Dust and Cool Gas Content and Distribution in Early-Type Galaxies

Abstract

This thesis considers the content and distribution of cool Interstellar Medium (ISM) in Early-Type Galaxies (ETGs), and how it can be used to infer the evolutionary mechanisms responsible for forming ETGs in the local Universe. High-resolution observations of molecular gas via carbon monoxide (CO) emission with the Atacama Large Millimetre /submillimetre Array (ALMA) interferometric telescope in five Dusty ETGs were analysed by fitting of kinematic models, to reveal any asymmetries or additional features. A new, clean and complete sample of galaxies was created within the Galaxy and Mass Assembly (GAMA) project equatorial regions to a redshift of 0.06, with visual morphological classifications assigned where possible. Masses and properties of cool dust for galaxies with suitable photometry in the sample were obtained by fitting a single modified blackbody (SMBB) model to sub-mm photometry from the Herschel-ATLAS project. New observations of molecular gas via CO emission for 32 Dusty ETGs from the clean and complete sample were also obtained using the IRAM 30m telescope.

The results of the work indicate that for dusty ETGs in the local Universe, in low density environments such as those in the GAMA equatorial regions, interactions and minor mergers play a significant role in their formation. These events are apparent for four of the five ALMA-observed Dusty ETGs, with the fifth affected by contamination with a background object. Molecular gas-to-dust mass ratios derived from IRAM 30m Telescope observations are less scattered at stellar masses <3 _ 1010 M_, indicating less introduction of ISM as a result of disturbance to form the ETGs. At higher stellar masses, gas-to-dust mass ratios are lower and more scattered, with minor mergers appearing to be more significant. The observed Dusty ETGs are mostly star-forming, and appear to have been observed at a stage where ISM has settled, allowing star formation to recommence.

The estimation of dust masses using Herschel-ATLAS photometry for lower stellar mass galaxies (_109:5 M_) appears to have been affected by lack of quantification of sub-mm excess emission, which may have led to under-estimation of dust mass. This is unavoidable without photometry at wavelengths greater than 500 _m, to constrain the longer-wavelength region of the dust emission spectrum. Nonetheless, dust masses were shown to be reduced in Dusty ETGs compared to Late-Type Galaxies (LTGs) above a stellar mass of _ 109:5 M_, while temperature and emissivity co effcient were found to be indistinguishable between Dusty ETGs and LTGs. Power-law relations from this work are shown for information between dust mass and star formation rate, and molecular gas mass and either dust mass or star formation rate, as expected if these properties in star-forming galaxies are linked.

Opportunities for further work have been identified. Possible projects include further observations with ALMA and the IRAM 30m telescope, to extend the results presented. Radiative transfer modelling for Dusty ETGs is also a possibility, to investigate nonequilibrium dust temperatures as an indicator of recent acquisition of dust.