Metallicity-based selection of streams. The left panel presents the two-dimensional metallicity distribution (Z_2D) for the VELA 7 galaxy at a redshift of z = 1.5, shown in a face-on orientation spanning 4R_d × 4R_d and centered on the galactic center. This map was derived by computing the mass-weighted average of the cold gas metallicities (Z) along the z-axis within the vertical range of −H_d to H_d. Overlaid on the map are dashed red and cyan circles representing radii of R_d and 2R_d, respectively. The image reveals a general decline in metallicity with increasing radial distance, indicating a negative metallicity gradient. The right panel displays the mass-weighted histogram of Z for cold gas cells located between r = R_d and r = 2R_d and vertically between z = −H_d and z = H_d normalized by the total mass of cold gas in the specified volume. The black dashed vertical line at Z = 0.009 marks the 99^th percentile of this distribution and serves as the threshold metallicity, Z_cut, used to distinguish between stream gas (Z < Z_cut) and non-stream gas (Z > Z_cut). This stream selection procedure is based on the assumption that the vast majority of cold gas located beyond R_d originates from recently accreted streams.