Structural Parameters of Galaxies in CANDELS

Abstract: We present global structural parameter measurements of 109,533 unique, H_F160W-selected objects from the CANDELS multi-cycle treasury program. Sersic model fits for these objects are produced with GALFIT in all available near-infrared filters (H_F160W, J_F125W and, for a subset, Y_F105W). The parameters of the best-fitting Sersic models (total magnitude, half-light radius, Sersic index, axis ratio, and position angle) are made public, along with newly constructed point spread functions for each field and filter. Random uncertainties in the measured parameters are estimated for each individual object based on a comparison between multiple, independent measurements of the same set of objects. To quantify systematic uncertainties we create a mosaic with simulated galaxy images with a realistic distribution of input parameters and then process and analyze the mosaic in an identical manner as the real data. We find that accurate and precise measurements -- to 10% or better -- of all structural parameters can typically be obtained for galaxies with H_F160W < 23, with comparable fidelity for basic size and shape measurements for galaxies to H_F160W ~ 24.5.

• The paper presents a comprehensive measurement of galaxy structures using two-dimensional Sérsic profile modeling on HST CANDELS images.
• It achieves high precision with parameters measured to m<25 and typical random uncertainties around 10% for most galaxies.
• The analysis links galaxy morphology with star formation properties, providing a robust dataset for advancing theories of galaxy evolution.

An Examination of Structural Parameters in CANDELS: Methodology and Measurements

The paper authored by van der Wel et al. is a comprehensive examination of the structural parameters of galaxies within the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). Through the use of the Hubble Space Telescope (HST) and its Wide Field Camera 3 (WFC3), the authors present a detailed analysis of 109,533 galaxies, including global structural parameter measurements in various near-infrared filters.

Core Contributions and Methodology

The primary contribution of this paper lies in the meticulous measurement and analysis of galaxy structures in the CANDELS fields. The survey covers a significant area with deep near-infrared data collected over 902 HST orbits. One of the key efforts articulated in the paper is the execution of two-dimensional parametric modeling using the Sérsic profile to fit the observed surface brightness distributions.

The methodological approach involves:

• Object Identification: Utilizing modified SExtractor software to generate segmentation maps and identify objects across CANDELS F160W mosaics.
• PSF Modeling: Construction of custom Point Spread Function (PSF) models crucial for accurate structural parameter inference, combining stacked stars and simulated models.
• Photometric and Structural Modeling: Application of GALFIT for fitting Sérsic profiles, outputting parameters such as magnitude, half-light radius, Sérsic index, axis ratio, and position angle.
• Error Analysis: Implementation of a dual strategy for error assessment, leveraging internal consistency and simulated image processing to estimate random and systematic uncertainties.

Results and Observations

The analysis reveals a depth of structural parameter measurements to magnitudes around 25 in the F160W filter with high precision, achieving typical random uncertainties of 10% for most parameters at m < 24.5. Systematic biases remain minor relative to the random uncertainty, albeit with notable dependencies based on galaxy morphology, particularly for galaxies with steep Sérsic profiles.

A major finding is the correlation between galaxies' structural traits and their star formation properties, affirming previous understandings while extending those insights to larger, higher precision datasets. The study elucidates the morphological transformation of galaxies from higher redshifts (z > 1), showcasing discrepancies from local galaxy populations, such as the presence of compact massive galaxies, which are hypothesized to be progenitors of present-day ellipticals.

Implications and Future Work

This work provides a critical dataset for subsequent studies in galaxy evolution, particularly for understudied redshift ranges (z > 2), allowing for precise morphological classification and analysis. The survey's structural dataset offers a robust foundation for exploring the relationship between galaxy morphology and other physical properties like star formation rates and the environment.

Speculatively, data from CANDELS may drive future advances in understanding the processes shaping early galaxies. The extensive suite of structural parameters should also encourage further refinement in theoretical models of galaxy formation.

In conclusion, the methodical assembly and release of structural parameters from CANDELS is a significant asset to the astrophysical community, poised to contribute deeply to ongoing research across various theoretical and observational domains in cosmic evolution.