CO J=2-1 line emission in cluster galaxies at z~1: fueling star formation in dense environments
Jeff Wagg, Alexandra Pope, Stacey Alberts, Lee Armus, Mark Brodwin, Robert S. Bussmann, Vandana Desai, Arjun Dey, Buell Jannuzi, Emeric Le Floc'h, Jason Melbourne, Daniel Stern
备注：
22 pages, 4 figures; accepted for publication in ApJ
利用IRAM Plateau de Bure干涉仪观测了红外发光星系团中z~1处的CO J=2-1谱线发射。我们的两个主要目标是位于两个大质量（> 10 ^14 Msun）星系团中心2 Mpc范围内的光学暗淡、尘埃遮蔽的星系（DOG）。在任一DOG中均未检测到CO线发射。我们计算了CO J=2-1谱线光度的3-sigma上限，L '_CO < 6.08x10^9和< 6.63x10^9 K km/s pc ^2。假设在本机宇宙中的超亮红外星系中导出了CO到H_2的转换因子，这就转化为M_H_2 < 4.86x10^9 Msun和M_H_2 < 5.30x10^9 Msun的冷分子气体质量的极限。这两个DOG都表现出中红外连续辐射，遵循幂律，这表明活动星系核有助于尘埃加热。因此，对这些DOG中恒星形成效率的估计是不确定的。第三个星系团成员的红外光度L_IR < 7.4x10^11 Lsun，是偶然发现的，它位于离星系团中心大约两维里半径的DOG的视场中，在CO J=2-1的谱线发射中。该天体的光谱表明，它可能是一个被遮蔽的活动星系核，测得的CO谱线光度为L '_CO =（1.94 +/-0.35）x10^10 K km/s pc ^2，由此估算出冷分子气体质量M_H_2 =（1.55+/-0.28 x10 ^10 Msun.远离星系团中心的z~1星系中存在大量分子气体，这表明燃料的存在可以驱动高红移星系团外围的恒星形成和活动星系核活动的增加。
英文摘要：
We present observations of CO J=2-1 line emission in infrared-luminous cluster galaxies at z~1 using the IRAM Plateau de Bure Interferometer. Our two primary targets are optically faint, dust-obscured galaxies (DOGs) found to lie within 2 Mpc of the centers of two massive (>10^14 Msun) galaxy clusters. CO line emission is not detected in either DOG. We calculate 3-sigma upper limits to the CO J=2-1 line luminosities, L'_CO < 6.08x10^9 and < 6.63x10^9 K km/s pc^2. Assuming a CO-to-H_2 conversion factor derived for ultraluminous infrared galaxies in the local Universe, this translates to limits on the cold molecular gas mass of M_H_2 < 4.86x10^9 Msun and M_H_2 < 5.30x10^9 Msun. Both DOGs exhibit mid-infrared continuum emission that follows a power-law, suggesting that an AGN contributes to the dust heating. As such, estimates of the star formation efficiencies in these DOGs are uncertain. A third cluster member with an infrared luminosity, L_IR < 7.4x10^11 Lsun, is serendipitously detected in CO J=2-1 line emission in the field of one of the DOGs located roughly two virial radii away from the cluster center. The optical spectrum of this object suggests that it is likely an obscured AGN, and the measured CO line luminosity is L'_CO = (1.94 +/- 0.35)x10^10 K km/s pc^2, which leads to an estimated cold molecular gas mass M_H_2 = (1.55+/-0.28)x10^10 Msun. A significant reservoir of molecular gas in a z~1 galaxy located away from the cluster center demonstrates that the fuel can exist to drive an increase in star-formation and AGN activity at the outskirts of high-redshift clusters.
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