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Astron. Astrophys. 351, 903-919 (1999)

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5. Summary

We have presented BVRI imaging of a sample of LSB galaxies with bulges. We find that these objects are well described as exponential disks with exponential bulges. The disk and bulge scale lengths of the galaxies have no preferred value. The central surface brightness does not depend on scale length or color, excluding fading scenarios. Despite the median central surface brightness of [FORMULA] = 23.04 mag arcsec-2 the median scale length ([FORMULA] = 12.6 kpc) is very large. The largest galaxies have sizes and luminosities comparable to giant LSB galaxies.

By decomposing the luminosity profiles using double exponentials we find that the disk scale lengths are correlated with the bulge scale lengths. This suggest a coupling in the formation of bulge and disk.

The bulge dominated LSB galaxies are observed to be redder than disk dominated LSB galaxies. On average the colors are still bluer than the colors of typical Freeman galaxies. The B/D ratios are observed to increase towards the longer wavelengths indicating that the bulges of LSB galaxies are redder than their disks.

The ratios between bulge and disk scale lengths scatter around [FORMULA] and the increase in B/D ratios towards longer wavelengths is consistent with observations of HSB spirals (dJ95). From the images shown in the appendix it is apparent that LSB galaxies exist in a wide range of morphological types. There seem to be roughly two types of bulge LSB galaxies in our sample. There are the ones with normal (round) bulges and the ones that look like they are undergoing some heavy evolution with blue rings and bars. But besides appearance no systematic differences show up in the rest of their investigated properties.

It remains unclear whether the bulge formed before or after the disk. More observations are needed to resolve this issue. We did not find any systematic differences in structural parameters or colors of the bulges between LSB and HSB galaxies.

To unravel the evolutionary history of bulge dominated LSB galaxies we need additional information on HI content and distribution, and chemical abundance information. Our sample has extended the fact that LSB galaxies cover a wide range in color, luminosity, size and morphology. The bulge dominated LSB galaxies fit in with the general trends defined by their HSB counterparts.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999