3 The catalogue: Parameters and measurements from the maps

For each galaxy, the following information was collected for constructing the catalogue:

1.

Optical parameters of the galaxy: name; coordinates; corrected diameter and morphological type, extracted from LEDA (Lyon-Meudon Extragalactic Database, first and second edition). For uncatalogued galaxies, the name and coordinates were taken as given by the authors or were estimated from the maps.

2.

HI observational parameters: the telescope used with its angular resolution; the velocity resolution of the map and the achieved sensitivity represented by the rms noise in the maps.

3.

The type of map, as described above and codified as described in the Read.me file.

4.

The value of the lowest contour level of the gas distribution, only for types of maps as described in 2-a). This contour level usually is related to the sensitivity, because it is, in general, 2, 3 or 4 times the rms noise of the observation. The usual unit of contour level is the column density ($n_{\rm HI}$) expressed in atoms cm^-2 (assuming optically thin gas), and this is the one used in this catalogue. When the levels are expressed in other units without any possibility of changing them into the $n_{\rm HI}$ values (e.g., when antenna temperatures or flux densities are used instead of brightness temperature), the value is lost and the condition is explained in a note. It is worth noticing that these surface densities are always beam-averaged, because in some way this effect will influence the HI diameter described in the next item.

5.

A measure of the HI extension spread by the gas, determined by the lowest isophote quoted in the previous item. These measurements were made by us and only for the type of maps described in 2-a). The outer regions of a disk are very fragile and can be severely affected by a violent environment. Since for statistical purposes, it is more convenient to deal with relatively unperturbed systems, the measurement of the extension intends to embrace the undisturbed gas. So it does not take into account those structures that probably have a tidal origin, like tails, plumes, HI streams and bridges extending outside the galaxy. Sometimes, however, it is not possible to distinguish these structures from the maps due to beam smearing, and then they are inevitably included in the measure of the HI extension. An example of this is NGC 4038/39, whose tail is resolved by van der Hulst (1979), but not by Rots (1980). Some maps show filamentary structures or HI clumps or "patchy structure" extending outside the general distribution of the galaxy. The HI extension does not include them, unless these structures are undoubtedly associated with the galaxy.

Finally, the measure of the gas extension has not, by itself, any sense unless it is referred to a sensitivity level. Therefore, when we were not able to find the $n_{\rm HI}$ value, the extension of the gas is not given.

With respect to the errors of these measurements, they come basically from two different sources: from the author of this catalogue and from the literature. That is, there are errors inherent to the measurements and possible errors in the maps published in the literature. For the first type of error, a favourable point is the fact that the measurements were made by only one person, who applied uniform criteria to do them. Whenever there was a doubt concerning the measurement (e.g. when the lower contour level is not well-defined or has distortions, plumes, warps, etc., or the outlying gas vanishes in a clumpy structure), we express it with the $\sim$ symbol. For the second type of error, we searched the literature for possible errata of the works.

6.

The hydrogen mass, as given in the reference and by integration of the HI map. This quantity was unaffected by distance effects, to be helpful in the comparison of the total gas emission collected by different telescopes.

7.

Footnotes that remark: HI appendages such as tails, plumes or bridges; references of works related to particular structures in a galaxy, such as HI holes or shells (particularly in nearby galaxies); lost entry parameters; references for errata; etc. These notes have the purpose of improving this catalogue.

Sometimes, we can see from the maps that two or more galaxies have a common envelope of gas. This envelope may be due to a genuine bridge of material joining the galaxies, or it may be due to the beam smearing; that is, it is not possible to remove the individual gas structures of the galaxies due to the low spatial resolution of the telescope. Other times, the author considers that more than one galaxy contributes to the gas emission. In these cases, the galaxies involved in the same HI structure are marked, and the parameters mentioned in previous items (1 to 7) correspond to all galaxies, as a whole.