M33的化学演化研究

Chemical Evolution of M33

作为本星系群中的旋涡星系之一,M33(NGC 598)是验证星系化学演化理论不可多得的目标,因为它具有适当的距离和角直径,使得现今对其气体、恒星形成率(SFR)及丰度进行精细观测成为可能。该文首先对:M33的盘建立了化学演化模型,同时考虑了内落和外流的影响,详细讨论了吸积和坍缩两种盘的内落模型。由于M33的质量较小、势阱较浅,容易受超新星爆发等过程的影响而引发外流,因此除了讨论内落时标的影响外,文中还讨论了与SFR成正比的外流的作用。得到了3条主要结论:(1)无论是坍缩模型还是吸积模型,在内落时标较长的时候,都可以给出较满意的结果,说明M33是通过慢速吸积过程形成的;(2)通过对M33的金属丰度研究表明,外流必不可少;(3)在采用较长的内落时标和引入外流后,与银河系相似的修正K-S定律可以较好地重现各个观测约束。

M33 （NGC 598）, as one of the spirals in Local Group, is an excellent target for testing galactic chemical evolution theory. Owing to its proper distance and angular size, detailed observations of present gas distribution, star formation rate （SFR）, and abundance properties are available. In this paper, the authors have established a chemical evolution model for M33 disk. They adopt two infall formulas, representing the collapse model and the accretion model respectively, to describe the growth of the disk with the advantage of ignoring complicated processes required in a Lambda CDM cosmology, and the influence of timescale τ has been discussed. Apart from adopting a simple assumption of the constant infall timescale τ they also examine the outflow which is assumed to be proportional to the star formation rate. Since M33 is less massive compared with the Milky Way, the lower potential well would very probably result in a significant outflow during the evolution of disk by supernova explosions or other perturbations. Their motivation for the present model is to discuss the different roles of infall and ourflow in building the M33 galaxy disk. They would like to know in what way the model could best reproduce the observed global properties. The main conclusions of this work are as follows： 1） Short infall timescale will lead to low gas fraction, low SFR and high abundance in ISM. Because most of gas will fall onto the disk at early time, and fuel the star formation. When stars are dying, they eject the newly produced elements and enrich the ISM quickly. No matter the collapse model or the accretion model, the results are all consistent with the observations except for the abundance gradient if a longer infall time-scale （15 Gyr） is adopted. These means M33 should form through the slow accreting process. 2） Outflow has significant influence on ISM enrichment, especially for the inner disk, but no obvious change in other model predictions. Due to the high SFR in the inner disk, more metals are carried out by outflowing gas and hence flatten the abundance gradient. The model results are consistent with observations when the outflow rate is comparable with SFR （b = 0.8）. Outflow can not be neglected owing to the low abundance in M33 disk. 3） The revised K-S SFR law which is similar to MW＇s can reproduce most of the observations by adopting long infall time-scale and considering outflow in our model. The ＂observed＂ SFR law doesn＇t work.