Tehran always was part of the Ethnic Azerbaijan
International Society for Azerbaijani Studies
Dear Khodadad if you are addressing me, i never said Turkish is spreading in the past 200 years. On the contrary all demographic data shows that Turkish spread in Iran has stopped especially in last 200 years. Last 200 years in fact is period of Farsi spread in Iran.
About Tehran, from the first day when it became capital of Turkish Azerbaijanian state of Qajar, majority of its population was Turkic origin and this city had a Turkish identity . At the beginning armies and bureaucrats from Qajar, Afshar, Shahseven, Kkhalaj, .... tribes, then others.
For example this is a Revsheni bek, an Ottoman diplomat observation about Tehran in the first years of 20 century: شهر طهران بيشتر بيك شهر ترك شبيه است، فارس در آنجا كم است. (Tehran resembles more to a Turkish city. There is little Fars people living there).
After 1945 Tehran experienced a flood of political immigrants and refugees from Pisheveri government. And today economical and social immigrants from all other Turkish populated regions of Iran.
So Tehran always was part of the Ethnic Azerbaijan, percentage of its majority Turkish population almost always was same and immigration of Turks to this old capital city of Turkic Seljuk empire is merely a matter of internal immigration in Azerbaijan.