How Robust Are the Size Measurements of High-redshift Compact Galaxies?

Abstract

Massive quiescent galaxies at $z\approx 2$ are apparently much more compact than galaxies of comparable mass today. How robust are these size measurements? We perform comprehensive simulations to determine possible biases and uncertainties in fitting single-component light distributions to real galaxies. In particular, we examine the robustness of the measurements of the luminosity, size, and other structural parameters. We devise simulations with increasing realism to systematically disentangle effects due to the technique (specifically using GALFIT) and the intrinsic structures of the galaxies. By accurately capturing the detailed substructures of nearby elliptical galaxies and then rescaling their sizes and signal-to-noise to mimic galaxies at different redshifts, we confirm that the massive quiescent galaxies at $z\approx 2$ are significantly more compact intrinsically than their local counterparts. Their observed compactness is not a result of missing faint outer light due to systematic errors in modeling. In fact, we find that fitting multi-component galaxies with a single Sérsic profile, the procedure most commonly adopted in the literature, biases the inferred sizes higher by up to 10% - 20%, which accentuates the amount of size evolution required. If the sky estimation has been done robustly and the model for the point-spread function is fairly accurate, GALFIT can retrieve the properties of single-component galaxies over a wide range of signal-to-noise ratios without introducing any systematic errors.

18 pages, 11 figures, 8 tables; Accepted for publication in ApJ

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• An Optimal Strategy for Accurate Bulge-to-disk Decomposition of Disk Galaxies
• The Decomposed Bulge and Disk Size-Mass Relations of Massive Galaxies at 1<z<3 in CANDELS
• The colors and sizes of recently quenched galaxies: a result of compact starburst before quenching
• The interstellar medium and feedback in the progenitors of the compact passive galaxies at z~2
• The VLA Frontier Field Survey: A Comparison of the Radio and UV/optical size of $0.3\lesssim z\lesssim 3$ star-forming galaxies
• JADES: Rest-frame UV-to-NIR Size Evolution of Massive Quiescent Galaxies from Redshift z=5 to z=0.5
• Multiwavelength Bulge-Disk Decomposition for the Galaxy M81 (NGC 3031). I. Morphology
• Lower mass normalization of the stellar initial mass function for dense massive early-type galaxies at z ~ 1.4
• The PSF Smoothing Effect on Concentration-Related Parameters of High Redshift Galaxies in HST and JWST
• Mock Observations: Morphological Analysis of Galaxies in TNG100 Simulations
• The JWST Emission Line Survey (JELS): The sizes and merger fraction of star-forming galaxies during the Epoch of Reionization