THE ABUNDANCE OF STAR-FORMING GALAXIES IN THE REDSHIFT RANGE 8.5-12: NEW RESULTS FROM THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN

Abstract

We present the results of the deepest search to date for star-forming\ngalaxies beyond a redshift z~8.5 utilizing a new sequence of near-infrared Wide\nField Camera 3 images of the Hubble Ultra Deep Field. This `UDF12' campaign\ncompleted in September 2012 doubles the earlier exposures with WFC3/IR in this\nfield and quadruples the exposure in the key F105W filter used to locate such\ndistant galaxies. Combined with additional imaging in the F140W filter, the\nfidelity of high redshift candidates is greatly improved. Using spectral energy\ndistribution fitting techniques on objects selected from a deep multi-band\nnear-infrared stack we find 7 promising z>8.5 candidates. As none of the\npreviously claimed UDF candidates with 8.5<z<10 is confirmed by our deeper\nmulti-band imaging, our campaign has transformed the measured abundance of\ngalaxies in this redshift range. Although we recover the candidate\nUDFj-39546284 (previously proposed at z=10.3), it is undetected in the newly\nadded F140W image, implying it lies at z=11.9 or is an intense emission line\ngalaxy at z~2.4. Although no physically-plausible model can explain the\nrequired line intensity given the lack of Lyman alpha or broad-band UV signal,\nwithout an infrared spectrum we cannot rule out an exotic interloper.\nRegardless, our robust z ~ 8.5 - 10 sample demonstrates a luminosity density\nthat continues the smooth decline observed over 6 < z < 8. Such continuity has\nimportant implications for models of cosmic reionization and future searches\nfor z>10 galaxies with JWST.\n