Two impact investing leaders have combined to create a $13.9 billion investment management firm that will also have expanded research and client services capabilities, Pax World Management LLC announced Monday.

Pax World Management, investment advisor to Pax World Funds with $4.5 billion in AUM, has been acquired by Impax Asset Management Group plc, a United Kingdom-based firm with $9.4 million in AUM. Both firms specialize in sustainable and impact investing.

The two firms partnered in the past to design and manage the Pax Global Environmental Markets Fund, which was launched in 2008 and today has more than $500 million in assets. The combination of the two firms will allow the group to offer enhanced resources and services to Pax’s and Impax’s existing and prospective shareholders and clients, Pax World said.

Under the terms of the transaction, Pax will be renamed Impax Investment Management (US) LLC and will continue to manage Pax World Funds as the U.S.-based mutual fund division of Impax’s global business.

Joe Keefe, Pax’s president and chief executive officer, will continue to lead the renamed company, reporting to Ian Simm, Impax’s chief executive officer. Keefe also will be named to Impax Asset Management Ltd.’s board of directors.