Astron. Astrophys. 354, 823-835 (2000)

2. Observations and reduction of the data

2.1. The H image

The observations of the image of NGC 3359 in H were carried out the night of February 12th, 1996, on the Isaac Newton Telescope, La Palma, in the context of the BARS international time project of the Canary Island Observatories. A CCD TeK-7 detector was used, with a projected pixel size of 0.59 0.59 arcsec. The observing conditions were good, with a seeing of 1 arcsec (FWHM measured in the final image), and the sky photometric. Two exposures were taken, one through a filter whose central wavelength (6594/44Å) coincided with the red-shifted H from the galaxy, the other, displaced in wavelength (6686/44Å) to permit precise continuum subtraction. The bandwidth of the filter (44Å) includes emission from the [NII ] emission lines. However, the contribution of these lines to the measured H fluxes accounting for the total intensity of [NII ] lines and the filter transmission at the given wavelengths, give place to a maximum contribution of 10% in the measured fluxes.

Standard image reduction routines were used: the bias was subtracted and the images corrected using the appropriate flats. Afterwards the images were aligned, and cleaned of cosmic ray effects, and the continuum image then subtracted from the other, leaving an image in H surface brightness. The scaling factor for the continuum subtraction was 0.93 with uncertainty less than 5%. The procedure is described in more detail in Rozas et al. (1996a). Finally, the astrometry of the image was carried out identifying our foreground stars in the Palomar Plates. The accuracy of the astrometry is better than 0.4 pix. Absolute flux calibration was performed via observations of standard stars from the list of Filippenko & Greenstein (1984). The H luminosity corresponding to an instrumental count is 2.61 10³³ erg s^-1 count^-1. In Fig 1 we show a grey scale representations of the H continuum-subtracted image of NGC 3359. As shown in this figure, it has well defined arms, well endowed with HII regions. The bar has a deprojected length of 2.9 arcmin (corresponding to a radial length of 9 kpc), and it dominates the total H emission of the galaxy, due to a chain of brilliant HII regions along its length. Outside the bar, the majority of the HII regions in the disc are concentrated within an annular zone, with fainter regions distributed along the spiral arms. The intensities of the latter are very disparate, with the western arm being much brighter, as well as bigger and better defined, with more vigorous star formation than the opposing arm. This asymmetry is not seen in HI , (Ball 1986), in which a far greater degree of large-scale symmetry is found. The slope of the H luminosity function (LF) of the HII regions (see Sect. 4) is within the range of values found previously for populations of galaxies of similar morphological type by Kennicutt et al. (1989) and by Rozas et al. (1996a).

Fig. 1. Grey scale representation of the continuum-subtracted H image of NGC 3359.

2.2. The U and I images

The images in the U and I bands were taken at the prime focus of the Isaac Newton Telescope, La Palma, also within the context of the BARS international time project. An EEV TeK-3 chip was used with a projected pixel size of 0.59" 0.59". Again the observing conditions were good, with a seeing of 1 arcsec (FWHM in the final image) and the sky photometric. The exposure times were 1800 s. for the U image and 600 s. for the I image. Standard reduction routines were used: the images were bias and flat-field corrected, sky subtracted, and cleaned of cosmic ray tracks. The calibration was performed with known standards.

The images in the two photometric bands were aligned by fitting Gaussian profiles to a number of field stars in the images. This was done in order to construct colour maps. We also obtained the exact resolution from these fits. This positioning was performed to a precision of better than 0.8 of a pixel, much better than the resolution of the images. The positions of some of the stars, as determined using the Hubble Space Telescope Guide Star Catalog, and the position of the center of the galaxy were used to place the images on a corrected R.A.-declination grid with an estimated precision of a few arcsec.

2.3. The K image

The K-band image was obtained on the Carlos Sánchez infrared telescope on the Observatorio del Teide, Tenerife, using the CAIN infrared camera. This has an array of 256256 pixels, with pixel size 1 1 arcsec. Thirty exposures of the galaxy, interlaced with thirty on the sky were summed and subtracted respectively to give the final image using a standard IR technique. The standard data reduction procedures of bias-subtraction, flat-fielding, and averaging were applied to produce the final frame. The resolution of the final image is some 1.8 arcsec. The image was calibrated with known standard stars.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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