Oldest Galaxy Ever Observed: Z ~ 10 Candidate UDFj-39546284 In The HUDF & XDF

The Hubble Ultra Deep Field (H-UDF) and the recently released eXtreme Deep Field (XDF) are perhaps the two most remarkable astronomical surveys ever conducted by humans. The light which has travelled from these regions has taken an eternity, light which has never been seen before by humans. In recent years, the identification of some of the oldest galaxies in the Universe with redshifts of z ~ 8 has become more routine (e.g. Lorenzoni et al. 2011; Bouwens et al. 2011b; Oesch et al. 2012b; Bradley et al. 2012).

Hence, the hunt for high-redshift objects is now focused on much more distant galaxies at redshifts > 9, with a small number of candidates being proposed (e.g. Bouwens et al. 2011a; Oesch et al. 2012a; Zheng et al. 2012; Coe et al. 2012; Bouwens et al. 2012a). This lack of observational prospect makes finding an object at extreme distances incredibly hard.

Still, a probable z  ~ 10 candidate “UDFj-39546284” (Bouwens et al. 2011a, Oesch et al. 2012a) was found in the original WFC3/IR observations of the Hubble Ultra Deep Field (Bouwens et al. 2011b). UDFj-39546284 is believed to be the oldest observed galaxy to date. UDFj-39546284 had all the expected properties of a star-forming galaxy at z  10, i.e., a demonstrated absence of flux blue-ward of 1.6μm and a blue spectral slope red-ward of the break as evidenced by its faintness in the available IRAC observations. However, the source was only detected in a single band (Ellis et al. 2013).

Fig.1: Gray-scale image of high redshift object UDFj-39546284 from HUDF WFC3/IR (Credit: NASA, ESA, G. Illingworth (University of California, Santa Cruz), R. Bouwens (University of California, Santa Cruz, and Leiden University), and the HUDF09 Team.)

This slight problem led to a new study. Using new, deep WFC3/IR F160W observations over the HUDF09/XDF field, Bouwens et al. (2012) tested the reality of the oldest galaxy ever known candidate. They found that UDFj-39546284 is detected at 4.3 in the new observations, and at 7.1 in the full 77-orbit H160-band image. This demonstrates at > 99.99% confidence that this z  ~ 10 candidate corresponds to a real source.

N.B. The previous record holder was UDFy-38135539 (a.k.a “HUDF.YD3″): [The Discovery Of UDFy-38135539: Z ~ 8 In The Hubble Ultra-Deep Field].

Journal References:

• Bouwens, R. J. et al. (2012) Confirmation Of The z~10 Candidate UDFj-39546284 Using Deeper WFC3/IR+ACS+IRAC Observations Over The HUDF09/XDF. ArXix: [http://arxiv.org/abs/1211.3105].

• Ellis, R.S. et al. (2013) The Abundance Of Star-Forming Galaxies In The Redshift Range 8.5-12: New Results From The 2012 Hubble Ultra Deep Field Campaign. The Astrophysical Journal: Letters, 763 (1): Article I.D.: L7.