A Radio Study of the Ultra-luminous FIR Galaxy NGC 6240

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Session 44 -- Radio Galaxies, Jets and Disks
Display presentation, Wednesday, 9:20-6:30, Pauley Room

[44.04] A Radio Study of the Ultra-luminous FIR Galaxy NGC 6240

E. Colbert (Univ.~of~Md.), A.S. Wilson (STScI \& Univ.~of~Md.), J. Bland-Hawthorn (Rice U.)

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A number of galaxies observed in the IRAS mission are noted to emit $\sim$99\% of their bolometric flux in the FIR, with FIR luminosities in excess of 10$^{11}$ L$_{\odot}$. The interacting galaxy NGC 6240 has often been referred to as the ``proto-typical'' ultra-luminous (L$_{\rm FIR}$ $\gapprox$ 10$^{12}$L$_{\odot}$) FIR galaxy. The origin of the FIR excess remains a disputed subject in the literature.

New observations of NGC 6240 were taken with the VLA at 20cm in the B-configuration, and at 3.6cm in the A-configuration. No significant radio emission was detected from or near the possible ultra-massive ``dark core'' hypothesized by Bland-Hawthorn et. al. (1991); however, approximately 30\% of Seyfert galaxies have 20 cm radio luminosities weaker than the upper limit derived from the radio maps.

The non-thermal radio emission from luminous FIR galaxies is tightly correlated with the FIR emission. Previous radio observations of NGC 6240 revealed two compact, steep-spectrum nuclear sources, nearly coincident with the two nuclear sources seen in optical images.

The 2 images from the new VLA observations and 5 images from previous VLA observations are used to identify the morphological and spectral features of the strong, compact components in the nuclear regions ($\lapprox$ 1.5 kpc; D$=$100 Mpc) and of the weaker ``clumps'' of diffuse emission south and west ($\gapprox$ 3 kpc) from the nucleus. Feasible explanations for the radio emission are discussed. The models that have been proposed in the literature for the FIR excess of NGC 6240 are evaluated for consistency with the observed radio emission.

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