2. THE SAMPLE

Several new catalogs of the distribution of HII regions in external galaxies have been published in the last decade. These come mainly from the surveys of H emission in Seyfert galaxies from Tsvetanov & Petrosian (1995), Evans et al. (1996) and González Delgado et al. (1997), from the H survey of southern galaxies from Feinstein (1997) and the H survey of ringed galaxies by Crocker et al. (1996). There have also been a considerable number of studies of the HII region distribution in individual galaxies: NGC 157, NGC 3631, NGC 6764 and NGC 6951 (Rozas et al. 1996); NGC 598 (Hodge et al. 1999): NGC 3198 (Corradi et al. 1991); NGC 3359 (Rozas et al. 2000b); NGC 4321 (Knapen 1998); NGC 4258 (Courtès et al. 1993); NGC 4736 (Rodrigues et al. 1998); NGC 5194 (Petit et al. 1996); NGC 5457 (Hodge et al. 1990, Scowen et al. 1992); NGC 6184 (Knapen et al. 1993); NGC 7331 (Petit 1998) and NGC 7479 (Rozas et al. 1999).

Figure 1. Distribution of galaxies as a function of number of HII regions.

From these studies we have retained all galaxies with an inclination less than 80° and with a sufficient number of HII regions listed. Indeed, more than 50 HII regions are necessary to allow a study of the spiral structure. Nevertheless, we included also five galaxies with fewer HII regions because we judged that they described fairly well the galaxy disc. In Fig 1 we show the number of catalogs as a function of the number of HII regions they have. The solid line corresponds to catalogs of the new sample, and the dashed line corresponds to the sample used in GGA (1991). From this figure we see that the quality of the two samples is roughly the same.

The properties of the galaxies selected for our study and of their HII region catalogs are listed in Table 1. Column 1 gives the galaxy name. Columns 2 and 3 the Hubble type (T) and the Hubble stage (S) from the RC3 catalog (de Vaucouleurs et al. 1991). Column 4 gives the arm class (AC) as defined in Elmegreen & Elmegreen (1987). Column 5 classifies the spiral structure (SS) seen in the HII region distribution, which we obtain by eye estimates as in GGA (1991). If we can see a clear grand design spiral we give a classification Y in Column 5 and N on the contrary. Intermediate cases are classified as R. Finally, in Column 6 we give the number of HII regions in the catalog (N) and in Column 7 a key for the reference of this particular catalog. The number distribution of galaxies as a function of galaxy type is shown in Fig 2. The solid line corresponds to the number distribution of types for the galaxies of the new sample, while the dashed line corresponds to the sample used in GGA (1991). As in the previous paper the peak is for galaxies of type Sbc. Nevertheless, this sample contains many more earlier type galaxies than the previous one. Indeed, in the previous sample most galaxies are of type Sbc and later, while in the new one most are Sbc and earlier.

Figure 2. Distribution of galaxies as a function of galaxy type.

In Fig 3 we compare the arm class classification of Elmegreen & Elmegreen (1987) with our classification of the arm structure present in the HII region distribution. The left panels refer to the galaxies in the new sample and the right panels to the galaxies in the GGA (1991) sample. The upper panels correspond to galaxies classified as Y, the second row of panels correspond to the galaxies classified as N and the third row contains the galaxies classified as R. The last row contains the distribution of all the galaxies according to arm class. The figure shows that galaxies classified with higher number in the arm class have a tendency to have well developed arms in the distribution of HII regions. There are, however, exceptions as e.g. NGC 5861 which has been given an arm class 12 by Elmegreen & Elmegreen (1987), but has no apparent spiral structure in the HII region distribution.

Figure 3. Comparison of the arm classification of Elmegreen & Elmegreen (1987) with our own classification of the arm structure present in the HII region distribution. The histograms give the number of galaxies as a function of the Elmegreen & Elmegreen (1987) arm class. Left panels are for the new sample and right ones for the old one. The upper panels contain galaxies with prominent spiral structure in their HII region distribution. The second row, galaxies with no such structure. The third row contains galaxies which are intermediate or unclassifiable, and the fourth row, all galaxies together.

We should also note that our new sample is formed mainly by barred and ringed galaxies. This is shown in Fig. 4 where we can see the number distribution of galaxies as a function of bar type as given in the RC3 catalog. In the upper panel we show the distribution of bar types of the galaxies of the GGA (1991) sample. In the middle panel we show the distribution for the new sample and in the lower panel the distribution of bar types of the two samples combined. While the first sample was biased towards non-barred galaxies (upper panel), the new catalog is biased towards barred and/or ringed galaxies (middle panel). This gives a quite uniform total distribution (lower panel).

Figure 4. Distribution of galaxies according to the bar type classification of the RC3 (1991) catalog.

Our final aim is to study the spiral structure outlined by the HII region distribution. Obviously, this cannot be done with a catalog with a number of HII regions lower than 50, and only the richer catalogs will be considered for this purpose in a forthcoming paper. Nevertheless, useful information on the galaxies can be obtained even from the less rich catalogs, as e.g. the orientation of the galaxies as seen in the sky and the radial scale of the distribution, which can be compared with the radial scale length of galaxy discs (Athanassoula et al. 1993).