1999A&A...344..779B

We present a complete FIR spectral energy distribution of a normal spiral disk. A full grating scan, from 43-189µm, centered on the nucleus of NGC 4414 was made with the ISO Long Wavelength Spectrometer (LWS). The mid-IR to millimeter continuum of NGC 4414 can be modeled with thermal dust emission from a warm component at 69K and a cool component at 24.5K. At a distance of 9.6Mpc, the LWS beam samples only the central ≃4kpc, 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≲20K at r>2kpc). The emission we detect comes from the inner disk (4">r>40") rather than the nucleus, which is poor in ionized and neutral gas. Three important cooling lines of the ISM, CII(158µm), NII(122µm), and OI(63µm) are strong and a weak OIII(88µm) line is detected as well. OI(146µm) is weak or not detected. No detections were found for OIII(52µm) and NIII(57µm). With a spectral resolution of ∼1500km/s (FWHM) in grating-scan mode, LWS was unable to resolve the FIR line emission. The LWS observations provide a good dust temperature estimate; combining this with our previous data allows us to constrain the N(H₂)/I_CO ratio to the range 5-8x10¹⁹cm2/(K.km/s) in the inner disk of this normal spiral. We present a 10GHz-10THz (3cm to 30µm) spectrum of the line and continuum emission of the ISM in NGC 4414. To the extent that the frequency ranges 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 entire Galaxy but can study single objects such as HII regions or molecular clouds. The sum of the individual features identified by ISO in the Galaxy plus 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 starbursts such as Arp 220.