BIBLIOGRAFÍA

a good correlation between the results of the two independent methods with a correlation coefficient of r= 0.64 and a p-value lower than 0.01.

We conclude that our method to recover the flux densities through aperture photometry is in good agreement with other methods, as has been shown in detail for the Hi-GAL com-pact source catalogue. This is further supported by our comparison to the dust temperatures ofGuzmán et al. (2015) implicitly requiring similar flux densities to result in the very good agreement of the dust temperatures of the two surveys. Finally the comparison with the am-monia kinetic dust temperatures shows that although comparing the results of two independent methods the results are in good agreement. These three comparisons show the reliability of our results, allowing us to compare the physical properties of 7861 sources of the ATLASGAL compact source catalogue.

4.4 Overview of main findings

In this section I will give an overview of the main results of the analysis conducted inUrquhart et al.(2018), as we will later build on these results, extending the analysis to the outer Galaxy.

Note that these conclusions were mostly drawn by the main author of the paper, but would not have been possible without the photometric data and the physical parameters derived from the SEDs which were my main contribution to the paper.

We investigated a complete sample of∼8000 dense clumps located between 5^◦ ≤ |`| ≤ 60^◦. We determined distances to∼97% of the sample, where the velocity measurements as described in Section4.2significantly contributed to the completeness, increasing the fraction of sources with velocity measurements from 84% to almost 98%. Investigating the distribution of the clumps, we find them to be strongly correlated with the spiral arms, with∼ 90% being located within 10 km s⁻¹of the spiral arm loci.

Furthermore, mid- to sub-millimetre photometry was performed on the whole sample as described in Section4.3, allowing to determine average dust temperatures, integrated fluxes and column densities for the clumps by fitting a two-component model to the SEDs.

With distances known, bolometric luminosities, clump masses and linear sizes could be derived from the SEDs fitted parameters. We find that the majority of the clumps are capable of forming high-mass stars and are unstable against gravity. Similarly, we find the peak column density strongly correlated with massive star formation, with 100% of clumps at 10²³cm⁻²or higher column densities forming high-mass stars.

Table 4.3: Summary of evolutionary types identified from the SED analysis and our previous work (Urquhart et al. 2014c). Adapted fromUrquhart et al.(2018).

Evolutionary Photometric Number of Fraction of total Notes

type type sources

MSF mid-IR bright 1222 0.16 )

0.88 Star forming

YSO mid-IR bright 4053 0.52

Protostellar mid-IR weak 1640 0.21

Quiescent 70 µm weak 946 <0.12 } <0.12 Non-star forming

0.0

Bolometric Luminosity (LO ^•)

0.0

Figure 4.5: Pannels show the distribution of the distance-limited subsamples of the four evo-lutionary types: Quiescent/pre-stellar (magenta), protostellar (green), YSOs (red) and MSF (blue). Figures fromUrquhart et al.(2018)

Using the photometric data, the evolutionary stages of the sources could be determined following the classification scheme established inKönig et al.(2017) (see Sect.3.3.1), with a slight alteration to the classes of mid-IR bright sources and HIIregions. We expand the group previously exclusively associated with HIIregions to include all massive star forming clumps (MSF) as identified by Urquhart et al.(2014c). The remaining mid-IR bright sources (i.e.

those not associated with a secondary tracer for high-mass star formation) are then referred to as young stellar object (YSO) forming clumps. A summary of sources associated with each class can be found in Table4.3

With the fractions of clumps in each evolutionary stage known, statistical lifetimes de-pendent on clump mass are derived. These indicate for the quiescent stage a lifetime of 5× 10⁴years for∼1,000 M clumps down to lifetimes of∼ 1 × 10⁴years for clumps around 10,000 Mand negligible lifetimes in the quiescent stage for even more massive clumps. We conclude that clumps form rapidly and are very unstable in their initial phase, which is fol-lowed quickly by star formation.

Furthermore, the evolutionary sequence reveals increasing temperature and luminosity but constant mass and column density (compare Fig4.5), in agreement with what was found for

4.4. OVERVIEW OF MAIN FINDINGS 69

W51 W43 W33 G351 G333 G305

10⁰

W51 W43 W33 G351 G333 G305

60.0 50.0 40.0 30.0 20.0 10.0 0.0 350.0 340.0 330.0 320.0 310.0 300.0

Galactic Longitude [deg]

W51 W43 W33 G351 G333 G305

Figure 4.6: Distribution of clump mass, bolometric luminosity and luminosity-to-mass ratio (top to bottom) as a function of galactic longitude. Crosses mark the mean over 2 degree with uncertainties estimated as the standard error of the mean. The red shaded area marks the

|`| < 5^◦region towards the Galactic centre not covered by (Urquhart et al. 2018).

the ATLASGAL Top100 sample (Chapter3). This means that the clumps form already with most of their mass assembled and either no significant amount of mass falls onto the clump during their evolution or infall and outflow cancel out each other. We conclude that once a clump is collapsing, the process is very rapid and almost independent from its larger scale environment.

