Anomaly distribution of quasar magnitudes: a test of lensing by a hypothetic supergiant molecular cloud in the Galactic halo

Laboratoire Univers et Particules, UMR5299 CNRS-In2p3/Montpellier University, 34095 Montpellier, France
Laboratoire d’Astrophysique de Marseille, UMR7236 CNRS-INSU/Aix-Marseille University, 13388 Marseille, France
e-mail: edmond.giraud@lpta.in2p3.fr

Received: 26 April 2012
Accepted: 12 June 2012

Abstract

Context. An anomaly in the distribution of quasar magnitudes based on the Sloan Digital Sky survey, was reported by Longo. The angular size of this quasar anomaly is on the order of ±15° on the sky. A smooth low surface brightness structure detected in γ-rays and at 408 MHz, coincides with the sky location and extent of the anomaly, and is close to the northern component of a pair of γ-ray bubbles discovered in the Fermi Gamma-ray Space Telescope survey. Molecular clouds are thought to be illuminated by cosmic rays. Molecular gas in the Galaxy, in the form of cold H₂, may be a significant component of dark matter as suggested by Pfenniger et al.

Aims. I test the hypothesis that the magnitude anomaly in the quasar distribution, is due to lensing by a hypothetical supergiant molecular cloud (SGMC) either in or falling into the Galactic halo.

Methods. A series of grid lens models are built by assuming that a SGMC is a fractal structure constructed with clumps of 10^-3 M_⊙, 10 AU in size, and considering various fractal dimensions. Local amplifications are computed by using the single-plane approximation.

Results. A complex network of caustics due to the clumpy structure is present. Our best single plane lens model capable of explaining Longo’s effect, at least in sparse regions, requires a mass (1.5−4.1) × 10¹⁰ M_⊙ within 8.7 × 8.7 × (5−8.6) kpc³ at a lens plane distance of 20 kpc, and is constructed from a molecular-cloud building-block of 5 × 10⁵ M_⊙ within a scale of 30 pc expanded by fractal scaling with dimension D = 1.8−2 out to 5−8.6 kpc for the SGMC. The mass budget depends on the cloud depth and on the fractal dimension.

Conclusions. If such a SGMC were found to exist, it may provide at least part of a lensing explanation for the luminous anomaly discovered in quasars and red galaxies.