Examples of Na I D fits of the stacked spectra from three different regions of the SFR-M⋆ plane, labelled in Fig. 4: low stellar mass (log(M)⋆ < 10.5 M⊙, top panels, bin A in Fig. 5), high stellar mass and high SFR (log(M)⋆ > 10.5 M⊙ log(SFR) > 1.1 M⊙ yr−1, central panels, bin B in Fig. 5), and low SFR and high stellar mass (log(M)⋆ > 10.5 M⊙, log(SFR) < 0.5 M⊙ yr−1, bottom panels, Bin C in Fig. 5). The expected wavelengths of Na I D are shown as vertical dashed lines. Left panels: black histogram illustrates the observed stacked spectrum, the red line the best-fit stellar continuum. Right panels: black histogram shows the residual flux and errors after the stellar continuum has been removed. The best-fit of the doublet is shown in blue. The dotted magenta curves are the single-Gaussian components. Only in bin B (central panel) do we detect a Doppler blueshift of the Na I D absorption feature with ΔV = −102 ± 41 km s−1. In the other two bins the Na I D line shows a zero-velocity emission (bin A, top panel) and absorption (bin C, bottom panel) below the instrumental resolution 70 km s−1.
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