Normalized flux, F_λ, as a function of wavelength (λ, in Å, left panel) and the EWs (in Å) or MAGs (in mag) of the Lick/IDS spectral absorption feature indices with the number (no., right panel) for NGC 6637. Left panel: all fluxes have been degraded to the resolution of FWHM_Lick/IDS (see Sect. 3.1). The U17 (red line) spectrum is limited in the range of 3500–7500 Å. The spectral flux of S05 (blue line, within the first observed region) has been multiplied by a const (equal to the ratio of the average flux within 4600–4800 Å of U17 to S05). The absorption feature passbands of the Balmer indices (spectral absorption feature indices 9 and 22–25) and the other Lick/IDS indices are expressed by grey and cyan graded regions, respectively. The number to the right of each graded region is the spectral absorption feature index number. Right panel: all spectral absorption feature indices are obtained at the resolution of FWHM_Lick/IDS. The spectral absorption feature indices from the U17 and S05’s integrated spectra are in red and blue solid points. The two vertical cyan lines are for Fe4531 and Fe5015 indices (7 and 10). The names of spectral absorption feature indices are given at the bottom of this panel.