WISE detections of known QSOs at redshifts greater than six
  We present WISE All-Sky mid-infrared (IR) survey detections of 55% (17/31) of
the known QSOs at z>6 from a range of surveys: the SDSS, the CFHT-LS, FIRST,
Spitzer and UKIDSS. The WISE catalog thus provides a substantial increase in
the quantity of IR data available for these sources: 17 are detected in the
WISE W1 (3.4-micron) band, 16 in W2 (4.6-micron), 3 in W3 (12-micron) and 0 in
W4 (22-micron). This is particularly important with Spitzer in its warm-mission
phase and no faint follow-up capability at wavelengths longwards of 5 microns
until the launch of JWST. WISE thus provides a useful tool for understanding
QSOs found in forthcoming large-area optical/IR sky surveys, using PanSTARRS,
SkyMapper, VISTA, DES and LSST. The rest-UV properties of the WISE-detected and
the WISE-non-detected samples differ: the detections have brighter i/z-band
magnitudes and redder rest-UV colors. This suggests that a more aggressive hunt
for very-high-redshift QSOs, by combining WISE W1 and W2 data with red observed
optical colors could be effective at least for a subset of dusty candidate
QSOs. Stacking the WISE images of the WISE-non-detected QSOs indicates that
they are on average significantly fainter than the WISE-detected examples, and
are thus not narrowly missing detection in the WISE catalog. The WISE-catalog
detection of three of our sample in the W3 band indicates that their mid-IR
flux can be detected individually, although there is no stacked W3 detection of
sources detected in W1 but not W3. Stacking analyses of WISE data for large AGN
samples will be a useful tool, and high-redshift QSOs of all types will be easy
targets for JWST.
