LOW CO LUMINOSITIES IN DWARF GALAXIES
LOW CO LUMINOSITIES IN DWARF GALAXIES
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DOI：
10.1088/0004-6256/143/6/138
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发表时间：
2012-03
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通讯作者：
A. Schruba;A. Leroy;F. Walter;F. Bigiel;E. Brinks;W. D. Blok;C. Kramer;E. Rosolowsky;K. Sandstrom;K. Schuster;A. Usero;A. Weiss;H. Wiesemeyer
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文献类型：
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作者：
A. Schruba;A. Leroy;F. Walter;F. Bigiel;E. Brinks;W. D. Blok;C. Kramer;E. Rosolowsky;K. Sandstrom;K. Schuster;A. Usero;A. Weiss;H. Wiesemeyer
我们目前的地图12 COJ = 2-1发射覆盖整个恒星形成盘附近的16个矮星系观测IRAM赫拉克勒斯调查。数据具有13″角分辨率，在我们的平均距离D = 4 Mpc时为250 pc，并通过10-1000分辨率元素对星系进行采样。我们采用堆叠技术进行第一次灵敏的搜索CO排放的矮星系以外的本地集团范围从个人的视线，堆叠在红外线明亮的区域嵌入式星星形成，并堆叠在整个星系。我们在CO中探测到五个星系，它们的总CO光度为LCO 2-1 =（3-28）× 106 K km s−1 pc 2。其他11个星系即使在叠加图像中也没有被探测到，它们的LCO为2-1 <$（0.4-8）× 106 K km s−1 pc 2。我们将联合收割机中的矮星系样本与文献中的大量旋涡星系样本结合起来，研究了LCO与MB和金属丰度的标度关系。我们发现，金属丰度为Z = 1 - 2-1/10 Z的矮星系的LCO比大质量旋涡星系小2-4个数量级，单位LB的LCO小1-2个数量级。与星星形成示踪剂（FUV和24 μm）的比较表明，矮星系中每单位星星形成率（SFR）的LCO要小1-2个数量级。一种可能的解释是矮星系形成恒星的效率要高得多：我们认为低LCO/SFR比率是由于CO到H2的转换因子αCO在低金属丰度环境中发生显着变化。假设矮星系中的H2耗尽时间为τdep = 1.8 Gyr，这意味着Z为1/2-1/10 Z的矮星系的αCO值比太阳金属丰度螺旋星系高出一个数量级。在低金属丰度下αCO的显著增加与先前的研究一致，特别是那些模拟尘埃排放以限制H2质量的本星系群矮星系。尽管根据现有数据很难确定αCO对金属丰度的依赖性，但结果表明，在较低的金属丰度下，CO越来越难以检测。这对高红移的恒星形成星系的可探测性有直接的影响，这些星系的平均金属丰度可能低于太阳。
We present maps of 12COJ = 2–1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13″ angular resolution, ∼250 pc at our average distance of D = 4 Mpc, and sample the galaxies by 10–1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarf galaxies outside the Local Group ranging from individual lines of sight, stacking over IR-bright regions of embedded star formation, and stacking over the entire galaxy. We detect five galaxies in CO with total CO luminosities of LCO 2–1 = (3–28) × 106 K km s−1 pc2. The other 11 galaxies remain undetected in CO even in the stacked images and have LCO 2–1 ≲ (0.4–8) × 106 K km s−1 pc2. We combine our sample of dwarf galaxies with a large sample of spiral galaxies from the literature to study scaling relations of LCO with MB and metallicity. We find that dwarf galaxies with metallicities of Z ≈ 1/2–1/10 Z☉ have LCO of 2–4 orders of magnitude smaller than massive spiral galaxies and that their LCO per unit LB is 1–2 orders of magnitude smaller. A comparison with tracers of star formation (FUV and 24 μm) shows that LCO per unit star formation rate (SFR) is 1–2 orders of magnitude smaller in dwarf galaxies. One possible interpretation is that dwarf galaxies form stars much more efficiently: we argue that the low LCO/SFR ratio is due to the fact that the CO-to-H2 conversion factor, αCO, changes significantly in low-metallicity environments. Assuming that a constant H2 depletion time of τdep = 1.8 Gyr holds in dwarf galaxies (as found for a large sample of nearby spirals) implies αCO values for dwarf galaxies with Z ≈ 1/2–1/10 Z☉ that are more than one order of magnitude higher than those found in solar metallicity spiral galaxies. Such a significant increase of αCO at low metallicity is consistent with previous studies, in particular those of Local Group dwarf galaxies that model dust emission to constrain H2 masses. Even though it is difficult to parameterize the dependence of αCO on metallicity given the currently available data, the results suggest that CO is increasingly difficult to detect at lower metallicities. This has direct consequences for the detectability of star-forming galaxies at high redshift, which presumably have on average sub-solar metallicity.