The origin and evolution of galaxies are closely tied to the cyclic feedback processes between stars and the interstellar medium (ISM). The aim of this thesis is to explore characteristics of the... Show moreThe origin and evolution of galaxies are closely tied to the cyclic feedback processes between stars and the interstellar medium (ISM). The aim of this thesis is to explore characteristics of the ISM, on global (galactic) scales down to sub-cloud (pc) scales. We explore new methods to investigate the ISM in external galaxies, through radio recombination line observations, and develop the tools and strategies needed to process new low-frequency observations with the Low Frequency Array. We also infer the presence of massive stars and characterize their properties and influence on the ISM. This thesis addresses the questions:- How does low-density ionized gas affect the evolution of the massive, galactic star-forming region, Cygnus X? Are the same fingerprints present in surveys of low-density ionized gas in our Galaxy?- What are the properties of star formation (star clusters) in the central starburst of the galaxy NGC 4945?- Can the ISM be explored outside of the local universe through radio recombination line observations? What are the ISM properties of a dwarf-like galaxy at z=1.1?- What techniques are best suited to detect faint radio recombination lines (at a previously unknown redshift) in extragalactic sources? Show less
The study of cosmic large-scale structure formation benefits from radio observations, because it provides an unbiased view on the early Universe. Distant radio galaxies and diffuse cluster sources... Show moreThe study of cosmic large-scale structure formation benefits from radio observations, because it provides an unbiased view on the early Universe. Distant radio galaxies and diffuse cluster sources generally have a steep spectrum, which implies an increased brightness towards lower frequencies (below 300 MHz). The quality of low-frequency radio observations is compromised by the propagation effects on cosmic radio waves passing through the ionosphere. In this thesis, we present a calibration method for low-frequency radio interferometric observations. This method significantly improves the quality of radio maps from archival observations as compared to other existing calibration methods. The method was used to produce one of the deepest high-resolution surveys at 153 MHz to date, including the detection of 16 candidate distant radio galaxies. Furthermore, the method was used in a study of the diffuse radio sources in the merging galaxy cluster Abell 2256. These observations support the theory of revival of old radio sources through cluster merger shock compression. Finally, we present a study of the cosmic large-scale structure near a radio galaxy in the early universe by using an optical selection technique for galaxies. The projected galaxy distribution appears to trace the cosmic structure during the assembly of galaxy clusters. Show less
In this thesis I discuss recent observations of distant (z > 2) radio galaxies. There is strong evidence that radio galaxies are the progenitors of the brightest cluster ellipticals and are... Show moreIn this thesis I discuss recent observations of distant (z > 2) radio galaxies. There is strong evidence that radio galaxies are the progenitors of the brightest cluster ellipticals and are among the most luminous and massive galaxies at any epoch, allowing relatively detailed studies of their formation process out to large distances. Submillimeter emission has been detected in many, implying star formation rates of order 1000 Msun/yr over scales of tens of kpc, consistent with a scenario in which these galaxies are currently forming the bulk of their eventual stellar population. Many radio galaxies also host giant (~ 150 kpc) emission line nebulae. We obtaind very deep narrow-band Ly-a images for a select few of these. The observations show spectacular detail with a wealth of morphological structure, such as extended (~ 80 kpc) filamentary structures, giving insight in feedback processes during their formation. I discuss the possible origin and the ionization source of these halos in connection with the sub-mm results, and discuss possible implications for theories of galaxy formation. Show less
