F. Vogt, E. Pérez, M. Dopita, L. Verdes-Montenegro, и S. Borthakur. (2017)cite arxiv:1701.01728Comment: 12 pages, 11 figures, accepted for publication in A&A. Supplementary movie assocociated with Fig. 8 is available (until publication) at: http://www.sc.eso.org/~fvogt/supp_mat/HCG91c/O_gradient.mp4.