Response approach to the integrated shear 3-point correlation function:
  the impact of baryonic effects on small scales
  The integrated shear 3-point correlation function $\zeta_{\pm}$ is a
higher-order statistic of the cosmic shear field that describes the modulation
of the 2-point correlation function $\xi_{\pm}$ by long-wavelength features in
the field. Here, we introduce a new theoretical model to calculate
$\zeta_{\pm}$ that is accurate on small angular scales, and that allows to take
baryonic feedback effects into account. Our model builds on the realization
that the small-scale $\zeta_{\pm}$ is dominated by the nonlinear matter
bispectrum in the squeezed limit, which can be evaluated accurately using the
nonlinear matter power spectrum and its first-order response functions to
density and tidal field perturbations. We demonstrate the accuracy of our model
by showing that it reproduces the small-scale $\zeta_{\pm}$ measured in
simulated cosmic shear maps. The impact of baryonic feedback enters effectively
only through the corresponding impact on the nonlinear matter power spectrum,
thereby permitting to account for these astrophysical effects on $\zeta_{\pm}$
similarly to how they are currently accounted for on $\xi_{\pm}$. Using a
simple idealized Fisher matrix forecast for a DES-like survey we find that,
compared to $\xi_{\pm}$, a combined $\xi_{\pm}\ \&\ \zeta_{\pm}$ analysis can
lead to improvements of order $20-40\%$ on the constraints of cosmological
parameters such as $\sigma_8$ or the dark energy equation of state parameter
$w_0$. We find similar levels of improvement on the constraints of the baryonic
feedback parameters, which strengthens the prospects for cosmic shear data to
obtain tight constraints not only on cosmology but also on astrophysical
feedback models. These are encouraging results that motivate future works on
the integrated shear 3-point correlation function towards applications to real
survey data.
