UKIDSS Research Articles

The clustering and abundance of star-forming and passive galaxies atz∼ 2

We use the UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Ultra-deep survey (UDS), currently the deepest panoramic near-infrared survey, together with deep Subaru optical imaging to measure the clustering, number counts and luminosity function of galaxies at z∼ 2 selected using the BzK selection technique. We find that both star-forming (sBzK) and passive (pBzK) galaxies, to a magnitude limit of KAB < 23, are strongly clustered. The passive galaxies are the most strongly clustered population, with scalelengths of r0= 15.0+1.9−2.2h−1Mpc compared with r0= 6.75+0.34−0.37h−1Mpc for star-forming galaxies. The direct implication is that passive galaxies inhabit the most massive dark matter haloes, and are thus identified as the progenitors of the most massive galaxies at the present day. In addition, the pBzKs exhibit a sharp flattening and potential turnover in their number counts, in agreement with other recent studies. This plateau cannot be explained by the effects of incompleteness. We conclude that only very massive galaxies are undergoing passive evolution at this early epoch, consistent with the downsizing scenario for galaxy evolution. Assuming a purely passive evolution for the pBzKs from their median redshift to the present day, their luminosity function suggests that only ∼2.5 per cent of present-day massive ellipticals had a pBzK as a main progenitor.

Exploring the substellar temperature regime down to ∼550 K

We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ULAS J133553.45+113005.2 (ULAS1335). We detail optical and near-infrared photometry for all three sources, and mid-infrared photometry for ULAS1335. We use near-infrared spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1238) and T9 (ULAS1335) to these objects. ULAS1017 is classed as a peculiar T8 (T8p) due to appearing as a T8 dwarf in the J-band, whilst exhibiting H and K-band flux ratios consistent with a T6 classification. Through comparison to BT-Settl model spectra we estimate that ULAS1017 has 750K T8 dwarfs to the rest of the T dwarf sequence, the suggestion of the Y0 spectral class for these objects is premature. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600K, possibly combined with low-gravity and/or high-metallicity. We find ULAS1335 to be extremely red in near to mid-infrared colours, with H [4.49] = 4.34 ± 0.04 . This is the reddest near to mid-infrared colour yet observed for a T dwarf. The near to mid-infrared spectral energy distribution of ULAS1335 further supports Teff < 600K, and we estimate Teff � 550 600K for ULAS1335. We estimate that ULAS1335 has an age of 0.6–5.3 Gyr, a mass of 15–31 MJ and lies at a distance of 8–12 pc.

Fifteen new T dwarfs discovered in the UKIDSS Large Area Survey

We present the discovery of fifteen new T2.5-T7.5 dwarfs (with estimated distances between ~24-93pc, identified in the first three main data releases of the UKIRT Infrared Deep Sky Survey. This brings the total number of T dwarfs discovered in the Large Area Survey (to date) to 28. These discoveries are confirmed by near infrared spectroscopy, from which we derive spectral types on the unified scheme of Burgasser et al. (2006). Seven of the new T dwarfs have spectral types of T2.5-T4.5, five have spectral types of T5-T5.5, one is a T6.5p, and two are T7-7.5. We assess spectral morphology and colours to identify T dwarfs in our sample that may have non-typical physical properties (by comparison to solar neighbourhood populations). The colours of the full sample of LAS T dwarfs show a possible trend to bluer Y-J with decreasing effective temperature beyond T8. By accounting for the main sources of incompleteness (selection, follow-up and spatial) as well as the effects of unresolved binarity and Malmquist bias, we estimate that there are 17+-4 >=T4 dwarfs in the J<=19 volume of the LAS second data release. Comparing this to theoretical predictions is most consistent with a sub-stellar mass function exponent alpha between -1.0 and 0. This is consistent with the latest 2MASS/SDSS constraint (which is based on lower number statistics), and is significantly lower than the alpha~1.0 suggested by L dwarf field populations, possibly a result of the lower mass range probed by the T dwarf class.

