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Issue A&A
Volume 434, Number 1, April IV 2005
Page(s) 149 - 161
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20035831



A&A 434, 149-161 (2005)
DOI: 10.1051/0004-6361:20035831

NIR spectroscopy of luminous infrared galaxies and the hydrogen recombination photon deficit

J. R. Valdés1, 2, S. Berta3, A. Bressan1, A. Franceschini3, D. Rigopoulou4 and G. Rodighiero3

1  INAF, Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
    e-mail: [valdes;bressan]@pd.astro.it
2  Instituto Nacional de Astrofísica, Optica y Electrónica, Apdos. Postales 51 y 216, CP 72000 Puebla, Pue., México
    e-mail: jvaldes@inaoep.mx
3  Dipartimento di Astronomia, vicolo dell'Osservatorio 5, 35122 Padova, Italy
    e-mail: [berta;rodighiero;franceschini]@pd.astro.it
4  Department of Physics (Astrophysics), University of Oxford, Denys Wilkinson Building, 1 Keble Road, Oxford OX1 3RH, UK
    e-mail: dar@astro.ox.ac.uk

(Received 10 December 2003 / Accepted 3 January 2005 )

Abstract
We report on near-infrared medium-resolution spectroscopy of a sample of luminous and ultraluminous infrared galaxies (LIRGs-ULIRGs), carried out with SOFI at the ESO 3.5 m New Technology Telescope. Because of wavelength dependence of the attenuation, the detection of the Pa$\alpha$ or Br$\gamma$ line in the Ks band should provide relevant constraints on SFR and the contribution of an AGN.

We find, however, that the intensities of the Pa$\alpha$ and Br$\gamma$ lines, even corrected for slit losses, are on average only 10% and 40%, respectively, of that expected from a normal starburst of similar bolometric luminosity. The corresponding star formation rates, after correcting for the attenuation derived from the NIR-optical emission line ratios, are 14% and 60% of that expected if the far infrared luminosity were entirely powered by the starburst.

This confirms the existence of a recombination photon deficit, particularly in the case of the Pa$\alpha$ line, already found in the Br$\gamma$ line in other infrared galaxies of similar luminosity.

In discussing the possible causes of the discrepancy, we find unlikely that it is due to the presence of an AGN, though two objects show evidence of broadening of the Pa$\alpha$ line and of the presence of coronal line emission. In fact, from our own observations and data collected from the literature we argue that the studied galaxies appear to be predominantly powered by a nuclear starburst.

Two scenarios compatible with the present data are that either there exists a highly attenuated nuclear star forming region, and/or that a significant fraction ($\simeq$80%) of the ionizing photons are absorbed by dust within the HII regions. We suggest that observations in the Br$\alpha$ spectral region could constitute a powerful tool to disentangle these two possibilities.


Key words: dust, extinction -- galaxies: starburst -- infrared: galaxies

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© ESO 2005

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