Dark Energy Survey Year 1 Results: Tomographic cross-correlations between Dark Energy Survey galaxies and CMB lensing from South Pole <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>Telescope</mml:mi><mml:mo>+</mml:mo><mml:mi>Planck</mml:mi></mml:mrow></mml:math>

We measure the cross-correlation between redMaGiC galaxies selected from the Dark Energy Survey (DES) year 1 data and gravitational lensing of the cosmic microwave background (CMB) reconstructed from South Pole Telescope (SPT) and Planck data over $1289\text{ }\text{ }{\mathrm{deg}}^{2}$. When combining measurements across multiple galaxy redshift bins spanning the redshift range of $0.15&lt;z&lt;0.90$, we reject the hypothesis of no correlation at $19.9\ensuremath{\sigma}$ significance. When removing small-scale data points where thermal Sunyaev-Zel'dovich signal and nonlinear galaxy bias could potentially bias our results, the detection significance is reduced to $9.9\ensuremath{\sigma}$. We perform a joint analysis of galaxy-CMB lensing cross-correlations and galaxy clustering to constrain cosmology, finding ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{m}}=0.27{6}_{\ensuremath{-}0.030}^{+0.029}$ and ${S}_{8}={\ensuremath{\sigma}}_{8}\sqrt{{\mathrm{\ensuremath{\Omega}}}_{\mathrm{m}}/0.3}=0.80{0}_{\ensuremath{-}0.094}^{+0.090}$. We also perform two alternate analyses aimed at constraining only the growth rate of cosmic structure as a function of redshift, finding consistency with predictions from the concordance $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. The measurements presented here are part of a joint cosmological analysis that combines galaxy clustering, galaxy lensing and CMB lensing using data from DES, SPT and Planck.