Compact groups in theory and practice – III. Compact groups of galaxies in the Sixth Data Release of the Sloan Digital Sky Survey

We present the largest publicly available catalogue of compact groups (CGs) of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6). We identify 2297 CGs down to a limiting magnitude of   r = 18 (∼0.24 groups  deg⁻²), and 74 791 CGs down to a limiting magnitude of   r = 21 (∼6.7 groups  deg⁻²). This represents 0.9 per cent of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14 per cent level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43 per cent completeness), and we find that the median redshift of these groups is   z _med= 0.09  . The median line-of-sight (LOS) velocity dispersion within the CGs from the bright catalogue is  σ_LOS≃ 230 km s⁻¹  , and their typical intergalactic separations are of the order of 50–100 kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select CGs such that the interloper fraction is well defined and minimized. This observational data set is ideal for large statistical studies of CGs, the role of environment on galaxy evolution and the effect of galaxy interactions in determining galaxy morphology.     

The Durham/UKST Galaxy Redshift Survey — V. The catalogue

We present the radial velocities and blue, optical magnitudes for all of the galaxies within the Durham/UKST Galaxy Redshift Survey. This catalogue consists of ∼2500 galaxy redshifts to a limiting apparent magnitude of B _J⋍17 mag, covering a ∼1500-deg² area around the South Galactic Pole. The galaxies in this survey were selected from the Edinburgh/Durham Southern Galaxy Catalogue and were sampled, in order of apparent magnitude, at a rate of one galaxy in every three. The spectroscopy was performed at the 1.2-m UK Schmidt Telescope in Australia using the FLAIR multi-object spectrograph. We show that our radial velocity measurements made with this instrument have an empirical accuracy of ±150 km s⁻¹. The observational techniques and data reduction procedures used in the construction of this survey are also discussed. This survey demonstrates that the UKST can be used to make a three-dimensional map of the large-scale galaxy distribution, via a redshift survey to b _J⋍17 mag, over a wide area of the sky.     

MegaZ-LRG: a photometric redshift catalogue of one million SDSS luminous red galaxies

We describe the construction of MegaZ-LRG, a photometric redshift catalogue of over one million luminous red galaxies (LRGs) in the redshift range  0.4 < z < 0.7  with limiting magnitude   i < 20  . The catalogue is selected from the imaging data of the Sloan Digital Sky Survey (SDSS) Data Release 4. The 2dF-SDSS LRG and Quasar (2SLAQ) spectroscopic redshift catalogue of 13 000 intermediate-redshift LRGs provides a photometric redshift training set, allowing use of ann z, a neural network-based photometric-redshift estimator. The rms photometric redshift accuracy obtained for an evaluation set selected from the 2SLAQ sample is  σ _z = 0.049  averaged over all galaxies, and  σ _z = 0.040  for a brighter subsample  ( i < 19.0)  . The catalogue is expected to contain ∼5 per cent stellar contamination. The ann z code is used to compute a refined star/galaxy probability based on a range of photometric parameters; this allows the contamination fraction to be reduced to 2 per cent with negligible loss of genuine galaxies. The MegaZ-LRG catalogue is publicly available on the World Wide Web from http://www.2slaq.info .     

The 2dF Galaxy Redshift Survey: luminosity functions by density environment and galaxy type

We use the 2dF Galaxy Redshift Survey to measure the dependence of the b _J-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius  8  h ⁻¹ Mpc  , and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than   M _{b J}−5log₁₀ h ≲−18.5  . In contrast, cluster regions have a relative excess of very bright early-type galaxies with   M _{b J}−5log₁₀ h ≲−21  . Differences in the mid- to faint-end population between environments are significant: at   M _{b J}−5log₁₀ h =−18  early- and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion that differs strikingly for early- and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation.     

