The keyword phrase "september 11th hashtags" functions as a noun phrase. The core or main point of this phrase is the plural noun "hashtags."
In this construction, "September 11th" acts as a compound adjectival modifier. It is a proper noun (the specific date and event) that describes and specifies the noun "hashtags." This grammatical structure clarifies which specific category of hashtags is being discussednamely, those related to the events of September 11th. The head of the phrase, which determines its primary part of speech, is "hashtags."
For practical application within an article, the entire phrase should be treated as a specific concept or a compound noun. It identifies a distinct digital and social phenomenon, making the noun "hashtags," as specified by the modifier "September 11th," the central subject of discussion.
