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

TL;DR

This study revisits the redshift determination of Hamilton's object, a gravitationally lensed galaxy, demonstrating the challenges and confirming its low redshift through re-analysis of spectroscopic data.

Contribution

It introduces a new data reduction pipeline and re-analyzes existing spectra to resolve redshift ambiguities in a complex lensing case.

Findings

Confirmed the galaxy's redshift as z=0.82 with high precision.

Demonstrated the difficulty of distinguishing between redshifts at z≈1 and z≈3 using current spectrographs.

Highlighted the need for new observations to verify the galaxy's unique lensing properties.

Abstract

Low-resolution spectrographs used to have difficulties to determine redshifts of galaxies at $z\approx1$ and $z\approx3$. Spectral emission and absorption lines of magnesium and iron redshifted to $z\approx1$ fall close to hydrogen, silicon, and oxygen lines at $z\approx3$. 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, its redshift was determined as $z=0.82$, while its MOIRCS spectrum hinted at $z=3.201$. To resolve the ambiguity, we completely re-analyse 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, signal enhancement, and line fitting by Python-routines. The re-evaluation confirms the previous result based on 6 absorption features, $z=0.820 \pm 0.001$ and 4 emission features, $z=0.821 \pm 0.002$. The alternative $z=3.199\pm 0.003$, based on 6 absorption and 2 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 in angular diameter distance between lensing cluster and source galaxy among the known cluster-scale strong lenses.