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

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1. Introduction

When studying the properties and evolution of galaxies it is necessary to have a proper census of all types of galaxies. Deep photographic surveys have shown the existence of a large number of galaxies with surface brightnesses much fainter than the night sky. The severe selection effects caused by the brightness of the night sky ensures that these galaxies are very much under-represented in conventional galaxy catalogs. However, in the last 20 years it has become clear that these low surface brightness (LSB) galaxies may constitute a major fraction of the total galaxy population. Clearly, LSB galaxies show us an alternative path of galaxy evolution which does not lead to the classical Hubble sequence and offer us a new window onto galaxy evolution (Impey & Bothun 1997; Bothun et al. 1997).

Most of the LSB galaxies investigated in any detail are either late-type and disk dominated (de Blok et al. 1995, hereafter dB95; McGaugh & Bothun 1994), or giant, Malin-1-like galaxies (Sprayberry et al. 1995; Pickering et al. 1997). The disk dominated LSB galaxies have only small traces of star formation (de Blok et al. 1996a), are very gas-rich (de Blok et al. 1996b), and appear quite unevolved (van der Hulst et al. 1993). The HI masses are a few times [FORMULA] (de Blok et al. 1996b) and the HI surface densities are usually close to the critical density for star formation (van der Hulst et al. 1993; de Blok et al. 1996b; Kennicutt 1989). They also have color gradients, the outer parts of the disks are bluer than the inner parts. Apart from this the LSB galaxies investigated by de Blok, McGaugh and van der Hulst tend to be bluer than classical Hubble sequence HSB galaxies. There has been some discussion on whether this was an intrinsic property of LSB galaxies or yet another selection effect, due to the blue-sensitive plates used for the LSB surveys. The discovery of a large number of red LSB galaxies (O'Neil et al. 1997) showed that there is also a red (B-V [FORMULA] 0.9) component of the LSB population. The continuous range of colors from the very blue to the very red clearly shows that LSB galaxies define a wide range of evolutionary states.

The fact that most LSB galaxies discovered so far are either bulgeless, late-type "normal-sized" galaxies or giant galaxies with a significant bulge component, raises the question whether there are any "normal-sized", bulge dominated LSB galaxies. Are there LSB galaxies with disks with scale lengths of a few kpc and a low surface brightness that have a significant bulge component?

If these galaxies do indeed exist then the fact that they are not turning up in LSB galaxy surveys can only mean that there are severe selection effects against them, or that we already found them and included them in our catalogs.

Because of their obvious bulge components it is hard to see which of the known selection effects could make us very much biased against them. We have therefore assumed that at least a fraction of the bulge dominated LSB galaxies have already been included in conventional catalogs, and have selected a small sample of what we can call bulge dominated LSB galaxies from the ESO-LV catalog (Lauberts & Valentijn 1989).

In the universe galaxies have surface brightnesses in a continuous range from low to high surface brightness, the low end being set by instrument sensitivity. Normally a LSB galaxy is defined to be a galaxy with a face-on disk central surface brightness more than one magnitude fainter than the Freeman (1970) value of [FORMULA] B mag arcsec-2. In the presence of a bulge the true central surface brightness of the galaxy will of course be much higher and it would be hard to call them LSB for that reason. However, that is not the point here. Bulge dominated galaxies can still have a LSB disk , and in that sense the central disk surface brightness is used as an indicator for the amount of evolution in the disk. For late-type galaxies one can roughly equate (for undisturbed galaxies) the central disk surface brightness with evolutionary stage. The possibility of having a bulge sitting in the middle of a LSB, possibly unevolved, disk raises interesting questions as how can an evolved component such as a bulge exist in an unevolved disk without affecting its evolution? Do bulges and disks evolve independently in LSB galaxies? Are bulge and disk surface brightnesses and sizes related? Galaxies towards the later types have a range in surface brightness and size. The early-type galaxies are mostly HSB or giant LSB galaxies. Do bulge dominated LSB galaxies form the "missing link" between HSB and giant LSB galaxies?

To explore some of these questions we selected and observed a sample of 20 LSB galaxies with bulges. This paper is organized in the following way: in Sect. 2 we discuss the sample selection and reduction. The structural parameters, central surface brightness and scale length, will be discussed in Sect. 3.1 and magnitudes in Sect. 3.2. The colors and color profiles are described in Sect. 3.3. These properties are then discussed and compared to the properties of disk dominated LSB and HSB galaxies in Sects. 4.1 and 4.2. A detailed look at the bulges is taken in Sect. 4.3. We will summarize the discussion and give concluding remarks in Sect. 5. We define a LSB galaxy to be a galaxy with a face-on disk central surface brightness one magnitude fainter than the Freeman value. In this paper we use [FORMULA] = 75 km s-1 Mpc-1.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999