We have searched for gravitational milli–lens systems by examining VLBI maps of ~ 300 flat–spectrum radio sources. So far we have followed up 7 candidates, with separations in the range 2–20 mas. None have been confirmed as lenses but several of them can not yet be definitively ruled out. If there are no milli-lenses in this sample then uniformly–distributed black holes of 10^6 to 10^8 M_⊙ cannot contribute more than ~ 1% of the closure density

A Kassiola

BJ Carr

JP Ostriker

NY Gnedin

RJ Nemiroff

TJ Pearson

WH Press

Symposium - International Astronomical Union

Wilkinson, P. N.

Henstock, D. R.

Browne, I. W. A.

Readhead, A. C. S.

Taylor, G. B.

Vermeulen, R. C.

Pearson, T. J.

Xu, W.

Caltech Authors - Main

A S E A R C H F O R G R A V I T A T I O N A L M I L L I - L E N S E S 
P.N. WILKINSON, D.R. HENSTOCK AND I.W.A BROWNE 
University of Manchester 
NRAL, Jodrell Bank, Macclesfield, 
Cheshire, SK11 9DL, UK 
AND 
A.C.S. READHEAD, G.B. TAYLOR, R.C. VERMEULEN, 
T.J. PEARSON AND W. XU 
California Institute of Technology 
OVRO, Robinson 105-24, Pasadena, CA 91125 USA 
A b s t r a c t . We have searched for gravitational milli-lens systems by ex-
amining VLBI maps of ~ 300 flat-spectrum radio sources. So far we have 
followed up 7 candidates, with separations in the range 2-20 mas. None 
have been confirmed as lenses but several of them can not yet be defini-
tively ruled out. If there are no milli-lenses in this sample then uniformly-
distributed black holes of 10 6 to 10 8 Μ Θ cannot contribute more than ~ 1% 
of the closure density. 
1. I n t r o d u c t i o n 
A few lens systems have been found with sub-arcsecond image separations 
but lensing by mass concentrations below ~ 1 0 9 M q has not yet been inves-
tigated because the image separation has been below the resolution limit 
of the V L A and the HST. VLBI imaging is the only direct way to push the 
image separation limit below ~ 100 mas. 
Searches for lensed systems with image separations in the range 1-100 
mas (so-called "milli-lenses"), are of particular cosmological interest. Such 
separations correspond to lensing masses in the range ^ 1 0 6 - 1 0 8 Μ Θ , com-
parable with the expected masses of pre-galactic compact objects (PCOs) . 
PCOs arise in a wide range of cosmogonie scenarios with a natural mass 
scale, set by the Jeans mass, of order 10 6 M@ (e.g. Carr 1990). Uniformly 
95 
M. Kafatos and Y. Kondo (eds.), Examining the Big Bang and Diffuse Background Radiations, 95-98. 
© 1996 IAU. Printed in the Netherlands. 
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96 
distributed PCOs could provide a significant fraction of the closure density 
of the universe and can only be detected by their gravitational lensing ef-
fects. Press & Gunn (1973) first calculated the optical depth of the Universe 
to lensing by point masses and their calculations have been extended by Os-
triker & Vietri (1986), Nemiroff and Bistolas (1990) and Kassiola, Kovner 
and Blandford (1991). Gnedin & Ostriker (1992) have recently suggested 
that radiation from an early generation of massive stars ( 1 O 6 , 5 M 0 ) , forming 
somewhat after decoupling, may have altered the light element abundances. 
Their hypothesis allows a larger amount of baryonic dark matter and does 
away with the need for non-baryonic forms. The massive stars collapse to 
black holes and with the required density, Ω ~ 0.15, ~ 5% of high-redshift 
quasars should be milli-lensed. 
2 . T h e V L B I Su rveys 
A series of VLBI surveys have now been made with resolutions of ~ 1 mil-
liarcsec (mas). The most recent was the second Caltech-Jodrell Bank (CJ2) 
VLBI survey—a global Mkll snapshot VLBI survey of 193 flat-spectrum 
radio sources at λ 6 cm (Taylor et al. 1994; Henstock et al. 1995). The aims 
of the CJ2 survey were to extend the morphological classification of the 
P R (Pearson & Readhead 1988) and CJ1 (Polatidis et al. 1994; Thakkar et 
al. 1994; Xu et al. 1994) VLBI surveys and to address several new cosmo-
logical questions. Amongst these was whether or not there is evidence for 
gravitational lensing on scales 2-200 mas. 
On the mas scale the great majority of flat-spectrum radio sources are 
asymmetric core-jets. Simulations of the lensing effect of a PCO show that 
the typical image configuration is likely to consist of the core-jet primary 
