Do the characters adjacent to the target need to be hit by the attack roll in order to take the splash damage or do they automatically get delt damage if the attack hits and damages the target?

You still need to check to see if they are hit.

2013 Core Rulebook: Area of Effect

Some powers and abilities use the term “area of effect.” An area of effect allows a power or game effect to target more than one character. Characters within the area of an effect are affected even though they may not be within the character’s range or line of fire. When it includes an attack, only one attack roll is made and the Attack Total is compared to each target character as well as any other characters specified by the area of effect. When a character is attacked exclusively as a result of being in an area of effect, it is not considered a target of the attack and it may be friendly to the attacking character. The game effect including an area of effect will define how damage is dealt to hit characters and may specify targeted characters’ damage differently from other hit characters. When a power or ability uses a ranged combat attack with an area of effect, if the game effect describes target characters, then the attacking character may multi-target normally, possibly creating multiple areas of effect.