With the positions and physical parameters known, their properties were investigated on kpc scales with respect to their Galactic environment. No variation was found for the L_bol/M_clump ratio either with Galactocentric radius nor with respect to the spiral arms, con-cluding that the arms play an important role in concentrating the material to form clumps, but do not have any impact on the star formation process. Care has to be taken though for the latter, as the small separation of the spiral arms in the inner Galaxy and the high uncertainty in heliocentric distance derived from kinematic distances make an analysis difficult. Similarly, the dust temperature T_duststays about constant in the inner Galaxy, but slightly increases out-side the Solar circle, likely due to lower density and hence decreased shielding against the interstellar radiation field.

Furthermore, clusters of star formation have been identified through a friends-of-friends analysis in order compare the properties of these larger scale complexes. We find that the 30 most massive complexes contain only 16% of all clumps, but make up for 36% of the total mass and 52% of the total bolometric luminosity of the sample. Furthermore, we find three outstanding clusters (W49, W51 and G351.598+001.89), that are responsible for almost 25%

of the total luminosity of all ATLASGAL sources. These also show a significantly increased

star forming activity, as their integrated L_bol/M_clump> 50 L/Mratio is at least twice as high as the remaining clusters, making them the best examples of ‘mini-starbursts’ within the Milky Way.

Finally, I will give an overview of the distribution of the clump masses, bolometric lumi-nosities and luminosity-to-mass ratio as a function of galactic longitude in Fig.4.6. As the luminosity-to-mass ratio is a distant independent property, 667 sources located in the|`| < 5<sup>◦</sup> region around the Galactic centre are also included here. For the masses and luminosities, we see a rather large scatter, with the bins containing W49 and W51 clearly outstanding. In con-trast, the average luminosity-to-mass ratio is found to be rather constant on large scales over the whole inner disk. An exception is the star formation activity (as indicated by Lbol/M<sub>clump</sub>) for the central|`| < 5 degree, which we find to be lower on average, with the central 2 degree being on the average level of the disk due to extreme Lbol/Mclumpvalues found for just a hand full of clumps towards the Galactic centre.

4.5 Summary

Using and refining the methods developed for the ATLASGAL Top100 paper, as well as com-pleting the distances needed to derive physical properties, we were able to extend the analysis to a complete sample of massive star-forming clumps above 1000 M throughout the inner Galaxy. With major findings for the inner part of the Milky Way, we can now ask how this compares to the outer Galaxy, where the environment is significantly different. With lower HI density, less intense UV radiation fields, smaller cosmic ray flux, lower metallicity and higher gas-to-dust ratio in the outer Galaxy, changes in the physical properties of star-forming clumps would not be surprising - which we will investigate and compare in the following two Chapters.

Part III

Outer Galaxy

73

Introduction

In the previous chapters we have investigated the physical properties of star-forming regions in the inner Galaxy based on the ATLASGAL survey. In the next two chapters we are going to extend our view to the outer part of the Milky Way, where the environment changes signif-icantly: in general we find lower HIdensity (Heyer & Dame 2015) and metallicity (Rudolph et al. 1997), less intense UV radiation fields, a smaller cosmic-ray flux (Bloemen et al. 1984), and a higher gas-to-dust ratio (Giannetti et al. 2017a). Differences between star-formation properties of the outer and inner Galaxy could therefore lead to a better understanding of the influence of the environment on the star-formation processes in general. To find variations of the physical properties with galactocentric radius, we will apply the methods developed for the ATLASGAL sample as described in the last two chapters to the outer Galaxy. For this pur-pose archival mid-infrared to submm dust continuum data is used in combination with newly obtained kinematic distances from CO(2–1) observations.

As physical properties can only be derived with reliable distances, the main goal of the observations in this part was to obtain these distances, which will be described in detail in Chapter5. We will then look at how they reveal the structure of the outer Galaxy in unprece-dented detail, investigating the large scale structures and the distribution of molecular clouds.

We are then able to determine the physical properties of the clumps and investigate these with respect to galactocentric radius, comparing them to the physical properties found for the inner Galaxy sample of ATLASGAL (Chapter6). At the end of the Chapter we will also briefly look at the influence of large scale structures found in the outer Galaxy on star-formation properties.

Figure 5.1: Image of SPIRE 250 µm emission showing an example of extracted sources around

` = 236^◦ and b=−1^◦. The image spans∼7^◦ in longitude and∼3^◦in latitude. Over-plotted are the sources identified by SExtractor (green ellipses).

C

5

Velocities, Distances and Structures