  The step edges on the Si(553)-Au surface undergo a 1 x 3 reconstruction at
low temperature which has recently been interpreted theoretically as the x3
ordering of spin-polarized silicon atoms at the edges of the graphitic Si
nanowires on this vicinal surface. This predicted magnetic ground state has a
clear spectroscopic signature - a silicon step-edge state at 0.5 eV above the
Fermi level - that arises from strong exchange splitting and hence would not
occur without spin polarization. Here we report spatially resolved scanning
tunneling spectroscopy data for these nanowires. At low temperature we find an
unoccupied state at 0.5 eV above every third step edge silicon atom, in
excellent agreement with the spin-polarized ground state predicted
theoretically. This spin-polarized state survives up to room temperature where
the position of the spins rapidly fluctuates among all Si step-edge sites.