The K-band Hubble diagram of submillimetre galaxies and hyperluminous galaxies

We present the K -band Hubble diagrams ( K – z relations) of submillimetre-selected galaxies and hyperluminous galaxies (HLIRGs). We report the discovery of a remarkably tight K – z relation of HLIRGs, indistinguishable from that of the most luminous radio galaxies. Like radio galaxies, the HLIRG K – z relation at   z ≲ 3  is consistent with a passively evolving ∼3 L _* instantaneous starburst starting from a redshift of   z ∼ 10  . In contrast, many submillimetre-selected galaxies are ≳2 mag fainter, and the population has a much larger dispersion. We argue that dust obscuration and/or a larger mass range may be responsible for this scatter. The galaxies so far proved to be hyperluminous may have been biased towards higher AGN bolometric contributions than submillimetre-selected galaxies due to the 60-μm selection of some, so the location on the K – z relation may be related to the presence of the most massive active galactic nucleus. Alternatively, a particular host galaxy mass range may be responsible for both extreme star formation and the most massive active nuclei.     

Radio galaxies in the 2SLAQ Luminous Red Galaxy survey – II. The stellar populations of radio-loud and radio-quiet LRGs

We present an analysis of the optical spectra of a volume-limited sample of 375 radio galaxies at redshift  0.4 < z < 0.7  from the 2dF-SDSS (Sloan Digital Sky Survey) Luminous Red Galaxy (LRG) and QSO (quasi-stellar object) (2SLAQ) redshift survey. We investigate the evolution of the stellar populations and emission-line properties of these galaxies. By constructing composite spectra and comparing with a matched sample of radio-quiet sources from the same survey, we also investigate the effect on the galaxy of the presence of an active nucleus.
The composite spectra, binned by redshift and radio luminosity, all require two components to describe them, which we interpret as an old and a younger population. We found no evolution with redshift of the age of the younger population in radio galaxies, nor were they different from the radio-quiet comparison sample. Similarly, there is no correlation with radio power, with the exception that the most powerful radio sources  ( P _1.4 > 10²⁶  W Hz⁻¹) have younger stars and stronger emission lines than the less powerful sources. This suggests that we have located the threshold in radio power where strong emission lines 'switch on', at radio powers of around 10²⁶ W Hz⁻¹. Except for the very powerful radio galaxies, the presence of a currently active radio active galactic nucleus (AGN) does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at   z ∼ 0.5  .     

Hubble Space Telescope imaging of the CFRS and LDSS redshift surveys – IV. Influence of mergers in the evolution of faint field galaxies from z∼1

Hubble Space Telescope images of a sample of 285 galaxies with measured redshifts from the Canada–France Redshift Survey (CFRS) and Autofib–Low Dispersion Spectrograph Survey (LDSS) redshift surveys are analysed to derive the evolution of the merger fraction out to redshifts z ∼1. We have performed visual and machine-based merger identifications, as well as counts of bright pairs of galaxies with magnitude differences δm ≤1.5 mag. We find that the pair fraction increases with redshift, with up to ∼20 per cent of the galaxies being in physical pairs at z ∼0.75–1. We derive a merger fraction varying with redshift as ∝(1+ z )^3.2±0.6, after correction for line-of-sight contamination, in excellent agreement with the merger fraction derived from the visual classification of mergers for which m =3.4±0.6. After correcting for seeing effects on the ground-based selection of survey galaxies, we conclude that the pair fraction evolves as ∝(1+ z )^2.7±0.6. This implies that an average L * galaxy will have undergone 0.8–1.8 merger events from z =1 to z =0, with 0.5 to 1.2 merger events occuring in a 2-Gyr time-span at around z ∼0.9. This result is consistent with predictions from semi-analytical models of galaxy formation. From the simple coaddition of the observed luminosities of the galaxies in pairs, physical mergers are computed to lead to a brightening of 0.5 mag for each pair on average, and a boost in star formation rate of a factor of 2, as derived from the average [O  ii ] equivalent widths. Mergers of galaxies are therefore contributing significantly to the evolution of both the luminosity function and luminosity density of the Universe out to z ∼1.     

