Stacking of interferometric data at sub-millimeter and radio wavelengths
Licentiate thesis, 2014
Understanding the procceses of galaxy evolution require observational constraints on the physical properties of galaxies at different times in the history of the Universe. Large and deep surveys at visible and near-infrared wavelengths have, during the past decade, been used to produce extensive catalogues of high-redshift galaxies, spanning a large range of the history of the Universe. However, to properly understand the properties of these galaxies, and the processes they are going through, we need study them across the full electromagnetic spectrum. Observing the galaxies at mm and radio wavelengths add additional probes of the star formation, and allow us to further constrain their properties. Current radio/mm/submm high-redshift galaxy surveys, however, primarily probe luminous starbursts and AGN. A way to probe the radio and
mm emission from less luminous galaxies is stacking. Stacking is a statistical approach to measure the average flux for known objects that are too faint to be detected individually at the target wavelength.
This thesis investigates stacking of interferometric data. Typically, stacking at different wavelengths ranges is done using deep imaging surveys observed with a single telescope. However, interferometry is not a direct imaging method, and this presents a number of challenges to stacking. We present a new stacking algorithm that works
directly on visibilities, which we refer to as uv-stacking. We compare this algorithm to an image-stacking algorithm, i.e., an algorithm that stacks the sources directly in
the imaged data. The uv-stacking algorithm is found to yield more robust results than the image-stacking algorithm. In the case of JVLA surveys, uv-stacking results in a post-stacking signal-to-noise ratio up to 40% higher compared to image-stacking. Furthermore, it is possible to use the algorithms to estimate an average size of the stacked source population. Stacking marginally extended sources of 1.5arcsec for JVLA, we find that the typical uncertainties of sizes measured with image-stacking are more than twice that of uv-stacking.
The stacking techniques are applied to colour-selected, high-redshift galaxies in the 344 GHz continuum survey ALESS (the ALMA survey of sources detected in the LABOCA Extended Chandra deep field south (ECDFS) Submillimetre Survey) and the VLA 1.4 GHz continuum of the ECDFS. Using the uv-stacking algorithm, the average sizes of the galaxies are measured to be around 1′′ , which correspond to a physical size of ∼ 8 kpc at z ∼ 2. Within the uncertainties, the stacked average sizes are consistent with sizes measured at visible and near-infrared wavelengths.
While the uv-stacking algorithm is designed primarily around stacking high-redshift galaxies in ALMA and VLA, it can easily be extended to work for other interferometric telescopes and use cases. In particular, the uv-stackning algorithm has been shown to alleviate the effects of high dynamic range.
methods: data analysis
galaxies: high- redshift
radio continuum: galaxies