Alex Woodbridge is a murdered man who used to work as a security guard at the Hickman Gallery. His body was found n the shoreline of the River Thames.
Sherlock examined the body and deduced that he worked at the gallery and that his death was related to the lost Vermeer painting somehow. It is later revealed by Sherlock Holmes that he was murdered by the Golem.
It is revealed, via his roommate, that he was an amateur astronomer who must have examined the new painting during his shift at the gallery and released it was a fake. He noticed that the Van Buren supernova was present in the painting even though it didn't appear until years after the date when the painting should have been created.
He contacted Professor of Astronomy Professor Cairns, and they were started investigating but Alex was killed before the professor had proved it.