Far-infrared wavelengths provide the opportunity to observe dust-enshrouded galaxies without large extinction effects and offer many diagnostics of the physical conditions in the interstellar medium of these galaxies. The Kuiper Airborne Observatory provided early data on the far-infrared fine structure lines that arise in photodissociation regions (PDRs) and H2 regions in galaxies. With the launch of the Infrared Space Observatory (ISO; Kessler et al. 1996; Kessler et al. 2003) the far-infrared properties of galaxies were observed with greater sensitivity than ever before. The Long Wavelength Spectrometer (LWS; Clegg et al. 1996; Gry et al. 2003) on ISO allowed the large-scale study of far-infrared atomic and molecular lines that supply new insight into the understanding of the interstellar medium of these sources. Our next opportunities for far-infrared spectroscopic studies of galaxies will come with the Stratospheric Observatory for Infrared Astronomy and the Herschel Space Observatory.
The LWS data presented in this paper were taken from the ISO archive. 3 A variety of extragalactic observing programs used the LWS to obtain spectra of the primary diagnostic lines of the interstellar medium in the far-infrared. These lines include [O III] 52 µm, [N III] 57 µm, [O I] 63 µm, [O III] 88 µm, [N II] 122 µm, [O I] 145 µm, and [C II] 158 µm. Among these atomic and ionic fine structure lines, [C II] 158 µm and [O I] 63 µm are the dominant cooling lines for neutral interstellar gas. Observations of [C II] 158 µm in NGC 6946 (Madden et al. 1993; Contursi et al. 2002) suggest that a significant fraction of the [C II] 158 µm emission might also originate in diffuse ionized gas in some galaxies, while the far-infrared emission lines from ionized species ([O III] 52 µm, [N III] 57 µm, [O III] 88 µm, and [N II] 122 µm) predominantly originate in H2 regions (see also Sauvage, Tuffs, & Popescu 2005). Combined with models of PDRs and H2 regions (e.g., Tielens & Hollenbach 1985; Rubin 1985; Wolfire, Tielens & Hollenbach 1990; Hollenbach, Takahashi, & Tielens 1991; Spinoglio & Malkan 1992; Rubin et al. 1994; Kaufman et al. 1999; Abel et al. 2005; Le Petit et al. 2006; Meijerink, Spaans, Israel 2007; Groves et al. 2008), these fine structure transitions can be used to derive gas temperatures, densities, and the intensity of the radiation fields in galaxies. The LWS was also used to observe a suite of molecular lines in galaxies (Fischer et al. 1999) including hydroxyl (OH; 53 µm, 65 µm, 79 µm, 84 µm, 119 µm, 163 µm), water (H2O; 59 µm, 67 µm, 75 µm, 101 µm, 108 µm), and the LWS range contains a plethora of high level rotational lines of carbon monoxide (Varberg & Evenson 1992). From the detections of multiple transitions of these molecules, the column densities and abundances for OH, H2O, and CO can be determined (e.g., Skinner et al. 1997; González-Alfonso et al. 2004; Spinoglio et al. 2005; González-Alfonso et al. 2008).
This contribution reports on LWS observations of seven far-infrared fine structure atomic and ionic lines, far-infrared lines from three molecular species, and the far-infrared continuum of 227 galaxies, in addition to serendipitous detections of Milky Way [C II] 158 µm emission. The collection of far-infrared line fluxes in this paper comprise the largest sample ever assembled and reduced in a uniform manner. These line fluxes are used to compare the relationship of the far-infrared fine structure lines, normalized to either another far-infrared line or the far-infrared continuum level, to two indicators of star formation activity: the 60 µm / 100 µm ratio and far-infrared-to-B ratio. The properties of these emission lines are compared to findings from previous LWS emission line studies (Malhotra et al. 1997, 2001; Leech et al. 1999; Fischer et al. 1999; Luhman et al. 1998; Negishi et al. 2001; Luhman et al. 2003). The LWS continuum fluxes derived in this work are compared to IRAS 60 µm and 100 µm fluxes, ISOPHOT 170 µm fluxes (Stickel et al. 2000), and infrared spectral energy distribution models for normal star-forming galaxies (Dale & Helou 2002).
These data can form an important framework for studies of global extragalactic interstellar media including the derivation of average gas temperatures, densities, abundances, and radiation fields integrated over entire galaxy systems. The line and continuum fluxes presented here can also supply the data for studies of the individual components (H2 regions, spiral arms, disk regions) of large galaxies resolved by the LWS. Contursi et al. (2002), for example, examine the physical conditions of these different galaxy components in NGC 1313 and NGC 6946 using PDR models (Kaufman et al. 1999), and Johnson et al. (in preparation) explore the relationships between the far- and mid-infrared cooling lines observed respectively by ISO and the Spitzer Space Telescope. LWS studies of individual galaxies have also been carried out for NGC 4038/4039, M 82, NGC 253, Cen A, NGC 1068, Arp 220, and Mrk 231 (Fischer et al. 1996; Colbert et al. 1999; Unger et al. 2000; Bradford et al. 1999; González-Alfonso et al. 2004; Spinoglio et al. 2005; González-Alfonso et al. 2008).
Section 2 describes the sample of galaxies, while Section 3 describes the observations and data analysis. In Section 4, the far-infrared continuum data are presented and assessed from comparisons to IRAS 60 and 100 µm data, ISOPHOT 170 µm data, and galaxy infrared spectral energy distribution models. The far-infrared line data and properties are presented in Section 5. In Section 6, the statistical trends seen in the line data are described and these trends are related to those found from previous studies. A summary of the main results is given in Section 7. The Appendix provides a description of the extended source correction and how it may be applied to the line and continuum fluxes for sources that are extended compared to the ~ 75" LWS aperture.
3 http://www.iso.vilspa.esa.es/ida Back.