贫HI星系中的分子氢缺乏及其对恒星形成的意义
Molecular hydrogen deficiency in HI-poor galaxies and its implications for star formation
摘要：
我们使用的样品47均匀和高灵敏度的CO图像从Nobeyama和BIMA调查证明，与普遍的看法相反，一个显着数量（~40%）的HI缺乏附近的螺旋星系也耗尽分子氢。虽然HI缺乏本身不是分子气体耗尽的充分条件，我们发现，H2还原与星系光盘内HI的去除有关。那些缺乏HI的星系具有正常的H2含量，主要是从它们的光盘外部失去HI，那里的H2含量在所有星系中都很低。这一发现与理论模型相一致，在理论模型中，星系中的分子比例主要由其气柱密度决定。我们的结果是支持的间接证据表明，分子缺乏星系形成恒星的速度较低，或有较暗的远红外线通量比气体丰富的星系，如预期的，如果恒星形成率是由分子氢含量。我们的结果是一致的情况下，当原子气柱密度降低到光盘内低于形成分子氢和恒星所需的临界值，螺旋成为静止和被动的演变系统。我们推测，这一过程将作用于由气体耗尽速率设定的时间尺度上，并且可能是在颜色-星等图中观察到的蓝色和红色序列之间过渡的第一步。
英文摘要：
We use a sample of 47 homogeneous and high sensitivity CO images taken from the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a significant number (~40%) of HI-deficient nearby spiral galaxies are also depleted in molecular hydrogen. While HI-deficiency by itself is not a sufficient condition for molecular gas depletion, we find that H2 reduction is associated with the removal of HI inside the galaxy optical disk. Those HI-deficient galaxies with normal H2 content have lost HI mainly from outside their optical disks, where the H2 content is low in all galaxies. This finding is consistent with theoretical models in which the molecular fraction in a galaxy is determined primarily by its gas column density. Our result is supported by indirect evidence that molecular deficient galaxies form stars at a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as expected if the star formation rate is determined by the molecular hydrogen content. Our result is consistent with a scenario in which, when the atomic gas column density is lowered inside the optical disk below the critical value required to form molecular hydrogen and stars, spirals become quiescent and passive evolving systems. We speculate that this process would act on the time-scale set by the gas depletion rate and might be a first step for the transition between the blue and red sequence observed in the color-magnitude diagram.
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