The clustering of colour-selected galaxies

We present measurements of the angular correlation function of galaxies selected from a B _J ∼23.5 multicolour survey of two 5°×5° fields located at high galactic latitudes. The galaxy catalogue of ∼4×10⁵ galaxies is comparable in size to catalogues used to determine the galaxy correlation function at low redshift. Measurements of the z ∼0.4 correlation function at large angular scales show no evidence for a break from a power law, although our results are not inconsistent with a break at ≳15 h⁻¹ Mpc. Despite the large fields-of-view, there are large discrepancies between the measurements of the correlation function in each field, possibly caused by dwarf galaxies within z ∼0.11 clusters near the South Galactic Pole.
Colour selection is used to study the clustering of galaxies from z ∼0 to z ∼0.4. The galaxy correlation function is found to depend strongly on colour, with red galaxies more strongly clustered than blue galaxies by a factor of ≳5 at small scales. The slope of the correlation function is also found to vary with colour, with γ∼1.8 for red galaxies and γ∼1.5 for blue galaxies. The clustering of red galaxies is consistently strong over the entire magnitude range studied, although there are large variations between the two fields. The clustering of blue galaxies is extremely weak over the observed magnitude range, with clustering consistent with r ₀∼2 h⁻¹ Mpc. This is weaker than the clustering of late-type galaxies in the local Universe, and suggests that galaxy clustering is more strongly correlated with colour than morphology. This may also be the first detection of a substantial low-redshift galaxy population with clustering properties similar to faint blue galaxies.     

Galaxy groups at 0.3 ≤z≤ 0.55 – II. Evolution to z∼ 0

We compare deep Magellan spectroscopy of 26 groups at  0.3 ≤ z ≤ 0.55  , selected from the Canadian Network for Observational Cosmology 2 field survey, with a large sample of nearby groups from the 2PIGG catalogue. We find that the fraction of group galaxies with significant [O  ii ]λ3727 emission (≥5 Å) increases strongly with redshift, from ∼29 per cent in 2dFGRS to ∼58 per cent in CNOC2, for all galaxies brighter than  ∼ M _*+ 1.75  . This trend is parallel to the evolution of field galaxies, where the equivalent fraction of emission-line galaxies increases from ∼53 to ∼75 per cent. The fraction of emission-line galaxies in groups is lower than in the field, across the full redshift range, indicating that the history of star formation in groups is influenced by their environment. We show that the evolution required to explain the data is inconsistent with a quiescent model of galaxy evolution; instead, discrete events in which galaxies cease forming stars (truncation events) are required. We constrain the probability of truncation ( P _trunc) and find that a high value is required in a simple evolutionary scenario neglecting galaxy mergers  ( P _trunc≳ 0.3 Gyr⁻¹)  . However, without assuming significant density evolution, P _trunc is not required to be larger in groups than in the field, suggesting that the environmental dependence of star formation was embedded at redshifts   z ≳ 0.45  .     

Recent arrival of faint cluster galaxies on the red sequence: luminosity functions from 119 deg2 of CFHTLS

The global star formation rate has decreased significantly since   z ∼ 1  , for reasons that are not well understood. Red-sequence galaxies, dominating in galaxy clusters, represent the population that have had their star formation shut off, and may therefore be the key to this problem. In this work, we select 127 rich galaxy clusters at  0.17 ≤ z ≤ 0.36  , from 119 deg² of the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS) optical imaging data, and construct the r '-band red-sequence luminosity functions (LFs). We show that the faint end of the LF is very sensitive to how red-sequence galaxies are selected, and an optimal way to minimize the contamination from the blue cloud is to mirror galaxies on the redder side of the colour–magnitude relation. The LFs of our sample have a significant inflexion centred at     , suggesting a mixture of two populations. Combining our survey with low-redshift samples constructed from the Sloan Digital Sky Survey, we show that there is no strong evolution of the faint end of the LF (or the red-sequence dwarf-to-giant ratio) over the redshift range  0.2 ≲ z ≲ 0.4  , but from   z ∼ 0.2  to ∼0 the relative number of red-sequence dwarf galaxies has increased by a factor of ∼3, implying a significant build-up of the faint end of the cluster red sequence over the last 2.5 Gyr.     

