Introduction

The extreme luminosities the dusty galaxy population which contributes the bulk of the extragalactic background in the far-infrared and submm has emphasised the importance of dust-obscured activity in the early evolution of massive galaxies and black holes (Blain et al. 1999; Cowie et al. 2002). The majority of the dusty, active systems detected by SCUBA appear to lie at z>1, their typical bolometric luminosities are 10¹¹-10¹³L_o and their space densities are ~10^-4Mpc^-3, 2-3 orders of magnitude higher than similar luminosity galaxies at z~0. However, little is known about the types of environment which these galaxies inhabit. Similar luminosity, dusty systems in the local Universe are typically found in low-density regions and they avoid the dense cores of rich clusters of galaxies (Tacconi et al. 2002). The situation is predicted to be very different at high redshifts where, if they truly represent the progenitors of massive ellipticals, the SCUBA galaxies should be clustered in the highest density regions (Ivison et al. 2000b).

Due to the small samples available in the submm waveband observational evidence of this clustering is tentative at the present-time (Scott et al. 2002; Webb et al. 2002). Nevertheless, there are suggestions that SCUBA galaxies are strongly clustered, in particular associations of SCUBA galaxies with other classes of clustered high-redshift sources, such as Lyman-break galaxies or X-ray sources, have been found (Chapman et al. 2001; Ledlow et al. 2002; Almaini et al. 2002). A more direct approach to tackle this question has been undertaken by Ivison et al. (2000b) in a targetted SCUBA survey of regions around high-redshift, powerful AGN. These are expected to preferentially inhabit high-density regions, a prediction which has some observational support from the discovery of excesses of both Extremely Red Objects and Ly-alpha emitters around some high-redshift radio galaxies. The first results from the Ivison et al. (2000b) survey show a significant overdensity of SCUBA galaxies around the signpost high-redshift AGN, but confirming this association with redshifts for the SCUBA sources has proved difficult.

One of the fields covered in the Ivison et al. (2000b) survey is that surrounding the z=2.39 steep-spectrum, narrow-line radio galaxy 53W002 (Windhorst et al. 1998). This region is especially interesting as it has been shown to contain an over-density of compact, Ly-alpha emission-line galaxies at z~2.4 (Pascarelle et al. 1996, 1998; Keel et al. 1999). Nine of these emission-line galaxies have been spectroscopically confirmed as companions to the radio galaxy, with a velocity dispersion of ~400 km s^-1 and a spatial extent of ~4Mpc (Keel et al. 1999). Here we report on the SCUBA observations of the 53W002 field which uncover four luminous, submm galaxies. By matching the submm source position using an astrometrically-precise 1.4-GHz map we show that one of these sources is coincident with a Ly-alpha-selected galaxy at z=2.39, 330kpc awy from the radio galaxy in projection. This confirms the presence of ultraluminous, dusty galaxies in the over-dense structure around 53W002 at a look-back time of 11Gyrs (we adopt a cosmology with q_o=0.5, and H_o=50kms^-1Mpc^-1). This work is described in full in Smail et al. (2002b).

Observations and Reduction

We observed a ~2.3'-diameter field centered on 53W002 at 450- and 850-um during 2001 March 3-6 using the SCUBA (Holland et al. 1999) on the James Clerk Maxwell Telescope (JCMT). The conditions were good and stable for the four nights when data was taken, with an 850-um opacity measured every hour of 0.12-0.23 for the first three nights, rising to 0.35 on the final night. The total exposure time was 33.3ks. The 450- and 850-um maps were created using SURF (Jenness & Lightfoot 1997). In order to extract reliable source positions and flux densities, the maps were deconvolved using the symmetric -1, +2, -1 zero-flux beam (which arises from chopping and nodding) and a modified version of the CLEAN algorithm (Hogbom 1974) as described by Ivison et al. (2000b). This process produces a restored map at the native resolution of the JCMT, 14'' FWHM, but without the negative sidelobes from the chopping of bright sources.

For the radio observations we employed the VLA in B configuration, obtaining a total of 3h of useful integration during 2001 May 14 and May 17, the resulting map has a noise level of 30uJy beam^-1, with a 5.3'' × 3.7'' beam.

