A Comparison of ¹²CO and ¹³CO Galactic Surveys

Abstract

Five square degrees of the first galactic quadrant have been mapped on a square 3′×3′ grid in both ¹²CO and ¹³CO. A comparison of the two sets of maps reveals the nature and extent of the influence on survey parameters caused by line saturation in the common species. Gross features of the ¹²CO and ¹³CO maps were found to be strikingly similar when the integrated line intensities of the two species were respectively plotted with linear and quasi-logarithmic contour intervals. In general, the two sets of maps yielded similar statistics, although the ~25% of clouds that are blended together were easier to separate in ¹³CO. The generally used rule-of-thumb that ¹²CO lines are 5X brighter than ¹³CO lines fit most of the data reasonably well. However, a quadratic fit: T*_A(¹³CO) = 0.037[T_A*(¹²CO)]² + 0.056T_A*(¹²CO) was a significantly better representation of the data. The curvature in this relation, induced by saturation of the ¹²CO line, explains the similarity of the quasi-logarithmic ¹³CO maps and the linear ¹²CO maps.