UKIDSS Research Articles

Galaxy Zoo: Morphologies Based on UKIDSS NIR Imaging for 71,052 Galaxies

We present morphological classifications based on Galaxy Zoo analysis of 71,052 galaxies with imaging from the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS). Galaxies were selected out of the Galaxy Zoo 2 (GZ2) sample, so also have gri imaging from the Sloan Digital Sky Survey. An identical classification tree, and vote weighting/aggregation was applied to both UKIDSS and GZ2 classifications enabling direct comparisons. With this Research Note we provide a public release of the GZ:UKIDSS morphologies and discuss some initial comparisons with GZ2.

A Partial Near-infrared Guide Star Catalog for Thirty Meter Telescope Operations

At first light, the Thirty Meter Telescope (TMT) near-infrared (NIR) instruments will be fed by a multiconjugate adaptive optics instrument known as the Narrow Field Infrared Adaptive Optics System (NFIRAOS). NFIRAOS will use six laser guide stars to sense atmospheric turbulence in a volume corresponding to a field of view of 2′, but natural guide stars (NGSs) will be required to sense tip/tilt and focus. To achieve high sky coverage (50% at the north Galactic pole), the NFIRAOS client instruments use NIR on-instrument wave front sensors that take advantage of the sharpening of the stars by NFIRAOS. A catalog of guide stars with NIR magnitudes as faint as 22 mag in the J band (Vega system), covering the TMT-observable sky, will be a critical resource for the efficient operation of NFIRAOS, and no such catalog currently exists. Hence, it is essential to develop such a catalog by computing the expected NIR magnitudes of stellar sources identified in deep optical sky surveys using their optical magnitudes. This paper discusses the generation of a partial NIR Guide Star Catalog (IRGSC), similar to the final IRGSC for TMT operations. The partial catalog is generated by applying stellar atmospheric models to the optical data of stellar sources from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) optical data and then computing their expected NIR magnitudes. We validated the computed NIR magnitudes of the sources in some fields by using the available NIR data for those fields. We identified the remaining challenges of this approach. We outlined the path for producing the final IRGSC using the Pan-STARRS data. We have named the Python code to generate the IRGSC as irgsctool, which generates a list of NGS for a field using optical data from the Pan-STARRS 3pi survey and also a list of NGSs having observed NIR data from the UKIRT Infrared Deep Sky Survey if they are available. irgsctool is available in the public domain on this GitHub public repository (https://github.com/sshah1502/irgsc), while the generated and validated IRGSC for the 20 test fields and additional Pan-STARRS Medium Deep Survey fields can be found on Zenodo.

A CCD search for variable stars in the open cluster NGC 6611

ABSTRACT We present the results of the UBVIC variability survey in the young open cluster NGC 6611 based on observations obtained during 34 nights spanning one year. In total, we found 95 variable stars. Most of these stars are classified as periodic and irregular pre-main sequence (PMS) stars. The analysis of the JHKS 2MASS photometry and four-colour IRAC photometry revealed 165 Class II young stellar sources, 20 of which are irregular variables and one is an eclipsing binary. These classifications, complemented by JHK UKIDSS photometry and riHα VPHAS photometry, were used to identify 24 candidates for classical T Tauri stars and 30 weak-lined T Tauri stars. In addition to the PMS variables, we discovered eight δ Scuti candidates. None of these were previously known. Furthermore, we detected 17 eclipsing binaries where two were previously known. Based on the proper motions provided by the Gaia EDR3 catalogue, we calculated the cluster membership probabilities for 91 variable stars. For 61 variables, a probability higher than 80 per cent was determined, which makes them cluster members. Only 25 variables with a probability less than 20 per cent were regarded to be non-members.

