We consider L-2-constraint minimizers of mass critical Hartree energy functionals in R-N with N >= 3. We prove that minimizers exist if and only if the parameter a > 0 satisfies a < a* =parallel to Q parallel to(2)(2), where Q is a positive radially symmetric ground state of Delta u - u + (integral(RN)|u(y)|(2)/|x-y|(2)dy)(u) = 0 in R-N. The blow-up behavior of minimizers as a approaches a* is also analyzed, for which all the mass concentrates at a global minimum point x(0) ...
