The Optical Counterparts to Galaxies in the Cen A Group Discovered by HIPASS
Patricia M. Knezek, PASA, 16 (1), in press.

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Discussion

The galaxy presented in Figure 2, HIPASS1321-31, is typical of the dwarfs which were discovered through the HIPASS. In general these dwarfs are very low surface brightness ( $\mu_0=24.2$ mag arcsec^-2 in B for HIPASS1321-31), with ``by-eye'' diameters of order 30

$^{\prime\prime}$ , which corresponds to $\sim$ 0.5 kpc at the distance on the Cen A group. This is comparable to the values found for Local Group dwarf galaxies, which have diameters between 0.18 and 3 kpc, based on their disk scale lengths (Mateo 1998).

The newly detected dwarfs are also represented by HIPASS1321-31 morphologically. Like HIPASS1321-31, most of the new members appear to be dwarf irregulars. This is not unexpected, as the HIPASS is going to select out galaxies which possess atomic gas, introducing a bias toward late-type systems. A few do have higher surface brightness inner regions, similar to some of the late-type galaxies with surface brightness ``steps'' discussed by Matthews & Gallagher (1997). Also similar to the Matthews & Gallagher systems, a few appear to have unresolved, point-like nuclei. In this respect, they differ from Local Group dwarfs, where only gas-poor, dwarf spheroidals, have nuclei. Apparently, even these small systems can have multi-component disks and/or a nuclear component, suggesting a complex evolutionary history.

The Cen A group also appears to differ from the Local Group, and other groups of galaxies, in the morphological composition of the group. It has already been noted by de Vaucouleurs (1979) that the Cen A group has the largest morphological variety seen in any nearby group. The dwarf population only has two members which Côté et al. (1997) claim can be classified as early-type dwarfs, NGC 5206 and NGC 5237, based strictly on optical morphology. Although detailed morphological classification has not yet been possible, the optical appearance of all the newly detected dwarfs is consistent with their being late-type dwarfs. Thus, while at least 30% of the Local Group members with M

$_{\rm B} < -11$ appear to be early-type systems, only $\sim$ 7% of the Cen A group members currently known are early-type, unless the two luminous S0s are included, NGC 5102 and NGC 5128. If the two groups actually have a similar morphological composition, this implies that the Cen A group contains at least 60 members.

In order to study the optical integrated properties of the Cen A group, we present a histogram of the distribution in absolute B magnitudes in Figure 3 for the members of the Cen A group within the Survey area (solid line) with B photometry, in comparison with a histogram of all known Local Group members (dotted line). We have chosen to display only the Cen A group members within our survey area since the correction for dwarfs outside the survey area is unknown. The absolute magnitudes for the Cen A group members were determined assuming a distance of 3.5 Mpc, and corrected for Galactic extinction. Internal extinction corrections have not been made. The Local Group data are from Mateo (1998), except for M 31, M 33, the LMC, the SMC, which are from de Vaucouleurs et al. (1991), scaled to the distances given in Mateo (1998), and the Milky Way, which is from van den Bergh (1992).

**Figure 3:** A histogram of the absolute blue magnitudes of the known Cen A group members within the HIPASS survey area and the known Local Group members.
$\begin{figure} \begin{center} \leavevmode \epsfysize=250pt \epsfbox{bmags.eps}\end{center}\end{figure}$

The dearth of galaxies in the Cen A group seen in Figure 3 at M

$_{\rm B} > -11$ is a selection effect. Most of the Local Group galaxies that are that faint are dwarf spheroidals with no detected HI gas. The three late-type Local Group members which do have such low luminosities have M $_{\rm HI}$ /L $_{\rm B}$ of $\sim$ 0.2 - 1.4. HIPASS would not detect galaxies that faint unless they had an ratio of M $_{\rm HI}$ /L $_{\rm B} > 7$ , while the Côté et al. (1997) study deliberately targeted late-type dwarf galaxies. Furthermore, at the distance of the Cen A group, a M

$_{\rm B} \sim -10$ implies an integrated apparent magnitude of m

$_{\rm B} > 17.7$ not including Galactic extinction, which is generally significant for this group of galaxies (values for A $_{\rm B}$ range from 0.14 - 2.12, with $\sim$ 1/3 of the galaxies having A

$_{\rm B} > 0.50)$ . Thus, the determination of membership at the faint end of the luminosity function will have to wait for more sensitive studies. Even though the newly detected galaxies all have M

$_{\rm B} < -10.5$ , however, they contribute very little to the integrated blue luminosity of the group. After correcting for Galactic extinction, the new members are found to add <4% to the total blue luminosity of the group.

