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

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1. Introduction

Deep surveys of the night-sky have uncovered a large population of disk galaxies with properties quite different from those of the extensively studied "normal" high surface brightness (HSB) galaxies. These so-called Low Surface Brightness (LSB) galaxies have, as their name already implies, surface brightnesses much lower than what was until quite recently assumed to be representative for disk galaxies (Freeman 1970).

The LSB galaxies we will be discussing are generally dominated by an exponential disk, with scale lengths of a few kpc. Morphologically they form an extension of the Hubble sequence towards very late-type galaxies. Observations suggest that LSB galaxies are unevolved galaxies, as shown by e.g. the sub-solar metallicities (McGaugh 1994) and the colors (McGaugh & Bothun 1994; de Blok et al. 1995; van den Hoek et al. 1997).

The evolutionary rate of a galaxy may in fact be reflected in its surface brightness. For example, the gas fraction ([FORMULA]) increases systematically with surface brightness, from a few percent for early type spirals to much higher values approaching unity for late type LSB galaxies (de Blok & McGaugh 1996). In many LSB galaxies the gas mass exceeds the stellar mass (even for such extreme assumptions as maximum disk).

It is still unclear what the physical driver is for the difference between HSB and LSB galaxies. Investigations of their dynamics, using H I observations (de Blok et al. 1996), suggest that LSB galaxies are low-density galaxies (de Blok & McGaugh 1996). This is one of the favored explanations for the low evolution rate of LSB galaxies (see e.g. van der Hulst et al. 1987), as this implies a large dynamical time-scale. Environment may also play a role. Tidal interactions with other galaxies increase star formation rates. This may not happen in LSB galaxies which are found to be isolated (Mo et al. 1994). In LSB galaxies a few star forming regions are usually found. These are distributed randomly over the galaxies and do not trace the spiral arms.

The main purpose of this paper is to investigate whether the low density alone can explain the properties of LSB galaxies. We use 3D numerical simulations to address this problem. The dynamical simulations include both stars and gas and incorporate a parameterized description of star formation, including feedback on the gas.

The prescription of star formation in numerical simulations is not straightforward, given the limited resolution of the models and the fragmentary knowledge about the physics governing star formation on kpc scales. In the literature various different star formation algorithms can be found. For instance, Friedli & Benz (1995) use a criterion based on the Toomre stability parameter Q (Toomre 1964) to study star formation in barred systems; Mihos & Hernquist (1994a,b, 1996) adopt a Schmidt law based on the gas density [star formation rate (SFR) [FORMULA]] to study mergers of galaxies; others require gas to be in a convergent and Jeans unstable flow to form stars (Katz 1992; Katz et al. 1996; Navarro & White 1993; Steinmetz 1996).

We employ the method of Gerritsen & Icke (1997, 1998), which uses a Jeans criterion to define star forming regions, coupled with an estimate of the cloud collapse time. In these simulations gas is treated fully radiative with allowed temperatures between 10 K and [FORMULA] K; cooling is described by standard cooling functions, heating is assumed to be provided by far-ultraviolet (FUV) radiation and mechanical heating from stars. In this way the simulated interstellar medium (ISM) mimics a multi-phase ISM (Field et al. 1969; McKee & Ostriker 1977). This is an improvement over other simulations which try to create a multi-phase ISM but do not allow radiative cooling below [FORMULA] K (e.g. Hernquist 1989; Katz et al. 1996). The multi-phase ISM allows us to restrict ourselves to considering only relatively cold ([FORMULA] K) regions as the sites for star formation.

Here we apply this method to study the ISM and star forming properties of LSB galaxies, and our results will be valid under the assumption that the physical processes governing star formation are the same for HSB and LSB galaxies. We test whether a low mass-density is sufficient to explain the properties of LSB galaxies by discussing two different models of a specific LSB galaxy: the first model has the cooling properties of a solar abundance ISM, while in the second model the cooling efficiency is lowered, thus mimicking a metal-poor ISM.

The structure of the paper is as follows. In Sect. 2 we describe the numerical techniques. The construction of a model LSB galaxy is presented in Sect. 3. This galaxy model is evolved for a few Gyr, using two different prescriptions for the cooling properties of the gas (Sect. 4). The implications of the simulations are discussed in Sect. 5. We conclude this paper with a summary in Sect. 6.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999