AAO Newsletter October 1996 - Page 10

We show here two approaches to line imaging with the TTF. One is charge shuffling between discrete frequencies. The scheme we adopted was to shuffle between a line and adjacent continuum. This was done for NGC 7130 in four emission lines and the results are presented in
Fig. 7

NGC 7130 is a luminous far-infrared galaxy (log[FIR/Lsun]=11.04) at redshift of 0.0159. Deep Kband imaging strongly suggests that NGC 7130 is undergoing an interaction (Bransford et al. 1996). Spectral mapping of the nuclear region (inner 6") of the galaxy by Shields & Filippenko (1990) shows that it has a composite (AGN+starburst) nature. The far-infrared spectral indices (alpha(25,60)= - 2.34 and alpha(60,100) = - 0.86) are consistent with a dominant star formation component. The new data will allow us to study the gas ionization over the face of the galaxy.

The second mode is to generate low (6-60Å) resolution spectra over the entire field by obtaining a series (~80) of narrowband images, contiguous in wavelength. In Fig. 8, we show the [S III]lambda9069 line flux distribution throughout the central regions of NGC 6240. Dopita & Sutherland (1996) have shown that this is an important diagnostic for identifying the source of ionization. The origin of the huge far-infrared flux in NGC 6240 (z = 0.0245) is highly controversial.

In Fig. 7, the background subtraction is extremely effective in charge shuffling mode as the on- and off-band images are interleaved in time. The method averages out subtle filter and CCD response variations, seeing and atmospheric transparency. There are additional advantages in using the same optical train for both on- and off-band exposures. The data cube mode is less effective for sky subtraction as the images are obtained sequentially in time (Fig. 8). However, the advantage here is that we have FORS-resolution spectra over the entire field.

J. Bland-Hawthorn (AAO), S. Lumsden (AAO),C. Heisler (MSO),
G. Cecil (UNC/SOAR) & M. Dopita (MSO)

Figure 8: TTF tunable imaging of the luminous far-infrared galaxy NGC 6240 in the emission line of [S III]lambda9069. The off-band continuum is overlaid as contours. The total exposure time is 3 mins and the field of view is 88". Ionized gas is seen over a 10 kpc region. The two knots 12 kpc SW of the nucleus are H II regions.

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