Hamilton's Object revisited: A challenging source redshift of a strong lensing configuration

Low-resolution spectrographs used to have difficulties in determining redshifts of galaxies at z and z Spectral emission and absorption lines of magnesium and iron redshifted to z fall close to hydrogen, silicon, and oxygen lines at z , signal enhancement, and line fitting by -routines. The reevaluation confirms the previous result based on six absorption features, z=0.820 ± 0.001, and four emission features, z=0.821 ± 0.002. The alternative z=3.199± 0.003, based on six absorption and two emission lines, is a worse fit, also compared to other spectra. Moreover, we find the MOIRCS spectrum inconclusive: observations cover two of three multiple images, with the slit for image C only covering its central bulge; furthermore the pixel-to-wavelength calibration requires a nightsky emission-line calibration due to a missing calibration arc lamp. New MOIRCS observations are needed to verify that Hamilton's Object has the smallest separation between lensing cluster and source galaxy among known cluster-scale strong lenses. Here, we demonstrate that even with modern, integrated field unit spectrographs, this task remains challenging. Hamilton's Object, a blue star-forming galaxy, gravitationally lensed into three multiple images by the galaxy cluster SDSS J223010.47-081017.8, is such a case. Using the Blue Keck Cosmic Web Imager (KCWI) its redshift was determined as z=0.82, while its MOIRCS spectrum hinted at z=3.201. To resolve the ambiguity, we completely reanalyse the Blue KCWI spectra of all three multiple images including the star-forming region in the outskirts. We employ a new data reduction pipeline, PypeIt Python in angular diameter distance the