Magnification of light from many distant quasars by gravitational lenses

Abstract

Exceptionally bright quasars with redshifts up to z=6.28 have recently been discovered 1 . Quasars are thought to be powered by the accretion of gas onto supermassive black holes at the centres of galaxies. Their maximum (Eddington) luminosity is proportional to the mass of the black hole, and so these bright quasars are inferred to have black holes with masses of more than a few billion solar masses. The existence of such massive black holes poses a challenge to models for the formation of structures in the early Universe, as it requires that the black holes would grow so massive in less than a billion years after the Big Bang. Here we show that up to a third of known quasars with z∼6 will have their observed flux magnified by a factor of 10 or more through gravitational lensing by galaxies along the line of sight. The inferred abundance of quasar host galaxies, as well as the luminosity density provided by the quasars, are therefore substantially overestimated.The four highest redshift quasars known 1 with z ∼ > 5.8 (SDSS 1044-0125 was later found 4 to have z = 5.73), were selected in the SDSS photometric system to have magnitudes z * < 20.2 and colors i * − z * > 2.2. The masses of the black holes powering these quasars are estimated to be ∼ > 3 × 10 9 M ⊙ , implying 3 that the mass of their host galaxies is ∼ > 10 13 M ⊙ . Such massive hosts lie on the steep exponential tail of the Press-Schechter 5

Four Quasars above Redshift 6 Discovered by the Canada-France High-zQuasar Survey

“…One possible explanation for this is that some of the most luminous z $ 6 quasars are gravitationally lensed (Wyithe & Loeb 2002;Comerford et al 2002). The high incidence of Mg ii absorption would then be related to above average mass density along the line of sight.…”

“…This method has been employed to argue that the highest redshift SDSS quasars are embedded in a highly neutral IGM ( f H i k 0:1; Wyithe & Loeb 2004;Wyithe et al 2005;Mesinger & Haiman 2007). However, there are many complicating factors, such as the uncertain density distribution close to luminous quasars and preionization by associated galaxies (Yu & Lu 2005).…”

“…For a quasar embedded in a uniform-density IGM, the size of the ionization front from the quasar is proportional to f À1 = 3 H i ,Ṅ 1 = 3 , and t 1 = 3 Q , where f H i is the fraction of hydrogen in the IGM that is neutral,Ṅ is the number of ionizing photons emitted by the quasar per second, and t Q is the age of the quasar activity (Cen & Haiman 2000;Wyithe & Loeb 2004). Note that the latter two terms could be combined into the total number of ionizing photons emitted so far by the quasar.…”

“…Cosmic variance in transmission due to large-scale density inhomogeneities is extremely large on proper scales up to 20 Mpc at z ¼ 5:5 and rapidly increases at earlier times before full reionization ( Wyithe & Loeb 2006). The SDSS currently contains only a handful of quasars at a high enough redshift (z > 6:1) to probe a large path length of the z > 6 IGM.…”

VERY LARGE ARRAY AND VERY LONG BASELINE ARRAY OBSERVATIONS OF THE HIGHEST REDSHIFT RADIO-LOUD QSO J1427+3312 ATZ= 6.12

“…Li et al (2007)). Wyithe & Loeb (2002) estimate that almost one third of known quasars at z ∼ 6 ought to be lensed by galaxies along the line of sight. If these quasars are indeed gravitationally lensed, the estimated masses of their associated spheroids could be smaller by up to an order of magnitude; this would allow a less efficient assembly process.…”