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Title: Local analogues to z ~ 5 Lyman break galaxies
Author: Greis, Stephanie M. L.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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Lyman break galaxies (LBGs), characterised by the eponymous spectral break at the electron transition energy of neutral hydrogen, are young, intensely star-forming, compact galaxies in the distant Universe. Due to the presence of hot, young stars within them, these galaxies are ideal candidates for the types of sources which contributed ionizing flux during the Epoch of Reionization by z ∼ 6. Not only can their study help us to learn more about the process of reionization, but LBGs are also primary sources for our understanding of the formation and evolution of galaxies. However, due to their large luminosity distance, and hence their apparent faintness and small projected sizes, they are difficult to study directly. A local analogue population, selected to reproduce the observed properties within their distant cousins, can be used to greatly improve the interpretation of distant LBGs. Such local Lyman break analogue (LBA) populations have been established for z ∼ 3 LBGs; however, there is significant evolution in the physical properties within galaxies between z ∼ 3 and z ∼ 5 − 7 (representing a difference of over one billion years in the age of the Universe), hence making it inappropriate to use the same analogue sample for both epochs. The establishment and study of an analogue sample of z ∼ 5 − 7 is described in this thesis. Building on the work of Stanway & Davies [2014], I selected an LBA candidate sample of 180 local (0.05 < z < 0.25) galaxies whose ultraviolet luminosity and colours matched those observed in the distant z ∼ 5 − 7 LBG population. I fit the spectral energy distribution (SED) of the candidates, deriving their stellar masses, dominant population ages, dust reddening, metallicities, star formation rates, and star formation rate densities. Comparing these properties with those found in the distant LBG sample, and depending on the age and mass cuts applied, approximately ∼ 40 − 70% of candidates are good local analogues. This confirms that galaxies exist in the local Universe whose physical properties are akin to those found in the distant cosmos. I have reduced and interpreted radio observations of a subset of the LBA candidates, confirming them to be young, star formation driven systems that do not host AGN. In fact, their stellar populations appear to be so young that in most sources no significant supernovae-driven radio continuum has been established. Their low ages also indicate that commonly used star formation rate (SFR) indicators, typically calibrated for stellar populations at ages > 100 Myr, are likely to underestimate their true SFRs. In order to determine the projected density of genuine LBA, I analysed low-resolution AAOmega spectroscopy for a sample of ∼ 230 photometrically selected LBA candidates. Combining this with the results of the SED fitting, the spatial density of genuine local Lyman break analogue galaxies lies between 24 and 40 per square degree. These findings have important implications for the distant LBG population, suggesting that galaxies with comparable star formation rate densities exhibit similar physical properties, and indicating that both LBAs and the distant LBG sources are undergoing bursty episodes of star formation (as opposed to a continuous star formation rate). From the high ionization parameters found in the local LBA sources, predictions can be made about potentially observable emission lines in the z ∼ 5 − 7 LBG population. Future surveys, such as the JWST and the LSST, are likely to shed further light on both this local analogue populations and their distant cousins.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy