WISE detections of known QSOs at redshifts greater than six

Abstract

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 mu m) band, 16 in W2 (4.6 mu m), 3 in W3 (12 mu m), and 0 in W4 (22 mu m). This is particularly important with Spitzer in its warm-mission phase and no faint follow-up capability at wavelengths longward of 5 mu m until the launch of James Webb Space Telescope (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 active galactic nucleus samples will be a useful tool, and high-redshift QSOs of all types will be easy targets for JWST.