2. Observation and data reduction
2.1. Observations with the Fabry-Perot interferometer
The two-dimensional velocity field of NGC 1084 was obtained on October 26, 1995 at the SAO 6m reflector, using a scanning Fabry-Perot interferometer (FPI) installed in the pupil plane of a focal reducer attached to the f/4 prime focus of the telescope. The detector was an intensified photon counting system (IPCS). The observational parameters are given in Table 1.
Table 1. FPI observations parameters.
An order separating filter with Å was used, centered at 6603 Å, close to the redshifted galactic emission line . The filter bandpass includes also the nitrogen emission line [NII]. This line falls into the interfringe of the etalon - very close to the next order line. Usually such situation complicates data processing and interpretation, but in our case the proximity of and [NII] interference rings was purposely used to compare gas kinematics in two emission lines from the same observational data set.
Observational data were converted into a cube of 32 images. A neon lamp spectrum was used for phase calibration. Reduction of the observational data was performed using the software ADHOC developed at the Marseille Observatory (Boulesteix 1993). It includes a phase map construction (wavelength calibration), subtraction of the night sky emission, spatial and spectral smoothing. The spatial resolution of our data, after smoothing, is , and the spectral resolution is close to . Uncertainty of velocity measurements depends mostly on calibration errors and is about .
After phase calibration, the first spectral channel corresponds to 6585.5 Å ( at the redshifted line). The [NII] line is observed in the -2 interference order relatively to the -line order and has a visible shift of () from the line position. Therefore, the emission lines are certainly separated since the spectral resolution of the FPI is about .
Fig. 1b shows the transmission curve of the narrowband order separating filter, while the and [NII] emission lines positions are marked as gray boxes. The width of these boxes corresponds to the full range of observable velocities (). The high velocity components of the line were included. The relative heights of the gray boxes have been set from normal emission lines ratio in HII regions. The flux from the [NII] line must then be 10 times lower than the flux from the [NII] line due to the filter transmission. Indeed, there is no traces of [NII] in our FPI spectra.
Relative intensities of the night sky emission lines from Osterbrock et al.(1996) are shown in Fig. 1. The FPI's mean night sky spectrum was obtained as an integrated emission on the detector's part which is free from emission of the galaxy and its ghost image. Then the mean night sky spectrum was subtracted from all Fabry-Perot spectra. The total mean night sky spectrum plotted in Fig. 1b and the individual sky lines from Fig. 1a are superimposed on the FPI spectrum. The relative intensities of these emission lines were multiplied on the filter bandpass transmission curve and the wavelengths of the night sky lines were converted to the wavelength scale for the interference order.
The night sky lines (label C in Fig. 1) and (label D) are the main contributors to the observed FPI spectra. Contribution from other lines is negligible (including which is the brightest in the filter bandpass but is located in the extreme blue wing of the filter). Discrepancies between the FPI night sky spectrum and the line intensities and positions from Osterbrock et al. (1996) are due to calibration errors and night sky brightness variations.
In Sect. 3 it will be shown that all non-circular components of the object's emission lines are brighter than the mean night sky spectrum. Therefore the errors due to subtraction of the night sky lines have no influence on the measurement of the high velocity motions of the gas.
The velocity map and monochromatic and red continuum images of the galaxy were constructed after sky subtraction and smoothing procedures. All spectral channels within (8 channels) from the channel with maximal intensity were summed in each pixel to obtain the image. The non-circular component of has a relative velocity larger than only in regions where its intensity is negligible in comparison with the main component (see below). Therefore the velocity range is optimal for measuring the total flux. The flux is calibrated using the integrated flux of +[NII] in NGC 1084 from Kennicutt & Kent (1983) and assuming a line ratio = 1.5.
Baricenters of the and [NII] lines were calculated to obtain the full-format velocity fields (the map of the first moment) in these lines. Fig. 2a. shows the image of the galaxy and isovelocities of the velocity field. For the regions with complex emission line profiles we used a multi-component gaussian analysis.
2.2. Photometric observations
Images of the galaxy were obtained on January 14, 1997, at the SAO 1m Zeiss reflector with a CCD camera through the V filter of Johnson's system and RC, IC filters of Cousin's system. The pixel size of CCD was 0.49", the seeing was about . Standard stars from Landolt (1992) were observed for flux calibration. The rms error in the determination of the photometric zero points was .
A map of the (V-IC) color index is shown in Fig. 3. A thick clumpy dust lane ("red" region in this image) appears in the SE inner part of NGC 1084 suggesting that this side of the galaxy is the closest to us. Since the SW part of the galaxy is redshifted and the NE part is blueshifted (see the velocity field in Fig. 2), the spiral arms are trailing. This situation is ordinary for spiral galaxies.
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000