The second data release of the Gaia mission contained astrometry and photometry for an incredible 1,692,919,135 sources, but how many sources did Gaia miss and where do they lie on the sky? The answer to this question will be crucial for any astronomer attempting to map the Milky Way with Gaia DR2. We infer the completeness of Gaia DR2 by exploiting the fact that it only contains sources with at least five astrometric detections. The odds that a source achieves those five detections depends on both the number of observations and the probability that an observation of that source results in a detection. We predict the number of times that each source was observed by Gaia and assume that the probability of detection is either a function of magnitude or a distribution as a function of magnitude. We fit both these models to the 1.7 billion stars of Gaia DR2, and thus are able to robustly predict the completeness of Gaia across the sky as a function of magnitude. We extend our selection function to account for crowding in dense regions of the sky, and show that this is vitally important, particularly in the Galactic bulge and the Large and Small Magellanic Clouds. We find that the magnitude limit at which Gaia is still 99% complete varies over the sky from $G=18.9$ to $21.3$. We have created a new Python package selectionfunctions (this https URL) which provides easy access to our selection functions.

中文翻译：

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Gaia verse II 的完整性：Gaia DR2 中缺少一颗恒星的几率是多少？

盖亚任务的第二次数据发布包含了令人难以置信的 1,692,919,135 个来源的天体测量和光度测量，但是盖亚错过了多少个来源，它们位于天空的哪个位置？这个问题的答案对于任何试图用 Gaia DR2 绘制银河系图的天文学家来说都是至关重要的。我们利用 Gaia DR2 仅包含至少有五个天体测量检测的源这一事实来推断 Gaia DR2 的完整性。源实现这五个检测的几率取决于观察的数量和对该源的观察导致检测的概率。我们预测每个源被 Gaia 观察到的次数，并假设检测概率是大小的函数或作为大小函数的分布。我们将这两个模型都拟合到 1。70 亿颗盖亚 DR2 恒星，因此能够可靠地预测盖亚在天空中的完整性作为星等的函数。我们扩展了我们的选择函数来解释天空密集区域的拥挤，并表明这非常重要，特别是在银河膨胀和大小麦哲伦云中。我们发现盖亚仍然完成 99% 的震级限制在天空中从 $G=18.9$ 到 $21.3$ 不等。我们创建了一个新的 Python 包选择函数（这个 https URL），它可以轻松访问我们的选择函数。特别是在银河隆起和大小麦哲伦云中。我们发现盖亚仍然完成 99% 的震级限制在天空中从 $G=18.9$ 到 $21.3$ 不等。我们创建了一个新的 Python 包选择函数（这个 https URL），它可以轻松访问我们的选择函数。特别是在银河隆起和大小麦哲伦云中。我们发现盖亚仍然完成 99% 的震级限制在天空中从 $G=18.9$ 到 $21.3$ 不等。我们创建了一个新的 Python 包选择函数（这个 https URL），它可以轻松访问我们的选择函数。