Microlensing-Based Estimate of the Mass Fraction in Compact Objects in Lens Galaxies

Abstract

We estimate the fraction of mass that is composed of compact objects in gravitational lens galaxies. This study is based on microlensing measurements (obtained from the literature) of a sample of 29 quasar image pairs seen through 20 lens galaxies. We determine the baseline for no microlensing magnification between two images from the ratios of emission line fluxes. Relative to this baseline, the ratio between the continua of the two images gives the difference in microlensing magnification. The histogram of observed microlensing events peaks close to no magnification and is concentrated below 0.6 magnitudes, although two events of high magnification, ∆m ∼ 1.5, are also present. We study the likelihood of the microlensing measurements using frequency distributions obtained from simulated microlensing magnification maps for different values of the fraction of mass in compact objects, α. The concentration of microlensing measurements close to ∆m ∼ 0 can be explained only by simulations corresponding to very low values of α (10% or less). A maximum likelihood test yields α = 0.05 +0.09 −0.03 (90% confidence interval) for a quasar continuum source of intrinsic size r s 0 ∼ 2.6 · 10 15 cm. This estimate is valid in the 0.1 − 10M ⊙ range of microlens masses. We study the dependence of the estimate of α with r s 0 , and find that α 0.1 for r s 0 1.3 · 10 16 cm. High values of α are possible only for source sizes much larger than commonly expected (r s 0 >> 2.6 · 10 16 cm). Regarding the current controversy about Milky Way/LMC and M31 microlensing studies, our work supports the hypothesis of a very low content in MACHOS (Massive Compact Halo Objects). In fact, according to our study, quasar microlensing probably arises from the normal star populations of lens galaxies and there is no statistical evidence for MACHOS in the dark halos.

Cosmic microwave background limits on accreting primordial black holes

“…In the mean time, there are a number of other interesting astrophysical probes in that mass range. These include future measurements of the stochastic gravitational-wave background [56][57][58][59][60] and of the mass spectrum [61], redshift distribution [62], and orbital eccentricies [63] for future binary-black-hole mergers; lensing of fast radio bursts by PBHs [64]; pulsar timing [65,66]; radio/x-ray sources [67] or the cosmic infrared background [68]; the dynamics of compact stellar systems [54]; strong-lensing systems [69]; and perhaps clustering of GW events [70][71][72][73]. The conclusions of our work suggest that it will be important to pursue vigorously these alternative avenues.…”

Microlensing of the broad line region in 17 lensed quasars

“…Our results may also be used to derive the fraction of smooth matter in lensing galaxies. A study of the probability of deriving the observed values of μ in sample of lensed quasars, varying the fraction of compact/smooth dark matter along the line-of-sight of the lensed images, makes possible to constrain the fraction of matter in compact form in these galaxies (Schechter & Wambsganss 2004;Pooley et al 2009;Mediavilla et al 2009;Bate et al 2011;Pooley et al 2012).…”

Flux and color variations of the quadruply imaged quasar HE 0435-1223

“…Furthermore, Mediavilla et al (2009) observed HE0435-1223 in the framework of a monitoring of 29 lensed quasars, and attributed eventual microlensing events to the normal stellar populations, while Blackburne & Kochanek (2010) focused on the quasar itself, applying a model with a time-variable accretion disk to the object. Mosquera et al (2011) found clear evidence of chromatic microlensing in the "A" component, and provided an estimate of the disk size in the R band in agreement with the simple thin-disk model.…”