We present the results of a search for galaxy substructures in a sample of 11 gravitational lens galaxies from the Sloan Lens ACS Survey .
We find no significant detection of mass clumps , except for a luminous satellite in the system SDSS J0956+5110 .
We use these non-detections , in combination with a previous detection in the system SDSS J0946+1006 , to derive constraints on the substructure mass function in massive early-type host galaxies with an average redshift \langle z _ { lens } \rangle \sim 0.2 and an average velocity dispersion \langle \sigma _ { eff } \rangle \sim 270 km s ^ { -1 } .
We perform a Bayesian inference on the substructure mass function , within a median region of about 32 kpc ^ { 2 } around the Einstein radius ( \langle R _ { ein } \rangle \sim 4.2 kpc ) .
We infer a mean projected substructure mass fraction f = ~ { } 0.0076 _ { -0.0052 } ^ { +0.0208 } at the 68 percent confidence level and a substructure mass function slope \alpha~ { } < 2.93 at the 95 percent confidence level for a uniform prior probability density on \alpha .
For a Gaussian prior based on cold dark matter ( CDM ) simulations , we infer f = ~ { } 0.0064 ^ { +0.0080 } _ { -0.0042 } and a slope of \alpha = ~ { } 1.90 ^ { +0.098 } _ { -0.098 } at the 68 percent confidence level .
Since only one substructure was detected in the full sample , we have little information on the mass function slope , which is therefore poorly constrained ( i.e .
the Bayes factor shows no positive preference for any of the two models ) .
The inferred fraction is consistent with the expectations from CDM simulations and with inference from flux ratio anomalies at the 68 percent confidence level .