Deep near-infrared survey towards the W40 and Serpens South region in the Aquila Rift: A comprehensive catalogue of young stellar objects

ABSTRACT Active star-forming regions are excellent laboratories for studying the origins and evolution of young stellar object (YSO) clustering. The W40–Serpens South region is such a region, and we compile a large near- and mid-infrared catalogue of point sources in it, based on deep near-infrared observations of Canada-France-Hawaii Telescope (CFHT) in combination with Two Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and Spitzer catalogues. From this catalogue, we identify 832 YSOs, and classify 15, 135, 647, and 35 of them to be deeply embedded sources, Class I YSOs, Class II YSOs, and transition disc sources, respectively. In general, these YSOs are well correlated with the filamentary structures of molecular clouds, especially the deeply embedded sources and the Class I YSOs. The W40 central region is dominated by Class II YSOs, but in the Serpens South region, half of the YSOs are Class I. We further generate a minimum spanning tree (MST) for all the YSOs. Around the W40 cluster, there are eight prominent MST branches that may trace the vestigial molecular gas filaments that once fed gas to the central natal gas clump. Of the eight, only two now include detectable filamentary gas in Herschel data and corresponding Class I YSOs, while the other six are populated exclusively with Class II YSOs. Four MST branches overlap with the Serpens South main filament, and where they intersect, molecular gas ‘hubs’ and more Class I YSOs are found. Our results imply a mixture of YSO distributions composed of both primordial and somewhat evolved YSOs in this star-forming region.

BFS 10: a nascent bipolar H ii region in a filamentary molecular cloud

ABSTRACT We present a study of the compact blister H ii region BFS 10 and its highly filamentary molecular cloud. We utilize 12CO observations from the Five College Radio Astronomy Observatory to determine the distance, size, mass, and velocity structure of the molecular cloud. Infrared observations obtained from the UKIRT Infrared Deep Sky Survey and the Spitzer Infrared Array Camera, as well as radio continuum observations from the Canadian Galactic Plane Survey, are used to extract information about the central H ii region. This includes properties such as the ionizing photon rate and infrared luminosity, as well as identifying a rich embedded star cluster associated with the central O9 V star. Time-scales regarding the expansion rate of the H ii region and lifetime of the ionizing star reveal a high likelihood that BFS 10 will develop into a bipolar H ii region. Although the region is expected to become bipolar, we conclude from the cloud’s velocity structure that there is no evidence to support the idea that star formation at the location of BFS 10 was triggered by two colliding clouds. A search for embedded young stellar objects (YSOs) within the molecular cloud was performed. Two distinct regions of YSOs were identified: one region associated with the rich embedded cluster and another sparse group associated with an intermediate-mass YSO.

Completing the protostellar luminosity function in Cygnus-X with SOFIA/FORCAST imaging

ABSTRACT We present a new SOFIA/FORCAST mid-infrared survey of luminous protostars and crowded star-forming environments in Cygnus X, the nearest million-solar mass molecular cloud complex. We derive bolometric luminosities for over 1000 sources in the region with these new data in combination with extant Spitzer and UKIDSS photometry, with 63 new luminous protostar candidates identified by way of the high-quality SOFIA/FORCAST data. By including FORCAST data, we construct protostellar luminosity functions (PLFs) with improved completeness at the high luminosity end. The PLFs are well described by a power-law function with an index of ∼−0.5. Based on the Herschel temperature and column density measurements, we find no obvious dependence of the PLFs on the local gas temperature, but PLFs in regions of high stellar density or gas column density exhibit some excess at higher luminosities. Through the comparison between our observed PLFs and existing accretion models, both the turbulent core and the competitive accretion models are consistent with our results, while the isothermal sphere model is disfavoured. The implications of these results on the star formation process are discussed.

Gas Dynamics in the Star-forming Region G18.148–0.283: Is It a Manifestation of Two Colliding Molecular Clouds?

Abstract We report the results obtained from a multiwavelength study of the H ii region G18.148−0.283 using the upgraded Giant Metrewave Radio Telescope at 1350 MHz, along with other archival data. In addition to the radio continuum emission, we have detected the H169α and H170α radio recombination lines toward G18.148−0.283 using a correlator bandwidth of 100 MHz. The moment-1 map of the ionized gas reveals a velocity gradient of approximately 10 km s−1 across the radio continuum peaks. The 12CO (J = 3−2) molecular line data from the CO High-Resolution Survey (COHRS) also show the presence of two velocity components that are very close to the velocities detected in the ionized gas. The spectrum and position–velocity diagram from CO emission reveal molecular gas at an intermediate-velocity range bridging the velocity components. We see mid-infrared absorption and far-infrared emission establishing the presence of a filamentary infrared dark cloud, the extent of which includes the targeted H ii region. The magnetic field inferred from dust polarization is perpendicular to the filament within the H ii region. We have also identified two O9 stars and 30 young stellar objects toward the target using data from the Two Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE). Cumulatively, this suggests that the region is the site of a cloud–cloud collision that has triggered massive star formation and subsequent formation of an H ii region.

