The spectral energy distribution of normal, starburst, and active galaxies

Abstract

We present the results of an extensive literature search of multiwavelength data for a sample of 59 galaxies, consisting of 26 Starbursts, 15 Seyfert 2's, 5 LINER's, 6 normal spirals, and 7 normal elliptical galaxies. The data include soft X-ray fluxes, ultraviolet, and optical spectra, near-, mid/far-infrared photometry, and radio measurements, selected to match as closely as possible the /UE aperture ( 10" X 20") . The galaxies are separated into 6 groups with similar characteristics, namely, ellipticals, spirals, LINER's, Seyfert 2's, starbursts of low and high reddening, for which we create average spectral energy distributions (SEDs). The individual groups SEDs are normalized to the À 7000 Á flux and compared, looking for similarities and differences among them. We find that the SEDs of normal spirals and ellipticals are very similar over the entire energy range, and fainter than those of all other groups. LINER's SEDs are similar to those of Seyfert 2's and Starbursts only in the visual to near-IR waveband, being fainter in the remaining wavebands. Seyfert 2's are similar to Starbursts in the radio to near-IR waveband, fainter in the visual to ultraviolet, but stronger in the X-rays. Low and high reddening Starbursts are similar along the entire SED, differing in the ultraviolet, where Low reddening Starbursts are stronger, and in the mid/far-IR where they are fainter. We have also collected multiwavelength data for 4 H n regions, a thermal supernova remnant, and a non-thermal supernova remnant (SNR), which are compared with the Starburst SEDs. The H n regions and thermal SNRs have similar SEDs, differing only in the X-ray and far infrared. The non-thermal SNR SED is a fiat continuum, different from all the other SEDs. Comparing the SEDs of Starbursts and H n regions we find that they are similar in the mid/far-IR parts of the spectrum, but H n regions are fainter in the radio and X-rays. Starbursts are also stronger than H n regions in the visual and near-IR parts of the spectrum, due to the contribution from old stars to starbursts. The bolometric fluxes of the different types of galaxies are calculated integrating their SEDs. These values are compared with individual waveband fiux densities, in order to determine the wavebands which contribute most to the bolometric flux. In Seyfert 2's, LINER's, and starbursts, the mid/far-IR emission are the most important contributers to the bolometric flux, while in normal spirals and ellipticals this flux is dorninated by the near-IR and visual wavebands. Linear regressions were performed between the bolometric and individual band fiuxes for each kind of galaxy. These fits can be used in the calculation of the bolometric flux for other objects of similar activity type, but with reduced waveband information.