Our analysis also exploits archival Hubble Space Telescope (HST) imaging of the 53W002 field in both the optical (WFPC2) and near-infrared (NICMOS), comprising three broadband exposures: 57.6-ks in F450W (B₄₅₀), 27.2-ks in F606W (V₆₀₆) and 20.4-ks in F814W (I₈₁₄), as well as a 40.5-ks exposure in the F410M medium band (which isolates redshifted Ly-alpha at z~2.4). The acquisition and reduction of the NICMOS imaging, 2.4-ks integrations in the F110W (J₁₁₀) and F160W (H₁₆₀) filters, is described in detail by Keel et al. (2002).

Near-infrared spectroscopy of several galaxies in the field of 53W002, in particular object #18, was obtained by Motohara et al. (2001a,b). These observations used the newly commissioned OH-airglow Suppression Spectrograph (OHS; Iwamuro et al. 2001) and Cooled Infrared Spectrograph and Camera for OHS, CISCO (Motohara et al. 2002) on the 8.2-m Subaru Telescope. A total of 4.8ks of integration in the K-band was obtained on the galaxy on 1999 May 3-4 with CISCO and 8ks in JH with OHS and CISCO on the night of 1999 May 21. An initial analysis of this spectrum is described in Motohara et al. (2001a).

Analysis and Results

We detect four significant sources in the 850-um SCUBA map of the 53W002 field above a 4-sigma limit of 3.7-mJy, this represents a modest excess over the blank-field expectation of 2 sources at this flux limit. One of the submm sources (SMMJ17142+5016), with an 850-um flux of 5.6+/-0.9 mJy, lies 1.3'' from a 260-uJy radio source. Using the positions of other radio sources in the field we comfirm that the radio emission arises within 1.0'' of an optical galaxy identified as #18 in Pascarelle et al. (1996). We conclude that #18 is the likely counterpart to SMMJ17142+5016 and in the following discussion we identify the galaxy as SMMJ17142+5016#18.

SMMJ17142+5016#18, is one of the brightest of the ~14 Ly-alpha emission-line sources detected at z~2.4 in a narrow-band imaging search around 53W002 (Pascarelle et al. 1996, 1998; Keel et al. 1999, 2002). As Figure 1 shows the galaxy consists of pair of components separated by 0.6'' (4.7Kpc at z=2.39). We compare the spatial distribution of the high-surface brightness Ly-alpha emission (from the F410M image) and UV continuum (given by the J₁₁₀-band, although this includes a small contribution from [OII]) in Figure 1b. This shows that a large fraction of the Ly-alpha emission from the system arises in the northern component, even though the underlying continuum in this feature is weak. This component may represent either lower-mass companion, tidal debris or scattered light from an obscured AGN. Deeper, ground-based Ly-alpha imaging of this galaxy by Keel et al. (1999) shows that it also possess a very extended Ly-alpha halo, >50kpc. The restframe optical morphology of this galaxy is thus strongly reminiscent of other well-studied SCUBA galaxies, showing merger/tidal features (Ivison et al. 1998, 2000a) which are probably related to the triggering of the ultraluminous activity in this system.

The restframe UV spectrum published by Pascarelle et al. (1996) shows a strong, but relatively narrow Ly-alpha line, weak CIV and possibly NV, confirming its redshift as z=2.393. We identify a number of strong emission lines in the OHS spectrum of SMMJ17142+5016#18 including H-alpha, H-beta, [OII]3727, [OIII]4959, 5007. All the lines are unresolved at the modest resolution of OHS (<1500kms^-1) and the [OIII] lines are particularly strong, the spectral properties are thus very similar to those of a Seyfert-2 (Motohara et al. 2001a).

Taking the 850-um flux of SMMJ17142+5016 and assuming a dust spectrum with T_d=38K and Beta=1.5, we estimate a dust mass of ~10⁸M_o, and a bolometric luminosity of (8+/-2)× 10¹²L_o. The presence of an AGN obviously provides the opportunity for some fraction of the far-infrared luminosity to come from AGN-heated dust and, as with z~0 ULIRGS, it is difficult to disentangle the contributions from star formation and AGN-heating to the overall SED. The existence of a large mass of dust, 10⁸M_o, suggests that massive star formation has occured, while the lack of X-ray detections for the general SCUBA population (Almaini et al. 2002), along with the constraints on AGN luminosities from a few well-studied SCUBA galaxies (e.g. Frayer et al. 1998; Bautz et al. 2000) both hint that only rarely does the AGN completely dominate the bolometric output from typical, luminous submm-selected sources. Hence we convert the observed bolometric luminosity into a star formation rate for >5M_o stars of ~750 M_oyr^-1 (Condon 1992), or ~4×10³M_oyr^-1 accounting for stars with masses of >0.1M_o using a Salpeter IMF. Even allowing for a dominant AGN-heated component, these estimates still suggest that the galaxy is forming stars at a rate of ~10³ M_oyr^-1 - a substantial star burst.

