Bibcode
Le Borgne, Jean-François; Schaerer, Daniel; Pelló, Roser; Laporte, N.; Hayes, Matthew
Bibliographical reference
Monthly Notices of the Royal Astronomical Society: Letters, Volume 425, Issue 1, pp. L19-L23.
Advertised on:
9
2012
Citations
26
Refereed citations
22
Description
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. Vacuum wavelengths are quoted.