4C23.56

This extraordinary triple at has an angular size of 55" or nearly 0.5 Mpc depending on your choice of cosmology. It is by far the largest known single object at high redshift. There are larger radio sources ( 1 Mpc), known at low redshift, but the general decrease in the angular size of radio sources with redshift (e.g. Rottgering et al 1994) makes 4C23.56 standout as an exceptional object. The object has a classic FRII morphology. (See Figures 9a - 9c.) Note that in order to fit the large size of this source on the page with out loss of detail for the optical images, we have rotated the image so that the axis is horizontal. The true PA of the source is 52 deg east of north, in the image north is at an angle of 38 deg counterclockwise from the vertical, as indicated by the tick marks. The opposite lobes and the core are perfectly co-linear, although the western arm length is twice the size of the eastern arm. The two sides have roughly the same intensity and remain roughly equivalent even at 2 cm. There is substantial extended emission from the jet leading to the western lobe A. The ridge line of this emission has an S shape at 20 cm that is remarkable in light of the co-linearity of the opposite lobes and core. The first curve outward to the south is not is not seen at 6 cm due to the spectral index. At 2 cm only the compact hot spot is detected. The eastern lobe B is also extended and has ripples along the jet. The ridge line along the eastern jet to the lobe rises smoothly all the way to the peak at 1.2" resolution. The core is unresolved and has a comparatively flat spectrum between 20 cm and 6 cm () but then becomes ultra steep by 2 cm ( see Figure 2g).

Both lobes have resolved depolarization structures at 20 cm and 6 cm; neither lobe is detected in the polarization image at 2cm (Figures 9d - 9e). The polarization angle rotates sharply at the total intensity peak. Each radio lobe is roughly bifurcated in the polarization image by the sharp depolarization.

The optical identification is faint, in a crowded field and 3.5" from a 21 magnitude red star. The relatively bright (17th magnitude) offset star (A in Figure 1d) is saturated and dominates the field to the southwest. (Note the presence of this star 30" away from this remarkable source makes this object an ideal candidate for study with adaptive optics.) The narrow band image shows an extraordinary bipolar Ly emission line nebula (Figure 9g). It is extremely large, with a bifurcated boxy appearance that is at least 14" (120 kpc) across the diagonal. The distinctly bisected nebula is centered on the radio core and perfectly aligned with radio axis. The opposing cones have faint asymptotic filaments with an opening angle of about 90 degrees. The nebula is diffuse, but has brighter knots within it. The bipolar morphology is highly symmetric, indicating that the source is near the plane of sky, consistent with the unusually large radio source. In V band the object is extended along the radio axis, with increasing brightness towards the A lobe to the southwest. In R band, the object is comparatively compact. In I band 4C23.56 is extended, aligned with radio axis and roughly symmetric about the peak in both directions. In J band the source appears bifurcated, with the southwestern of the two components being brighter and more aligned. The K band morphology is much more sharply peaked with diffuse emission towards the southwest lobe, and possibly to the northeast as well.

The astrometry places the radio core to the east of the peak in the K band; they are plausibly coincident. However, the bipolar emission line nebula has a distinct center that is spatially coincident with the bifurcated morphology in J band. If we adopt this common bifurcation as the position of the radio core, then the aligned UV continuum extends towards the southwestern lobe A.