Holes in Dwarfs vs. Holes in Spirals

Having convinced ourselves that the most distinctive features of the ISM in dwarf galaxies are H I holes and shells, at least provided they are not participating in strong interactions, the question is now how their physical properties compare to the holes found in more massive, spiral galaxies. A detailed comparison has been performed by Brinks & Walter (1998) and only the main results are highlighted here. In what follows the observed H I hole properties of M31 (Brinks & Bajaja 1986), an example of a massive spiral galaxy similar to our own; M33 (Deul & den Hartog 1990), a less massive spiral; IC2574 (Walter & Brinks 1998a) and HoII (Puche et al. 1992), two dwarf galaxies in the M81 group of galaxies (note that HoII is four times less massive than IC2574) will be compared. In other words, the sequence M31 - M33 - IC2574 - HoII spans a large range of different Hubble types from massive spirals to low-mass dwarfs.

Despite the fact that a similar study, and at considerably higher sensitivity, is now available for the SMC (Staveley-Smith et al. 1997), we have decided not to include their results, the main reason being that the linear scales (or spatial frequencies) sampled by the ATCA only just overlap with those observed in the galaxies listed above. The linear resolution, at 28 pc, is almost four times higher than, for example, the VLA maps of IC2574. At the other end of the spectrum, because of the lack of short spacing information, structures larger than a few hundred parsec will have been missed. Moreover, the SMC is a very disturbed system, being torn apart by tidal forces due to interactions with the LMC and the Galaxy. An additional argument for leaving out the SMC is that Staveley-Smith et al. used a different approach in searching and identifying the holes which makes a direct comparison difficult.

Other objects with catalogued H I holes, such as DDO47 (Walter & Brinks 1998b) and IC10 (Wilcots & Miller 1998) have so few holes that a statistical analysis is not warranted. Limiting the comparison to the four galaxies listed above has some further advantages. The linear resolutions are very similar, as are the velocity resolutions with which they have been observed (see Walter & Brinks 1998a for details). In addition, all four galaxies were examined in more or less the same fashion, one of the authors (E. Brinks) having taken part in the analysis of three of the four objects. The results for the four galaxies suffer partially from low statistics and incompleteness due to personal bias and observational constraints (such as the beamsize). However, these effects, to first order, affect a comparison in a similar way and that it is valid to try to find global trends as a function of Hubble type. In order to remove the human factor, it would be interesting to apply an automated object recognition package such as that developed by Thilker et al. (1998) to all galaxies with sufficiently detailed observations.

Fig. 2 (left) shows an overlay of the relative size distribution of the holes found in the four galaxies. In this plot the bins are on a linear scale. Note that there is a clear sequence with Hubble type! The size distribution for holes in M31 and M33 cuts off sharply near 600 pc. In contrast, holes in IC2574 and HoII reach sizes of 1200 to 1500 pc, respectively. The lack of holes with sizes smaller than pc is due to our resolution limit. As explained in the previous section, holes are larger for ``later'' Hubble types because these smaller galaxies have lower masses and hence a lower mass surface density. So, for the same amount of energy deposited, an H I shell can grow much larger, both because of a lower gravitational potential and a lower ambient density. Because the H I layer is much thicker in dwarfs as well (Puche et al. 1992; Walter & Brinks 1998a, b), shells take longer to break out of the disk.

>From the observed hole properties such as the expansion velocities and diameters, one can try to estimate the amount of energy which was needed to produce the holes, based on numerical simulations. Here we use the numerical model developed by Chevalier (1974) to calculate the total mechanical energy needed to create the holes. Note that many assumptions enter the calculations and that the results should only be taken to be order of magnitude estimates. A plot of the results of this analysis is shown in Fig. 2 (right). Gratifyingly, the energies needed to produce the holes are the same for all galaxies - this suggests that, whatever the underlying physical mechanism for the creation of the holes is, it seems to be the same in all galaxies, at least to first order.

In addition to the objects listed in the introduction and the maps shown here, several more dwarf galaxies have been observed. Walter & Brinks are working on data on HolmbergI and M81dwarfA. Van Dyk et al. (1998) present data for SextansA. Wilcots and collaborators have data on three more galaxies, IC1613, IC10, and NGC4449, the former object being completely dominated by H I holes and shells, much like IC2574 and HoII. Hence, within the coming year, a lot more material should thus become public, allowing for better comparative studies to be performed.

Figure 2: Left: Comparison of the relative distribution, in percentage, of the diameters of the H I holes in IC2574, M31, M33 and HoII. right: Comparison of the relative distribution, in percentage, of the energies required to produce the H I holes in IC2574, M31, M33 and HoII.

© Copyright Astronomical Society of Australia 1997