Detailed analysis of the poorly studied northern open cluster NGC 1348 using multi-color photometry and GAIA EDR3 astrometry

Abstract The membership determination for open clusters in noisy environments of the Milky Way is still an open problem. In this paper, our main aim is to provide the membership probability of stars using proper motions and parallax values of stars using Gaia Third Data Release astrometry. Apart from the Gaia astrometry, we have also used other photometric data sets like the UKIRT Infrared Deep Sky Survey, WISE, the American Association of Variable Star Observers (AAVSO) Photometric All-Sky Survey, and Pan-STARRS1 in order to understand cluster properties from optical to mid-infrared regions. We selected 438 likely members with membership probabilities higher than $50\%$ and G ≤ 20 mag. We obtained the mean value of proper motion as μx = 1.27 ± 0.001 and μy = −0.73 ± 0.002 mas yr−1. The cluster’s radius is determined as ${7{^{\prime }_{.}}5}$ (5.67 pc) using radial density profile. Our analysis suggests that NGC 1348 is located at a distance of 2.6 ± 0.05 kpc. The mass function slope is found to be 1.30 ± 0.18 in the mass range 1.0–4.1 M⊙, which is in fair agreement with Salpeter’s value within the 1σ uncertainty. The present study validates that NGC 1348 is a dynamically relaxed cluster. We computed the apex coordinates (A, D) for NGC 1348 as (A°, D°) = (−23.°815 ± 0.°135, −22.°228 ± 0.°105). In addition, calculations of the velocity ellipsoid parameters (VEPs), matrix elements μij, direction cosines (lj, mj, nj), and the galactic longitude of the vertex have been conducted in this analysis.

The Araucaria Project: Deep Near-infrared Photometric Maps of Local and Sculptor Group Galaxies. I. Carina, Fornax, and Sculptor

Abstract Deep near-infrared (NIR) J- and K-band photometry of three Local Group dwarf spheroidal galaxies—Fornax, Carina, and Sculptor—is made available for the community. Until now, these data have only been used by the Araucaria Project to determine distances using the tip of the red giant branch and RR Lyrae stars. Now, we present the entire data collection in the form of a database, consisting of accurate J- and K-band magnitudes, sky coordinates, ellipticity measurements, and timestamps of observations, complemented by stars’ loci in their reference images. The depth of our photometry reaches about 22 mag at the 5σ level, and is comparable to NIR surveys, such as the UKIRT Infrared Deep Sky Survey (UKIDSS) or the VISTA Hemisphere Survey (VHS). Small overlap with VHS and no overlap with UKIDSS makes our database a unique source of quality photometry.

Compact galaxies and the size–mass galaxy distribution from a colour-selected sample at 0.04 < z < 0.15 supplemented by ugrizYJHK photometric redshifts

ABSTRACT The size–mass galaxy distribution is a key diagnostic for galaxy evolution. Massive compact galaxies are potential surviving relics of a high-redshift phase of star formation. Some of these could be nearly unresolved in Sloan Digital Sky Survey (SDSS) imaging and thus not included in galaxy samples. To overcome this, a sample was selected from the combination of SDSS and UKIRT Infrared Deep Sky Survey (UKIDSS) photometry to r < 17.8. This was done using colour–colour selection, and then by obtaining accurate photometric redshifts (photo-z) using scaled flux matching (SFM). Compared to spectroscopic redshifts (spec-z), SFM obtained a 1σ scatter of 0.0125 with only 0.3 per cent outliers (|Δln (1 + z)| > 0.06). A sample of 163 186 galaxies was obtained with 0.04 < z < 0.15 over $2300\, {\rm deg}^2$ using a combination of spec-z and photo-z. Following Barro et al. log Σ1.5 = log M* − 1.5log r50, maj was used to define compactness. The spectroscopic completeness was 76 per cent for compact galaxies (log Σ1.5 > 10.5) compared to 92 per cent for normal-sized galaxies. This difference is primarily attributed to SDSS ‘fibre collisions’ and not the completeness of the main galaxy sample selection. Using environmental overdensities, this confirms that compact quiescent galaxies are significantly more likely to be found in high-density environments compared to normal-sized galaxies. By comparison with a high-redshift sample from 3D-HST, log Σ1.5 distribution functions show significant evolution, with this being a compelling way to compare with simulations such as EAGLE. The number density of compact quiescent galaxies drops by a factor of about 30 from z ∼ 2 to log (n/Mpc−3) = − 5.3 ± 0.4 in the SDSS–UKIDSS sample. The uncertainty is dominated by the steep cut off in log Σ1.5, which is demonstrated conclusively using this complete sample.

