Universität zu Köln

Kabanovic, Slawa ORCID: 0000-0002-2207-9982 (2023). The interstellar medium revealed by the [CII], CO and HI lines. PhD thesis, Universität zu Köln.

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Abstract

This thesis is based on observational data of the [CII] 158 μm line as an ideal tracer of molecular cloud formation and stellar feedback effects on the interstellar medium (ISM). Ionizing radiation and winds from massive stars provide stellar feedback, which can either destroy molecular clouds, the birthplaces of stars, or trigger the formation of new stars by expanding HII regions. These processes govern the star formation rate, so it is essential to quantify the various feedback processes for a better understanding of the evolution of the ISM in galaxies. The [CII] line is one of the most important cooling lines of the ISM. It originates in gas at various physical conditions, from low-density, cool, atomic gas to dense and warm photodissociation regions (PDRs). Most importantly, it traces gas kinematics at high spectral resolution, i.e., well below 1 km/s, which was possible to observe using the GREAT receiver (Guesten et al., 2003), followed by upGREAT (Risacher et al., 2018), on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). [CII] data from the SOFIA C+ Squad Orion A mapping project and the legacy program FEEDBACK are presented. I focus on results for the RCW 120 HII region and the Cygnus X massive star-forming region. For analyzing these enormous data sets (several million spectra), various numerical tools were developed and included in an open-source python-based library named “astrokit”. Machine learning algorithms such as the Gaussian mixture model were used to disentangle regions of various physical properties in the data cube. I also determined the physical properties of these regions using a two-layer multi-velocity component radiative transfer model, which separates the warm emitting background material from the cold absorbing layer. With a dendrogram-based approach, I disentangled the faint [13CII] emission from the noise. A comparison of the observed [12CII] to [13CII] intensity indicates significant optical depth effects in Orion A and RCW 120. The two-layer multicomponent model showed that a substantial amount of C+ (column density of 10^18 cm^-2) is located in an external cold absorbing layer. A study of atomic hydrogen (HI) self-absorption (HISA) confirmed an atomic gas origin of the [CII] emission and revealed that RCW 120 is embedded in an extended HI envelope. To derive the physical conditions of the cold, dense molecular cloud associated with RCW 120, I used 12CO and 13CO data observed with the Atacama Pathfinder Experiment (APEX). Both isotopic lines have a high S/N that allowed to use an automated version of the two-layer model fit on the entire spectral data cube. The derived physical properties of the warm emitting and the cold absorbing layer suggest a column density deficit towards the center of RCW 120. This implies that the parental molecular cloud is flattened along the line of sight, and the expanding HII region is breaking out into the surrounding cold atomic envelope. Studying the dynamics of the [CII] data in Orion A and the FEEDBACK sources revealed fast, expanding [CII] shells in most of the sources, i.e., the Orion Veil, RCW 120, RCW 49, RCW 36, and NGC 7538. Energy calculations suggest that these shells are wind-driven by O-type or Wolf-Rayet stars and that they are a common feature in HII regions, undetected in previous observations of molecular line emission. The [CII] line also serves as a tracer for the assembly of molecular clouds. In the Cygnus X region, we were able to show that [CII] reveals CO-dark molecular gas and the highly dynamic interaction between atomic envelopes of molecular clouds that lead to the formation of dense regions where massive stars form. This is the first time that this process, which was predicted by simulations, was observed in the ISM.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Kabanovic, Slawa kabanovic@ph1.uni-koeln.de orcid.org/0000-0002-2207-9982 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-718109
Date:	December 2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language Interstellar Medium, Stellar Feedback, Self-Absorption, Molecular Cloud Formation, HII Regions English
Date of oral exam:	5 June 2023
Referee:	Name Academic Title Stutzki, Jürgen Prof. Dr. Jolie, Jan Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71810