Spectral analysis of galaxies in the ultraviolet

  • Claire Le Cras

Student thesis: Doctoral Thesis


Galaxy SEDs contain a wealth of information about their stellar populations with the UV region tracing their hot component. In young populations this hot component comes from luminous O and B-type stars whereas, in old populations a hot component can be produced after sufficient mass loss, a phenomenon known as the UV upturn.

The UV region of galaxies remains relatively unexplored and models lack calibration due to the paucity of observational data. However, by investigating features seen in the UV region it may be possible to explore both types of population with the potential to find indicators that can differentiate between the two.

I use a large sample (∼ 275,000) of z ≥ 0.6 massive (log(M/M) >∼ 11) galaxies taken from the Sloan Digital Sky Survey (SDSS) - III Baryon Oscillation Spectroscopic Survey (BOSS). I use both individual spectra and stacks and employ a suite of models including a UV contribution from old populations, spanning various effective temperatures, fuel consumptions, and metallicities.

By investigating the effect of the UV upturn on the strength of mid-UV indices I find a subset that are able to differentiate between old and young UV ages; Mg I, Fe I, and BL3096.

I find evidence for old stars contributing to the UV in massive galaxies, rather than star formation. The data favour models with low/medium upturn temperatures (10,000 - 25,000K) consistent with local galaxies, depending on the assumed metallicity, and with a larger fuel ( f ∼ 6.5 · 10−2M). Models with only one temperature are favoured over models with a temperature range, which would be typical of an extended horizontal branch

Old UV-bright populations are found in the whole working sample (92%), with a mass fraction peaking around 10 - 20%. Upturn galaxies are massive and have redder colours, in agreement with findings in the local Universe. I find that the upturn phenomenon appears at z ∼ 1 and its frequency increases towards lower redshift, as expected by the stellar evolution of low mass stars. These findings will help to constrain stellar evolution in the little explored UV upturn phase.

The highest redshift galaxies are young and hence exhibit a pronounced UV spectrum due to their massive star components. The UV rest-frame is also what is actually sampled via optical and near-IR observations at these high redshifts. However, a comprehensive study of UV absorption lines, which may provide useful indicators for physical properties such as stellar age and metallicity, is still lacking.

I exploit stellar population models of absorption line indices in the far-UV (1200 - 1900Å) to study the spectra of young high-z galaxies. Using high-z spectra from Sommariva et al. (2012), Erb et al. (2010), and VVDS, the central aim of this analysis is to assess the ability of the model indices to recover the stellar ages and metallicities found in the literature.

Using a set of far-UV indices I fit both SSPs and CSPs to the strength of the absorption features found in the data as well as fitting the full far-UV spectral region. A simple test using mock galaxies shows the effect of dust to be negligible when fitting the indices in combination however, there may be more complicated effects that are not modelled well.

The analysis shows that currently it is not possible to reliably derive the stellar ages or metallicities of high-z galaxies using the methods explored in this work. The large range of χ2 values for the model fitting is likely due to the errors of the spectral indices being underestimated and the low quality of the UV data. However, issues may also lie on the model side. Emission lines are known to effect the far-UV region and could contaminate the absorption features investigated in this work. The spectra analysed have not been "cleaned" of emission lines and such features are not included in theoretical modelling.
Date of AwardMay 2017
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorDavid Bacon (Supervisor)

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