Overall, the two groups have similar luminosity distributions, at least to M

$_{\rm B} \sim -11$ . This is consistent with the luminosity function being the same in both groups. For the Local Group, Mateo (1998) found the best fitting Schechter (1976) function to have

$\alpha = -1.16$ , and M

$_{\rm B} = -21.42$ . It is interesting that the luminosity distribution of the two groups is so similar, while the morphological composition is so different. As noted above, for M

$_{\rm B} < -11$ , about 30% of the Local Group are early-type galaxies, whereas <15% of the Cen A group appear to be early-types, and the percentage is only that high if you include the luminous S0s NGC 5102 and NGC 5128. If the HI PASS and Côté et al. (1997) survey techniques have truly missed the corresponding early-type, gas poor galaxies in the Cen A group, then the luminosity functions of the two groups may differ significantly. If the luminosity functions are actually the same, then, at least for M

$_{\rm B} < -11$ , the morphological composition of the two groups differs significantly.

Another way of stating the difference between the two groups is to look at the ratio of gas mass to blue luminosity for the individual members. The ratio of atomic gas mass-to-blue luminosity, M $_{\rm HI}$ /L $_{\rm B}$ , is shown in Figure 4 versus the absolute blue magnitude, M $_{\rm B}$ , for all Cen A group members for which optical photometry was available, and for the Local Group. The HI mass of NGC 5128 is from Richter et al. (1994). Local Group HI data are from Mateo (1998), except for M 31, M 33, the LMC, the SMC, and the Milky Way, which are from Lang (1980), scaled to the distances given in Mateo (1998). The error bar shown in the upper right corner of Figure 4 is a representative error bar for the new galaxies identified in this study, assuming a magnitude error of 0.3 mags and an HI mass error of 50%. This is very likely an overestimate of the errors. The mean M $_{\rm HI}$ /L $_{\rm B}$ for these new galaxies is 2.7, while the median is $\sim$ 1.5. Thus, these galaxies have typical atomic gas to blue light ratios which are $\sim$ 10 times higher than that found in late-type disk galaxies, and which are comparable to the highest values found by Matthews & Gallagher (1997) in their study of extreme late-type galaxies.

**Figure 4:** A graph of M $_{\rm HI}$ /L $_{\rm B}$ versus M $_{\rm B}$ of the known Cen A group members and the Local Group galaxies. The errorbar indicates a typical uncertainty for the *new* galaxies detected in this study. The uncertainty is smaller for the previously known galaxies.
$\begin{figure} \begin{center} \leavevmode \epsfysize=250pt \epsfbox{mhlb.eps}\end{center}\end{figure}$

From Figure 4 it can be seen that the ratio of gas mass to blue luminosity is similar for the brightest galaxies in both the Cen A group and the Local Group. However, for

$-16 < {\rm M_{B}} < -11$ , a significant number of the galaxies in the Cen A group appear to be gas rich. In fact, more than three times as many galaxies in the Cen A group have M $_{\rm HI}$ /L $_{\rm B} > 2$ as in the Local Group. If Cen A has recently acquired a population of dwarfs which have not yet been significantly affected by the group dynamics, as suggested by Graham (1979) and van Gorkom et al. (1990), this might explain the population of extremely gas-rich dwarfs. However, the line-of-sight velocity dispersion of the group is only $\sim$ 135 km ${\rm s}^{-1}$ within the HIPASS survey region, and only $\sim$ 140 km ${\rm s}^{-1}$ including all known members. As noted by Côté et al. (1997), while this implies that the Cen A group is not virialized, the velocity dispersion is quite typical of groups of galaxies of this size. It is not obvious, then, that this group of galaxies has accreted a new, gas rich population of dwarfs, while other similar groups of galaxies apparently have not.

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