4 Discussion

A great majority (73%) of our galaxies are of type Im (26) and Ir (162), about 20% are of type Sph/Ir (12) and Sph (39), while the rest of 8% is a collection of different types from spiral to Im/Sm and BCD. The detection rate of our sample galaxies depends on the morphological type. 75% of the spirals (type S0 to Sm/Im and BCD) were detected; the detection rate for types Im and Ir is very similar close to 60%, whereas the detection rate for types Sph/Ir and Sph is considerable lower at 33 and 23%, respectively. The detection rate depends on the optical surface brightness (SB) class, too. From high SB to low, very low, and extremely low SB the detection rate decreases from 70% to 58%, 49%, and 43%, respectively. This trend reflects the type dependence and the fact that we deal with fainter galaxies as we descend from high SB to very low SB, the median absolute magnitudes for the detected galaxies change from -15.43 (H) to -13.92(VL) for our brightness classes. A number of the galaxies within the present sample are associated with nearby groups of galaxies (e.g. Tully 1988) according to their position, radial velocity and relative resolution:

NGC672 group: kk13, kk14, kk15;

NGC784 group: kk16, kk17;

Maffei group: kk19, kk21, kk22, kk23, kk35 kk44;

Orion group: kk49;

M81 group: kk81, kk83, kk85, kk89, kk89, kk91;

Leo group: kk94;

CVn cloud: kk109, UGC7298, kk137, kk141, kk144, kk148, kk149, kk151, kk154, kk158, kk160, kk191, kk206, kk220, kk230;

Centaurus group: kk170, kk179, kk182, kk190, kk191, kk195, kk197, kk200, kk211, kk217, kk218;

NGC6946 group: kk250, kk251, kk252;

Virgo cluster: kk111, kk127, kk128, kk140, NGC4523, IC3517, kk164, kk168, kk169, kk172, kk173, U8091.

There are a few cases of high $M_{\rm HI}/L_{B}$ values in Table 2. Four of the five galaxies with $M_{\rm HI}/L_{B} \ge 5$ are actually found to be confused by emission from nearby galaxies (see footnotes to Table2).

The present sample of galaxies as presented in Tables 1 and 2 will be discussed now in some detail with the help of global parameters. The distribution of radial velocity (v₀, corrected for the rotation of our galaxy) is given in Fig.4. Apart from a few background objects most of the galaxies belong to the local supercluster, about 25% are within the Local Volume. From this situation it is clear that the great majority of the galaxies in the present sample are dwarfish in nature. This will be shown more convincingly below when we compare several other global parameters of these objects.

Figure 6: The total mass of neutral hydrogen $M_{\rm HI}$ of the galaxies in our sample is plotted versus the linear extent (in kpc). The full line represents the regression line for the KKT sample (Huchtmeier & Richter 1988)

Next we will look at the optical linear diameter A₀ (in kpc). The histogram in Fig.5 presents the number of galaxies binned in intervals of 0.5 kpc width. The distribution of the optical linear diameters of our galaxies extends from 0.2 kpc to 26 kpc, yet the great majority is smaller than 8 kpc in diameter (in the de Vaucouleurs D₂₅ system). Galaxies in the Local Volume (indicated by shaded areas) are even smaller with a median value of 1.4 $\pm$ 0.2 kpc.

Now we will use the correlation of two global parameters to compare the present sample of galaxies with the previously known galaxies in the Local Volume. In Fig.6 the total mass of neutral hydrogen $M_{\rm HI}$ of the galaxies is plotted versus their linear extent A₀ for this sample of galaxies. The full line is the regression line for the KKT sample (Huchtmeier & Richter 1988). This regression line seems to be an excellent fit for the present sample, too. The average HI mass of the galaxies in the Local Volume is 4.6 10⁷ $M_{\hbox {$\odot $ }}$.

The HI masses in Fig.6 cover a range from 10⁶ to 10¹⁰ solar masses. The HI luminosity function for galaxies has been studied with galaxies of 10⁷ and more solar masses in HI so far. With the data of the new dwarf galaxies within the Local Volume we will be able in the end to discuss the HI luminosity function starting from 10⁶ solar masses.

Figure 7: The distribution of line widths of our galaxy sample is given for the observed values (dv) in the upper panel and for the (for inclination corrected values (dvi) in the lower panel. Galaxies within the Local Volume (i.e. within 10 Mpc) are marked by the shaded areas

The galaxies in our sample have small line widths on the average. In Fig.7 we present the distribution of observed line widths in the upper panel and the (for inclination) corrected line widths in the lower panel. The optical axial ratio has been used here to derive the inclination. Galaxies within the Local Volume are indicated by the shaded areas. The peak of the line width distribution of the galaxies within the Local Volume is 39 kms^-1 for the uncorrected and 47 kms^-1 for the corrected line widths.

The three global parameters we have considered so far point altogether toward the dwarfish character of the Local Volume objects in our sample: the average linear diameter of $1.4 \pm 0.2$ kpc (Fig. 5), the mean total HI mass of 4.6 10⁷ $M_{\hbox {$\odot $ }}$ and the small line width of less than 50 km s^-1.

Figure 8: The pseudo column density of neutral hydrogen ( $\Sigma _{\rm HI}$ in $M_{\hbox {$\odot $ }}$pc-2) of our sample as plotted versus the relative HI-content ( $M_{\rm HI}/M_{\rm T}$)

Two more global parameters are shown in Fig.8, pseudo HI surface density $\Sigma _{\rm HI}$ and the relative HI content $M_{\rm HI}/M_{\rm T}$. The pseudo HI surface density is obtained by dividing the total HI mass $M_{\rm HI}$ of the galaxy by the disk area of the galaxy as defined by its optical diameter A₀. This quantity is given in units of solar mass per square parsec as well as in the usual HI column density $N_{\rm HI}$ in atoms cm^-2. This quantity is plotted versus the relative HI content $M_{\rm HI}/M_{\rm T}$. Our galaxies fill the usual range in HI surface density as well as in relative HI content as observed for normal galaxies (e.g. HR). The present sample of galaxies is relatively rich in HI. Some of the scatter in the diagram is due to uncertainties in observed quantities, especially the inclination which is used to correct the line width which itself enters the total mass calculation by the square. The optical diameters are uncertain for galaxies at low galactic latitudes due to the high foreground extinction, e.g. Cas2, ESO 137-G27, BK12, ESO 558-11. If we exclude the confused galaxies and those with heavy galactic extinction all entries in Fig.8 with $\Sigma _{\rm HI}$ $\ge$ 100  $M_{\hbox {$\odot $ }}$pc^-2 are gone. Low values of the HI surface density are not only due to the uncertainties of observational data, the gas content of dwarf galaxies is very sensitive to outside influences (tidal interactions) due to their shallow gravitational potential.

Finally we plot the HI surface brightness versus the optical surface brightness (Fig.9). The surface brightness class (Table 1, Col. 7) has been coded from 4 to 1 from high SB to extremely low SB in steps of 1. The different errors of the mean values of each class essentially depend on the different population size of each SB class. However, there is a definite trend of the HI surface density to grow with increasing optical SB by a factor of 2 to 4 (e.g. van der Hulst et al. 1993; de Blok 1997).

Figure 9: This figure presents a correlation between the pseudo HI column density with the optical surface brightness of the galaxy in our actual sample. The surface brightness class is taken from KK98; 1 = extremely low, 2 = very low, 3 = low, 1 = high SB. The error bars correspond to twice the rms error of the mean of each SB class