Selected H I emission spectra corresponding the positions indicated in Fig. 1. The black lines represent the H I spectra, the colors correspond to the HISA. The gray spectra indicate the mean of the five off-positionspresented in Fig. 5. The blue and red dashed lines represent the fits of the background to the HISA spectrum for a polynomial of second and fourth order, respectively, using the velocity range of v_LSR = 40−50 and 60−70 km s⁻¹ for the baseline of the fit. The blue and red solid lines show the difference between fitted H I spectra and the measured H I spectra (T_on-off in Eq. (4)) for a polynomial of second and fourth order, respectively. We fitted Spectra 1, 2, and 5 using a Gaussian function, shown by the black solid curve on top of the HISA spectra.