Detecting low-mass haloes with strong gravitational lensing I: the
  effect of data quality and lensing configuration
  This paper aims to quantify how the lowest halo mass that can be detected
with galaxy-galaxy strong gravitational lensing depends on the quality of the
observations and the characteristics of the observed lens systems. Using
simulated data, we measure the lowest detectable NFW mass at each location of
the lens plane, in the form of detailed \emph{sensitivity maps}. In summary, we
find that: (i) the lowest detectable mass $M_{\rm low}$ decreases linearly as
the signal-to-noise ratio (SNR) increases and the sensitive area is larger when
we decrease the noise; (ii) a moderate increase in angular resolution (0.07" vs
0.09") and pixel scale (0.01" vs 0.04") improves the sensitivity by on average
0.25 dex in halo mass, with more significant improvement around the most
sensitive regions; (iii) the sensitivity to low-mass objects is largest for
bright and complex lensed galaxies located inside the caustic curves and lensed
into larger Einstein rings (i.e $r_{E}\geq1.0"$). We find that for the
sensitive mock images considered in this work, the minimum mass that we can
detect at the redshift of the lens lies between $1.5\times10^{8}$ and
$3\times10^{9}M_{\odot}$. We derive analytic relations between $M_{\rm low}$,
the SNR and resolution and discuss the impact of the lensing configuration and
source structure. Our results start to fill the gap between approximate
predictions and real data and demonstrate the challenging nature of calculating
precise forecasts for gravitational imaging. In light of our findings, we
discuss possible strategies for designing strong lensing surveys and the
prospects for HST, Keck, ALMA, Euclid and other future observations.
