![]() | Annu. Rev. Astron. Astrophys. 1994. 32:
531-590 Copyright © 1994 by Annual Reviews. All rights reserved |
One of the most useful signatures of baryonic dark matter candidates is
undoubtedly their gravitational lensing effects. Indeed, it is
remarkable that
lensing could permit their detection over the entire mass range of
10-7
M
to 1012
M
. All
sorts of astronomical objects can serve as lenses
(Blandford & Narayan 1992)
but the crucial advantage of Population III objects is that they
are compact and spherically symmetric, which makes their effects very clean.
To search for them, one requires sources that are numerous, small,
bright, and have predictable intrinsic variations
(Nemiroff 1991a).
The most useful sources
to date have been quasars, galaxies, radio jets, gamma-ray bursts, and
stars; all
of these are discussed below. Other possibilities include radio sources
(Blandford & Jarosynski
1981),
supernovae
(Schneider & Wagoner 1987,
Linder et al 1988,
Rauch 1991),
and pulsars
(Krauss & Small 1991).
There are two distinct
lensing effects and these probe different but nearly overlapping mass
ranges:
macrolensing (the multiple-imaging of a source) can be used to
search for objects larger than 104
M
, while
microlensing (modifications to the intensity of
a source) can be used for objects smaller than this. The current
constraints on the density
c of
compact objects in various mass ranges are brought together
in Figure 5.
![]() |
Figure 5. Macrolensing and microlensing constraints on thc density parameter for compact objects of mass M. Current limits are shown by shaded lines and potential ones by broken lines. (a) VLA limit of Hewitt (1986); (b) optical and HST limit of Surdej et al (1993); (c) VLBI limit of Kassiola et al (1991); (d) and (e) potential speckle interferometry and VLBA limits; (f) region required to explain the quasar variations claimed by Hawkins (1993); (g) Dalcanton et al (1994) quasar line-continuum limit; (h) gamma-ray burst limit of Nemiroff et al (1993), assuming these are at a cosmological redshift; (i) corresponds roughly to the range of values required to explain the MACHO and EROS microlensing results; (j) potential limit associated with the null results from the EROS CCD study. |