Luminous K-band selected quasars from UKIDSS

Abstract The largest K-band flux-limited sample of luminous quasars to date has been constructed from the UKIRT (UK Infrared Telescope) Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release, covering an effective area of 12.8 deg2. Exploiting the K-band excess (KX) of all quasars with respect to foreground stars, including quasars experiencing dust reddening and objects with non-standard spectral energy distributions (SEDs), a list of targets suitable for spectroscopic follow-up observations with the AAOmega multi-object spectrograph is constructed, resulting in more than 200 confirmed active galactic nuclei (AGN). KX selection successfully identifies as quasar candidates objects that are excluded from the Sloan Digital Sky Survey (SDSS) quasar selection algorithm due to their colours being consistent with the stellar locus in optical colour space (with the space density of the excluded objects agreeing well with results from existing completeness analyses). Nearly half of the KX-selected quasars with K≤ 17.0 at z &amp;lt; 3 are too faint in the i band to have been targeted by the SDSS quasar selection algorithm, revealing a large population of quasars with red i−K colours. The majority of these objects have significant amounts of host galaxy light contributing to their K-band magnitudes, consistent with previous predictions. The remaining objects are morphologically stellar and have colours consistent with quasars experiencing Small Magellanic Cloud (SMC)-type reddening with 0.10 &amp;lt; E(B−V) &amp;lt; 0.25. The i−K colour distribution indicates that &amp;lt;10 per cent of the quasar population is missing from this K-band selected sample due to dust reddening, and comparisons with simulations strongly favour an obscured fraction of &amp;lt;20 per cent. Photometric redshifts and classifications are computed for the candidates that were not observed spectroscopically. For the extended objects whose colours are consistent with those of a reddened quasar, models of galaxy surface brightness profiles appropriate for each object are used to eliminate the possibility of the presence of a nuclear source bright enough for inclusion in a K≤ 17.0 quasar sample. The effectiveness of near-infrared colour selection of quasars has been demonstrated by this modest-sized sample, and it will only become more apparent as the amount of available data increases.

ULAS J234311.93-005034.0: a gravitational lens system selected from UKIDSS and SDSS

We report the discovery of a new gravitational lens system. This object, ULAS J234311.93-005034.0, is the first to be selected by using the new UKIRT Infrared Deep Sky Survey (UKIDSS), together with the Sloan Digital Sky Survey (SDSS). The ULAS J234311.93-005034.0 system contains a quasar at redshift 0.788 which is doubly imaged, with separation 1.4 arcsec. The two quasar images have the same redshift and similar, though not identical, spectra. The lensing galaxy is detected by subtracting point spread functions from R-band images taken with the Keck telescope. The lensing galaxy can also be detected by subtracting the spectra of the A and B images, since more of the galaxy light is likely to be present in the latter. No redshift is determined from the galaxy, although the shape of its spectrum suggests a redshift of about 0.3. The object's lens status is secure, due to the identification of two objects with the same redshift together with a lensing galaxy. Our imaging suggests that the lens is found in a cluster environment, in which candidate arc-like structures, that require confirmation, are visible in the vicinity. Further discoveries of lenses from the UKIDSS survey are likely as part of this programme, due to the depth of UKIDSS and its generally good seeing conditions.

A SURVEY OFz∼ 6 QUASARS IN THE SLOAN DIGITAL SKY SURVEY DEEP STRIPE. I. A FLUX-LIMITED SAMPLE ATzAB&lt; 21

W e present the discovery ofve quasars at z � 6 selected from 260 deg 2 ofthe Sloan Digital Sky Survey (SDSS)southern survey,a deep im aging survey obtained by repeatedly scanning a stripe along the CelestialEquator. Theve quasarswith 20 < zA B < 21 are 1{2 m agnitudesfainterthan the lum inous z � 6 quasarsdiscovered in the SDSS m ain survey. One ofthem was independently discovered by the UKIRT Infrared Deep Sky Survey. These quasars,com bined with another z � 6 quasarknown in thisregion,m ake a com plete ux-lim ited quasarsam ple at zA B < 21. The sam ple spanstheredshiftrange5:85 � z � 6:12 and the lum inosity range 26:5 � M 1450 � 25:4 (H0 = 70 km s 1 M pc 1 ,m = 0:3,and � = 0:7).W eusethe 1=Va m ethod to determ ine thatthe com oving quasarspatialdensity at hzi = 6:0 and hM 1450i = 25:8 is (5:0 � 2:1) � 10 9 M pc 3 m ag 1 . W e m odelthe bright-end quasarlum inosity function (QLF)at z � 6 asa powerlaw �(L1450) / L � . The slopecalculated from a com bination ofoursam ple and the lum inousSDSS quasarsam ple is 3:1 � 0:4,signicantly steeper than the slope ofthe QLF at z � 4. Based on the derived QLF, wend thatthe quasar/AGN population cannotprovide enough photonsto ionize the intergalactic m edium (IGM )at z � 6 unlessthe IGM isvery hom ogeneousand the lum inosity (L � )atwhich theQLF powerlaw breaksisvery low. Subjectheadings: galaxies:active| quasars:em ission lines| quasars:general

