Published online by Cambridge University Press: 27 November 2019
We study quotients of mapping class groups
${\Gamma _{g,1}}$
of oriented surfaces with one boundary component by the subgroups
${{\cal I}_{g,1}}(k)$
in the Johnson filtrations, and we show that the stable classifying spaces
${\mathbb {Z}} \times B{({\Gamma _\infty }/{{\cal I}_\infty }(k))^ + }$
after plus-construction are infinite loop spaces, fitting into a tower of infinite loop space maps that interpolates between the infinite loop spaces
${\mathbb {Z}} \times B\Gamma _\infty ^ + $
and
${\mathbb {Z}} \times B{({\Gamma _\infty }/{{\cal I}_\infty }(1))^ + } \simeq {\mathbb {Z}} \times B{\rm{Sp}}{({\mathbb {Z}})^ + }$
. We also show that for each level k of the Johnson filtration, the homology of these quotients with suitable systems of twisted coefficients stabilises as the genus of the surface goes to infinity.