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arXiv 0807.3674astro-ph
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z~2处亚毫米级微弱射电选择星暴星系的干涉CO观测

Interferometric CO Observations of submillimeter-faint, radio-selected starburst galaxies at z~2

S. C. Chapman, R. Neri, F. Bertoldi, Ian Smail, T. R. Greve, D. Trethewey, A. W. Blain, P. Cox, R. Genzel, R. J. Ivison, A. Kovacs, A. Omont A. M. Swinbank

摘要:

高红移、尘埃遮蔽的星系--在射电中被选择为发光的,但在850微米处相对较暗--似乎代表了与超亮亚毫米(submm-)亮星系群不同的星系群。它们可能是恒星形成的星系,具有更热的尘埃温度,或者它们可能具有较低的远红外光度和来自模糊的活动星系核(AGN)的较大贡献。在这里,我们提出了三个z~2的例子,我们称之为亚毫米微弱射电星系(SFRGs)在CO(3-2)使用IRAM高原德布雷干涉仪的观察,以研究它们的气体和动力学性质。我们估计了三个SFRG中每一个的分子气体质量(分别为8.3 × 10 ^{9} M_odot、<5.6 × 10 ^{9} M_odot和15.4 × 10 ^{9} M_odot),并且在RG 163655的情况下,通过测量CO(3-2)线的宽度来估计动力学质量(8 × 10 ^{10} csc^2i M_odot)。虽然这些气体质量很大,但平均而言,它们比亚毫米选择星系(SMG)低4倍。射电推断的恒星形成速率(<SFR_radio>=970 M_odot \yr)表明恒星形成效率比SMG高得多,气体耗尽时间尺度(~ 1100万年)比形成当前恒星质量所需的时间(~160 Myr; ~10^{11} M_odot)短得多。相比之下,SFRs可能被高估了几个因素,使效率与其他超亮恒星形成星系通常测量的效率一致,并表明SFRGs更像紫外线(UV)选择的恒星形成星系,具有增强的无线电发射。在350微米处对\rga的初步探测表明,尘埃温度更高,因此气体与尘埃的质量分数与SMG相似。我们的结论是SFRGs的射电光度大于自然规模从本地ULIRGs给定的气体质量或气体分数。

英文摘要:

High-redshift, dust-obscured galaxies -- selected to be luminous in the radio but relatively faint at 850um -- appear to represent a different population from the ultra-luminous submillimeter- (submm-) bright population. They may be star-forming galaxies with hotter dust temperatures or they may have lower far-infrared luminosities and larger contributions from obscured active galactic nuclei (AGN). Here we present observations of three z~2 examples of this population, which we term submm-faint radio galaxies (SFRGs) in CO(3-2) using the IRAM Plateau de Bure Interferometer to study their gas and dynamical properties. We estimate the molecular gas mass in each of the three SFRGs (8.3x10^{9} M_odot, <5.6x10^{9} M_odot and 15.4x10^{9} M_odot, respectively) and, in the case of RG163655, a dynamical mass by measurement of the width of the CO(3-2) line (8x10^{10} csc^2i M_odot). While these gas masses are substantial, on average they are 4x lower than submm-selected galaxies (SMGs). Radio-inferred star formation rates (<SFR_radio>=970 M_odot\yr) suggest much higher star-formation efficiencies than are found for SMGs, and shorter gas depletion time scales (~11 Myr), much shorter than the time required to form their current stellar masses (~160 Myr; ~10^{11} M_odot). By contrast, SFRs may be overestimated by factors of a few, bringing the efficiencies in line with those typically measured for other ultraluminous star-forming galaxies and suggesting SFRGs are more like ultraviolet- (UV-)selected star-forming galaxies with enhanced radio emission. A tentative detection of \rga at 350um suggests hotter dust temperatures -- and thus similar gas-to-dust mass fractions -- as the SMGs. We conclude that SFRGs' radio luminosities are larger than would naturally scale from local ULIRGs given their gas masses or gas fractions.

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