The WFCAM Science Archive

We describe the WFCAM Science Archive, which is the primary point of access for users of data from the wide-field infrared camera WFCAM on the United Kingdom Infrared Telescope (UKIRT), especially science catalogue products from the UKIRT Infrared Deep Sky Survey. We describe the database design with emphasis on those aspects of the system that enable users to fully exploit the survey data sets in a variety of different ways. We give details of the database-driven curation applications that take data from the standard nightly pipeline-processed and calibrated files for the production of science-ready survey data sets. We describe the fundamentals of querying relational databases with a set of astronomy usage examples, and illustrate the results.

On the evolution of clustering of 24-μm-selected galaxies

This paper investigates the clustering properties of a complete sample of 1041 24-μm-selected sources brighter than F24 μm= 400 μJy in the overlapping region between the Spitzer Wide-Area Infrared Extragalactic (SWIRE) and UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) surveys. With the help of photometric redshift determinations we have concentrated on the two interval ranges z=[0.6–1.2] (low-z sample) and z≥ 1.6 (high-z sample) as it is in these regions were we expect the mid-infrared (IR) population to be dominated by intense dust-enshrouded activity such as star formation and black hole accretion. Investigations of the angular correlation function produce an amplitude A∼ 0.010 for the high-z sample and A∼ 0.0055 for the low-z one. The corresponding correlation lengths are r0= 15.9+2.9−3.4 and 8.5+1.5−1.8 Mpc, showing that the high-z population is more strongly clustered. Comparisons with physical models for the formation and evolution of large-scale structure reveal that the high-z sources are exclusively associated with very massive (M≳ 1013M⊙) haloes, comparable to those which locally host groups-to-clusters of galaxies and are very common within such (rare) structures. Conversely, lower z galaxies are found to reside in smaller haloes (Mmin∼ 1012M⊙) and to be very rare in such systems. On the other hand, mid-IR photometry shows that the low-z and high-z samples include similar objects and probe a similar mixture of active galactic nucleus (AGN) and star-forming galaxies. While recent studies have determined a strong evolution of the 24-μm luminosity function between z∼ 2 and 0, they cannot provide information on the physical nature of such an evolution. Our clustering results instead indicate that this is due to the presence of different populations of objects inhabiting different structures, as active systems at z≲ 1.5 are found to be exclusively associated with low-mass galaxies, while very massive sources appear to have concluded their active phase before this epoch. Finally, we note that the small-scale clustering data seem to require steep (ρ∝r−3) profiles for the distribution of galaxies within their haloes. This is suggestive of close encounters and/or mergers which could strongly favour both AGN and star formation activity.

A wide deep infrared look at the Pleiades with UKIDSS: new constraints on the substellar binary fraction and the low-mass initial mass function

Abstract We present the results of a deep wide-field near-infrared survey of 12 deg2 of the Pleiades conducted as part of the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) Galactic Cluster Survey (GCS). We have extracted over 340 high-probability proper motion (PM) members down to 0.03 M⊙ using a combination of UKIDSS photometry and PM measurements obtained by cross-correlating the GCS with data from the Two Micron All Sky Survey, the Isaac Newton Telescope and the Canada–France–Hawaii Telescope. Additionally, we have unearthed 73 new candidate brown dwarf (BD) members on the basis of five-band UKIDSS photometry alone. We have identified 23 substellar multiple system candidates out of 63 candidate BDs from the (Y−K, Y) and (J−K, J) colour–magnitude diagrams, yielding a binary frequency of 28–44 per cent in the 0.075−0.030 M⊙ mass range. Our estimate is three times larger than the binary fractions reported from high-resolution imaging surveys of field ultracool dwarfs and Pleiades BDs. However, it is marginally consistent with our earlier ‘peculiar’ photometric binary fraction of 50 ± 10 per cent presented by Pinfield et al., in good agreement with the 32–45 per cent binary fraction derived from the recent Monte Carlo simulations of Maxted &amp; Jeffries and compatible with the 26 ± 10 per cent frequency recently estimated by Basri &amp; Reiners. A tentative estimate of the mass ratios from photometry alone seems to support the hypothesis that binary BDs tend to reside in near equal-mass ratio systems. In addition, the recovery of four Pleiades members targeted by high-resolution imaging surveys for multiplicity studies suggests that half of the binary candidates may have separations below the resolution limit of the Hubble Space Telescope or current adaptive optics facilities at the distance of the Pleiades (a ∼7 au). Finally, we have derived luminosity and mass functions from the sample of photometric candidates with membership probabilities. The mass function is well modelled by a lognormal peaking at 0.24 M⊙ and is in agreement with previous studies in the Pleiades.

The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z similar or equal to 2 rom the UKIDSS Ultra Deep Survey Early Data Release

The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z similar or equal to 2 rom the UKIDSS Ultra Deep Survey Early Data Release

The colour selection of distant galaxies in the UKIDSS Ultra Deep Survey Early Data Release

Abstract We investigate colour selection techniques for high-redshift galaxies in the UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra Deep Survey Early Data Release (UDS EDR). Combined with very deep Subaru optical photometry, the depth (KAB= 22.5) and area (0.62 deg2) of the UDS EDR allow us to investigate optical/near-infrared selection using a large sample of over 30 000 objects. By using the B−z′, z′−K colour–colour diagram (the BzK technique) we identify over 7500 candidate galaxies at z &amp;gt; 1.4, which can be further separated into passive and star-forming systems (pBzKs and sBzKs respectively). Our unique sample allows us to identify a new feature not previously seen in BzK diagrams, consistent with the passively evolving track of early-type galaxies at z &amp;lt; 1.4. We also compare the BzK technique with the R−K colour selection of extremely red objects (EROs) and the J−K selection of distant red galaxies (DRGs), and quantify the overlap between these populations. We find that the majority of DRGs at these relatively bright magnitudes are also EROs. Since previous studies have found that DRGs at these magnitudes have redshifts of z∼ 1, we determine that these DRGs/EROs have spectral energy distributions consistent with being dusty star-forming galaxies or active galactic nuclei at z &amp;lt; 2. Finally, we observe a flattening in the number counts of pBzK galaxies, similar to other studies, which may indicate that we are sampling the luminosity function of passive z &amp;gt; 1 galaxies over a narrow redshift range.

Eight new T4.5-T7.5 dwarfs discovered in the UKIDSS Large Area Survey Data Release 1

We present eight new T4.5-T7.5 dwarfs identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) Data Release 1 (DR1). In addition we have recovered the T4.5 dwarf SDSS J020742.91+000056.2 and the T8.5 dwarf ULAS J003402.77-005206.7. Photometric candidates were picked up in two-colour diagrams over 190 square degrees (DR1) and selected in at least two filters. All candidates exhibit near-infrared spectra with strong methane and water absorption bands characteristic of T dwarfs and the derived spectral types follow the unified scheme of Burgasser et al. (2006). We have found 6 new T4.5-T5.5 dwarfs, one T7 dwarf, one T7.5 dwarf, and recovered a T4.5 dwarf and a T8.5 dwarf. We provide distance estimates which lie in the 15-85 pc range; the T7.5 and T8.5 dwarfs are probably within 25 pc of the Sun. We conclude with a discussion of the number of T dwarfs expected after completion of the LAS, comparing these initial results to theoretical simulations.

Two T dwarfs from the UKIDSS early data release

We report on the first ultracool dwarf discoveries from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey Early Data Release (LAS EDR), in particular the discovery of T dwarfs which are fainter and more distant than those found using the 2MASS and SDSS surveys. We aim to show that our methodologies for searching the ~27 sq degs of the LAS EDR are successful for finding both L and T dwarfs $via$ cross-correlation with the Sloan Digital Sky Survey (SDSS) DR4 release. While the area searched so far is small, the numbers of objects found shows great promise for near-future releases of the LAS and great potential for finding large numbers of such dwarfs. Ultracool dwarfs are selected by combinations of their YJH(K) UKIDSS colours and SDSS DR4 z-J and i-z colours, or, lower limits on these red optical/infrared colours in the case of DR4 dropouts. After passing visual inspection tests, candidates have been followed up by methane imaging and spectroscopy at 4m and 8m-class facilities. Our main result is the discovery following CH4 imaging and spectroscopy of a T4.5 dwarf, ULASJ 1452+0655, lying ~80pc distant. A further T dwarf candidate, ULASJ 1301+0023, has very similar CH4 colours but has not yet been confirmed spectroscopically. We also report on the identification of a brighter L0 dwarf, and on the selection of a list of LAS objects designed to probe for T-like dwarfs to the survey J-band limit. Our findings indicate that the combination of the UKIDSS LAS and SDSS surveys provide an excellent tool for identifying L and T dwarfs down to much fainter limits than previously possible. Our discovery of one confirmed and one probable T dwarf in the EDR is consistent with expectations from the previously measured T dwarf density on the sky.

