The 10 GHz 10 THz spectrum of a normal spiral galaxy

We present a complete FIR spectral energy distribution of a normal spiral d isk. A full grating scan, from 43-189 mu m, centered on the nucleus of NGC 4414 was made with the ISO Long Wavelength Spectrometer (LWS). The mid-IR t o millimeter continuum of NGC 4414 can be modeled with thermal dust emissio n from a warm component at 69 K and a cool component at 24.5 K. At a distan ce of 9.6 Mpc, the LWS beam samples only the central approximate to 4 kpc, which includes most of the FIR-bright disk but excludes dust emission from a cooler phase of the ISM in the outer regions of the galaxy (e.g. T less t han or similar to 20 K at r > 2 kpc). The emission we detect comes from the inner disk (4 " greater than or similar to r greater than or similar to 40 ") rather than the nucleus, which is poor in ionized and neutral gas. Thre e important cooling lines of the ISM, CII(158 mu m), NII(122 mu m), and OI( 63 mu m) are strong and a weak OIII(88 mu m) line is detected as well. OI(1 46 mu m) is weak or not detected. No detections were found for OIII(52 mu m ) and NIII(57 mu m). With a spectral resolution of similar to 1500 km s(-1) (FWHM) in grating-scan mode, LWS was unable to resolve the FIR line emissi on. The LWS observations provide a good dust temperature estimate; combinin g this with our previous data allows us to constrain the N(H-2)/I-CO ratio to the range 5-8 x 10(19) cm(-2) (K km s(-1))(-1) in the inner disk of this normal spiral. We present a 10 GHz-10 THz (3 cm to 30 mu m) spectrum of the line and conti nuum emission of the ISM in NGC 4414. To the extent that the frequency rang es overlap, the FIR continuum and spectral line properties of NGC 4414 are similar to the COBE data for the Galaxy. The ISO LWS cannot survey the enti re Galaxy but can study single objects such as HII regions or molecular clo uds. The sum of the individual features identified by ISO in the Galaxy plu s the diffuse gas should yield a spectrum similar to the template presented here. In contrast, the spectrum of NGC 4414 is very different from that of early starbursts such as NGC 4038/9 and Arp 299 and from more evolved star bursts such as Arp 220.