Nature of Hα selected galaxies at z > 2. I. Main-sequence and dusty star-forming galaxies

Abstract

We present the results from our narrow-band imaging surveys of Hα emitters (HAEs) at z = 2.2 and z = 2.5 in the Subaru/XMM-Newton Deep survey Field with near-infrared camera MOIRCS on the Subaru Telescope. We have constructed a clean sample of 63 star-forming galaxies at z = 2.2 and 46 at z = 2.5. For 12 (or 92%) out of 13 HAEs at z = 2.2, their Hα emission lines have been successfully detected by the spectroscopy. While about 42% of the red, massive HAEs with M * > 1010.8 M contain active galactic nuclei (AGNs), most of the blue, less massive ones are likely to be star-forming galaxies. This suggests that the AGN may play an important role in galaxy evolution at the late stage of truncation. For the HAEs excluding possible AGNs, we estimate the gas-phase metallicities on the basis of [N II]/Hα ratios, and find that the metallicities of the Hα selected galaxies at z = 2.2 are lower than those of local star-forming galaxies at fixed stellar mass, as shown by previous studies. Moreover, we present and discuss the so-called main sequence of star-forming galaxies at z > 2 based on our unique sample of HAEs. By correlating the level of dust extinction with the location on the main sequence, we find that there are two kinds/modes of dusty star-forming galaxies: starbursting galaxies and metal-rich normal star-forming galaxies. © 2013. The American Astronomical Society. All rights reserved.