Multiwavelength study of the nuclei of a volume-limited sample of galaxies – I. X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 10³⁸ erg s⁻¹. The galaxies are selected from a complete, volume-limited sample of 46 galaxies with     for which we have extensive multiwavelength data. For an almost complete subsample with     (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ∼35 per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity – there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H  ii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population – out of 29 galaxies we find nine sources with a luminosity greater than 10³⁹ erg s⁻¹. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.     

Kinematic properties and stellar populations of faint early-type galaxies – I. Velocity dispersion measurements of central Coma galaxies

We present velocity dispersion measurements for 69 faint early-type galaxies in the core of the Coma cluster, spanning  −22.0 ≲ M_R ≲−17.5 mag  . We examine the   L –σ  relation for our sample and compare it to that of bright elliptical galaxies (Es) from the literature. The distribution of the the faint early-type galaxies in the   L –σ  plane follows the relation   L ∝σ^2.01±0.36  , which is significantly shallower from   L ∝σ⁴  as defined for the bright Es. While increased rotational support for fainter early-type galaxies could account for some of the difference in slope, we show that it cannot explain it. We also investigate the colour–σ relation for our Coma galaxies. Using the scatter in this relation, we constrain the range of galaxy ages as a function of their formation epoch for different formation scenarios. Assuming a strong coordination in the formation epoch of faint early-type systems in Coma, we find that most had to be formed at least 6 Gyr ago and over a short 1-Gyr period.     

Spatial clustering of Ultra Steep Spectrum sources and galaxies

We present measurements of the clustering properties of galaxies in the field of redshift range 0.5 ≲ z ≲ 1.5 Ultra Steep Spectrum radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories Very Large Array Sky Survey. Galaxies in these USS fields were identified in deep near-infrared observations, complete down to   K _s= 20  , using the IRIS2 instrument at the Anglo-Australian Telescope. We used the redshift distribution of   K _s < 20  galaxies taken from Cimatti et al. (2002) to constrain the correlation length r ₀. We find a strong correlation signal of galaxies with   K _s < 20  around our USS sample. A comoving correlation length   r ₀= 14.0 ± 2.8  h ⁻¹ Mpc  and γ= 1.98 ± 0.15 are derived in a flat cosmological model universe.
We compare our findings with those obtained in a cosmological N -body simulation populated with galform semi-analytic galaxies. We find that clusters of galaxies with masses in the range   M = 10^13.4–14.2  h ⁻¹ M_⊙  have a cluster–galaxy cross-correlation amplitude comparable to those found between the USS hosts and galaxies. These results suggest that distant radio galaxies are excellent tracers of galaxy overdensities and pinpoint the progenitors of present day rich clusters of galaxies.     

Further constraints on the evolution of Ks-selected galaxies in the GOODS/CDFS field

We have selected and analysed the properties of a sample of  2905 K_s < 21.5  galaxies in  ∼131 arcmin²  of the Great Observatories Origins Deep Survey (GOODS) Chandra Deep Field South (CDFS), to obtain further constraints on the evolution of K_s -selected galaxies with respect to the results already obtained in previous studies. We made use of the public deep multiwavelength imaging from the optical B through the infrared (IR) 4.5-μm bands, in conjunction with available spectroscopic and COMBO17 data in the CDFS, to construct an optimized redshift catalogue for our galaxy sample. We computed the K_s -band luminosity function and determined that its characteristic magnitude has a substantial brightening and a decreasing total density from   z = 0  to  〈 z 〉= 2.5  . We also analysed the colours and number density evolution of galaxies with different stellar masses. Within our sample, and in contrast to what is observed for less massive systems, the vast majority (∼85–90 per cent) of the most massive  ( M > 2.5 × 10¹¹ M_⊙)  local galaxies appear to be in place before redshift   z ∼ 1  . Around 65–70 per cent of the total assemble between redshifts   z = 1  and 3 and most of them display extremely red colours, suggesting that plausible star formation in these very massive systems should mainly proceed in obscured, short-time-scale bursts. The remaining fraction (up to ∼20 per cent) could be in place at even higher redshifts   z = 3–4  , pushing the first epoch of formation of massive galaxies beyond the limits of current near-IR surveys.     

