Infrared observations of the most luminous quasar

Abstract

The QSO Q 0420–388 is one of many discovered by Osmer and Smith in their objective prism survey of the southern sky, and one of ∼10 now known with z>3. Its brightness and large redshift indicate a luminosity at rest wavelength 1,475 Å somewhat larger than any other measured. The source is particularly noteworthy because of the steepness of its observed visual spectrum over the range 3,600–6,800 Å, where Fν ∼ ν^−2±1; this suggests that appreciably more power might be radiated in the infrared (IR). The near IR photometric observations reported here of Q 0420–388 (z = 3.12) show that it is 13.5±0.2 mag at 2.2 µm. This observation and the optical observations of Osmer and Smith¹ indicate a bolometric luminosity ≥5 × 10¹⁴L_⊙, for H₀ = 50 km s⁻¹ Mpc⁻¹ and q₀ = 0.1 confirming Q 0420–388 as the most luminous quasar for cosmologies with small q₀.