Source Properties: Colours
Colour-Colour Diagrams
For catalogued sources detected in three or more colours we can
produce colour-colour distributions. The two examples presented in the
upper two panels of Figure 1 display the UVW2−UVM2 -
UVM2−UVW1 plane and the U−B - B−V plane, both with
relatively distinctive features. No attempt has been made to de-redden
or k-correct the colours. In this section, our task is to understand
the colour-colour distributions.
Fig 1: UV and optical filter colour-colour diagrams of
catalogued sources. Intensity scales are logarithmic. The red line
represents synthesized stellar spectra from the ATLAS9
project, ranging within photospheric temperatures 3,000 <
T < 50,000 K. The three curves correspond to logg =
0.0, 3.0 and 5.0 cm s-2. The red dots refer to the Pickles
(1998) standard stars of spectral types ranging from M6 to O5 and
luminosity types ranging from I to V. The various galaxy curves are
evolutionary tracks for Starbursts, ellipticals, Spirals and
Irregulars taken from the spectral models of Rocca-Volmerange
and Guiderdoni (1988) which include stellar evolution, nebular
emission and internal extinction. Characteristic AGN colours are taken
from the STSDAS
composite AGN library, constructed from IUE
spectra.
A family of spectral models and standard sources, with no Galactic extinction or redshifts applied, have been folded through the XMM-OM filter bandpasses and displayed in the two lower panels of figure 1. The red tracks and pink dots correspond to various stellar models for the Galactic population and spectral standards stars. In optical colours, the distribution of this sample is easily understood with stars revealing ever-bluer colours, right-to-left and top-to-bottom as we travel from cool M to hot O stars. The UV colours reveal a different trend in the cool stars however, stars initially become redder as we travel from M stars to hotter types before turning a corner at approximately solar spectral types. This cool-star behaviour result from a combination of negligible UV flux and the wings of the UV transmission curves extending into the optical bands. Figure 2 reveals the extent of these wings and how more M star blue photons are collected through the UVW2 filter relative to UVM2.
Fig 2: The upper panel plots both the effective area curves
for the UV filters and an ATLAS9 synthetic M6 V spectrum. The lower
panel shows the wavelength distribution of photons, as a fraction of
the total, detected through each filter. For cool stellar sources, the
bluest UVW2 filter has a better optical response than the UVM2 filter,
explaining the turnover in the UV colour-colour
distribution.
Extragalactic models and standard spectra have been included in the lower panels of figure 1. Using these particular combinations of colours, the optical set provide a more sensitive diagnostic for discriminating Galactic from Extragalactic sources, in the absence of dust and cosmological expansion.
Comparing the UV and optical colour distributions, the stand-out feature is the much larger population of blue sources in the UV. This is dominated by selection effects. The Source Properties: Sky Coverage section reveals a deficit of optical sources in the catalogue from the Galactic Plane and Magellanic Clouds. These fields did not meet our v1.0 screening criteria because they were crowded, with source confusion biasing both source detection and photometry. Future releases of the catalogue will include these rejected data once suitable algorithms for crowded fields have been developed.
Galactic Extinction
The observed colour-colour distributions are broader than those of the combined models and standard sources caused in part to photometric measurement uncertainty. Also we have yet to consider source reddening due to dust within our galaxy. In figure 3 we take the logg = 5.0 stellar sequence from the ATLAS9 models and extinguish them with increasing columns of Galactic dust content before folding the spectra through the XMM-OM effective area curves.
Fig 3: Colours from the ATLAS9 stellar spectra reddened by
varying columns of Galactic dust. Spectral extinction coefficients are
taken from Pei
(1992), with Rv = 3.08.
UVW1−UVM2 colours naturally tend towards the red as dust extinction increases. UVW2−UVM2 colours however evolve towards the blue with increased redenning. This is a consequence of the broad 2175 graphite feature in the Galactic extinction curve. Due to the intrinsically red nature of cool stars there is degeneracy in the UV colours between M dwarfs and e.g. highly redenned G stars, a degeneracy which does not exist in optical colours, highlighting the importance of associated optical observations.
Cosmological Sources
Figure 3 takes the family of cold elliptical galaxy models from
Rocca-Volmerange and Guiderdoni (1988) and rebins the spectra to
discrete redshifts out to z = 0.5. The absence of far-UV
information for the oldest elliptical populations prevents us from
extending the models accurately to higher redshifts.
Fig 4: Cold elliptical galaxy colours from Rocca-Volmerange
and Guiderdoni (1988), artificially redshifted to discrete distances
between 0.0 < z < 0.5.
For most sources along the track of cold ellipticals, UV colours
between 0.00 < z < 0.15 indicate a general trend towards
redder UVW2−UVM2 colours with redshift which, however, reverses
towards bluer colours at The optical colours from the elliptical galaxy simulation evolve
towards the top-left of the U−B - B−V diagram between 0.00
< z < 0.35. At redshifts > 0.35 the previous trend is
reversed. By far the strongest spectral feature within ellipticals at
z < 0.5, is the Balmer limit at a rest wavelength
λ3646Å which passes from the XMM-OM B filter to the V
filter z ~ 0.35. It is this transition across filter bandpasses
which causes the switch in colour trend.