That appears to be a joist sitting on top of a double plate. You then have some OSB, and another 2x4 being the bottom of your next set of walls.
Behind that joist is probably nothing. However it could be a stack- but I'd guess even if there are two there's still going to be a gap of about 1/2" to 3/4" in the center of it, straight through.
You can drill straight down and fish up your wires that way.
Now when you do this, even with a small hole, you're going to create a cavity for fire to travel up. Please make sure you stuff insulation around it to prevent that activity from occurring. And you're going to make it hard to get at when you're done, so since it's a stairwell you might want to make sure (where the railing goes) you have a piece of wood to cover the wires from getting spiked.
Just don't go cutting it, OK?