Technically, "authority" is defined as a person or group's legitimate power over others. It is the power to influence the opinions of others, esp. because of one's recognized knowledge or scholarship, authoritative opinion, or acknowledged expertise.  Regarding research, "authority" relates to whether or not the author(s) has established their expertise. This expertise may come in many forms, from special skills (training, learned abilities, longevity at a position, etc.); to knowledge derived from formal education and current/continual research.
You can establish the authority of your sources by researching the author(s).
An expert is: