Overview

UDFy-38135539, catalogued also as HUDF.YD3, is an extremely faint galaxy candidate first identified in deep imaging of the Hubble Ultra Deep Field. It was proposed as one of the earliest known galaxy candidates because its photometric colors and a tentative spectroscopic feature suggested a very high redshift. The object is discussed in the literature as an example of how sensitive imaging and follow-up spectroscopy probe the first few hundred million years after the Big Bang; see general material on distant galaxies.

Redshift and distance

Early follow-up work reported a redshift near z≈8.6, which corresponds to light that began its journey roughly 13.1 billion years ago and places the source within the epoch astronomers call the reionization era. This epoch marks the transition of the intergalactic medium from neutral to ionized as the first generations of stars and galaxies formed. The claimed distance and epoch are often summarized in reviews of the Hubble Ultra Deep Field and of observational efforts in the direction of the Fornax region.

Detection and properties

UDFy-38135539 is very faint in visible and near-infrared bands and was detected primarily through deep space-based imaging and careful photometric selection. A weak emission line detected in early ground-based spectroscopy was interpreted as Lyman‑alpha at high redshift; such a line would indicate active star formation and the presence of hot, young stars producing ultraviolet photons. Photometric measurements also suggest a compact, low-mass system typical of early galaxies.

Confirmation and scientific debate

The spectroscopic claim for UDFy-38135539 proved difficult to reproduce: the reported emission line was faint and near the detection limits of the instruments used, so subsequent analyses questioned whether it arose from the target or from noise, sky features, or a lower-redshift interloper. Because of that uncertainty, the object is often described as a candidate high-redshift galaxy rather than an unambiguous, secure spectroscopic detection. The case illustrates broader challenges in confirming extremely distant sources.

Significance

Whether fully confirmed or not, UDFy-38135539 helped motivate deeper surveys and improvements in analysis techniques to study galaxy formation during the first billion years. Candidates like this inform theoretical models of early star formation, feedback, and the timing of reionization, and they help set priorities for future observations with more sensitive facilities.

Further reading