Discussion and Conclusions

To summarise: we confirm the identification of SMMJ17142+5016#18 as an ultraluminous infrared galaxy at z=2.4, within the protocluster containing the radio galaxy 53W002. The compact, southern component of SMMJ17142+5016#18 appears to host a Seyfert-2 nucleus, while the extended emission to the north and east is most easily explained as tidal debris which is ionised by either local star formation or radiation from the AGN. Perhaps the most interesting feature of SMMJ17142+5016#18 is the extended Ly-alpha halo around this galaxy (Keel et al. 1999, 2002). This low-surface brightness features is on a much larger scale than the high-surface brightness Ly-alpha emission in Figure 1 and extends out east from the galaxy at least 6'' (50kpc). The combination of strong submm emission and an extended Ly-alpha cloud is somewhat surprising, given the propensity for dust to absorb Ly-alpha photons, although it has been seen before (Chapman et al. 2001). Taniguchi & Shioya (2001) have suggested that this phenomenum arises from an extended superwind driven by a highly-obscured starburst, the wind then shocks and ionises the gas halo which surrounds the system (Chapman et al. 2002). This model can explain several properties of these Ly-alpha halos (although other processes are also feasible, see Chapman et al. 2001 and Francis et al. 2001) and hence deserves continued investigation, in particular studying the kinematics of the extended Ly-alpha emission around SMMJ17142+5016#18 using an integral-field spectrograph on an 8-m class telescope may identify the dynamical signatures of a superwind.

Finally, we note that the confirmation of a luminous SCUBA galaxy in a protocluster environment strengthens their associated with the formation phase of the massive ellipticals, which dominate rich clusters at the present-day. Moreover, it appears that SMMJ17142+5016#18 contributes significantly (compared to the Ly-alpha-selected population) to the star formation occuring in the structure around 53W002. We can estimate a crude lower limit to the star formation density in this region, taking the rate calculated above and assuming SMMJ17142+5016#18 is the only ultraluminous galaxy in the entire structure, which conservatively contains a volume of 4×4×4Mpc³ (Keel et al. 1999), giving a star formation density of >60M_oyr^-1Mpc^-3. Even allowing for a significant contribution from the AGN to the bolometric luminosity, this is still one to two orders of magnitude higher than the mean star formation density at this epoch (Smail et al. 2002a). This is a higher contrast than the 2-3× overdensity of Ly-alpha emitters in this region (several of which are also AGN, Keel et al. 1999) and suggests that vigorous, obscured star formation may be enhanced in the protocluster environment. The intense winds and strong radiation fields produced by this activity (which may be the cause of the extended Ly-alpha emission around this galaxy) will have profound consequences for the evolution of the other galaxies within this structure.

Acknowledgements

We thank Fumihide Iwamuro and Toshinori Maihara for obtaining the OHS spectra of SMMJ17142+5016#18, Clare Jenner for help with the SCUBA survey and Ian Waddington for the NICMOS imaging. We are indebted to all staff members of the Subaru telescope, NAOJ, who helped with the OHS observations. We acknowledge useful conversations with Andrew Blain, Scott Chapman, Cedric Lacey and Alice Shapley, as well as support from the Royal Society (IRS), the Leverhulme Trust (DGG, IRS) and PPARC (JSD, JAS).

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Figure 1: Two images demonstrating the varied morphology of SMMJ17412+5016#18 in the restframe optical/UV. These show a 9''× 9'' (70kpc at z=2.39) true-color representation constructed from the HST/WFPC2 B₄₅₀V₆₀₆I₈₁₄ imaging (left) and on the right a 3''× 3'' view which contrasts the morphology of the J₁₁₀-band, restframe near-UV continuum emission (shown as a grayscale) with the morphology in the F410M filter which is dominated by the Ly-alpha emission and is shown as a logarithmic contour plot. From these images we see that the optical counterpart to SMMJ17412+5016 comprises a compact red component to the South and a more diffuse blue structure to the North-East with an extension to the West. At z=2.39, the K-band magnitude of SMMJ17142+5016#18 corresponds to an apparent magnitude of M_V~-23, however, its present-day luminosity depends critically on the competing effects of current dust extinction and subsequent star formation and hence the z=0 absolute luminosity of this galaxy is extremely uncertain.

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