NEAR-INFRARED IMAGING OF MARKARIAN-231 - EVIDENCE FOR A DOUBLE NUCLEUS

Near-infrared (1.2-2.4 mum) images are presented for the central 10 ar csec of the Seyfert 1 galaxy Markarian 231. We find a faint, but intri nsically luminous (M(K) approximately -20.7) secondary peak in the nea r-infrared light distribution approximately 3.5 arcsec (2.7 kpc) south of the primary Seyfert 1 nucleus. Since there is no optical or infrar ed evidence for ongoing star formation at the location of this seconda ry peak, and its near-infrared luminosity and color are comparable to slightly reddened spiral bulges or elliptical nuclei, we identify this peak with the stripped nucleus of the companion galaxy involved in th e Mrk 231 merger event. Depending upon the exact ratio of the masses o f the primary and secondary nucleus in the Mrk 231 system we estimate a merger time scale of less-than-or-equal-to 10(9) yr. The morphology of the southern nucleus suggests that it may have recently survived a close passage (r<200 pc) with the Seyfert 1 nucleus on a highly ellipt ical orbit, in which case the merger time scale may be significantly s horter (approximately 10(7) yr). We re-calculate the average merger ti me scale for the seven ultraluminous infrared galaxies with double nuc lei in the Bright Galaxy Sample (the BGS) of Soifer et al. [AJ, 98, 76 6 (1989)] and derive a value of approximately 10(8) yr. Since seven of ten of the ultraluminous infrared galaxies in the BGS are now known t o be double, we estimate the ultraluminous ''phase'' may be close to t his value. Along with Arp 220 and Mrk 273, Mrk 231 is the third member of the class to posess a high brightness temperature non-thermal radi o core and a double nucleus, suggesting the time scale for the generat ion or fueling of the active nucleus can be much less than the dynamic al time scale for the merger of the progenitor nuclei.