A spatially resolved study of ionized regions in galaxies at different ...

44 3 • IFS of a GEHR in NGC 6946
Object Offset (α, δ) Exptime Grating GROT Spec. Range Dispersion Date
(arcsec) (s) (Å) (Å pix −1 )
NGC 6946 0,0 3 × 500 V300 -75 3700-7000 3.2 09/07/2008
NGC 6946 1.56,0.78 3 × 500 V300 -75 3700-7000 3.2 09/07/2008
NGC 6946 1.56,-0.78 3 × 500 V300 -75 3700-7000 3.2 09/07/2008
NGC 6946 0,0 3 × 500 V300 -72 7000-10100 3.2 09/08/2008
NGC 6946 1.56,0.78 3 × 500 V300 -72 7000-10100 3.2 09/08/2008
NGC 6946 1.56,-0.78 3 × 500 V300 -72 7000-10100 3.2 09/08/2008
Table 3.2: Journal of observations and instrumental configuration.
taken. These nine exposures were divided in 3 groups corresponding to different pointings.
Each of these three pointings have a small offset in position from the other ones in order to
perform a dithering mosaic. This allows to have a final mosaic with a filling factor of 1, so
that there is no flux loss. This procedure was also followed in the case of the standard stars
(see section 3.3.6). The observed spectrophotometric standard stars were BD +28 ◦ 4241 for
the blue part of the spectrum (GROT=-75) and BD +17 ◦ 4708 for the red one (GROT=-
72). They were used to obtain the characteristic sensitivity function of the telescope and
spectrograph for the spectral flux calibration. Calibration images were obtained following the
science exposures and consisted of emission line lamps spectra (HgCdHe) or ARC exposures,
and spectra of a continuum lamp needed for the wavelength calibration and to locate the
spectra on the CCD, respectively.
Figure 3.3 shows an Hα (not continuum-subtracted) of NGC 6946 showing the position
of the PPak field at the NE arm of the galaxy. The center of the pointing is located at 4 ′
(6.85 kpc) from the center of the galaxy.
3.3 Data Reduction
Integral Field Spectroscopy (IFS) is a technique for obtaining spectroscopic data of astronomical
objects, in particular objects in crowded fields and extended objects like galaxies,
planetary nebulae, star-forming regions, etc, collecting the spectra of many different regions
of the object under identical instrumental and atmospheric conditions.
In contrast to long-slit spectroscopy, IFS offers the opportunity to obtain three-dimensional
(two spatial and one spectral dimension) information of an object from a single observation.
To store three-dimensional information of a two-dimensional plane of a detector like a
Charge-Coupled Device (CCD), it is necessary to discretize at least one dimension, done by
the discretization of the focal surface in spatial elements. In most cases, the object is wider
than the width of the slit of the long-slit spectrograph, loosing a significant amount of light.
This is not the case of an Integral Field Unit (IFU) with lenslet arrays or fiber re-formatters
combined with dithering methods in order to have a filling factor of one. 1
1 The difference among the major techniques of IFS reside, basically, in the fact that they just vary the way