A far-infrared view of the Lockman hole from ISO 95-μm observations – II. Optical identifications and insights into the nature of the far-infrared sources

We present the optical identifications of a 95-μm ISOPHOT sample in the Lockman hole over an area of approximately half a deg². The Rodighiero et al. catalogue includes 36 sources, making up a complete flux-limited sample for   S _95 μm≥ 100 mJy  . Reliable sources were detected, with decreasing but well-controlled completeness, down to   S _95 μm≃ 20 mJy  . We have combined mid-infrared (IR) and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15-μm associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed spectral energy distributions (SEDs). Four of these 95-μm sources have been classified as faint IR (FIR) galaxies  ( L _FIR < 1. e 11 L_⊙)  , six as luminous IR galaxies (LIRGs) and only one as an ultraluminous IR galaxy (ULIRG). We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 μm. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at   z < 0.2  ), their luminosities and star formation rates (SFR; median value  ∼ 10 M_⊙ yr⁻¹  ), the sources unveiled by ISOPHOT at 95 μm seem to correspond to the low redshift  ( z < 0.3)  FIRBACK 175-μm population, composed of dusty, star-forming galaxies with moderate SFRs. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes.     

Galaxy groups at 0.3 ≤z≤ 0.55 – I. Group properties

The evolution of galaxies in groups may have important implications for the evolution of the star formation history of the Universe, since many processes which operate in groups may suppress star formation and the fraction of galaxies in bound groups grows rapidly between   z = 1  and the present day. In this paper, we present an investigation of the properties of galaxies in galaxy groups at intermediate redshift  ( z ∼ 0.4)  . The groups were selected from the Canadian Network for Observational Cosmology Redshift Survey (CNOC2) redshift survey as described by Carlberg et al., with further spectroscopic follow-up undertaken at the Magellan telescope in order to improve the completeness and depth of the sample. We present the data for the individual groups, and find no clear trend in the fraction of passive galaxies with group velocity dispersion and group concentration. We stack the galaxy groups in order to compare the properties of group galaxies with those of field galaxies at the same redshift. The groups contain a larger fraction of passive galaxies than the field, this trend being particularly clear for galaxies brighter than   M _{B J} < −20  in the higher velocity dispersion groups. In addition, we see evidence for an excess of bright passive galaxies in the groups relative to the field. In contrast, the luminosity functions of the star-forming galaxies in the groups and the field are consistent. These trends are qualitatively consistent with the differences between group and field galaxies seen in the local Universe.     

K-band imaging of strong Ca ii-absorber host galaxies at z∼1

We present K -band imaging of fields around 30 strong Ca  ii absorption-line systems, at  0.7 < z < 1.2  , three of which are confirmed damped Lyman α systems. A significant excess of galaxies is found within 6.0 arcsec (≃50 kpc) from the absorber line of sight. The excess galaxies are preferentially luminous compared to the population of field galaxies. A model in which field galaxies possess a luminosity-dependent cross-section for Ca  ii absorption of the form  ( L / L *)^0.7  reproduces the observations well. The luminosity-dependent cross-section for the Ca  ii absorbers appears to be significantly stronger than the established  ( L / L *)^0.4  dependence for Mg  ii absorbers. The associated galaxies lie at large physical distances from the Ca  ii -absorbing gas; we find a mean impact parameter of 24 kpc  ( H ₀= 70 km s⁻¹ Mpc⁻¹)  . Combined with the observed number density of Ca  ii absorbers the large physical separations result in an inferred filling factor of only ∼10 per cent. The physical origin of the strong Ca  ii absorption remains unclear, possible explanations vary from very extended discs of the luminous galaxies to associated dwarf galaxy neighbours, remnants of outflows from the luminous galaxies, or tidal debris from cannibalism of smaller galaxies.     

