Discovery of a Luminous Quasar in the Nearby Universe

Abstract

In the course of the Pico dos Dias survey (PDS), we identified the stellar-like object PDS 456 at coordinates α = 17h28m19796, δ = -14°15'5587 (epoch 2000), with a relatively nearby (z = 0.184) and bright (B = 14.69) quasar. Its position at Galactic coordinates l = 104, b = +112, near the bulge of the Galaxy, may explain why it was not detected before. The optical spectrum of PDS 456 is typical of a luminous quasar, showing a broad (FWHM ~ 4000 km s-1) Hβ line, very intense Fe II lines, and a weak [O III] λ5007 line. PDS 456 is associated to the infrared source IRAS 17254-1413 with a 60 μm infrared luminosity L60 = 3.8 × 1045 ergs s-1. The relatively flat slopes in the infrared [α(25, 60) = -0.33 and α(12, 25) = -0.78] and a flat power index in the optical (Fν ∝ ν-0.72) may indicate a low dust content. A good match between the position of PDS 456 and the position of the X-ray source RXS J172819.3-141600 implies an X-ray luminosity LX = 2.8 × 1044 ergs s-1. The good correlation between the strength of the emission lines in the optical and the X-ray luminosity, as well as the steep optical to X-ray index estimated (αOX = -1.64), suggests that PDS 456 is radio quiet. A radio survey previously performed in this region yields an upper limit for radio power at about 5 GHz of approximately 2.6 × 1030 ergs-1 Hz-1. We estimate the Galactic reddening in this line of sight to be AB ≃ 2.0, implying an absolute magnitude MB = -26.7 (using H0 = 75 km s-1 Mpc-1 and q0 = 0). In the optical, PDS 456 is therefore 1.3 times more luminous than 3C 273 and the most luminous quasar in the nearby (z ≤ 0.3) universe.