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Issue A&A
Volume 421, Number 1, July I 2004
Page(s) 115 - 127
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20035679



A&A 421, 115-127 (2004)
DOI: 10.1051/0004-6361:20035679

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

O. R. Prouton1, A. Bressan2, 3, M. Clemens1, A. Franceschini1, G. L. Granato2, 3 and L. Silva4

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

(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  $1~\rm\mu m$.

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  $\mu$m) 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.


Key words: ISM: dust, extinction -- galaxies: stellar content -- infrared: galaxies -- radio continuum: galaxies

Offprint request: A. Bressan, bressan@pd.astro.it

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© ESO 2004


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