A catalogue and analysis of X-ray luminosities of early-type galaxies

We present a catalogue of X-ray luminosities for 401 early-type galaxies, of which 136 are based on newly analysed ROSAT PSPC pointed observations. The remaining luminosities are taken from the literature and converted to a common energy band, spectral model and distance scale. Using this sample we fit the L _X  :  L _B relation for early-type galaxies and find a best-fit slope for the catalogue of ∼2.2. We demonstrate the influence of group-dominant galaxies on the fit and present evidence that the relation is not well modelled by a single power-law fit. We also derive estimates of the contribution to galaxy X-ray luminosities from discrete-sources and conclude that they provide     . We compare this result with luminosities from our catalogue. Lastly, we examine the influence of environment on galaxy X-ray luminosity and on the form of the     relation. We conclude that although environment undoubtedly affects the X-ray properties of individual galaxies, particularly those in the centres of groups and clusters, it does not change the nature of whole populations.     

The 2dF-SDSS LRG and QSO Survey: the spectroscopic QSO catalogue

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep,  18 < g < 21.85  (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at   z ≲ 2.6  . The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg² and contains new spectra of 16 326 objects, of which 8764 are QSOs and 7623 are newly discovered [the remainder were previously identified by the 2dF QSO Redshift Survey (2QZ) and SDSS]. The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12 702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission-line galaxies with a median redshift of   z = 0.22  .
We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at   g ≃ 21.85  QSO colours are significantly affected by the presence of a host galaxy up to redshift   z ∼ 1  in the SDSS ugriz bands. In particular, we see a significant reddening of the objects in   g − i   towards the fainter g -band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2–3 Gyr.
The full catalogue, including completeness estimates, is available on-line at http://www.2slaq.info/ .     

Clustering of galaxies at 3.6 μm in the Spitzer Wide-area Infrared Extragalactic legacy survey

We investigate the clustering of galaxies selected in the 3.6 μm band of the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular two-point correlation function is calculated for 11 samples with flux limits of S _3.6≥ 4–400 μJy, over an 8 deg² field. The angular clustering strength is measured at >5σ significance at all flux limits, with amplitudes of A = (0.49–29) × 10⁻³ at 1°, for a power-law model, A θ^−0.8. We estimate the redshift distributions of the samples using phenomological models, simulations and photometric redshifts, and so derive the spatial correlation lengths. We compare our results with the Galaxies In Cosmological Simulations (GalICS) models of galaxy evolution and with parametrized models of clustering evolution. The GalICS simulations are consistent with our angular correlation functions, but fail to match the spatial clustering inferred from the phenomological models or the photometric redshifts. We find that the uncertainties in the redshift distributions of our samples dominate the statistical errors in our estimates of the spatial clustering. At low redshifts (median z ≤ 0.5), the comoving correlation length is approximately constant,   r ₀= 6.1 ± 0.5  h ⁻¹  Mpc, and then decreases with increasing redshift to a value of 2.9 ± 0.3  h ⁻¹ Mpc for the faintest sample, for which the median redshift is z ∼ 1. We suggest that this trend can be attributed to a decrease in the average galaxy and halo mass in the fainter flux-limited samples, corresponding to changes in the relative numbers of early- and late-type galaxies. However, we cannot rule out strong evolution of the correlation length over  0.5 < z < 1  .