3. Line ratios and diagnostic diagrams
For all galaxies listed in Table 1 emission lines are traced down to S/N . Detected spectral features for all galaxies are , and . Line ratios are measured using a block average algorithm. The block size was fixed by the achieved S/N (at least 2) to reach simultaneously the best possible spatial resolution. For the disk and the halo of a galaxy we have chosen block sizes of 1 - 2 and 3 - 4 rows, respectively.
For the galaxies NGC 1963 and NGC 3044 additional emission lines could be detected from the halo DIG, namely 1, , , , and . Thus the extraplanar DIG in these objects can be discussed for the first time in the framework of diagnostic diagrams. A typical spectrum of the DIG in NGC 1963 is given in Fig. 2.
A comparison with photoionization models of Ma86 and DM94 can clarify some properties of the sources which keep the DIG ionized. Only the line ratios of / and have to be corrected for interstellar extinction. The precise values are obtained following Osterbrock (1989). Unreddened line ratios can be computed by assuming an average interstellar extinction curve and adopting as nebular reference emission line. The extinction can be determined from the measured Balmer decrement. It is assumed that such a correction is negligible for the remaining line ratios because only pairs of emission lines in the red spectral region had been chosen which are located in a narrow wavelength region ( Å) so that differential reddening is not significant.
It is noteworthy that an extinction correction does not influence a clear separation of different excitation mechanisms significantly (Baldwin et al. 1981). The dereddening for / and amounts to 11 for the disk emission and 4 for the halo emission. For the remaining line ratios the reddening correction is less than 1 (disk and halo).
In extragalactic objects basically four different ionization mechanisms can be clearly distinguished. Firstly "normal" photoionization by O stars which is present mainly in HII regions (star forming regions). Secondly shock ionization present in objects which are excited by supernova remnants (SNR) or by superwinds. Finally we can also distinguish between photoionization by extremely hot stars such as the central stars of planetary nebulae and objects which are photoionized by a power-law continuum source (AGN). To probe the ionization mechansism(s) of the extraplanar DIG it is useful to plot line ratios by pairs on a logarithmic scale (Baldwin et al. 1981, Veilleux & Osterbrock 1987). Since the above mentioned mechanisms produce characteristically different spectra, all objects cover different locations when plotted into a single diagnostic diagram.
In order to verify our findings obtained from line ratios and diagnostic diagrams we examined also the broadening of line widths at extraplanar latitudes. Therefore we plotted a kinematical diagnostic diagram (line ratio vs. line width (FWHM)) which indicates the presence of additional excitation mechanisms, such as shocks or stellar winds.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100