NGC 3108是不是从一个早型星系转变成晚型星系--一个天文学上的雌雄同体？
Is NGC 3108 transforming itself from an early to late type galaxy -- an astronomical hermaphrodite?
摘要：
分层星系形成模型的一个共同特征是“逆”形态转化过程：一个以星系核团为主的星系吸积一个气体盘，显著降低系统的星系核团与星系盘的质量比。在它们的形成过程中，今天的星系可能会在哈勃图上执行许多这样的循环。NGC 3108是这样一个“雌雄同体”星系的一个很好的候选者：它是一个尘埃带早型星系，在一个大尺度盘中分布着大量的HI气体。我们给出了窄带H_α和R-波段成像，并与HI分布的结果进行了比较。发射是在两个组成部分：一个核酒吧和扩展盘组件符合HI分布。这表明，一个恒星盘目前正在形成的HI气体。H_α和HI发射区以及HII区的空间分布与棒旋结构一致，半径约为20 kpc。我们测量的消光校正SFR为0.42兆太阳/年。HII区的光度函数与其他旋涡星系相似，幂律指数为-2.1，表明恒星形成机制与其他旋涡星系相似。我们测量了目前的磁盘质量，发现它太大了，已经形成了目前的SFR在过去几个Gyr。NGC 3108的SFR可能在过去更高。根据目前的SFR，NGC 3108的盘将在未来5.5 Gyr内增长到约6.2x10^9 Msun。虽然这是相当大的，但与大的球质量相比，盘将是微不足道的：最终的恒星质量盘与球质量比将是~0.02。如果没有SFR的提升和额外的气体供应，NGC 3108将无法转变为任何类似螺旋的东西。
英文摘要：
A common feature of hierarchical galaxy formation models is the process of "inverse" morphological transformation: a bulge dominated galaxy accretes a gas disk, dramatically reducing the system's bulge-to-disk mass ratio. During their formation, present day galaxies may execute many such cycles across the Hubble diagram. A good candidate for such a "hermaphrodite" galaxy is NGC 3108: a dust-lane early-type galaxy which has a large amount of HI gas distributed in a large scale disk. We present narrow band H_alpha and R-band imaging, and compare the results with the HI distribution. The emission is in two components: a nuclear bar and an extended disk component which coincides with the HI distribution. This suggests that a stellar disk is currently being formed out of the HI gas. The spatial distributions of the H_alpha and HI emission and the HII regions are consistent with a barred spiral structure, extending some 20 kpc in radius. We measure an extinction- corrected SFR of 0.42 Msun/yr. The luminosity function of the HII regions is similar to other spiral galaxies, with a power law index of -2.1, suggesting that the star formation mechanism is similar to other spiral galaxies. We measured the current disk mass and find that it is too massive to have been formed by the current SFR over the last few Gyr. It is likely that the SFR in NGC 3108 was higher in the past. With the current SFR, the disk in NGC 3108 will grow to be ~6.2x10^9 Msun in stellar mass within the next 5.5 Gyr. While this is substantial, the disk will be insignificant compared with the large bulge mass: the final stellar mass disk-to-bulge ratio will be ~0.02. NGC 3108 will fail to transform into anything resembling a spiral without a boost in the SFR and additional supply of gas.
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