Systematics in Metallicity Gradient Measurements I : Angular Resolution, Signal-to-Noise and Annuli Binning

TL;DR

This study examines how angular resolution, signal-to-noise ratio, and binning affect metallicity gradient measurements in high-redshift galaxies, revealing systematic biases and critical resolution limits.

Contribution

It provides a systematic analysis of measurement biases in metallicity gradients due to observational parameters, highlighting the importance of resolution and S/N in accurate gradient determination.

Findings

Measured gradients flatten with decreasing angular resolution.

There exists a critical resolution below which gradients are significantly biased.

A minimum S/N of ~5 is needed for reliable gradient constraints with 3-annuli binning.

Abstract

With the rapid progress in metallicity gradient studies at high-redshift, it is imperative that we thoroughly understand the systematics in these measurements. This work investigates how the [NII]/Halpha ratio based metallicity gradients change with angular resolution, signal-to-noise (S/N), and annular binning parameters. Two approaches are used: 1. We downgrade the high angular resolution integral-field data of a gravitationally lensed galaxy and re-derive the metallicity gradients at different angular resolution; 2. We simulate high-redshift integral field spectroscopy (IFS) observations under different angular resolution and S/N conditions using a local galaxy with a known gradient. We find that the measured metallicity gradient changes systematically with angular resolution and annular binning. Seeing-limited observations produce significantly flatter gradients than higher angular resolution observations. There is a critical angular resolution limit beyond which the measured metallicity gradient is substantially different to the intrinsic gradient. This critical angular resolution depends on the intrinsic gradient of the galaxy and is < 0.02 arcsec for our simulated galaxy. We show that seeing-limited high-redshift metallicity gradients are likely to be strongly affected by resolution-driven gradient flattening. Annular binning with a small number of annuli produces a more flattened gradient than the intrinsic gradient due to weak line smearing. For 3-annuli bins, a minimum S/N of ~ 5 on the [NII] line is required for the faintest annulus to constrain the gradients with meaningful errors.