2. THE SAMPLE

Our sample consists of 64 spiral galaxies with a distribution of HII regions not too irregular and with an inclination angle of less than 80° for which catalogues with more than 50 HII region positions are available in the literature. They are listed in Table 1. Column 1 gives the galaxy name, Column 2 the number of HII regions in the published catalogue and Column 3 gives a number referencing to the source of the catalogue. For 8 galaxies two catalogues were available and for 2 galaxies three. Columns 4 and 5 give the type of the galaxy according to the Second Reference Catalogue (de Vaucouleurs et al. 1976, hereafter RC2) and to the Shapley-Ames catalogue (Sandage & Tamman 1981, hereafter SA).

Depending on whether the HII regions outline what could be called a grand design spiral, or not, we have classified the galaxies in our samples as Y or N. Intermediate cases, i.e. when the galaxy has some trace of spirality but of short extent or badly defined, are labelled R in Column 6. The arm type classification of Elmegreen & Elmegreen (1987) is given in Column 7, while Column 8 mentions whether there is flux information in the catalogue.

It is obvious that a better analysis can be made of galaxies for which the catalogue contains many HII regions, yet some information can be obtained with as low as 50 HII regions. We have thus grouped galaxies according to the number of HII regions in their catalogue in Table 2. This shows that a complete study, including the analysis of the spiral structure whenever existent, can be made for the 19 galaxies with more than 200 HII regions, while a reasonable one for the 21 galaxies with 100 N 200.

Table 2.

NUMBER OF HII REGIONS DETECTED	NUMBER OF GALAXIES
50 < N 100	24
100 < N 150	17
150 < N 200	4
200 < N 250	3
250 < N 300	3
300 < N 400	3
400 < N 500	2
500 < N	8

Figure 1 shows a histogram of the number of galaxies of a given type, as classified in the RC2. We note that more than half of our sample are of type bc or c. This is due on the one hand to the lack of HII regions in early-type galaxies and on the other to the preference of observers to "pretty" galaxies and to the relative scarcity of large angular diameter in very late-type galaxies.

Figure 1. Histogram of the number of galaxies as a function of type according to the second reference catalogue. The number of galaxies is given also on top of each bin.

Our galaxies were classified as Y, N or R based on their HII region distribution, not the galaxy image as for the classification of Elmegreen & Elmegreen (1987). It is thus of interest to compare the two classifications. For this we plot in Figure 2 histograms of the number of galaxies of a given Elmegreen & Elmegreen arm class. We have done this for all our sample (a), the Y ones (b), the R ones (c) and the N ones (d) separately. We note that the correspondence is very good. There are no galaxies with arm classes higher than 7 in our N subsample, while most galaxies in our Y subsample have arm class larger or equal to 5. ⁽¹⁾ One galaxy, NGC 925, is in our Y subsample, while being classified as chaotic by Elmegreen & Elmegreen. However the difference is not between the blue light and the HII region distribution, but rather between the classifiers, since we would say that this galaxy has a structure.

Figure 2. Histogram of the number of galaxies as a function of arm class according to Elmegreen & Elmegreen (1987). a) All galaxies in our sample, b) galaxies with clear structure in their HII region distribution, c) intermediate cases, d) galaxies with no structure in their HII region distribution.

¹ Class 5 according to Elmegreen & Elmegreen (1987) corresponds to galaxies with two symmetric, short arms in the inner regions and with irregular outer arms and class 6 to galaxies with two symmetric inner arms and feathery ringlike outer structure. Back.