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

110 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies
1e−15
A
Continuum
Difference
Hα
1.5
cm−2 )
Flux (erg s−1
1.0
C
B
0.5
0.0
−10 0 10 20
arcsecs
Figure 4.2: Spatial profile of the light distribution along the slit for the observed Hα emission
(dashed red line), the adjacent continuum (thick green line), and the difference between the two (thin
blue line).
IIZw71 and He-Ar for J1657. The images were processed and analysed with IRAF 1 routines
in the usual manner. This procedure includes the removal of cosmic rays, bias subtraction,
division by a normalised flat field, and wavelength calibration. Typical wavelength fits were
performed using 30-35 lines in the blue and 20-25 lines in the red and polynomials of second
to third order. These fits were made at 117 different locations along the slit in each arm
for the WHT-ISIS observations (beam size of 10 pixels) and at 80 different locations for the
CAHA-Twin observations (beam size of 10 pixels) obtaining rms residuals between ∼0.1 and
∼0.2 pix for the WHT data and between ∼0.1 and ∼0.3 pix for the CAHA data.
In the last step, the spectra were corrected for atmospheric extinction and flux-calibrated.
For both arms of WHT, two standard star observations were used, allowing a good spectrophotometric
calibration with an estimated accuracy of about 5%. For the CAHA data,
four standard star observations were performed each night at the same time for both arms,
allowing a good spectrophotometric calibration with an estimated accuracy of about 3%
between the different standard star flux calibration curves.
Figure 4.2 and the left panel of Figure 4.3 show the spatial distribution of the Hα flux
along the slit for IIZw71 and J1657. The emission line profile in these plots have been generated
by collapsing 11 pixels of the spectra in the direction of the resolution at the central
1 IRAF: the Image Reduction and Analysis Facility is distributed by the National Optical Astronomy
Observatories, which is operated by the Association of Universities for Research in Astronomy, In. (AURA)
under cooperative agreement with the National Science Foundation (NSF).