AA- and I´m helpless

[ToomuchCoffee]

First post,great forum! I played MS limit and start NL now and I´m lost OOP! No reads on MP2.

Poker Stars - No Limit Hold'em Cash Game - $0.25/$0.50 Blinds - 9 Players - (LegoPoker HH Converter)

SB: $103.45
BB: $63.40
Hero (UTG): $54.30
UTG+1: $49.25
MP1: $50.00
MP2: $72.90
MP3: $129.45
CO: $31.15
BTN: $43.45

Preflop: Hero is dealt A[img]/images/graemlins/spade.gif[/img] A[img]/images/graemlins/heart.gif[/img] (9 Players)
<font color="red">Hero raises to $2.00</font>, 2 folds, MP2 calls $2.00, 5 folds

Flop: ($4.75) 4[img]/images/graemlins/spade.gif[/img] J[img]/images/graemlins/heart.gif[/img] 8[img]/images/graemlins/club.gif[/img] (2 Players)
<font color="red">Hero bets $4.00</font>, MP2 calls $4.00

Turn: ($12.75) 9[img]/images/graemlins/club.gif[/img] (2 Players)
<font color="red">Hero bets $8.00</font>, MP2 calls $8.00

River: ($28.75) 3[img]/images/graemlins/diamond.gif[/img] (2 Players)
Hero checks, MP2 checks

Pot Size: $28.75

When I checked the river, I already thought: What now if he bets more than half pot?