Long-term NIR variability in the UKIDSS Ultra Deep Survey: a new probe of AGN activity at high redshift

ABSTRACT We present the first attempt to select active galactic nuclei (AGN) using long-term near-infrared (NIR) variability. By analysing the K-band light curves of all the galaxies in the UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra Deep Survey, the deepest NIR survey over ∼1 deg2, we have isolated 393 variable AGN candidates. A comparison to other selection techniques shows that only half of the variable sources are also selected using either deep Chandra X-ray imaging or Infrared Array Camera (IRAC) colour selection, suggesting that using NIR variability can locate AGN that are missed by more standard selection techniques. In particular, we find that long-term NIR variability identifies AGN at low luminosities and in host galaxies with low stellar masses, many of which appear relatively X-ray quiet.

Faint Standards for ZYJHK from the UKIDSS and VISTA Surveys

ABSTRACT The currently defined ‘United Kingdom Infrared Telescope (UKIRT) Faint Standards’ have JHK magnitudes between 10 and 15, with Kmedian = 11.2. These stars will be too bright for the next generation of large telescopes. We have used multi-epoch observations taken as part of the UKIRT Infrared Deep Sky Survey (UKIDSS) and the Visible and Infrared Survey Telescope for Astronomy (VISTA) surveys to identify non-variable stars with JHK magnitudes in the range 16–19. The stars were selected from the UKIDSS Deep Extragalactic Survey and Ultra Deep Survey, the WFCAM calibration data (WFCAMCAL08B), the VISTA Deep Extragalactic Observations (VIDEO), and UltraVISTA. Sources selected from the near-infrared databases were paired with the Pan-STARRS Data Release 2 of optical to near-infrared photometry and the Gaia astrometric Data Release 2. Colour indices and other measurements were used to exclude sources that did not appear to be simple single stars. From an initial selection of 169 sources, we present a final sample of 81 standard stars with ZYJHK magnitudes, or a subset, each with 20 to 600 observations in each filter. The new standards have Ksmedian = 17.5. The relative photometric uncertainty for the sample is <0.006 mag and the absolute uncertainty is estimated to be ≲ 0.02 mag. The sources are distributed equatorially and are accessible from both hemispheres.

A Tale of Two Clusters: An Analysis of Gas-phase Metallicity and Nebular Gas Conditions in Proto-cluster Galaxies at z ∼ 2

Abstract The ZFIRE survey has spectroscopically confirmed two proto-clusters using the MOSFIRE instrument on Keck I: one at z = 2.095 in COSMOS and another at z = 1.62 in UKIRT Infrared Deep Sky Survey (UDS). Here, we use an updated ZFIRE data set to derive the properties of ionized gas regions of proto-cluster galaxies by extracting fluxes from emission lines Hβ 4861 Å, [O iii] 5007 Å, Hα 6563 Å, [N ii] 6585 Å, and [S ii] 6716,6731 Å. We measure gas-phase metallicity of members in both proto-clusters using two indicators, including a strong-line indicator relatively independent of the ionization parameter and electron density. Proto-cluster and field galaxies in both UDS and COSMOS lie on the same Mass–Metallicity Relation with both metallicity indicators. We compare our results to recent IllustrisTNG results, which report no significant gas-phase metallicity offset between proto-cluster and field galaxies until z = 1.5. This is in agreement with our observed metallicities, where no offset is measured between proto-cluster and field populations. We measure tentative evidence from stacked spectra that indicate UDS high-mass proto-cluster and field galaxies have differing [O iii]/Hβ ratios; however, these results are dependent on the sample size of the high-mass stacks.

