GALEX far-ultraviolet observations of metal-poor subdwarfs

Far-ultraviolet photometry derived from the GALEX satellite observatory has been compiled for a sample of metal-poor subdwarfs with (mathrm{[Fe/H]} < -1.0). The FUV properties of these subdwarfs are compared with those of a set of Population I dwarfs that are known to have low levels of chromospheric activity. Comparisons are made via a number of photometric plots, including an absolute FUV magnitude versus ((V-K_{s})) diagram, two-colour diagrams involving both ((m_{ mathrm{FUV}}-B)) and ((m_{mathrm{FUV}}-V)) versus (B-V), and a two-colour diagram composed of ((m_{mathrm{FUV}}-V)) versus ((V-K_{s})). The warmest subdwarfs with ((V-K_{s}) sim1.2mbox{--}1.4) show FUV excesses ranging from (sim2mbox{--}3~mbox{mag}) relative to the Population I dwarfs, with the amount of FUV enhancement decreasing among subdwarfs of decreasing effective temperature. The coolest dwarfs that are compared have ((V-K_{s}) sim1.8), and among these stars the subdwarfs with (-2.0 leq{mathrm{[Fe/H]}} leq-1.0) approach the locus of low activity Population I dwarfs in the ((m_{mathrm{FUV}}-V, V-K_{s})) diagram. In the ((m_{mathrm{FUV}}-B, B-V)) diagram the subdwarfs in this metallicity range overlap the Population I dwarf sequence for ((B-V) > 0.6). The behaviour of the subdwarfs is consistent with their FUV fluxes being determined by a combination of a photospheric FUV spectrum, the strength of which diminishes towards cooler effective temperatures, and a spectrum of emission lines arising from a chromosphere and/or transition region which are of comparable strength between the coolest dwarfs and subdwarfs.