Roommate refusing to pay her half of gas bill?

I had been living in a duplex from August til the end of February. My landlord finally found someone to move in to help split the costs of the bills in mid December. Before she moved in, I had the heat set to 62 and was doing everything I could to keep the bills down. When my roommate moved in, she wanted the heat set to 70 or 72. I changed it, but when we got the gas bill for usage in January it had increased 2.5 times from what I was used to. When I approached her to get a check for half of gas and electricity, she initially refused to pay and wanted me to take her to court to get the money. She finally did give me a check a few days before it was due. I moved out at the end of February because she was verbally abusive and things were just getting uncomfortable. She switched the gas bill into her name, but I got the final bill for February. I sent her the bill by letter and asked for half of the amount. I even enclosed a SASE so she could mail it back to me easier. I hadn't heard anything back from her. Tuesday I sent a copy of the letter, plus another copy of the bill and another envelope to her by certified mail. Still haven't heard anything. So I believe she is going to ignore me and refuse to pay. My question is this: is there anything I can legally do about this? My lease said that tenants must split the utilities. My landlord however says he can't legally get involved in this situation. If I don't pay, what will happen? I don't know if it's possible, but I will call the gas company and see what would happen, and if they will put half of the amount onto her bill or shut off services until the bill is paid. Any help?