Distribution of neutral hydrogen in the Perseus Arm

A statistical analysis is applied to 21-cm data to derive models of the density distribution of interstellar gas in small volumes of spiral arms. After subtracting a background model, the density fluctuations are statistically represented by superpositions of randomly located spherical clouds. The expected number of these model clouds per unit volume and their central gas densities are both derived as functions of cloud diameter by a correlation analysis, using a Monte Carlo technique. We analyzed many small volumes within four large segments of the Perseus arm and one segment of an interarm feature, each observed with the 140-foot (43 m) telescope and 400-channel receiver at NRAO. The resulting cloud distributions are presented; they vary rapidly between small volumes, but the average distributions over each entire segment are remarkably consistent. The cloud mass spectrum is very well described by a power lawJ(m) m , with p = 1.75 + 0.15 for the Perseus arm segments and p = 2.0 l 0.15 for the interarm feature, for cloud masses 10 < m < 106 Al0. There is a slight surplus of the smallest clouds on the farther side of the arm. Attempts to obtain relative distances to the segments were partially successful.