Comparison of the photospheric velocity of SN 2012aa with that of SNe Ibc and SLSNe. Velocities ofFe ii λλ4924, 5018, 5169 absorption in SN 2012aa are indicated with filled squares, circles, and triangles, respectively. The dashed line represents the average photospheric velocity of SN 2012aa calculated from theFe ii lines. All of the transients labelled in the left column are SNe Ibc, while the SLSNe Ic are labelled in the right column. For all SLSNe, the velocities at individual epochs have been connected with dotted lines. The photospheric velocities (measured fromFe ii andSi ii lines) of other transients in the plot have been adopted from the literature: SN 2007bi from Young et al. (2010); all other SLSNe are from Nicholl et al. (2015); SNe 2002ap, 2006aj, and 2008D are from Mazzali et al. (2008); SNe 1994I and 1998bw are from Patat et al. (2001); SNe 2003jd, 2007gr, and PTF10qts are from Valenti et al. (2008a), Hunter et al. (2009), and Walker et al. (2014), respectively.