with the demagnified, hence fainter, secondary off to one side. When the 
images are convolved with a 1 mas restoring beam the secondary may ap-
pear unresolved even if the primary is well resolved. Any source with a 
compact companion component no larger than the strongest component 
was therefore treated as a lens candidate. 
About 300 flat-spectrum radio sources have now been mapped in the 
PR, CJ1 and CJ2 VLBI surveys; the typical resolution is 1 mas and the 
dynamic range is > 100 : 1. We are confident of detecting compact lensed 
components 30 times weaker than the brightest component in the map out 
to ±200 mas. However, for image separations < 1.5 mas, i.e. comparable 
with the beam size, we are only sensitive to compact components with flux 
ratios < 10 : 1. 
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97 
3. T h e Redshi f t Dis t r ibu t ion 
The majority ( ~ 80%) of the ~ 300 flat-spectrum sources in the PR, CJ1 
and CJ2 surveys now have measured redshifts. The redshift distribution is 
shown in Fig. 1. The fraction of high redshift sources is a strong function 
of the limiting flux density of the sample. In the combined P R sample 
(Seem > 1*3 Jy) and CJ1 sample (1.3 Jy < Seem < 0.7 Jy) only 3 objects 
(rsj 3%) have ζ > 2.5 whereas in the CJ2 sample (0.7 Jy < S 6 c m < 0.31 Jy) 
18 objects ( ^ 9%) have ζ > 2.5. As yet we have redshifts for only ~ 70% of 
the CJ2 sample but expect to increase this to ~ 90% with further optical 
observations of the faint objects. 
C J F l a t S p e c t r u m S a m p l e r e d s h i f t d i s t r i b u t i o n 
1 1 . 5 
Figure 1. The redshift distribution of flat-spectrum sources in P R + C J 1 + C J 2 
4 . F o l l o w - u p Obse rva t ions 
For the seven lens candidates we are made initial follow-up observations 
with 6 telescopes of the VLBA during February and March 1994. We made 
total intensity maps at λ 18cm, λ 3.6cm and λ 2cm to complement the 
existing λ 6cm maps. Lensed sources must contain two or more mas com-
ponents which have simply-related structures and identical radio spectra. 
This multi-wavelength attack enabled us to show that about half of the 
candidates are conventional core-jet sources. For example Fig. 2 shows the 
CJ2 survey map of 0740+768; our higher resolution VLBA map at λ 2cm 
reveals the compact double source to be a core with a jet in p.a. —100°. A 
few candidates cannot yet be definitively ruled out. For these we must await 
the results of higher sensitivity VLBA observations at Λ2 cm scheduled for 
the Winter of 1994/5. 
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98 
Figure 2. The lens candidate 0740+768: left) CJ2 map at A6cm (resolution ~ 1 mas) 
made with a "global" array; right) V L B A map at Λ2 cm (resolution ~ 0.5 mas) 
It we fail to detect milli-lensing in ~ 300 flat-spectrum radio sources 
then uniformly-distributed PCOs can only contribute < 1% to the closure 
density. In order to place tighter limits on the cosmological density of PCOs, 
and specifically to confront Gnedin and Ostriker's (1992) prediction that 
~ 5% of high redshift quasars might be lensed, at least 1000 radio-selected 
flat-spectrum sources must be searched. The VLBA is well-suited to this 
new surveying task. 
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available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0074180900109970
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A Search for Gravitational Milli–Lenses

P. N. Wilkinson

D. R. Henstock

I. W. A. Browne

A. C. S. Readhead

G. B. Taylor

R. C. Vermeulen

T. J. Pearson

W. Xu

Crossref

A Search for Gravitational Milli-Lenses

We have searched for gravitational milli–lens systems by examining VLBI maps of ~ 300 flat–spectrum radio sources. So far we have followed up 7 candidates, with separations in the range 2–20 mas. None have been confirmed as lenses but several of them can not yet be definitively ruled out. If there are no milli-lenses in this sample then uniformly–distributed black holes of 106to 108M⊙cannot contribute more than ~ 1% of the closure density.</jats:p

P.N. Wilkinson

D.R. Henstock

I.W.A. Browne

A.C.S. Readhead

G.B. Taylor

R.C. Vermeulen

T.J. Pearson

English

https://authors.library.caltech.edu/91650/1/search_for_gravitational_millilenses.pdf

A Search for Gravitational Milli–Lenses

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