The discovery of ERO counterparts to faint submillimetre galaxies

Abstract

We have used deep ground‐based imaging in the near‐infrared (near‐IR) to search for counterparts to the luminous submillimetre (submm) sources in the catalogue of Smail et al. For the majority of the submm sources the near‐IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10 per cent of the sample) we find a relatively bright near‐IR source close to the submm position, sources that were unidentified in the deep Hubble Space Telescope (HST) and ground‐based R‐band images used by Smail et al. We place limits on colours of these sources from deep high‐resolution Keck II imaging and find they have 2σ limits of (I−K)≳6.8 and (I−K)≳6.0, respectively. Both sources thus class as extremely red objects (EROs). Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. This connection provides important insights into the nature of the enigmatic ERO population and faint submm galaxies in general. From the estimated surface density of these submm‐bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide useful insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank‐field submm surveys will be extremely difficult owing to their faintness (K∼20.5, I≳26.5). Finally, we discuss the radio and submm properties of the two submm‐bright EROs discovered here and suggest that both galaxies lie at z≳2.

GRB 010222: A Burst within a Starburst

“…The red colors of ERO galaxies may be due to an old stellar population or a highly reddened population of young stars. Their space density, although not well determined, is comparable to that of SCUBA sources at these Ñux levels (Cowie et al 1994 ;Moustakas et al 1997 ;Smail et al 1999).…”

Faint Submillimeter Counts from Deep 850 Micron Observations of the Lensing Clusters A370, A851, and A2390

“…Our simulations (see x 4) suggest that interpretation of individual sources and attempts to identify counterparts are best restricted to at least the !4 subsample (see also Scott et al 2002). In the last three columns of Table 3 we give the previous submillimeter flux measurements from Smail et al (1997Smail et al ( , 1998 for the sources detected in their shallow survey, along with the R.A. and decl. offsets between our measured source positions and theirs.…”

“…Thus, in order to search for this population with SCUBA, one must observe fields with massive cluster lenses to take advantage of both gravitational amplification by the lens and reduced confusion noise. Smail, Ivison, & Blain (1997) and Smail et al (1998) pioneered this method, making the first SCUBA observations of cluster lenses. They studied seven clusters with well-constrained lens models for which it is possible to correct the observed source fluxes for lens amplification.…”

The extended counterpart of submm source Lockman 850.1

“…SMM02399, Ivison et al 1998) to fainter very red objects (e.g. Smail et al 1999;Gear et al 2000;Bertoldi et al 2000) and may include objects that are optically blank to the limit of the deepest HST images Muxlow et al 2001). Accurate source positions and identifications also test hypotheses suggesting a local nature of part of the submm sources (Lawrence 2001).…”