Modelling the Spectral Energy Distribution of compact luminous infrared galaxies: Constraints from high frequency radio data

¹ Dipartimento di Astronomia, Università degli Studi di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
² INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
³ SISSA, Strada Costiera, 34131 Trieste, Italy
⁴ INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy

Corresponding author: A. Bressan, bressan@pd.astro.it

Received: 14 November 2003
Accepted: 16 March 2004

Abstract

We have performed 23 GHz VLA observations of 7 compact, luminous infrared galaxies, selected as showing evidence of starburst activity. New and published multi-frequency data are combined to obtain the spectral energy distributions of all 7 galaxies from the near-infrared to the radio (at 1.4 GHz). These SEDs are compared with new models, for dust enshrouded galaxies, which account for both starburst and AGN components.
In all 7 galaxies the starburst provides the dominant contribution to the infrared luminosity; in 4 sources no contribution from an AGN is required. Although AGN may contribute up to 50 percent of the total far-infrared emission, the starbursts always dominate in the radio. The SEDs of most of our sources are best fit with a very high optical depth of ~50 at .
The scatter in the far-infrared/radio correlation, found among luminous IRAS sources, is due mainly to the different evolutionary status of their starburst components. The short time-scale of the star formation process amplifies the delay between the far-infrared and radio emission. This becomes more evident at low radio frequencies (below about 1 GHz) where synchrotron radiation is the dominant process. In the far-infrared (at wavelengths shorter than 100 μm) an additional source of scatter is provided by AGN, when present. AGN may be detected in the near-infrared by the absence of the knee, typical of stellar photospheres. However, near-infrared data alone cannot constrain the level at which AGN contribute because the interpretation of their observed properties, in this wave-band, depends strongly on model parameters.