大银河系半径恒星形成的性质
On the Nature of Star Formation at Large Galactic Radii
摘要：
我们比较了21个星系中恒星形成区的远紫外（FUV）、近紫外（NUV）和Halpha测量结果，以确定其盘在主光学半径（R25）以外的半径处的性质。在我们的扩展和非扩展UV盘的代表性样品中，我们发现一半的扩展UV盘也表现出扩展的Halpha发射。我们发现，扩展的UV光盘分为两类，那些在Halpha光盘接近光学边缘（R25）的尖锐截断，和那些在Halpha以及在紫外线的扩展发射。虽然大多数在R25附近具有强Halpha截断的星系在紫外辐射方面表现出显著的相应衰减（因子10- 100），但过渡往往比Halpha平滑得多，并且显著的紫外辐射通常远远超出这个半径，证实了Thilker等人（2007）和其他人的早期结果。在对尘埃衰减进行校正后，R25以外地区的FUV总排放量的中位数为1.7%，但在最极端的情况下可能高达35%。相应的Halpha发射分数平均约为一半。这种差异反映了在外盘的HII区域中，Halpha与紫外线辐射的比率略低，以及显示HII区域的星团比例较低。扩展盘中的大多数HII区域的通量与少数电离O型星一致，并且这种小星团中上初始质量函数的不良采样可能可以解释发射特性的差异，与Zaritsky & Christlein（2007）的早期结论一致，而不需要调用恒星IMF本身的显着变化。一致的Ha/FUV比和最亮的HII区域总的Ha流量在我们整个星系样本的内部和扩展光盘没有证据表明在低气体密度外盘的集群光度函数或IMF的变化。
英文摘要：
We have compared far-ultraviolet (FUV), near-ultraviolet (NUV), and Halpha measurements for star forming regions in 21 galaxies, in order to characterise the properties of their discs at radii beyond the main optical radius (R25). In our representative sample of extended and non-extended UV discs we find that half of the extended UV discs also exhibit extended Halpha emission. We find that extended UV discs fall into two categories, those with a sharp truncation in the Halpha disc close to the optical edge (R25), and those with extended emission in Halpha as well as in the ultraviolet. Although most galaxies with strong Halpha truncations near R25 show a significant corresponding falloff in UV emission (factor 10--100), the transition tends to be much smoother than in Halpha, and significant UV emission often extends well beyond this radius, confirming earlier results by Thilker et al. (2007) and others. After correcting for dust attenuation the median fraction of total FUV emission from regions outside of R25 is 1.7%, but it can be as high as 35% in the most extreme cases. The corresponding fractions of Halpha emission are approximately half as large on average. This difference reflects both a slightly lower ratio of Halpha to UV emission in the HII regions in the outer discs, as well as a lower fraction of star clusters showing HII regions. Most HII regions in the extended disc have fluxes consistent with small numbers of ionising O-type stars, and this poor sampling of the upper initial mass function in small clusters can probably account for the differences in the emission properties, consistent with earlier conclusions by Zaritsky & Christlein (2007), without needing to invoke a significant change in the stellar IMF itself. Consistent Ha/FUV ratios and brightest HII region to total Halpha fluxes in the inner and extended discs across our whole galaxy sample demonstrate no evidence for a change in the cluster luminosity function or the IMF in the low gas density outer disc.
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