标题： CARMA CO观测三种极端贫金属、恒星形成星系 显示英文标题

标题： CARMA CO Observations of Three Extremely Metal-Poor, Star-Forming Galaxies

摘要： 我们呈现了对三个金属贫乏的矮不规则星系Leo P（Z ~ 3% Z_Solar）、Sextans A（Z ~ 7.5% Z_Solar）和Sextans B（Z ~ 7.5% Z_Solar）的敏感CO（J = 1 - 0）发射线观测，所有观测均使用联合毫米波天文学阵列（CARMA）干涉仪获得。 虽然没有检测到CO发射，但这三个系统的接近性使我们能够对这些金属贫乏星系中的CO光度（L_CO）设定非常严格的（4 sigma）上限。 我们发现CO光度分别为Leo P的L_CO < 2900 K km/s pc^2，Sextans A的L_CO < 12400 K km/s pc^2，以及Sextans B的L_CO < 9700 K km/s pc^2。 将我们的结果与最近关于将L_CO转换为分子氢质量的因子的观测估计以及理论模型进行比较，进一步表明，要么CO到H_2的转换因子随着金属量的减少而急剧增加，要么这些三个星系中的恒星形成非常高效，所需的分子氢很少。 显示英文摘要

摘要： We present sensitive CO (J = 1 - 0) emission line observations of three metal-poor dwarf irregular galaxies Leo P (Z ~ 3% Z_Solar), Sextans A (Z ~ 7.5% Z_Solar), and Sextans B (Z ~ 7.5% Z_Solar), all obtained with the Combined Array for Millimeter-wave Astronomy (CARMA) interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4 sigma) upper limits on the CO luminosity (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO < 2900 K km/s pc^2 for Leo P, L_CO < 12400 K km/s pc^2 for Sextans A, and L_CO < 9700 K km/s pc^2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H_2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.