XM Satellite Radio Holdings and Sirius Satellite Radio have agreed to merge, the two companies said today.

The deal would consolidate the only two companies in the emerging business of subscription-only satellite radio, and is sure to face scrutiny from federal regulators. Investors and analysts have been speculating about a deal for months.

The two companies said in a statement that Mel Karmazin, the chief executive of Sirius, would become chief executive of the new company while Gary Parsons, the chairman of XM, would remain in that role.

Under terms of the deal, stockholders of XM will get 4.6 shares of Sirius for each share of XM, valuing XM at $4.57 billion.