Variable stars in young open cluster NGC 2244

ABSTRACT We present results of a UBVIC variability survey in the young open cluster NGC 2244. In total, we found 245 variable stars. Most of them, 211 stars, are variables with irregular variations. Furthermore, 23 periodic variables were found. We also detected four candidates for δ Scuti stars and seven eclipsing binaries. Based on the mid-infrared Spitzer and WISE photometry and near infrared JHKS 2MASS photometry, we classified 104 young stellar sources among our variables: 1 Class I object, 1 Class I/flat spectrum object, 4 flat spectrum objects, 91 Class II objects, and 7 transition disc objects. This classification, together with r′i′Hα IPHAS photometry and JHK UKIDSS photometry, were used for identification of pre-main sequence stars among irregular and periodic variables. In this way, 97 CTTS candidates (96 irregular and one periodic variable), 68 WTTS candidates (54 irregular and 14 periodic variables), and 6 Herbig Ae/Be stars were found. For 223 variable stars we calculated membership probability based on proper motions from Gaia DR2 catalogue. Majority of them, 143 stars, are cluster members with probability greater than 70 per cent. For only 36 variable stars the membership probability is smaller than 20 per cent.

Primeval very low-mass stars and brown dwarfs – VI. Population properties of metal-poor degenerate brown dwarfs

ABSTRACT We presented 15 new T dwarfs that were selected from UKIRT Infrared Deep Sky Survey, Visible and Infrared Survey Telescope for Astronomy , and Wide-field Infrared Survey Explorer surveys, and confirmed with optical to near-infrared spectra obtained with the Very Large Telescope and the Gran Telescopio Canarias. One of these new T dwarfs is mildly metal-poor with slightly suppressed K-band flux. We presented a new X-shooter spectrum of a known benchmark sdT5.5 subdwarf, HIP 73786B. To better understand observational properties of brown dwarfs, we discussed transition zones (mass ranges) with low-rate hydrogen, lithium, and deuterium burning in brown dwarf population. The hydrogen burning transition zone is also the substellar transition zone that separates very low-mass stars, transitional, and degenerate brown dwarfs. Transitional brown dwarfs have been discussed in previous works of the Primeval series. Degenerate brown dwarfs without hydrogen fusion are the majority of brown dwarfs. Metal-poor degenerate brown dwarfs of the Galactic thick disc and halo have become T5+ subdwarfs. We selected 41 T5+ subdwarfs from the literature by their suppressed K-band flux. We studied the spectral-type–colour correlations, spectral-type–absolute magnitude correlations, colour–colour plots, and HR diagrams of T5+ subdwarfs, in comparison to these of L–T dwarfs and L subdwarfs. We discussed the T5+ subdwarf discovery capability of deep sky surveys in the 2020s.

A homogeneous sample of 34 000 M7−M9.5 dwarfs brighter than J = 17.5 with accurate spectral types

The space density of late M dwarfs, subtypes M7–M9.5, is not well determined. We applied the photo-type method to iz photometry from the Sloan Digital Sky Survey and YJHK photometry from the UKIRT Infrared Deep Sky Survey, over an effective area of 3070 deg2, to produce a new, bright J(Vega) < 17.5, homogeneous sample of 33 665 M7–M9.5 dwarfs. The typical S/N of each source summed over the six bands is > 100. Classifications are provided to the nearest half spectral subtype. Through a comparison with the classifications in the BOSS Ultracool Dwarfs (BUD) spectroscopic sample, the typing is shown to be accurately calibrated to the BUD classifications and the precision is better than 0.5 subtypes rms; i.e. the photo-type classifications are as precise as good spectroscopic classifications. Sources with large χ2 > 20 include several catalogued late-type subdwarfs. The new sample of late M dwarfs is highly complete, but there is a bias in the classification of rare peculiar blue or red objects. For example, L subdwarfs are misclassified towards earlier types by approximately two spectral subtypes. We estimate that this bias affects only ∼1% of the sources. Therefore the sample is well suited to measure the luminosity function and investigate the softening towards the Galactic plane of the exponential variation of density with height.

