IRAS F21013-0739: a possible evolutionary successor of an ultraluminous infrared galaxy

Abstract

We present a stellar population synthesis study of a type II luminous infrared galaxy, IRASF21013-0739. Optical images show clear characteristics of a merger remnant. The H-band absolute magnitude is M_H = −25.1, which is ~2 times as luminous as L* galaxies. Stellar populations are obtained through the stellar synthesis code STARLIGHT. We find that it experienced a recent starburst (SB) phase ~ 100 Myr ago. By reconstructing the ultraviolet-to-optical spectrum, and adopting Calzetti et al. and Leitherer et al.'s extinction curves, we estimate the past infrared (IR) luminosities of the host galaxy and find it may have experienced an ultraluminous infrared galaxy phase which lasted for about 100 Myr. Its i-band absolute magnitude is M_i = −22.463, and its spectral type shows type 2 active galactic nucleus (AGN) characteristics. The mass of the supermassive black-hole is estimated to be M_BH = 1.6 × 10⁷ M_⊙ (lower-limit). The Eddington ratio L_bol/L_Edd is 0.15, which is typical of Palomar-Green (PG) quasars. Both the nuclear SB and AGN contribute to the present IR luminosity budget, and the SB contributes ~67%. On the diagram of IR color versus IR/optical excess, it is located between IR quasars and PG quasars. These results indicate that IRASF21013-0739 has probably evolved from a ULIRG, and it can possibly evolve into an AGN.