2. THE SAMPLE AND DATA

The ISO LWS sample of galaxies selected from the ISO archive for this paper is presented in Table 1 and lists the galaxy positions, recession velocities, morphologies, optical sizes, and the flux densities of these galaxies in the four IRAS bands along with the IRAS 60 µm / 100 µm ratios. The positions, optical sizes, and velocities were taken from the NASA Extragalactic Database (NED) in mid-2004.

The sample includes both normal and Seyfert galaxies that were initially selected by identifying galaxies in the IRAS Cataloged Galaxies and Quasars Observed in the IRAS Survey (CGQ; Fullmer & Lonsdale 1989). The galaxies identified from the CGQ range in 60 µm and 100 µm flux density from 1 to 1300 Jy. The ISO Data Archive was queried using this list, from which 198 galaxies were observed in the LWS L01 "spectral range" or L02 "line" observing modes. Later, galaxies with IRAS fluxes less than 1 Jy or those with no cataloged IRAS flux were added to the sample in order to enlarge the sample. With these considerations, another 29 galaxies were identified within the ISO Archive. Photometric mode L02 observations in which the grating remained in a fixed position are excluded from this sample. The large, nearby galaxies M 31 and the Small and Large Magellanic Clouds are excluded from this sample because the size of these three galaxies is over 100 times larger than the LWS beam.

Among these 227 galaxies there are two distinct subsets, distinguished by the far-infrared size of the galaxy. The 181 galaxies in the first subset are unresolved in the far-infrared with respect to the ~ 75" LWS beam. This unresolved subset of galaxies is an extension of the combined sets of smaller samples observed with the LWS (Malhotra et al. 2001; Pierini et al. 1999; Luhman et al. 2003; Negishi et al. 2001) with additional sources added from the ISO Data Archive. The second subset consists of 46 galaxies resolved by the LWS beam in the far-infrared. The data from this resolved subset of galaxies can be used to complement past studies (Stacey et al. 1991; Madden et al. 1993, 1997) of large galaxies with data taken from the Kuiper Airborne Observatory and ISO. The resolved galaxies are denoted as such in Table 1. The IRAS flux densities presented in Table 1 are selected from either Rice et al. (1988) or Dale et al. (2000) for the large, nearby galaxies in the resolved subset. For the unresolved subset of galaxies, the IRAS fluxes in Table 1 are taken from SCANPI co-additions of the IRAS survey scans.

The galaxies in this sample are distributed across the entire sky. Figure 1 displays the galaxy distribution in Galactic coordinates. The clump of galaxies at (l, b) ~ (280°, 74°) is the Virgo Cluster. Approximately 40 galaxies lie within the Zone of Avoidance, where |b| < 20°. Although there may be some serendipitous Galactic line and continuum emission in all directions of the sky, this serendipitous contamination is more likely toward galaxies within the Zone of Avoidance in either the galaxy spectra or the spectra of an off-source position taken during these observations. This contamination of the observed line measurements by Milky Way emission is a concern, and a discussion of detected Galactic emission lines is found in Section 5.

Figure 1. Aitoff projection of the galaxies selected for this sample. The galaxies in this sample are distributed across the sky. Galaxies unresolved by the LWS in the far-infrared are displayed with crosses. Resolved galaxies by the LWS in the far-infrared are shown with open squares. The clump of galaxies at RA, Dec (70°, 280°) are members of the Virgo Cluster.

The Palomar Observatory Sky Survey plates and other observations found within NED were used to reexamine the optical morphology for each galaxy (Harold Corwin, private communication). In Figure 2, the distribution of the optical morphological types for the two subsets is shown. Both the resolved and unresolved subsets span the range of early- to late-type galaxies. The unresolved subset contains a relatively large number of S0 galaxies while the resolved subset contains no elliptical, S0/a, or peculiar galaxies.

Figure 2. The distributions of the optical morphologies of the resolved and unresolved subsets of galaxies. The resolved subset is cross-shaded.

Most of the galaxies in both the unresolved and resolved subsets are relatively nearby, and Figure 3 shows the redshift distribution for both subsets. All galaxies in the resolved subset have an absolute redshift less than 2000 km s^-1. With the exception of a large bin (22/181) of galaxies with redshifts greater than 10000 km s^-1, most galaxies in the unresolved subset have redshifts less than 6000 km s^-1.

Figure 3. The distributions of redshifts for the resolved and unresolved subsets. The resolved galaxy redshifts are cross-shaded.

Figure 4 shows the distribution of measured flux densities at 60 µm and 100 µm for the resolved and unresolved galaxies in this contribution, with IRAS detections taken from Table 1. At 60 µm, the resolved galaxies span a more elevated flux density range (~ 1-1300 Jy) than the unresolved galaxies (~ 0.2-150 Jy), with the median flux density for unresolved galaxies smaller by a factor of roughly 4.5. At 100 µm, the unresolved and resolved galaxy subsets have similar distributions over flux density, covering the range ~ 0.5 to 1000 Jy with the median flux density of unresolved galaxies smaller by only a factor of 1.6.

Figure 4. The distributions of IRAS 60 µm and 100 µm flux densities. The resolved galaxy subset is cross-shaded.

The distributions of the 60 µm / 100 µm ratio and far-infrared flux are displayed in Figure 5. The 60 µm / 100 µm ratio is an indicator of the typical heating intensity of dust in galaxies and may also suggest the relative star formation activity level of a galaxy. Lower 60 µm / 100 µm ratios typically correspond to quiescent galaxies, whereas higher 60 µm / 100 µm ratios indicate either a higher rate of star formation or perhaps the presence of an AGN (Helou 1986). The far-infrared flux is defined as FIR = 1.26 × 10^-14 [2.58 f(60) + f(100)] W m^-2, where f(60 µm) and f(100 µm) are the 60 µm and 100 µm IRAS flux densities in Jy (Helou et al. 1988). The unresolved and resolved galaxy subsets peak near 60 µm / 100 µm ratios of 0.5 and 0.4, respectively, and the resolved subset does not contain many warm galaxies with 60 µm / 100 µm ratios greater than 0.7. It is not surprising that resolved galaxies are on average closer and more quiescent than unresolved galaxies. The larger subset of unresolved (~ distant) systems should include more galaxies exhibiting extreme luminosities and activity levels. The distribution of FIR values for the two subsets spans five orders of magnitude with a peak between 10^-12 and 10^-13 W m^-2. The resolved galaxies reach FIR values as large as 10^-10 W m^-2, an order of magnitude larger than the brightest unresolved galaxies. In Figure 6, the IRAS 12 µm / 25 µm ratio is plotted against the 60 µm/100 µm ratio for the resolved and unresolved subsets. The sequence of infrared colors in Figure 6 is associated with a sequence of star formation activity in galaxies (Helou 1986) and dust-heating intensity (Boulanger et al. 1988), with the upper left populated by quiescent galaxies and the lower right by warmer, more actively star-forming galaxies.

Figure 5. The distributions of IRAS 60 µm / 100 µm ratio and FIR. The resolved galaxy subset is cross-shaded.

Figure 6. The IRAS color-color diagram for the galaxies in this sample. Quiescent galaxies are located towards the upper left and actively star-forming galaxies towards the lower right. Filled circles are galaxies in the unresolved subset. Open circles are galaxies in the resolved subset.