Bachelor’s thesis: Multi-scale Clean in Radio Interferometric Imaging

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In this bachelor thesis, I present the basic theory of imaging in interferometric radio astronomy and show the application in imaging data of The HI Nearby Galaxy Survey (THINGS). All analyses is done using the software packages CASA, plus IDL and GIPSY for additional analyses. The main part of this thesis deals with multi-scale cleaning and the comparison with its progenitor, the Högbom clean algorithm using a subset of 3 galaxies of THINGS. Various tests demonstrate that the capabilities of multi-scale clean clearly exceed those of the ’traditional’ clean, as e.g. demonstrated by very deep cleaning of the galaxy NGC5055.

This thesis examines the advantages of multi-scale clean as implemented in CASA‘s msclean over conventional (point source) clean algorithms when applied to THINGS.

BSc_Krieger.pdf (8 MB)

Master’s thesis: Star Formation in the Galactic Center

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The Central Molecular Zone (CMZ) in the Galactic Center (GC) contains large amounts of dense molecular gas and exhibits high star formation rate densities. Such high densities are not frequently encountered in the local universe, but appear to be more common at high redshift. Recent work by Longmore et al. (2013) and Kruijssen et al. (2015) suggest a star formation sequence (SFS) in gas streams that is triggered by a close passage to the Galactic Center, Sgr A*. This thesis describes the data reduction and imaging of the radio interferometric Survey of Water and Ammonia in The Galactic Center (SWAG), especially the targeted metastable ammonia hyperfine structure lines, which are used to test the SFS hypothesis.
SWAG is currently mapping the central 500 pc of the Milky Way at 1 pc resolution with the Australia Telescope Compact Array (ATCA) in 42 spectral lines at 21-25GHz. The range -1.3 < l < 2.0, -0.6 < b < 0.4 in Galactic longitude and latitude is observed in ∼6500 pointings over 525 hours in three parts in 2014-2016. This thesis covers observation and reduction of data taken in 2014 and 2015 that targeted the inner ∼200 pc. In this context a pipeline was developed as part of this work that will be applied to the full survey after its completion in mid 2016. This survey results in maps and spectra of all 42 lines at unprecedented resolution (both spectrally and spatially), sensitivity, and areal coverage. Using ammonia hyperfine structure line fits, maps of line-of-sight velocity, line width, column density, opacity and four gas temperature measures were calculated.
Based on this data and the kinematic model of Kruijssen et al. (2015), the absolute time dependence of kinematic gas temperature is inferred along the molecular clouds’ orbit. It was found that gas temperatures increase as a function of time in both regimes before and after the corresponding cloud passes pericenter where its collapse is triggered. Other investigated quantities (line width, column density, opacity) show no strong sign of time dependence but dominating cloud-to-cloud variations. The results are discussed in the framework of tidal triggering of cloud collapse and orbital kinematics and found to generally match the predictions, i.e. the existence of a tidally triggered star formation sequence in the Galactic center can be confirmed. It must now be tested if the model fulfills, or needs adaptions to, these new constraints regarding temperature evolution of molecular clouds during collapse and star formation.

My master’s thesis explains the steps of data reduction, imaging and ammonia hyperfine structure fitting of the Survey of Water and Ammonia in the Galactic Center (SWAG), and investigates gas kinematics and temperature of clouds in the Central Molecular Zone. It is found that they match the implications of the triggered star formation hypothesis of Longmore et al. (2013) and Kruijssen et al. (2015).

42 spectral lines were observed in SWAG and their images presented in appendix A. This heavily increases the file size to 46MB. The short version without App. A is much lighter at only 22 MB.

MSc_Krieger_complete.pdf (45 MB)

MSc_Krieger_short.pdf (21 MB)

PhD thesis: Zooming into the Blast Furnace – A close look into the Molecular Gas in the NGC253 starburst

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Starburst galaxies are characterized by intense star formation at high star formation rate surface densities and short gas depletion times. Strong stellar feedback drives galaxy-scale winds and outflows in all gas phases, i.e. ionized, neutral and molecular. The extreme conditions in local starburst galaxies are thought to be similar to those in typical high-redshift star forming galaxies, e.g. at the peak of the cosmic star formation history.
In this thesis, we present and analyze 0.15″ (∼2.5 pc) resolution ALMA CO(3-2) observations of the nuclear starburst in NGC253. Using this data, we study the molecular outflow in unprecedented detail, zoom into NGC253’s super star clusters (SSCs) and compare the starburst to the similar but more quiescent center of the Milky Way.
Firstly, we kinematically decompose the molecular gas emission in NGC253 into a disk and non- disk component to then separate out the molecular outflow. We systematically improve on previous measurement and obtain mass outflow rates dM/dt∼14-39 M yr-1 for the starburst. The kinetic energy and momentum of the molecular outflow dominates over the other gas phases and is consistent with being supplied by the starburst at a few percent efficiency.
Secondly, we study the physical and chemical conditions in the molecular gas in the (proto-)SSCs in NGC253, the places where future outflows will be launched from. The SSCs differ significantly in chemical complexity and show up to 55 lines belonging to 14 different chemical species. Spectral modelling allows us to infer spectral line ratios and physical properties. The molecular gas in the SSCs is hot, consistent with UV photon-dominated chemistry and permeated by intense infrared radiation.
Thirdly, we compare the molecular cloud properties in the starbursting center of NGC253 and the Milky Way Galactic Center (GC), that shares similar properties as NGC253. Using a structure identification algorithmon resolution-, area- and noise-matched datasets allows for a direct comparison of the kinematic structure. Through common cloud scaling relations, we infer a high external pressure (Pext∼107-7.5 K cm-3) in NGC253 and a significant amount of unbound (non-self-gravitating) molecular gas that is characterized by high velocity dispersion.
In summary, in this thesis we could follow the life cycle of a molecular outflow from an actively star forming molecular cloud before the launching of an outflow all the way out to distances that are hundreds of parsecs above the starburst disk where the outflow fades away.

In my PhD thesis I study several aspects of the dense molecular gas in the center of the starburst galaxy NGC253. Parsec scale resolution ALMA data resolves the molecular outflow, allows to zoom into super star clusters and enables us to expand Galactic studies to nearby galaxies.

PhD_Krieger_compressed.pdf (18 MB)