Multi-conjugated adaptive optics imaging of distant galaxies -- A comparison of Gemini/GSAOI and VLT/HAWK-I data

TL;DR

This study demonstrates that multi-conjugated adaptive optics significantly improves the signal-to-noise ratio and depth of distant galaxy imaging at 2 micrometers, outperforming traditional seeing-limited observations despite increased background noise.

Contribution

First empirical comparison of MCAO and natural seeing data for distant galaxy imaging, showing substantial S/N improvements with MCAO at 8m class telescopes.

Findings

MCAO compensates for higher thermal background with wavefront correction.

S/N gain of 40% for typical galaxy sizes, up to 2.5 times for smaller objects.

MCAO enhances near-diffraction-limited imaging depth for distant galaxies.

Abstract

Multi-conjugated adaptive optics (MCAO) yield nearly diffraction-limited images at 2$\mu$m wavelengths. Currently, GeMS/GSAOI at Gemini South is the only MCAO facility instrument at an 8m telescope. Using real data and for the first time, we investigate the gain in depth and S/N when MCAO is employed for $K_{\rm s}$-band observations of distant galaxies. Our analysis is based on the Frontier Fields cluster MACS J0416.1-2403, observed with GeMS/GSAOI (near diffraction-limited) and compared against VLT/HAWK-I (natural seeing) data. Using galaxy number counts, we show that the substantially increased thermal background and lower optical throughput of the MCAO unit are fully compensated for by the wavefront correction, because the galaxy images can be measured in smaller apertures with less sky noise. We also performed a direct comparison of the signal-to-noise ratios (S/N) of sources detected in both data sets. For objects with intrinsic angular sizes corresponding to half the HAWK-I image seeing, the gain in S/N is 40 per cent. Even smaller objects experience a boost in S/N by a up to a factor of 2.5 despite our suboptimal natural guide star configuration. The depth of the near diffraction limited images is more difficult to quantify than that of seeing limited images, due to a strong dependence on the intrinsic source profiles. Our results emphasize the importance of cooled MCAO systems for $K_{\rm s}$-band observations with future, extremely large telescopes.