Carnegie Observatories Astrophysics Series, Vol. 2:
"Measuring and Modeling the Universe"
ed. W. L. Freedman (Cambridge: Cambridge Univ. Press)
For a PDF version of the article, click
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THE HUBBLE CONSTANT FROM GRAVITATIONAL LENS TIME DELAYS
Christopher S. Kochanek
Harvard-Smithsonian
Center for Astrophysics
Paul L. Schechter
Massachusetts Institute of
Technology
Abstract. There are now 10 firm time delay measurements in
gravitational lenses. The
physics of time delays is well understood, and the only important variable
for interpreting the time delays to determine H0 is
the mean surface mass density
<
> (in units of
the critical density for gravitational
lensing) of the lens galaxy at the radius of the lensed images. More
centrally concentrated mass distributions with lower
<> predict higher
Hubble constants, with
H0 
1 - <> to lowest
order. While we cannot determine
<> directly given
the available data on the current time delay lenses, we find
H0 = 48 ± 3 km s-1 Mpc-1
for the isothermal (flat
rotation curve) models, which are our best present estimate for the mass
distributions of the lens galaxies. Only if we eliminate the dark matter
halo of the lenses and use a constant mass-to-light ratio (M /
L) model to find
H0 = 71 ± 3 km s-1 Mpc-1
is the result consistent with local estimates. Measurements of time
delays in
better-constrained systems or observations to obtain new constraints on the
current systems provide a clear path to eliminating the
<> degeneracy
and making estimates of H0 with smaller uncertainties
than are possible
locally. Independent of the value of H0, the time
delay lenses provide
a new and unique probe of the dark matter distributions of galaxies and
clusters because they measure the total (light + dark) matter surface
density.
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