Using K-and R-band imaging of the ELAIS N2 field, we investigate the number counts, clustering, morphology and radio/X-ray emission of extremely red objects (EROs), defined as galaxies with R − K > 5.0. This criterion will select old, passive ellipticals at z > 0.9. To K = 21 we identify a total of 158 EROs in 81.5 arcmin 2 . The ERO number counts are lower than predicted by pure luminosity evolution models, but higher than predicted by current cold dark matter-based hierarchical models. The ERO counts are consistent with a non-evolving model and also with a luminosity evolution model incorporating moderate merging and a decrease with redshift in the comoving number density of passive galaxies ('M-DE').We investigate the clustering of the EROs by calculating their angular correlation function, ω(θ ), and obtain a >2σ detection of clustering at K =19-20 limits. The ω(θ ) amplitude of these EROs is much higher than that of full K-limited samples of galaxies, and is best-fitted by models with a comoving correlation radius r 0 10-13 h −1 Mpc. These results, which are in agreement with Daddi et al., suggest that the intrinsic clustering of at least the brighter EROs is even stronger than that of present-day giant ellipticals.We estimate seeing-corrected angular sizes and morphological types for a K 19.5 subsample of EROs (31 galaxies) and find a ∼3:2 mixture of bulge and disc profiles. Of these EROs ∼ 1 4 appear to be interacting, disturbed or otherwise irregular, and two are visible mergers. We find the angular sizes of the bulge-profile EROs are consistent with passively evolving ellipticals in the M-DE model, at the expected z ∼ 0.9-2. The ERO mean radii are smaller than the non-evolving prediction, implying surface brightness evolution.Seven of the 31 bright EROs are detected as F(1.4GHz) 30 µJy radio sources in a VLA survey. The strongest, at 5 mJy, is also a Chandra X-ray detection, and lies at the centre of a significant overdensity of EROs -it is probably an FRI radio galaxy in a z ∼ 1 cluster. Of the other, much fainter, sources, five are point-like and may be weak active galactic nuclei, while the sixth is elongated and aligned with the optical axis of an extended, low-surface brightness ERO, suggesting a powerful but dust-reddened starburst.A possible interpretation is discussed in which the EROs are a mixture of (i) 'pEROs', strongly clustered passively evolving giant ellipticals, formed at high redshifts, the oldest EROs and (ii) 'dsfEROs', dusty post-interaction galaxies, with a few active starbursts (ULIRGs), and less strongly clustered. With time, the younger dsfEROs are continually assimilated into the ERO class, diluting the clustering and increasing the comoving number density. Both types ultimately evolve into present-day early-type galaxies.

The SCUBA 8-mJy survey is the largest submillimetre (submm) extragalactic mapping survey undertaken to date, covering 260 arcmin 2 to a 4σ detection limit of 8 mJy at 850 µm, centred on the Lockman Hole and ELAIS N2 regions. Here, we present the results of new 1.4-GHz imaging of these fields, of the depth and resolution necessary to reliably identify radio counterparts for 18 of 30 submm sources, with possible detections of a further 25 per cent. Armed with this greatly improved positional information, we present and analyse new optical, near-infrared (near-IR) and XMM-Newton X-ray imaging to identify optical/IR host galaxies to half of the submm-selected sources in those fields. As many as 15 per cent of the submm sources detected at 1.4 GHz are resolved by the 1.4-arcsec beam and a further 25 per cent have more than one radio counterpart, suggesting that radio and submm emission arise from extended starbursts and that interactions are common. We note that less than a quarter of the submm-selected sample would have been recovered by targeting optically faint radio sources, underlining the selective nature of such surveys. At least 60 per cent of the radio-confirmed optical/IR host galaxies appear to be morphologically distorted; many are composite systems -red galaxies with relatively blue companions; just over one half are found to be very red (I − K > 3.3) or extremely red (I − K > 4); contrary to popular belief, most are sufficiently bright to be tackled with spectrographs on 8-m telescopes. We find one submm source which is associated with the steep-spectrum lobe of a radio galaxy, at least two more with flatter radio spectra typical of radio-loud active galactic nuclei (AGN), one of them variable. The latter is amongst four sources (≡15 per cent of the full sample) with X-ray emission consistent with obscured AGN, though the AGN would need to be Compton thick to power the observed far-IR luminosity. We exploit our well-matched radio and submm data to estimate the median redshift of the S 850 µm ∼ 8 mJy submm galaxy population. If the radio/far-IR correlation holds at high redshift, and our sample is unbiased, we derive a conservative limit of z 2.0, or 2.4 using spectral templates more representative of known submm galaxies.

For early-type galaxies, the correlations between stellar mass and size, velocity dispersion, surface brightness, colour, axial ratio and colour gradient all indicate that two mass scales, M * = 3 × 10 10 and 2 × 10 11 M , are special. The smaller scale could mark the transition between wet and dry mergers, or it could be related to the interplay between supernovae (SNe) and active galactic nuclei (AGNs) feedback, although quantitative measures of this transition may be affected by morphological contamination. At the more massive scale, mean axial ratios and colour gradients are maximal, and above it, the colours are redder, the sizes larger and the velocity dispersions smaller than expected based on the scaling at lower M * . In contrast, the colour-σ relation, and indeed, most scaling relations with σ , are not curved: they are well described by a single power law, or in some cases, are almost completely flat. When major dry mergers change masses, sizes, axial ratios and colour gradients, they are expected to change the colours or velocity dispersions much less. Therefore, the fact that scaling relations at σ > 150 km s −1 show no features, whereas the size-M * , b/a-M * , colour-M * and colour gradient-M * relations do, suggests that M * = 2 × 10 11 M is the scale above which major mergers dominate the assembly histories of early-type galaxies.