The optical and near-infrared properties of 2837 quasars in the United Kingdom Infrared Telescope Infrared Deep Sky Survey

The United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of hemispheric imaging projects to extend the work of the Two-Micron All-Sky Survey (2MASS) by reaching three magnitudes deeper in YJHK imaging, to K = 18.2 (5σ , Vega) over wide fields. Better complementing existing optical surveys, such as the Sloan Digital Sky Survey (SDSS), the resulting public imaging catalogues provide new photometry of rare object samples too faint to be reached previously. The first data release of the UKIDSS has already surpassed the 2MASS in terms of photons gathered, and using this new data set we examine the near-infrared (NIR) properties of 2837 quasars found in the SDSS and newly catalogued by the UKIDSS in ∼189 deg 2 . The matched quasars include the right ascension range 22 h ‐4 h on the Southern Equatorial Stripe (SDSS Stripe 82), an area of significant future follow-up possibilities with deeper surveys and pointed observations. The sample covers the redshift and absolute magnitude ranges 0.08 z 5.03 and −29.5 Mi −22.0, and 98 per cent of SDSS quasars have matching UKIDSS data. We discuss the photometry, astrometry, and various colour properties of the quasars. We also examine the effectiveness of a quasar‐star separation using the NIR passbands. The combination of SDSS ugriz photometry with the YJHK NIR photometry from the UKIDSS over large areas of the sky has enormous potential for advancing our understanding of the quasar population.

The United Kingdom Infrared Telescope Infrared Deep Sky Survey First Data Release

The First Data Release (DR1) of the UKIRT Infrared Deep Sky Survey (UKIDSS) took place on 2006 July 21. UKIDSS is a set of five large near-infrared surveys, covering a complementary range of areas, depths, and Galactic latitudes. DR1 is the first large release of survey-quality data from UKIDSS and includes 320 sq degs of multicolour data to (Vega) K=18, complete (depending on the survey) in three to five bands from the set ZYJHK, together with 4 sq degs of deep JK data to an average depth K=21. In addition the release includes a similar quantity of data with incomplete filter coverage. In JHK, in regions of low extinction, the photometric uniformity of the calibration is better than 0.02 mag. in each band. The accuracy of the calibration in ZY remains to be quantified, and the same is true of JHK in regions of high extinction. The median image FWHM across the dataset is 0.82 arcsec. We describe changes since the Early Data Release in the implementation, pipeline and calibration, quality control, and archive procedures. We provide maps of the areas surveyed, and summarise the contents of each of the five surveys in terms of filters, areas, and depths. DR1 marks completion of 7 per cent of the UKIDSS 7-year goals.

The UKIRT Infrared Deep Sky Survey Early Data Release

This paper defines the UKIRT Infrared Deep Sky Survey (UKIDSS) Early Data Release (EDR). UKIDSS is a set of five large near-infrared surveys being undertaken with the United Kingdom Infrared Telescope Wide Field Camera (WFCAM). The programme began in 2005 May and has an expected duration of 7 yr. Each survey uses some or all of the broad-band filter complement ZY JHK. The EDR is the first public release of data to the European Southern Observatory (ESO) community. All worldwide releases occur after a delay of 18 months from the ESO release. The EDR provides a small sample data set, ∼50 deg2 (about 1 per cent of the whole of UKIDSS), that is a lower limit to the expected quality of future survey data releases. In addition, an EDR+ data set contains all EDR data plus extra data of similar quality, but for areas not observed in all of the required filters (amounting to ∼220 deg2). The first large data release, DR1, will occur in mid-2006. We provide details of the observational implementation, the data reduction, the astrometric and photometric calibration and the quality control procedures. We summarize the data coverage and quality (seeing, ellipticity, photometricity, depth) for each survey and give a brief guide to accessing the images and catalogues from the WFCAM Science Archive.

