STAR FORMATION RATES AND METALLICITIES OF K-SELECTED STAR-FORMING GALAXIES AT z similar to 2

arXiv:0809.5100v1 [astro-ph] 30 Sep 2008 Star Formation Rates and Metallicities of K-selected Star Forming Galaxies at z ∼ 2 1 Masao Hayashi 2 , Kentaro Motohara 3 , Kazuhiro Shimasaku 4 , Masato Onodera 5,8 , Yuka Katsuno Uchimoto 3 , Nobunari Kashikawa 6 , Makiko Yoshida 2 , Sadanori Okamura 4 , Chun Ly 7 , and Matthew A. Malkan 7 hayashi@astron.s.u-tokyo.ac.jp ABSTRACT We present spectroscopy of 15 star-forming BzK galaxies (sBzKs) with K AB . 23 in the Subaru Deep Field, for which Hα and some other emission lines are detected in 0.9 to 2.3 µm spectra with a resolution of R=500. Using Hα luminosities, we obtain star formation rates (SFRs), and then specific SFRs (SSFRs) dividing SFRs by stellar masses, which are derived from SED fitting to BV Ri ′ z ′ K photometry. It is found that sBzKs with higher stellar masses have larger SFRs. A negative correlation is seen between stellar mass and SSFR, which is consistent with the previous results for z ∼ 2 galaxies. This implies that a larger growth of stellar mass occurs in less massive galaxies. In addition, gas-phase oxygen abundances, 12+log(O/H), are derived from the ratio of [N II ](λ6584) to Hα using the N2 index method. We have found a correlation between stellar mass and oxygen abundance in the sense that more massive sBzKs tend to be more metal rich, which is qualitatively consistent with the relation for UV-selected z ∼ 2 galaxies. However, the metallicity of the sBzKs is ∼ 0.2 dex higher than that of UV-selected galaxies with similar stellar masses, which is significant considering the small uncertainties. The sBzKs in our sample have redder R − K colors than the UV-selected galaxies. This galaxy color- dependence in the oxygen abundance may be caused by older or dustier galaxies having higher metallicities at z ∼ 2. Subject headings: formation galaxies: abundances — galaxies: high-redshift — galaxies: starburst — galaxies: Introduction Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Use of the UKIRT 3.8-m telescope for the observa- tions is supported by NAOJ. 2 Department of Astronomy, Graduate School of Sci- ence, University of Tokyo, Tokyo 113-0033, Japan 3 Institute of Astronomy, Graduate School of Science, University of Tokyo, Mitaka, Tokyo 181-0015, Japan 4 Department of Astronomy and Research Center for the Early Universe, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan 5 Department of Astronomy, Yonsei University, Sinchon- dong 134, Seodaemun-gu, Seoul, Korea 6 Optical and Infrared Astronomy Division, National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan 7 Department of Physics and Astronomy, University of California at Los Angeles, P. O. Box 951547, Los Angeles, CA 90095-1547, USA 8 Service d’Astrophysique, CEA Saclay, Orme des Recent studies suggest that the era of z ∼ 2 is a turning point in galaxy formation and evolution. The cosmic star formation rate (SFR) begins to drop at z ∼ 1 − 2 from a flat plateau at higher redshifts (Dickinson et al. 2003; Fontana et al. 2003; Ly et al. 2007). Significant evolution of the Hubble sequence occurred at z ∼ 1 − 2 (Kajisawa & Yamada 2001). It is also found that the number density of QSOs has a peak at z ∼ 2 (Richards et al. 2006). These facts suggest that dramatic changes in the galaxy population oc- curred at z ∼ 2, which are important to study in Merisiers, 91191 Gif-sur-Yvette Cedex, France