GALFIT-ing AGN Host Galaxies in COSMOS: HST vs. Subaru

TL;DR

This study compares galaxy morphology measurements of AGN hosts from HST and Subaru imaging, revealing that Sersic index estimates from ground-based data are unreliable for high-redshift galaxies, impacting morphological classifications.

Contribution

It demonstrates the limitations of ground-based Subaru imaging for accurate Sersic index measurements of AGN host galaxies compared to HST, informing future morphological analyses.

Findings

Sersic index estimates differ significantly between HST and Subaru data.

Magnitude and radius measurements are consistent across both telescopes.

Sersic index variations have little impact on fit quality, unlike other parameters.

Abstract

The COSMOS field has been extensively observed by most major telescopes, including Chandra, HST, and Subaru. HST imaging boasts very high spatial resolution and is used extensively in morphological studies of distant galaxies. Subaru provides lower spatial resolution imaging than HST but a substantially wider field of view with greater sensitivity. Both telescopes provide near-infrared imaging of COSMOS. Successful morphological fitting of Subaru data would allow us to measure morphologies of over $10^4$ known active galactic nucleus (AGN) hosts, accessible through Subaru wide-field surveys, currently not covered by HST. For 4016 AGN between $0.03<z<6.5$, we study the morphology of their galaxy hosts using GALFIT, fitting components representing the AGN and host galaxy simultaneously using the i-band imaging from both HST and Subaru. Comparing the fits for the differing telescope spatial resolutions and image signal-to-noise ratios, we identify parameter regimes for which there is strong disagreement between distributions of fitted parameters for HST and Subaru. In particular, the S\'ersic index values strongly disagree between the two sets of data, including sources at lower redshifts. In contrast, the measured magnitude and radius parameters show reasonable agreement. Additionally, large variations in the S\'ersic index have little effect on the $\chi^2_\nu$ of each fit whereas variations in other parameters have a more significant effect. These results indicate that the S\'ersic index distributions of high-redshift galaxies that host AGN imaged at ground-based spatial resolution are not reliable indicators of galaxy type, and should be interpreted with caution.