Field M 31-1a centered on RA: 00:42:44.1 and Dec: +41:15:42 (J2000) observed with IRAM-30 m/HERA. The center is indicated with a cross and the circle displays the 12′′ beam. The Y scale of each spectrum is in main-beam temperature (T_mb between –0.01 and 0.02 mK, as indicated on the top right corner), while the X-axis shows the velocity for the range between –600 and 0 kms^-1 smoothed to 13 kms^-1 velocity resolution. A thin line indicates the systemic velocity at  −310 kms^-1. The color-coding (displayed in the top right corner) of the velocity is used for the spectra with 3σ detections.