1 Introduction

Mechanisms for heating interstellar gas include collisions, radiation from stars, shocks, and cosmic rays. Examination of the spectral lines that cool the gas can help determine the dominant excitation mechanisms and conditions. Studies of particular regions in our Galaxy and observations of external galaxies have suggested that stellar ultraviolet radiation can ionize vast volumes of a galaxy and that the far-ultraviolet (FUV) radiation impinging on neutral cloud surfaces is responsible for a large fraction of the observed far-infrared (FIR) spectral line emission that cools the gas (Crawford et al. 1985; Stacey et al. 1991; Shibai et al. 1991). Tielens & Hollenbach (1985) defined photodissociation regions (PDRs) as "a neutral region where FUV radiation dominates the heating and/or some important aspect of the chemistry". Thus PDRs include most of the atomic gas in a galaxy, both in diffuse clouds and in dense regions (for a recent review, Hollenbach & Tielens 1997).

[C II]158 $\mu$m and [O I]63 $\mu$m lines are important coolants in PDRs, while gas heating in PDRs is thought to be dominated by energetic photoelectrons ejected from dust grains following FUV photon absorption (Watson 1972; de Jong 1977). For galactic nuclei and star-forming regions in the spiral arms, most of the observed [C II] line emission arises from PDRs on molecular cloud surfaces. However, integrated over the disks of spiral galaxies, a substantial fraction may also arise from "standard" atomic clouds, i.e., the cold neutral medium gas regions (CNM) (Madden et al. 1993; Bennett et al. 1994) or from extended low-density warm ionized gas regions (ELDWIM) (Heiles 1994). Contributions from the various gas phases can be estimated by observations of several FIR forbidden lines (Carral et al. 1994; Luhman et al. 1998).

With the LWS on board the ISO (Clegg et al. 1996) it now becomes possible to measure far-infrared lines not only from infrared-bright galaxies but also from normal galaxies (for latest reviews, Genzel & Cesarsky 2000; Fischer 2000). Lord et al. (1996) made observations of several FIR fine-structure forbidden lines in a normal galaxy NGC 5713 and found a fairly high [C II]158 $\mu$m to the FIR intensity ratio. Smith & Madden (1997) made observations of [N II]122 $\mu$m and [C II]158 $\mu$m lines for five spiral galaxies in the Virgo cluster and found enhanced ratios of [C II] to CO(J=1-0) intensities in two of the five galaxies. They interpreted the results in terms of either low metallicity or an increase in the contribution from the CNM. Braine & Hughes (1999) obtained a complete FIR spectrum of a normal disk galaxy NGC 4414 and investigated the physical conditions of the interstellar medium. Leech et al. (1999) presented observations of [C II]158 $\mu$m in 19 quiescent galaxies in the Virgo cluster. They indicated a good correlation between [C II] and far-infrared intensities and a trend of increasing [C II]-to-far-infrared intensity ratio with increasing galaxy lateness, which has been shown to be related to the star-formation rate (Pierini et al. 1999). Malhotra et al. (2001a) reported observations of 4 early-type galaxies with ISO/LWS and interpreted the observed low ratio of [C II]158 $\mu$m to far-infrared intensities in terms of the soft radiation field in the target galaxies. Hunter et al. (2001) presented observations of 5 irregular galaxies with ISO/LWS and found strong [C II]158 $\mu$m emission relative to the far-infrared continuum.

Malhotra et al. (1997, 2001b) have investigated the far-infrared properties of 60 nearby normal galaxies based on line-spectroscopic observations mainly of [C II]158 $\mu$m and [O I]63 $\mu$m line emissions. They complemented their line data with IRAS photometry to estimate the far-infrared continuum intensity FIR (Helou et al. 1988) and found a trend that the ratio of the [C II] line intensity to FIR decreases with the far-infrared color becoming bluer. Several interpretations have been proposed for the trend (Malhotra et al. 1997; Genzel & Cesarsky 2000; Helou et al. 2001). Malhotra et al. (2001b) favor the interpretation of the decrease in the photoelectron yield owing to the increase in positive charges of dust grains under strong ultraviolet radiation.

The FIR continuum emission shorter than 60 $\mu$m is dominated by the emission from stochastically heated very small grains (Desert et al. 1986; Dwek et al. 1997; Onaka 2000; Dale et al. 2001; Li & Draine 2001). The spectral energy distribution (SED) longer than 100 $\mu$m is crucial to correctly estimate the thermal emission from submicron dust grains and understand the FIR SED of galaxies. In this paper we investigate the FIR properties of nearby galaxies based on LWS full grating spectra from 43 to 197 $\mu$m including both line and continuum emission. The continuum spectra longer than 100 $\mu$m enable better estimates of the average temperature of submicron dust grains as well as the strength of the interstellar radiation field. Because the aperture size of LWS is large ($\sim$80 $^{\prime\prime}$), the spectra of galaxies include contributions from various interstellar regions within the galaxies. This paper investigates the mechanism of gas heating with the aim of a better understanding of the global physical conditions of the interstellar medium in galaxies.