The optical cut-off source 0500+019: A background quasar seen through a foreground galaxy?

The optical spectrum of the extremely red (α_R-B ≈ -7) optical counterpart of the radio source 0500+019 shows an unidentified emission line at 6543 Å in addition to absorption and emission lines of Ca H&K λλ 3933,3968, [O II] λ 3727, and [O III] λλ 4959,5007 at a redshift of z = 0.583. The optical (R-band) morphology of 0500+019 consists of an asymmetric galaxy with no obvious point source. In contrast, the infrared (K_s) morphology is dominated by a point source, while the galaxy component seen in the R-band is clearly visible only after image restoration. Since the emission line at 6543 Å cannot be identified within the z = 0.583 redshift system, it is interpreted as being due to a second system of currently unknown redshift. The optical and infrared morphologies then naturally lead to a picture, where a background source, likely a quasar seen in the infrared, is located behind a foreground galaxy, which is most clearly detected in the optical. In this case the steep optical spectral index may be the result of foreground absorption rather than being a source intrinsinc property.