing through the Galactic disc, disrupting the Perseus arm in the 3rd Galactic quadrant, and causing the observed expanding supershell. Investigating the impact of the supershell on the distribution of the CO clouds and dust clumps, we find the number-density of the sources increased within the walls of the shell. We find this to be in agreement withMcClure-Griffiths et al.(2006), interpreting the steep walls of the shell as an indication of compression and being associated with a shock.

Finally large scale complexes were identified using a friends-of-friends analysis towards the outer Galaxy, hinting at a hierarchical structure at the largest scales reaching sizes up to the order of 250 pc. These complexes seem not to be necessarily associated with the spiral arms, and therefore future study of much larger regions is necessary to understand these structures.

With the distances and positions known for the selected dust clumps, we can now investi-gate their physical properties and compare these to the results of the previous work, in order to get a view on star formation throughout the whole Milky Way.

C

6

Physical Properties

6.1 Introduction

In this chapter we are going to investigate the physical properties that can be derived from archival mid-infrared to sub-millimetre continuum emission data in combination with the dis-tances obtained in Chapter5. First we will discuss how we obtained the dust spectral energy distributions for the outer Galaxy. We will then present the derived physical properties and investigate how consistent they are, followed by a detailed look at the results in the following sections. Here we will mainly investigate the dependence on the distance to the Galactic cen-tre (Galactocentric radius) and differences found in the physical parameters of star-forming clumps between the environments of the inner and outer Galaxy. For this purpose we use the properties derived for ATLASGAL byUrquhart et al.(2018; reviewed in Chapter4) as a sam-ple for the inner Galaxy and the sources of the present work for the outer Galaxy. At the end of this chapter, we will briefly look at how the spiral arms and the supershell, that happens to be located in the survey area, influence the properties of the observed clumps.