A Dedicated z ∼ 5 Analogue Sample. As shown in section 1.3, there is significant evolution of Xxxxx break galaxies be- tween z ∼ 5 and z ∼ 3 which must be taken into account when determining the appropriate selection criteria for local analogues to z ∼ 5 LBGs. Xxxxxxx & Xxxxxx [2014] present a pilot sample of 21 compact star-forming galaxies in the local (0.05 < z < 0.25) Universe, whose ultraviolet luminosities match those of z ≥ 5 LBGs. The candidates were selected using DR7 of the Xxxxx Digital Sky Survey (SDSS) [Xxxxxxxxx et al., 2009] and GALEX (GR6) [Xxxxxx et al., 2005] photometry, as well as spectroscopic measurements by SDSS. Analysing the optical emission lines within these sources, the galaxies are found to be metal-poor with typical metal- licities of a few tenth Solar, consistent with the distant z ∼ 5 LBG population. Additionally, radio continuum observations at 5500 and 9000 MHz of 13 sources indicate radio spectral slopes which preclude the presence of strong dust-obscured AGN in all radio-observed sources. The sample established by Xxxxxxx & Xxxxxx [2014] forms the basis of the work described in the following chapters. By com- paring the star formation densities of the Xxxxxx et al. [2007] sample of z ∼ 2 − 3 Xxxxx break analogues with those of typical values for LBGs at higher redshifts, Xxxxxxx & Xxxxxx [2014] indicate that only ∼ 3% of the Xxxxxx et al. [2007] xxx- ple of UVLGs have a star formation density comparable to that observed in distant Xxxxx break galaxies. This discrepancy only becomes exacerbated as the Xxxxx break samples are pushed to higher redshifts. It is also worth noting that, while such an analogue sample is selected to produce similar physical conditions in both low (z < 0.25) and high (z ∼ 5) galaxies, the nature of these galaxies is likely to be very different. The z ∼ 5 LBGs are presumably among the first structures to collapse in the Universe, and thus likely reside in massive, though rare, dark matter haloes. Due to this dark matter overdensity, these galaxies begin star formation only a Gyr after the Big Bang. On the other hand, the collapse of, and star formation in, the z < 0.25 sources has been delayed for a further ∼ 10 Gyrs, suggesting that they reside in much lower mass dark matter haloes. It may also be that star formation in these younger galaxies was surpressed by photoionization by the IGM, which might explain the observed lack of large numbers of dwarf galaxies compared to ΛCDM predictions.
