Astron. Astrophys. 344, 779-786 (1999)

1. Introduction

Until recently, identifying the important cooling lines in the interstellar medium (ISM) of galaxies has been restricted, for reasons of sensitivity, to luminous starburst galaxies (e.g. Carral et al. 1994). Furthermore, complete spectral scans are not possible even from airborne observatories due to the Earth's atmosphere. The Infrared Space Observatory Long Wavelength Spectrometer ¹ (ISO LWS; Clegg et al. 1996) gives us the opportunity to measure the FIR emission of weak sources such as ordinary galaxies from 40 to 190 microns with no breaks. Theoretical modelling and observations of the Milky Way and other galaxies (Crawford et al. 1985; Tielens & Hollenbach 1985a; Bennett et al. 1994) show that the strongest cooling lines of the ISM are not the CO lines that dominate the millimeter emission, although these are important for the cool, dense ISM, but lines in the FIR, particularly the CII line at 158 µm.

Observations of the Galaxy provide unique information about individual sources such as HII regions, Photo-dissociation regions (PDRs), molecular clouds,... but developing a face-on view of the Galaxy or even calculating total luminosities from our in-plane observing site is quite complicated. Using COBE data, Wright et al. (1991) fit the FIRAS observations to a 3 component model of the galaxy defined as a central gaussian peak, an exponential disk, and a "molecular" ring with a gaussian radial profile. Bloemen et al. (1990) fit the IRAS 60 and 100 µm to the HI and CO surveys of the Galaxy; they use the HI/CO velocities (assuming rotation curve and circular rotation) to derive distances, average over radial bins a few kpc in size, and assume that the dust emission can be expressed as a linear combination of the HI and CO emission. Sodroski et al. (1994) represent the COBE DIRBE 140 and 240 µm emission from the galactic plane as a linear combination of the HI, CO, and 5 GHz continuum emission to derive FIR emissivities for the molecular, atomic, and ionized gas; these are then multiplied by previous estimates of the mass of each gas component to derive the total FIR luminosity. It is in fact much more difficult to derive global emission characteristics for the Galaxy than for local spirals, where all components are at the same distance and no assumptions about the distribution are necessary.

The FIR NII (122 and 205 µm), CII (158 µm), and possibly OI(145 µm) lines were detected in the Galaxy by the COBE FIRAS instrument (Wright et al. 1991; Bennett et al. 1994). Other FIR observations of the Galaxy pinpoint specific regions such as Orion or the Galactic Center with the exception of the unbiased LWS parallel mode observations which detected the OI (63 µm) and CII lines with a preliminary ratio of CII/OI few (Caux & Gry 1997).

Table 1. Basic information about NGC 4414. is calculated by integrating the two-temperature fit to the ISO LWS spectrum. H-magnitude (total, not ) estimated from Condon et al. (1987)

We selected NGC 4414 as a good representative of a "normal" galaxy whose emission averaged over the ISO LWS beam should be representative of the inner disk or molecular ring of a spiral such as our own. It is a relatively isolated Sc galaxy near the North Galactic Pole (b=83^o). The highly extended ¹²CO, ¹³CO, and HI emission (Braine et al. 1997 - hereafter Paper II) shows that no recent interaction with another galaxy has occurred. Furthermore, very little ionized or neutral gas is present in the nucleus (Pogge 1989; Braine et al. 1993 - hereafter Paper I; Sakamoto 1996; Braine et al. in prep.), enabling us to measure the emission of a quiescent galactic disk with minimal contribution from any nuclear emission.

The properties of the neutral ISM in galactic nuclei are different - denser, warmer, non-negligible tidal shear - from those in galactic disks. Given the poor spatial resolution of ISO in the FIR ( kpc at a distance of 10 Mpc), the LWS beam cannot separate the nuclear and disk emission except for the very closest spiral galaxies. However, due to the absence of CO, HI, or H emission in the center of NGC 4414, the FIR emission we detect comes from the disk. We are thus dealing with a simpler system than, say, observations of the centers of NGC 891 or NGC 6946 where the nuclear ISM emits strongly and cannot be separated from disk emission.

At the assumed distance of 9.6 Mpc ( Mpc^-1), in keeping with our previous work, the ISO LWS beam ( FWHM) corresponds to a galactocentric radius of 2 kpc and closely matches the CO-bright part of the disk of NGC 4414 (see Fig. 2 in Paper I). The most logical comparison is thus with the molecular ring, excluding the nucleus, in the Milky Way. The gas surface density (i.e. CO brightness) in this part of NGC 4414 is several times that of the molecular ring of the Galaxy but lower than that of the disk of M 51 or mild/moderate starbursts and orders of magnitude below that of Ultra-luminous IR galaxies. The LWS observations presented here complete the radio-optical spectral energy distribution for this galaxy.

Pre-ISO FIR spectroscopy of galaxies was limited to the CII line except for the local starbursts M 82 and NGC 253 and the IR-luminous merger NGC 3256. ISO LWS spectral scans have been presented for NGC 4038/9 (Fischer et al. 1996), Arp 299 (Satyapal et al. 1998), Circinus (Genzel 1997), Arp 220 (Fischer et al. 1997), and the major lines were observed in NGC 5713 by Lord et al. (1996a). All of these galaxies are more actively forming stars than NGC 4414. Furthermore, the emission cannot be interpreted as coming from a more-or-less quiescent disk due to the nuclear or starburst components. The other (unpublished) LWS spectral scans of galactic disks were made with much lower integration times (factor 3-8) than what we present here.

The goal of the LWS observations of NGC 4414 is to derive cooling rates and dust temperatures, thereby providing a high-quality template for the FIR emission of the ISM, integrated over several kpcs, of a "normal" spiral disk.

© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999
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