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arXiv 0806.0888astro-ph
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z~2处的密切关系:亚毫米星系和BzK-selected星系

A close relationship at z~2: submillimetre galaxies and BzK-selected galaxies

Toshinobu Takagi, Yoshiaki Ono, Kazuhiro Shimasaku, Hitoshi Hanami

摘要:

本文研究了红移z~2的两个大质量恒星形成星系群,即亚毫米星系(SMG)和BzK选择星系(BzKs)之间的关系。在斯巴鲁/XMM-牛顿深场发现的60个SMG中,我们收集了24个SMG的28个无线电对应物的光学-近红外测光,基于35个SMG的无线电地图的精确天空位置(Ivison等人,2007)。我们发现它们的K波段幅度和BzK [=(z-K)-(B-z)]颜色之间存在相关性:几乎所有K-微弱(K_AB> 21.3)无线电探测到的SMG都有BzK>-0.2,因此有BzKs。这一结果为通过搜索SMG周围的BzK来执行SMG的直接光学识别提供了强有力的支持。我们计算了无线电未检测到的SMG周围的每个BzK关联的形式显著性(P '值),并找到6个新的鲁棒标识,包括一个双重标识。通过这项分析,我们获得了目前对BzK选择的SMG表面密度的最佳估计,这表明只有1%的BzK是SMG。如果BzKs是z~2处的正常盘状星系,如它们的恒星形成率(SFR)和恒星质量之间的相关性以及动力学性质所示,SMG很可能正在合并BzKs。在这种情况下,由于合并,SFR的典型增强仅为~3的因子,这比局部ULIRG低一个数量级。这可以指示大多数合并BzK可以被观察为SMG。考虑到在z~2处可能有很高比例的合并(至少比z> 1处的比例高10%),在祖先种群(即BzKs)中发现如此低比例的SMG是相当令人困惑的。

英文摘要:

We investigate the relationship between two massive star-forming galaxy populations at redshift z~ 2; i.e. submillimetre galaxies (SMGs) and BzK-selected galaxies (BzKs). Out of 60 SMGs found in the Subaru/XMM-Newton deep field, we collect optical--NIR photometry of 28 radio counterparts for 24 SMGs, based on refined sky positions with a radio map for 35 SMGs (Ivison et al. 2007). We find a correlation between their K-band magnitudes and BzK [= (z-K)-(B-z)] colours: almost all of the K-faint (K_AB > 21.3) radio-detected SMGs have BzK>-0.2, and therefore BzKs. This result gives strong support to perform direct optical identification of SMGs by searching for BzKs around SMGs. We calculate the formal significance (P' value) for each of the BzK associations around radio-undetected SMGs, and find 6 new robust identifications, including one double identification. From this analysis, we obtain the current best estimate on the surface density of BzK-selected SMGs, which indicates that only ~1 per cent of BzKs are SMGs. If BzKs are normal disk-like galaxies at z~ 2 as indicated by the correlation between their star formation rate (SFR) and stellar mass and also by dynamical properties, SMGs are likely to be merging BzKs. In this case, a typical enhancement of SFR due to merging is only a factor of ~ 3, which is an order of magnitude lower than that of local ULIRGs. This may indicate that most of the merging BzKs could be observed as SMGs. Considering a possible high fraction of mergers at z~ 2 (at least it would be higher than the fraction at z> 1 of ~ 10 per cent), it is rather puzzling to find such a low fraction of SMGs in the progenitor population, i.e. BzKs.

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