矮星系中4.5微米的过剩的起源
The Origin of the 4.5 micron Excess from Dwarf Galaxies
摘要：
矮星系往往比螺旋星系有更红的[3.6微米] - [4.5微米]斯皮策宽带颜色。为了研究这种效应，对于一个大样本的矮星系，我们结合斯皮策通量与其他波长的数据，并比较人口合成模型。金属丰度较低的系统平均颜色较红[3.6] - [4.5]，但分散性很大。观测到的[3.6] - [4.5]颜色范围太大，不能仅仅用年龄或金属丰度差异引起的恒星颜色变化来解释;星际效应也有一定的贡献。对于最红的系统，4.5微米的光度可能不是恒星质量的良好示踪剂。我们确定了三个因素，使这种颜色变红的侏儒。首先，在某些系统中，强Br-α发射对4.5微米发射有显著贡献。第二，在某些情况下，高的光学深度会导致斯皮策波段的星光强烈变红。第三，在某些星系中，星云连续体主导着4.5微米的流量，在极端情况下，也是3.6微米的流量。在较低的金属丰度系统中，较硬的紫外辐射场产生更多的红外气体连续谱和更多的Br-α/恒星形成率。这三个因素的结合可以解释我们的样本星系中4.5微米的过量，因此没有必要援引热尘埃对4.5微米波段的主要贡献。然而，考虑到不确定性，我们不能完全排除4.5微米的热尘排放。需要在3 - 5微米范围内进行更多的光谱观测来解开这些影响。
英文摘要：
Dwarf galaxies tend to have redder [3.6 micron] - [4.5 micron] Spitzer broadband colors than spirals. To investigate this effect, for a large sample of dwarf galaxies we combine Spitzer fluxes with data at other wavelengths and compare to population synthesis models. Lower metallicity systems are found to have redder [3.6] - [4.5] colors on average, but with considerable scatter. The observed range in [3.6] - [4.5] color is too large to be accounted for solely by variations in stellar colors due to age or metallicity differences; interstellar effects must contribute as well. For the reddest systems, the 4.5 micron luminosity may not be a good tracer of stellar mass. We identify three factors that redden this color in dwarfs. First, in some systems, strong Br-alpha emission contributes significantly to the 4.5 micron emission. Second, in some cases high optical depths lead to strong reddening of the starlight in the Spitzer bands. Third, in some galaxies, the nebular continuum dominates the 4.5 micron flux, and in extreme cases, the 3.6 micron flux as well. The harder UV radiation fields in lower metallicity systems produce both more gaseous continuum in the infrared and more Br-alpha per star formation rate. The combination of these three factors can account for the 4.5 micron excess in our sample galaxies, thus it is not necessary to invoke a major contribution from hot dust to the 4.5 micron band. However, given the uncertainties, we are not able to completely rule out hot dust emission at 4.5 micron. More spectroscopic observations in the 3 - 5 micron range are needed to disentangle these effects.
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