THE ORIGIN OF THE 4.5 μm EXCESS FROM DWARF GALAXIES
THE ORIGIN OF THE 4.5 μm EXCESS FROM DWARF GALAXIES
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DOI：
10.1088/0004-6256/138/1/130
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发表时间：
2009-04
期刊：
影响因子：
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通讯作者：
Beverly J. Smith;M. Hancock
中科院分区：
文献类型：
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作者：
Beverly J. Smith;M. Hancock
矮星系倾向于比螺旋星系有更红的[3.6 μm] − [4.5 μm]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，我们结合联合收割机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]，但分散性相当大。观测到的[3.6] − [4.5]色的范围太大了，不能仅仅用恒星颜色的变化来解释，因为年龄或金属丰度的差异;星际效应也有一定的贡献。对于最红的系统，4.5 μm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，在某些体系中，强Brα发射对4.5 μm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，星云连续统支配着4.5 μm的流量，在极端情况下，也支配着3.6 μm的流量。在较低金属丰度的系统中，较硬的紫外辐射场产生更多的红外气体连续谱和更多的Brα/星星形成率。这三个因素的结合可以解释我们样本星系中4.5 μm的过量，因此没有必要援引热尘埃对4.5 μm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 μm的热尘埃排放。需要在3-5 μm范围内进行更多的光谱观测来解开这些影响。
Dwarf galaxies tend to have redder [3.6 μm] − [4.5 μm] 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 μm 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 Brα emission contributes significantly to the 4.5 μm 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 μm flux, and in extreme cases, the 3.6 μm flux as well. The harder UV radiation fields in lower metallicity systems produce both more gaseous continuum in the infrared and more Brα per star formation rate. The combination of these three factors can account for the 4.5 μm excess in our sample galaxies, thus it is not necessary to invoke a major contribution from hot dust to the 4.5 μm band. However, given the uncertainties, we are not able to completely rule out hot dust emission at 4.5 μm. More spectroscopic observations in the 3–5 μm range are needed to disentangle these effects.