Galaxy clusters at 0.6 &amp;lt; z &amp;lt; 1.4 in the UKIDSS Ultra Deep Survey Early Data Release

Abstract We present the first cluster catalogue extracted from the UKIRT Infrared Deep Sky Survey (UKIDSS) Early Data Release. The catalogue is created using UKIDSS Ultra Deep Survey infrared J and K data combined with 3.6- and 4.5-μm Spitzer bands and optical BVRi′z′ imaging from the Subaru Telescope over 0.5 deg2 in the Subaru XMM–Newton Deep Field. We have created a new cluster detection algorithm, based on the friends-of-friends and Voronoi tessellation methods, which utilizes probability distribution functions derived from a photometric redshift analysis. We employ mock catalogues to understand the selection effects and contamination associated with the algorithm. The cluster catalogue contains 13 clusters at redshifts 0.61 ≤z≤ 1.39 with luminosities 10 L*≲Ltot≲ 50 L*, corresponding to masses 5 × 1013≲Mcluster≲ 3 × 1014 M⊙ for (M/M⊙)/(L/L⊙) = 75 h. The measured sky surface density of ∼10 clusters deg−2 for high-redshift (z= 0.5–1.5), massive (&amp;gt; 1014 M⊙) clusters is precisely in line with theoretical predictions presented by Kneissl et al.

New members in the Upper Scorpius association from the UKIRT Infrared Deep Sky Survey Early Data Release*†

We present the results of a 9.3-deg 2 infrared (ZYJHK) survey in the Upper Scorpius association extracted from the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Cluster Survey Early Data Release. We have selected a total of 112 candidates from the (Z - J, Z) colour-magnitude diagram over the Z = 12.5-20.5 magnitude range, corresponding to M = 0.25-0.01 M ⊙ at an age of 5 Myr and a distance of 145 pc. Additional photometry in J and K filters revealed most of them as reddened stars, leaving 32 possible members. Among them, 15 have proper motion consistent with higher-mass members from Hipparcos and optical spectra with strong Ha in emission and weak gravity features. We have also extracted two lower-mass candidate members for which no optical spectra are in hand. Three members exhibit strong Ha equivalent widths (>20 A), suggesting that they could still undergo accretion, whereas two other dwarfs show signs of chromospheric activity. The likelihood of the binarity of a couple of new stellar and substellar members is discussed as well.

The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours

The UKIRT Infrared Deep Sky Survey is a set of five surveys of complementary combinations of area, depth, and Galactic latitude, which began in 2005 May. The surveys use the UKIRT Wide Field Camera (WFCAM), which has a solid angle of 0.21deg^2. Here we introduce and characterise the ZYJHK photometric system of the camera, which covers the wavelength range 0.83-2.37 microns. We synthesise response functions for the five passbands, and compute colours in the WFCAM, SDSS and 2MASS bands, for brown dwarfs, stars, galaxies and quasars of different types. We provide a recipe for others to compute colours from their own spectra. Calculations are presented in the Vega system, and the computed offsets to the AB system are provided, as well as colour equations between WFCAM filters and the SDSS and 2MASS passbands. We highlight the opportunities presented by the new Y filter at 0.97-1.07 microns for surveys for hypothetical Y dwarfs (brown dwarfs cooler than T), and for quasars of very--high redshift, z>6.4.

The possible detection of high-redshift Type II QSOs in deep fields

The colours of high-redshift Type II quasi-stellar objects (QSOs) are synthesized from observations of moderate-redshift systems. It is shown that Type II QSOs are comparable to starbursts at matching the colours of z850-dropouts and i775-drops in the Hubble UltraDeep Field, and more naturally account for the bluest objects detected. Type II QSOs may also account for some of the i775-drops detected in the Great Observatories Origins Deep Survey (GOODS) fields. It is shown that by combining imaging data from the Hubble Space Telescope and the James Webb Space Telescope, it will be possible to clearly separate Type II QSOs from Type I QSOs and starbursts based on their colours. Similarly, it is shown that the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) ZYJ filters may be used to discriminate high-redshift Type II QSOs from other objects. If Type II QSOs are prevalent at high redshifts, then active galactic nuclei (AGNs) may be major contributors to the re-ionization of the intergalactic medium.