It's the word "to" that establishes this as a cost to pay. It doesn't say "do A and B", it say "do A to do B". Therefore, if you don't do A then B will not occur.
It's section 2.4 in the FAQ:
(2.4) Paying Costs
Costs are any resources paid in order to play a card, as well as anything before the “to” part of a card ability. For example, Slavering Gug (Core Set F124) reads “Action: Pay 4 to choose and wound a character.” Draining a domain with at least 4 resources is the cost for that ability.
You may only pay costs with cards you control. You may never exhaust a card, drain a domain, drive insane, sacrifice, or otherwise use or alter the game state of any card you do not control, as the means of a paying the cost of an effect.
Note that sacrificing a card to pay for an effect is also considered paying a cost. For example, if a player wishes to activate Predatory Byakhee (Core Set F89), he must sacrifice the Byakhee. If the effect were canceled, the Byakhee would still be sacrificed.
If an effect is canceled, any costs that were paid to trigger that effect are still lost. For example, if Player A drains a domain to play Shotgun Blast, but Player B plays Power Drain (which cancels the Shotgun Blast). Player A’s domain remains drained, even though the Shotgun Blast effects were canceled.
When any player has a sacrifice effect, that cost must be paid, if able. If said sacrifice does not happen, then the cost has not been paid and the effect does not resolve.