HST F160W Imaging of Very Massive Galaxies at 1.5 < z < 3.0: Diversity of Structures and the Effect of Close Pairs on Number Density Estimates

Abstract We present a targeted follow-up Hubble Space Telescope WFC3 F160W imaging study of very massive galaxies ( ) selected from a combination of ground-based near-infrared galaxy surveys (UltraVISTA, NEWFIRM Medium Band Survey-II , UKIRT Infrared Deep Sky Survey (UKIDSS) Ultra-Deep Survey (UDS) at 1.5 < z < 3). We find that these galaxies are diverse in their structures, with ∼1/3 of the targets being composed of close pairs, and span a wide range in sizes. At 1.5 < z < 2.5, the sizes of both star-forming and quiescent galaxies are consistent with the extrapolation of the stellar mass–size relations determined at lower stellar masses. At 2.5 < z < 3.0, however, we find evidence that quiescent galaxies are systematically larger than expected based on the extrapolation of the relation derived using lower stellar mass galaxies. We used the observed light profiles of the blended systems to decompose their stellar masses and investigate the effect of the close pairs on the measured number densities of very massive galaxies in the early universe. We estimate correction factors to account for close-pair blends and apply them to the observed stellar mass functions (SMFs) measured using ground-based surveys. Given the large uncertainties associated with this extreme population of galaxies, there is currently little tension between the (blending-corrected) number density estimates and predictions from theoretical models. Although we currently lack the statistics to robustly correct for close-pair blends, we show that this is a systematic effect that can reduce the observed number density of very massive galaxies by up to a factor of ∼1.5, and should be accounted for in future studies of SMFs.

Broadband Spectral Energy Distributions of SDSS-selected Quasars and of Their Host Galaxies: Intense Activity at the Onset of AGN Feedback

Abstract We present the mean spectral energy distribution (SED) of a sample of optically selected quasars (QSOs) at redshifts of 1 ≤ z ≤ 5. To derive it, we exploit photometric information from the Sloan Digital Sky Survey, UKIRT Infrared Deep Sky Survey, and Wide-field Infrared Survey Explorer surveys in combination with a stacking analysis of Herschel, AKARI, and Planck maps at the location of the QSOs. The near-UV and optical parts of the reconstructed mean rest-frame SED are similar to those found in other studies. However, the SED shows an excess at 1–2 μm (when compared to the aforementioned SEDs normalized in the near UV) and a prominent bump around 4–6 μm, followed by a decrease out to ∼20 μm and a subsequent far-IR bump. From the fitted SEDs, we estimate the average active galactic nucleus (AGN) luminosity L AGN and star formation rate (SFR) as a function of cosmic time, finding typical erg s−1 and SFR ∼50–1000 M ⊙ yr−1. We develop mid-IR-based criteria to split the QSO sample, finding that these allow us to move along the average relationship in the SFR versus L AGN diagram toward increasing AGN luminosities. When interpreted in the context of the in situ coevolution scenario presented by Lapi et al., our results suggest that the detection in the far-IR band is an effective criterion to select objects where the star formation is on the verge of being affected by energy/momentum feedback from the central AGN.

Verification of Photometric Parallaxes with Gaia DR2 Data

Results of comparison of Gaia DR2 parallaxes with data derived from a combined analysis of 2MASS (Two Micron All-Sky Survey), SDSS (Sloan Digital Sky Survey), GALEX (Galaxy Evolution Explorer), and UKIDSS (UKIRT Infrared Deep Sky Survey) surveys in four selected high-latitude | b | > 48° sky areas are presented. It is shown that multicolor photometric data from large modern surveys can be used for parameterization of stars closer than 4400 pc and brighter than g S D S S = 19 . m 6 , including estimation of parallax and interstellar extinction value. However, the stellar luminosity class should be properly determined.

Primeval very low-mass stars and brown dwarfs – IV. New L subdwarfs, Gaia astrometry, population properties, and a blue brown dwarf binary

We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of these new objects were followed up with the X-shooter instrument on the Very Large Telescope. We studied the photometric properties of the population of known L subdwarfs using colour-spectral type diagrams and colour-colour diagrams, by comparison with L dwarfs and main-sequence stars, and identified new colour spaces for L subdwarf selection/study in current and future surveys. We further discussed the brown dwarf transition-zone and the observational stellar/substellar boundary. We found that about one-third of 66 known L subdwarfs are substellar objects, with two-thirds being very low-mass stars. We also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude corrections, and tangential velocities of 20 known L subdwarfs observed by the Gaia astrometry satellite. One of our L subdwarf candidates, ULAS J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a ~L6p dwarf and a ~T4p dwarf. This binary is likely a thick disc member according to its kinematics.