Bimodality of the population of RRLs in velocity and metallicity. The first column shows the histogram of V_ϕ^* for the RRLs with measured (Li et al. 2023b) photometric metallicity (black) and those with assigned metallicities (green, see Sect. 2); the Gaussian mixture for the total sample solution is shown for illustration (dotted grey line). The second column shows the period-amplitude (V-band) diagram where red dots are fast rotating RRLs with V_ϕ^* < −170 km s⁻¹ and blue dots are slower rotating RRLs with V_ϕ^* > −170 km s⁻¹. The third column shows the V_ϕ^*, [Fe/H] plane for the RRLs with measured metallicities, with the black and blue vertical lines indicating V_ϕ^* = −230 km s⁻¹ and V_ϕ^* = −170 km s⁻¹, respectively for all panels. From top to bottom, each row corresponds to a different radial bin: R ∈ (9,11), (11,13), (13,15), (15,17) kpc.