We identify a very deep sample of 198 extremely red objects (EROs) in the Chandra Deep Field South, selected on the basis of I775−Ks > 3.92, to a limit Ks≃ 22 using the public European Southern Observatory (ESO)/Great Observatories Origins Deep Survey (GOODS) survey. The ERO number counts flatten from a slope of γ≃ 0.59 to 0.16 at K > 19.5, where they remain below the predictions for pure luminosity evolution, and fall below even a non‐evolving model. This suggests there is a significant decrease with redshift in the comoving number density of passive/very red galaxies. We investigate the angular correlation function, ω(θ), of these EROs and detect positive clustering for Ks= 20.5–22.0 sources. The EROs show stronger clustering than other galaxies at the same magnitudes. The ω(θ) amplitudes are best‐fitted by models in which the EROs have a comoving correlation radius r0≃ 12.5 ± 1.2 h−1 Mpc, or r0≃ 21.4 ± 2.0 h−1 Mpc in a stable clustering model. We find a 40‐arcsec diameter overdensity of 10 EROs, centred on the Chandra X‐ray source (and ERO) XID:58. On the basis of colours we estimate that about seven, including XID:58, belong to a cluster of EROs at z≃ 1.5. The 942‐ks Chandra survey detected 73 X‐ray sources in the area of our ERO sample, 17 of which coincide with EROs. Of these sources, 13 have X‐ray properties indicative of obscured active galactic nuclei (AGN), while the faintest four may be starbursts. In addition, we find evidence that Chandra sources and EROs are positively cross‐correlated at non‐zero (∼2–20 arcsec) separations, implying that they tend to trace the same large‐scale structures. In conclusion, these findings appear consistent with a scenario where EROs are the z > 1 progenitors of elliptical/S0 galaxies, some forming very early as massive spheroids, which are strongly clustered and may evolve via an AGN phase, others more recently from mergers of disc galaxies.

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We investigate the angular correlation function, ω(θ), and third‐ and fourth‐order hierarchical moments of a large sample of ∼70 000 galaxies with apparent magnitudes 18.5≤R≤23.5. The data consist of 47 red‐band INT Wide Field Camera CCD images, forming two widely separated fields of total areas 1.01 and 0.74 deg2. Galaxy clustering is detected with a high significance of ∼10σ. Over the R=21 to 23.5 range of magnitude limits, the angular correlation functions approximately follow θ−0.8 power laws at θ>5 arcsec, with amplitudes consistent with previous surveys and best fitted by a luminosity evolution model in which galaxy clustering is approximately stable in proper coordinates (ɛ=0). Assuming the redshift distribution from our pure luminosity evolution (PLE) model and the present‐day galaxy correlation radii from Loveday et al., we estimate the clustering evolution as for both fields combined, or for the larger of the two fields which is thought to be the better in data quality. On the larger of our fields, ω(θ) at 2≤θ≤5 significantly exceeds a ω(θ)∝θ−0.8 power law of the amplitude measured at θ>5 arcsec. If this excess of close pairs is due to interacting and merging galaxies, we estimate that it is consistent with the local fraction of galaxies in close (<19 h−1 kpc) pairs combined with a merger/interaction rate evolving as (1+z)m with . We derive the hierarchical moments s3(θ) and s4(θ) from the counts‐in‐cells of the 18.5≤R≤23.5 galaxies. We find relatively steep slopes of approximately θ−0.4 for s3(θ) and θ−0.6 for s4(θ), similar to those of the hierarchical moments of 17≤B≤20 galaxies in the EDSGCS at comparable physical scales. The s3(θ) and s4(θ) of the 18.5≤R≤23.5 galaxies show only a small reduction in normalization relative to those of the less deep EDSGCS survey, consistent with the expectations from N‐body simulations of the evolving mass distribution. This indicates there is little change in the mean linear or non‐linear biasing of galaxies from z∼0 to ∼1, and hence appears to favour luminosity evolution for galaxies over ‘transient starburst dwarf’ models, and supports the interpretation of the stable galaxy clustering as evidence for a low‐Ω universe.

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We present estimated redshifts and derived properties for a sample of 1663 galaxies with Ks <= 22 (Vega), selected from 50.4 sq.arcmin of the GOODS/CDFS field with deep ISAAC imaging, and make an extensive comparison of their properties with those of the extremely red galaxies (ERGs) selected in the same field. We study in detail the evolution of Ks-selected galaxies up to redshifts z ~ 4 and clarify the role of ERGs within the total Ks-band galaxy population. We compute the total Ks-band luminosity function (LF) and compare its evolution with the ERG LF. Up to =2.5, the bright end of the Ks-band LF shows no sign of decline, and is progressively well reproduced by the ERGs with increasing redshift. We also explore the evolution of massive systems present in our sample: up to 20%-25% of the population of local galaxies with assembled stellar mass M>1x10^11 Msun has been formed before redshift z ~ 4, and contains ~ 45% to 70% of the stellar mass density of the Universe at that redshift. Within our sample, the comoving number density of these massive systems is then essentially constant down to redshift z ~ 1.5, by which point most of them have apparently evolved into (I-Ks)-selected ERGs. The remaining massive systems observed in the local Universe are assembled later, at redshifts z <= 1.5. Our results therefore suggest a two-fold assembly history for massive galaxies, in which galaxy/star formation proceeds very efficiently in high mass haloes at very high redshift.Comment: Revised version accepted by MNRAS. 17 pages, 15 figure