It is known to historians and the linguists that the Persians (old Persian "Parsa"), belong to the Iranic branch of the Indo-Iranic subgroup of the Indo-European language family, and were the culturally dominating tribe in the Iranian plateau. The Indo-Iranics called themselves "Aryans" from the old Iranic "Airyau", but because of a misconception due to some scholars (starting with Friedrich Schlegel) and the later political implications associated with the word Aryan, this word is no longer in scholarly use and the majority of linguists and historians prefer the term Indo-Iranic. Let us start by reviewing some facts about the relationships of the Indo-Iranics to other Indo-European tribes.
It is widely believed in error that the Indo-Iranics were culturally very similar and closely related to the Celts. A reason, for this misconception is the similarity between the very names of "Iran" and "Ireland". Linguistically, the resemblance between the words Iran and Ireland is coincidental, as Iran comes from the Iranic "Airyanam Vaeja" meaning "of the Aryan land", "Ireland" stems from the Celtic "Iwer-ion" meaning "fertile land".
All cultural connections that exist between the Celts and the Indo-Iranics come from the time span, roughly (300-200 B.C.E to maybe 200-300 A.C.E), when the Celts were the next-door neighbors of the Iranic Scythians, who at that time dwelled in eastern or central Europe and Anatolia. The Scythians were the various Iranic tribes living to the north of Caspian Sea, in Ukraine, and in the steppes of Central Asia. Their Persian relatives knew them as the "Saka" or "Sak". This is an oversimplification, as there were indeed various Iranic tribes that lived in the vast area mentioned before and it is quite likely that these tribes were not more related to each other than the Persians were related to Sakas. It is evident that the Celts got a lot of cultural influence from the Scythians, but the similarities between the two cultures are historically recent. We mention finally that the people closest related to the Celts, linguistically and culturally, were the Indo-European Italics.
The fact is that: There were no Indo-European people closer to the Indo-Iranics than the northern European Baltic people i.e. the Lithuanians, the Latvians and the Prussians. They share almost all of the cultural traits of ancient Iranics (Fires, Equinoxes etc.). The second closest to the Indo-Iranics (linguistically and culturally) were the Slavic Indo-Europeans. There are some connections to the Germanic Indo-Europeans also. Among the Iranic tribes, some extinct and some still around, the most well known are, the Medes, Scythians (e.g. Sakas, Parthians), Persians, Sarmatians (e.g. Alans, Ossetians, the Roxolanis), Bactrians, Sogdians, Kushans, Tajiks, Pashtons, Balochis, and Kaferistanis (Nuristanis nowadays). Some of these tribes might have been a subgroup of the other ones. All of these people shared many common linguistic, cultural, and religious traits such as a passion for horses and natural elements such as the sun, the fire, and the water. While many of them were warlike, they were mostly nomadic and isolated groups, and the urbanization through agriculture came later, and in some cases many of these people still keep the nomadic life styles.
Curiously, many use the word Persian as a synonym for Iranic or Iranian. Strictly speaking the Persians (from old Persian "Parsa") were just an Iranic tribe that settled in the centre and South of what is now Iran. According to some historians, the Persian tribe in turn contained a few smaller tribes, like the Pasargadae, the Maraphii and the Maspii. This is again by far not the complete picture of the Persian sub-tribes. In the south the Persians were not alone, as they had or eventually got some other Iranic neighbors e.g. Scythians and perhaps even some Sogdians and Pashtons.
Persians managed to create the very first Indo-European global empire which was the vastest at that time. In a separate post, we shall discuss more about the Persians in particular, and their contribution to the Iranic culture.