We study a sample of 39 massive early-type lens galaxies at redshift z \lesssim 0.3 to determine the slope of the average dark-matter density profile in the innermost regions .
We keep the strong lensing and stellar population synthesis modeling as simple as possible to measure the galaxy total and luminous masses .
By rescaling the values of the Einstein radius and dark-matter projected mass with the values of the luminous effective radius and mass , we combine all the data of the galaxies in the sample .
We find that between 0.3 and 0.9 times the value of the effective radius the average logarithmic slope of the dark-matter projected density profile is -1.0 \pm 0.2 ( i.e. , approximately isothermal ) or -0.7 \pm 0.5 ( i.e. , shallower than isothermal ) , if , respectively , a constant Chabrier or heavier , Salpeter-like stellar IMF is adopted .
These results provide positive evidence of the influence of the baryonic component on the contraction of the galaxy dark-matter halos , compared to the predictions of dark matter-only cosmological simulations , and open a new way to test models of structure formation and evolution within the standard \Lambda CDM cosmological scenario .