Astron. Astrophys. 333, L1-L4 (1998)

4. The most massive stars in the Antennae are not visible at optical wavelengths

We now show that one of the inconspicuous regions at optical wavelengths harbors the most massive stars in the Antennae. For this we use the imaging capability of the CVFs obtained with pixel field of view, which offers a unique opportunity to study the variation of spectral features from one area of the galaxy to the other.

In Fig. 1 we show as an example, a comparison between the CVF spectra for the brightest 15 µm peak in the overlap region and the two nuclei. The distinct characteristic of this optically obscured starburst knot is the [Ne III] line at 15.5 µm above the relatively enhanced continuum beyond 10 µm. Vigroux et al. (1996) have shown that in the Antennae the 15 µm continuum intensity is well correlated with the [Ne II] and the [Ne III] line intensities. This confirms that the enhancement of the continuum 10 µm is due to the thermal emission of hot dust heated by the absorption of the ionizing photons emitted by young stars. This is strengthened by the fact that the 15 µm intensity increases together with the [Ne III]/[Ne II] ratio (Vigroux et al. 1996).

For a given physical size, the [Ne III]/[Ne II] ratio is a measure of the hardness of the UV flux. The [Ne III] to [Ne II] ratio is 1 in the brightest area of the overlap region, and decreases to 0.1 or below in the central regions of NGC 4038 and in NGC 4039 (Vigroux et al. 1996). Using a [Ne III]/[Ne II] ratio of 1 and the diagnostic arguments by Kunze et al. (1996), a single star equivalent effective temperature of 44000 K is derived. This would correspond to an O5 main sequence star with a mass of 50-60 . A more adequate treatment of star cluster evolution leads to a young cluster ( 7 10⁶ yr) starburst with IMF extending up to 100 (Kunze et al. 1996). Therefore, using observations at optical wavelengths only, could lead to biased low values for the high mass cut-off in the IMF of interacting, luminous galaxies.

Multiwavelength observations of nearby starburst systems are instructive when deriving the morphologies of galaxy populations at high redshifts. Simulations that take into account band-shifting and surface brightness dimming by Hibbard & Vacca (1997) have shown that nearby interacting systems that are luminous in the infrared, are the best local analogs of the highest redshift galaxies found in the Hubble Deep Field (HDF). Nevertheless, at z 1.5 the HDF is sensitive to the rest-frame UV emission, and due to the presence of an old population and/or dust it may be difficult to recover the global morphology of the underlying systems (O'Connell, 1997). If, as in the Antennae, the most intense starburst galaxies at high redshifts have substantial amounts of dust (Franceschini et al 1994; Guiderdoni et al. 1997), high sensitivity observations in the far-infrared and submillimeter wavelength bands will be needed to reveal the true global morphology of very distant galaxies.

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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