What constitutes authorship?

Author X submitted a paper to another journal, and included author Y, a student in the same institute, as a courtesy. Author Y had drawn two figures for the paper and discussed some of the observations (all made by author X) with author X but the paper did not deal with the thesis research of author Y.

After the original paper was returned, requiring extensive revisions, author X revised the paper and submitted it to our journal. Author X did not include author Y as an author, but listed Y’s contribution in the acknowledgments. Following notification of acceptance and online publication, author Y complained to the editor, insisting that he should be added as an author by the journal without reference to author X. The editor contacted author X who indicated that author Y did not make a substantial scientific contribution to the article and that the notice in the acknowledgments was appropriate.

Authors X and Y hold similar views as to what author Y's contribution was, but different views on whether this is sufficient for authorship. The editor notified authors X, Y and the administrative head of the institute, and recommended that the parties resolve the matter among themselves. Author Y indicated that this suggested approach was unacceptable, repeated that he should be added as an author on the paper regardless of the opinion of author X and did not accept the editor’s position that this matter could not be resolved arbitrarily in author Y's favour by the editor or the publisher. Author X has insisted on publication as the single author. The editor has continued to attempt to make contact with the parties, without recent success.