用H-α发射线探测z=2.2的宇宙恒星形成活动
Cosmic Star Formation Activity at z=2.2 Probed by H-alpha Emission Line Galaxies
摘要：
我们提出了一个试点窄带调查的H-α发射在z=2.2的大天文台起源深调查北（GOODS-N）领域与MOIRCS仪器的斯巴鲁望远镜。这次测量达到了23.6（NB 209）的3西格玛极限星等，这相当于在56弧^[2]的连续区域（不包括较浅的区域）上的3西格玛极限线通量为2.5 x 10^-17 erg s^-1 cm^-2。从这次调查中，我们已经确定了11个H-α发射体和一个活动星系核在z=2.2的基础上的窄带过剩和光度红移。我们获得了其中七个新的对象，包括一个活动星系核的光谱，并检测到3西格玛以上的发射线从所有的。我们估计了单个星系的恒星形成率（SFR）和恒星质量（M_star）。SFR和M_star的平均值分别为27.8 M_solar yr^-1和4.0 × 10^10 M_solar。它们的特定恒星形成率与它们的恒星质量成反比。拟合到Schechter函数得到H-α光度函数，log L = 42.82，log phi = -2.78，alpha =-1.37。根据H-α光度与恒星形成率之间的肯尼库特关系，测量体积中的平均恒星形成率密度估计为0.31 M_solar yr^-1 Mpc ^-3。我们比较我们的H-α发射器在z=2.2在GOODS-N与窄带线发射器在其他领域和集群，看看他们的时间演化和环境依赖性。我们发现，在星团环境中，恒星形成活动从z=2.5迅速减少到z=0.8，而在场环境中，它只是适度的变化。这一结果表明，不同环境下星系形成的时间尺度不同，高密度区的恒星形成活动最终会超过低密度区，这是高红移下“星系形成偏差”的结果。
英文摘要：
We present a pilot narrow-band survey of H-alpha emitters at z=2.2 in the Great Observatories Origins Deep Survey North (GOODS-N) field with MOIRCS instrument on the Subaru telescope. The survey reached a 3 sigma limiting magnitude of 23.6 (NB209) which corresponds to a 3 sigma limiting line flux of 2.5 x 10^-17 erg s^-1 cm^-2 over a 56 arcmnin^2 contiguous area (excluding a shallower area). From this survey, we have identified 11 H-alpha emitters and one AGN at z=2.2 on the basis of narrow-band excesses and photometric redshifts. We obtained spectra for seven new objects among them, including one AGN, and an emission line above 3 sigma is detected from all of them. We have estimated star formation rates (SFR) and stellar masses (M_star) for individual galaxies. The average SFR and M_star is 27.8M_solar yr^-1 and 4.0 x 10^10M_solar, respectivly. Their specific star formation rates are inversely correlated with their stellar masses. Fitting to a Schechter function yields the H-alpha luminosity function with log L = 42.82, log phi = -2.78 and alpha = -1.37. The average star formation rate density in the survey volume is estimated to be 0.31M_solar yr^-1Mpc^-3 according to the Kennicutt relation between H-alpha luminosity and star formation rate. We compare our H-alpha emitters at z=2.2 in GOODS-N with narrow-band line emitters in other field and clusters to see their time evolution and environmental dependence. We find that the star formation activity is reduced rapidly from z=2.5 to z=0.8 in the cluster environment, while it is only moderately changed in the field environment. This result suggests that the timescale of galaxy formation is different among different environments, and the star forming activities in high density regions eventually overtake those in lower density regions as a consequence of "galaxy formation bias" at high redshifts.
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