Keck-I MOSFIRE spectroscopy of the z~12 candidate galaxy UDFj-39546284

TL;DR

Deep spectroscopic observations of a high-redshift galaxy candidate near z~12 yield inconclusive results, highlighting the need for more sensitive data to confirm its nature and redshift.

Contribution

This study provides the deepest H-band spectroscopy of a z~12 galaxy candidate, setting constraints on its emission lines and redshift, and discusses the challenges in confirming such distant objects.

Findings

No definitive emission line detection at z~12

A tentative line candidate at lower flux levels

Deeper observations are necessary for confirmation

Abstract

We report the results of deep (4.6h) H band spectroscopy of the well studied z~12 H-band dropout galaxy candidate UDFj-39546284 with MOSFIRE on Keck-I. These data reach a sensitivity of 5-10 x 10^-19 erg s^-1 cm^-2 per 4.4 Angstrom resolution element between sky lines. Previous papers have argued this source could either be a large equivalent width line emitting galaxy at 2<z<3.5 or a luminous galaxy at z~12. We find a 2.2 sigma peak associated with a line candidate in deep Hubble-Space-Telescope Wide-Field-Camera 3 Infrared grism observations, but at a lower flux than what was expected. After considering several possibilities we conclude these data can not conclusively confirm or reject the previous line detection, and significantly deeper spectroscopic observations are required. We also search for low-redshift emission lines in ten other 7<z<10 z, Y, and J-dropout candidates in our mask and find no significant detections.