Sarmatian Auxiliaries in Britannia

Salve, I have a question about the Sarmatian Auxiliary detachments that served in Britannia. If I remember correctly they were sent there under Hadrian's reign and guarded Hadrian's Wall, I've been wondering why these Sarmartians were serving under Hadrian. I mean, where they part of a treaty obligation or where they mercenaries? I'm just curious if anyone here has an answer.

This is my first post here, I've been reading through the wealth of information that this great site has to offer for the past few months and glad I found this place.

Welcome to the site. It's a very good question, and I'd be interested in an answer to that one. I don't recall any of the hadrian's wall literature covering histories of the auxiliary cohorts.

Cassius Dio (LXXI.16) says that a treaty negotiated by Marus Aurelius in 175AD, ending the war with the Iazyges (a Sarmation people who lived in Eastern Hungary), required them to provide 8000 troops to the Roman Army.  5500 of them were sent to Britain.  One unit, the ala primae Sarmatorum, has been identified by inscriptions at its station at Ribchester in Lancashire.

Ah thank you, I forgot about the expedition against the lazyges. That makes sense; I was sure it was part of a treaty obligation, but I wasn't sure what treaty.

Auxillaries were pretty much like the Foreign Legion is for the French today, or the Ghurkas to the British, except they probably weren't  expendable despite being considered lower grade troops, reflected in different equipment and lower pay. It meant a ready source of military manpower from volunteers who looked forward to becoming Roman citizens as a reward for their service. Treaties weren'r necessary nor did the Romans consider them mercenaries as they were led by Roman officers.

Ah thank you all who posted, I had been wondering about this for sometime.