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Astron. Astrophys. 342, 655-664 (1999)

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6. Conclusions

What causes the low evolution rate for LSB galaxies? The low density has often been invoked to explain this, since the dynamical time scales with [FORMULA]. This scenario is exactly what is tested in simulation H. The only difference with a normal HSB galaxy is the scale length of the galaxy, which is for instance three times larger than the scale length for the equally bright galaxy NGC 6503. The result is striking: adopting "standard" values for the star formation process results in a SFR identical to the rates of HSB galaxies.

Thus the low density in itself seems not capable of doing the job, and we have to rely on a scarcity of heavy elements to reproduce a true LSB galaxy. This fits in logically with the notion that stars are the producers of these elements; the low star formation activity prevents metal enrichment of the ISM. It implies that the SFR has been low throughout the evolution of LSB galaxies, and that these galaxies are "trapped" in their current evolutionary state: low density prevents rapid star formation, which prevents enrichment of the ISM, which prevents cooling, resulting in a warm one-phase ISM. So although the lack of metals is directly responsible for the low SFR, the low density may ultimately determine the fate of LSB galaxies.

In summary we reach the following conclusions on the physical properties of LSB galaxies.

  1. Dominant halo: The dominance of the dark matter halos in LSB galaxies (de Blok & McGaugh 1997) results in very thin and stable disks. The major over minor axis ratios of the stellar disks are larger than 15. Lack of dust in LSB galaxies will result in edge-on LSB galaxies having apparent surface brightnesses equal to those of edge-on HSB galaxies of similar size. The lack of truly LSB edge-on galaxies may indicate that disk galaxies with [FORMULA] mag arcsec-2 are rare.

  2. Low metallicity: Modeling the low SFR found in LSB galaxies, demands that the metallicity in the ISM must be approximately 0.2 solar, which is consistent with observation by McGaugh (1994) and de Blok & van der Hulst (1998a). These low metallicities prevent effective cooling so that the ISM in LSB galaxies primarily consists of warm ([FORMULA] K) gas. The amount of cold (molecular) gas is probably less than 5% of the H I mass, supporting claims by Schombert et al. (1990) and de Blok & van der Hulst (1998b) who derive small molecular gas fractions.

  3. Low average SFR: Due to the fluctuations in the SFR in LSB galaxies and their large contrast with the average SFR, the spread in colors among LSB galaxies will be larger than among HSB galaxies. From the distribution of birthrate parameters in our simulations we deduce that, if the currently known blue gas-rich LSB galaxies are the most actively star forming LSB galaxies, they constitute over 80 percent of the total population of gas-rich field LSB disk galaxies. This implies that there is at most an additional 20 percent of quiescent, gas-rich LSB disk galaxies. This does not preclude the existence of an additional red, gas-poor population. However this population must have an evolutionary history quite different from that described in this work.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999