We present the deepest optical spectrum acquired to date of Balmer-dominated shocks in the NW rim of SN 1006 .
We detect the broad and narrow components of H \alpha , H \beta and H \gamma and report the first detection of the He I \lambda 6678 emission line in this supernova remnant .
We may have detected , at the 1.5 \sigma level , faint He II \lambda 4686 emission .
We measure a full width half maximum of 2290 \pm 80 km { s } ^ { -1 } in the broad component H \alpha line , with broad-to-narrow flux ratios of 0.84 ^ { +0.03 } _ { -0.01 } and 0.93 ^ { +0.18 } _ { -0.16 } in H \alpha and H \beta , respectively .
To match these observations , our nonradiative shock models require a low degree of electron-proton equilibration at the shock front , T _ { e } / T _ { p } \leq 0.07 , and a shock speed of 2890 \pm 100 km { s } ^ { -1 } .
These results agree well with an earlier analysis of ultraviolet lines from SN 1006 .
The He I/H \alpha and He I/He II flux ratios also indicate low equilibration .
Furthermore , our models match the observations for mostly ionized ( \sim 90 % ) preshock H and mostly neutral ( \gtrsim 70 % ) preshock He , respectively .
We conclude that the high H ionization fraction can not be explained by either photoionization from the reverse shock or relic ionization from EUV photons released in the 1006 A.D. supernova .
The most plausible explanation appears to be photoionization from the Galactic Lyman continuum .