A peculiar galaxy appears at redshift 11: properties of a moderate‐redshift interloper

• Published in: Monthly notices of the Royal Astronomical Society. Letters 2012-09, Vol.425 (1), p.L19-L23
• Main Authors: Hayes, Matthew, Laporte, Nicolas, Pelló, Roser, Schaerer, Daniel, Le Borgne, Jean‐François
• Format: Article
• Language: English
• Subjects: galaxies: evolution; galaxies: high‐redshift; galaxies: individual: A2667‐J1; galaxies: photometry; galaxies: star formation
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1111/j.1745-3933.2012.01293.x

ABSTRACT In 2011 Laporte et al. reported a very high redshift galaxy candidate: a lensed J‐band dropout (A2667‐J1). J1 has a photometric redshift of z= 9.6–12, the probability density function for which permits no low‐ or intermediate‐z solution. We here report new spectroscopic observations of this galaxy with Very Large Telescope/X‐Shooter, which show clear [O iii] λ5007 Å, Lyα, Hα and Hβ emission and place the galaxy firmly at z= 2.082. The oxygen lines contribute only ∼25 per cent to the H‐band flux and do not significantly affect the dropout selection of J1. After correcting the broad‐band fluxes for line emission, we identify two roughly equally plausible natures for A2667‐J1: it is either a young heavily reddened starburst or a maximally old system with a very pronounced 4000‐Å break, upon which a minor secondary burst of star formation is superimposed. Fits show that to make a 3σ detection of this object in the B band (V band), imaging of depth AB = 30.2 (29.5) would be required – despite the relatively bright near‐infrared (NIR) magnitude, we would need optical data of equivalent depth to the Hubble Ultra Deep Field to rule out the mid‐z solution on purely photometric grounds. Assuming that this stellar population can be scaled to the NIR magnitudes of recent Hubble Space Telescope/Wide Field Camera 3 (WFC3) IR‐selected galaxies, we conclude that unfeasibly deep optical data (reaching AB ∼ 32) would be required for the same level of security. There is a population of galaxies at z≈ 2 with continuum colours alone that mimic those of our z= 7–12 candidates.