Discovery of a Luminous Quasar in the Nearby Universe

In the course of the Pico dos Dias survey (PDS), we identified the stellar-like object PDS 456 at coordinates α = 17 h 28 m 19 796, δ = -14°15'55 87 (epoch 2000), with a relatively nearby ( z = 0.184) and bright ( B = 14.69) quasar. Its position at Galactic coordinates l = 10 4, b = +11 2, 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 L 60 = 3.8 × 10 45 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 ( F ν ∝ 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 L X = 2.8 × 10 44 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 (α OX = -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 × 10 30 ergs -1 Hz -1 . We estimate the Galactic reddening in this line of sight to be A B ≃ 2.0, implying an absolute magnitude M B = -26.7 (using H 0 = 75 km s -1 Mpc -1 and q 0 = 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.