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The SDSS catalogue also contains up to two independent measurements of source redshift, a spectroscopic value and a less accurate photometric value. The redshift distributions of XMM-OM sources with SDSS DR6 extragalactic counterparts (nearest neighbour within 2 arcsec) are plotted in Fig 1 with sampling of Δz = 2 x 10-3. The spectroscopic redshift limit at z = 0.2 is a systematic consequence of the Sloan survey rather than intrinsic to the SUSS sample. It is not clear how real the photometric limit at z = 0.8 is, although the correlation between photometric and spectroscopic redshifts within the inset of Fig 1 indicates that photometric redshifts can be trusted to within few x 10-2 between 0.0 < z < 0.25.
In section Source Properties: Colours, we predicted galaxy evolution tracks within UV and optical colour-colour diagrams over a range of redshifts. We can test whether the XMM-OM colours and SDSS redshifts combine to provide the predicted evolutionary tracks within colour-colour distributions. Two tracks from the Rocca-Volmerange and Guiderdoni (1988) family of spectral models are plotted in Fig 2. These include the effects of stellar evolution, nebular emission and internal extinction. The dots are XMM-OM sources with three observed UV or optical colours and an SDSS extragalactic counterpart. The colour of the dot references either the SDSS spectroscopic redshift (upper panels) or photometric redshift (lower panels). No attempt has been made to correct for Galactic extinction but the majority of sources have Galactic latitudes of b > 50°.
Figure 1: Photometric (orange) and spectroscopic (green) redshifts of the SDSS DR6 counterparts of XMM-OM SUSS sources. The inset provides the photometric-to-spectroscopic distribution of sources with redshift measured by both methods.
Figure 2: XMM-OM UV and optical colours of sources in XMM-SUSS compared with SDSS DR6 counterparts. Colour indicates redshift where the top two panels contain spectroscopic redshifts, zsp, and the bottom two panels photometric redshifts, zph. Yellow dots in the upper panels reveal sources with no spectroscopic redshift. The two model tracks represent galaxy spectral evolution models of Rocca-Volmerange and Guiderdoni (1988) at redshift z = 0.0 and 0.2.
There are not enough sources in the colour-colour sample with spectroscopic redshifts to make a good comparison with the model tracks. The photometric distribution does however reveal a trend consistent with the z = 0 and z = 0.2 expectations, revealing general consistency between XMM-OM source colours, SDSS redshifts and synthesized galaxy models.
Unlike UBV colours, UV-only colours do not provide an independent, diagnostic of redshift at 0.0 < z < 0.2. The majority of sources in the sample are consistent with young stellar populations, providing a picture of on-going star formation across a range of cosmological redshift. The UV colours reveal a tail towards older stellar populations. It remains to be determined whether this is sampling the galaxy tracks where more mature stars dominate the UV light or whether the tail consists of mis-identified main sequence stars.