If they were slower than light we would have "seen" them as light travels to us faster than they would. Slower than light would have meant many thousands of years travelling space; the pure maintenance of a spacecraft for that duration would be phenominal. If their biology is anything like a human (genetics being passed from one generation to another), then they would have had to have about 5,000 on-board to avoid breeding issues, death, disease, accidents, etc. - these are all issues humans face, for example "let's colonise mars" - okay, so we need to send fresh people all the time as they can't procreate too much (odd child is okay) as our genetics need at least 500 childless individuals to reduce disabilities, and 5,000 to be safe for repopulation.You assume they traveled faster than light. Maybe they lived a long time or maybe they were in suspended animation or maybe their voyage was multi-generational or maybe not. Who knows.