Abstract
I present a thesis on exploration of the cold and dense circumgalactic medium (CGM) of BX610, a massive star forming galaxy at z ∼ 2. The study is performed with the Atacama Large Millimeter Array (ALMA), on a deep data set with ∼ 13.8 hrs of integration time, providing a mean spatial resolution of 0.164 arcsec (synthesized beam), and and including the shortest interferometric baselines available for ALMA (15 m). The goal of the thesis is to constrain the presence and extent of cold molecular gas beyond interstellar medium (ISM) scales and, possibly, in the CGM. Specifically, the analysis considers two molecular emission lines, CO(4-3) and [CI ](1-0). These emission lines are mapped and show spatially resolved gas structures dominated by rotation, as revealed by an analysis of the moment maps.
To investigate the extent of such molecular gas reservoirs, I used several methods. First, I applied a uv tapering tool, which consists in imaging the data by weighting more the shortest baselines, hence providing lower resolution and higher sensitivity to large scale structures. At first glance, this method seemed to map a larger extent for both emission lines, suggesting that the native synthesised beam is not sensitive enough to detect the extent of the gas correctly.
Secondly, I measured the size of the emission through a 2D Gaussian fit of the gasses, which revealed that the higher extent from the tapering is not a robust result, much due to the higher RMS of the tapered maps, as well as the inherit errors in the fitting process. The results from the 2D Gaussian fitting of the non-tapered maps found an estimated extent for CO(4-3) at ∼ 3.5 ± 0.1 × 3.2 ± 0.1 marcsec for major- and minor axis FWHM respectively. For the [CI ](1-0) we find for major- and minor axis FWHM respectively.
Thirdly I examined the possible gas emission outside the galactic disk region, with a curve of growth analysis. Here the total flux was calculated as a function of aperture. This analysis revealed, for both emission lines, that there was no evidence for additional flux beyond observation maximum recoverable scale (MRS).
Finally I performed an analysis based on the uv data, where the complex visibility amplitude is plotted as a function of baseline. The real part of the visibility was binned and modelled with a Gaussian, an exponential function and and exponential function with a point source. From the Gaussian fit I measured the FWHM size of the Gaussian model equal to 382.4 ± 3.30 marcsec (3.22 ± 0.03 kpc) for CO(4-3) and 392.7 ± 10.8 marcsec (3.30 ± 0.09 kpc) for [CI](1-0). These results are in agreement with the 2D Gaussian fit of the emission maps. Therefore, since all methods deliver consistent results, I conclude that there is no detection of cold and dense components beyond ISM scales of BX610. In fact, the analysis also shows that the MRS of the observation is too small, even with the smallest possible ALMA configuration, to detect large-scale components beyond ISM scales. The uv analysis was also used to recalculate the total flux for the CO(4-3) and [CI ](1-0) emission. This was done by extrapolating the best-fit model (exponential function) to uv distance = 0, where the amplitude gives the flux for the selected velocity range. From this, I found that the total flux of CO(4-3) is 1.83 ± 0.03 Jy km s−1 , and 0.66 ± 0.02 Jy km s−1 for the [CI ](1-0) emission.
For further research, the CGM of BX610 should be observed with a more compact interferometric configuration. Any large-scale molecular structures of the CGM would possibly be detected with ACA or ATCA. For an accurate observation of the cold CGM components, a single dish telescope with a large aperture size would be the optimal tool, making BX610 an interesting target for studies with the planned AtLAST project.
Supervisors: Associate Professor Claudia Cicone, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor Øystein Elgarøy, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Research Fellow Chiara Circosta, European Space Astronomy Centre