Nature of the Strongly Lensed Submillimeter Galaxy SMM J14011+0252

Abstract

We have carried out near-infrared JHK spectroscopy of the gravitationally lensed submillimeter galaxy SMM J14011+0252 at z = 2.565, using the OH-airglow suppressor and the Cooled Infrared Camera and Spectrograph for OHS on the Subaru Telescope. This object consists of two optical components, J1 and J2, which are lensed by the cluster Abell 1835. J1 suffers additional strong lensing by a foreground galaxy at z = 0.25 in the cluster. The rest-frame optical Hα, Hβ, and [O II] λ3727 lines are detected in both J1 and J2, and [N II] λλ6548, 6583 lines are also detected in J1. A diagnosis of emission-line ratios shows that the excitation source of J1 is stellar in origin, consistent with previous X-ray observations. The continua of J1 and J2 show breaks at rest-frame 4000 A, indicating a relatively young age. Combined with optical photometry, we have carried out model-spectrum fitting of J2 and find that it is a very young (~50 Myr) galaxy of rather small mass (~108 M⊙) that suffers some amount of dust extinction. A new gravitational lensing model is constructed to assess both the magnification factor and contamination from the lensing galaxy of the component J1, using a Hubble Space Telescope F702W image. We have found that J1 suffers strong lensing with magnification of ~30, and its stellar mass is estimated to be 109 M⊙. These results suggest that SMM J14011+0252 is a major merger system at high redshift that undergoes intense star formation but is not a formation site of a giant elliptical galaxy. Still having plenty of gas, it will transform most of the gas into stars and will evolve into a galaxy of 1010 M⊙. Therefore, this system is possibly an ancestor of a present-day, less massive galaxy such as a midsized elliptical galaxy or a spiral galaxy.