IAM is the broader identity and access discipline; IGA is the governance and lifecycle component within IAM.
IAM includes directories, authentication, SSO, federation, authorization, privileged access and governance. IGA focuses on who should receive access, how access is approved and provisioned, whether it remains appropriate and when it must be removed. It adds identity lifecycle automation, access requests, certifications, role and entitlement management, separation-of-duties controls, risk analysis and audit evidence.