Dark Energy Survey Year 1 results: constraints on intrinsic alignments and their colour dependence from galaxy clustering and weak lensing

We perform a joint analysis of intrinsic alignments and cosmology using tomographic weak lensing, galaxy clustering, and galaxy–galaxy lensing measurements from Year 1 (Y1) of the Dark Energy Survey. We define early- and late-type subsamples, which are found to pass a series of systematics tests, including for spurious photometric redshift error and point spread function correlations. We analyse these split data alongside the fiducial mixed Y1 sample using a range of intrinsic alignment models. In a fiducial non-linear alignment model analysis, assuming a flat cold dark matter cosmology, we find a significant difference in intrinsic alignment amplitude, with early-type galaxies favouring A_IA = 2.38⁺₋⁰_0.³²₃₁ and late-type galaxies consistent with no intrinsic alignments at 0.05⁺₋⁰_0.¹⁰₀₉. The analysis is repeated using a number of extended model spaces, including a physically motivated model that includes both tidal torquing and tidal alignment mechanisms. In multiprobe likelihood chains in which cosmology, intrinsic alignments in both galaxy samples and all other relevant systematics are varied simultaneously, we find the tidal alignment and tidal torquing parts of the intrinsic alignment signal have amplitudes A₁ = 2.66⁺₋⁰_0.⁶⁷₆₆, A₂ = −2.94⁺₋¹_1.⁹⁴₈₃, respectively, for early-type galaxies and A₁ = 0.62⁺₋⁰_0.⁴¹₄₁, A₂ = −2.26⁺₋¹_1.³⁰₁₆ for late-type galaxies. In the full (mixed) Y1 sample the best constraints are A₁ = 0.70⁺₋⁰_0.⁴¹₃₈, A₂ = −1.36⁺₋¹_1.⁰⁸₄₁. For all galaxy splits and IA models considered, we report cosmological parameter constraints consistent with the results of the main DES Y1 cosmic shear and multiprobe cosmology papers.