I've always been very much interested in the origin of these people and the way they served Iran. However, the origin and fate of these people has always been a mystery for me and despite reading about them I couldn't get my answers. Therefore I pose my questions about them here:
- Origin of Parthians: As far as I know they were the third wave of Iranians who reached the Iranian plateau. However, despite them being depicted in base-reliefs of Persepolis, they seem somehow "absent" during the era of the Achaemenid Empire (in the sense that there are not mentioned as the glorious warriors who appeared later in facing Romans). Do we know if they were really absent in the sense of not many of them living under Achaemenid rule?
- Seen as Persians?: Later, after the collapse of the empire they somehow got absorbed into the Sassanian court in the sense that they maintained the Seven Parthian clans as part of their feudal aristocracy. But back then were they seen as Persians? Like was it different to live in the Sassanian empire as a Parthian man or as a Persian man?
- Where are they now?: After the muslim invasion, the persians were defending their lands side by side with parthians but step by step they had to go deeper into the eastern lands of the empire as they were receiving defeats after defeats. But then after the final fall of the Sassanids, the parthians also disappeared with them from the historical texts (I know this sentence is not fully accurate but to me it seems as if they finally somehow merged with the persians into one). So the question is: where are they now? are they Tajiks or some Afgans? or are they simply reffered to as Persians nowadays?
Please let me know if some parts of the questions have to be put in a more accurate way.
Re. your first question, the somewhat traditional account seems to be that the Achaemenids indeed had a province/satrapy called Parthia (e.g. mentioned here and here). But the people who would later found the Parthian empire did not live in Parthia at that time yet! Rather, towards the end of the Seleucid empire that area was invaded from the north by Iranian steppe peoples (Dahae or Parni) and these invaders went on to found the ruling circles of the Arsacid or "Parthian" empire. So if this account is correct, it seems that the predecessors of the ruling class of the Parthian empire lived outside of Achaemenid control.
Re. your second question, there are several bilingual inscriptions from the third century (but apparently not later). There is also the observation that only two of the seven ancient houses of Iran are actually attested in Arsacid-era sources and the other five may just have made up their genealogies. Both of which IMHO implies that, at least for a while, being Parthians was not a disadvantage in the Sasanian empire.
Re. your third question, there seems to have been a post-Islamic regional dynasty in Northern Iran that traced its origin back to the House of Karen. If I understand correctly, Christensen in L'Iran sous les Sassanides, p.42 asserts that the language (or dialect) of the Parthians is related to those dialects spoken in northern and central Iran today.
This is all just googled and I am by no means a historian, so please take it with a grain of salt.