CLAUDS (see here for overview paper) is a major Canada-France-Hawaii Telescope (CFHT) program (~68 dark-time nights plus archival data) that uses the blue-sensitive CCDs of CFHT’s MegaCam mosaic imager to obtain U-band imaging of four representative regions of the sky to an unprecedented combination of depth and area. CLAUDS covers 18.60 deg2 to a median depth of U=27.1 AB mag (5σ in 2” apertures) and with median seeing of FWHM = 0.92 arcsec. Within this, selected “ultra-deep” sub-areas in the COSMOS and SXDS fields total 1.36 deg2 and reach a median depth of U=27.7 AB mag (5σ in 2” apertures).

CLAUDS images overlap with comparably-deep grizy (and narrow-band) imaging from the Deep and UltraDeep layers of Subaru’s Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). The combination of CLAUDS U and HSC-SSP optical data allows two redshift windows to be explored in great statistical detail: 

  1. z = 2-3 (“Cosmic Noon”) is made accessible through the U-dropout and BX/BM techniques,

  2. z < 1 (the epoch of declining cosmic star formation) using photometric redshifts, for which U-band is essential at these redshifts.

The combination of depth and area of the CLAUDS and HSC surveys will be unmatched until well into the LSST era and enables a wide range of studies of galaxy formation and evolution in a wide range of environments over the past 85% of the history of the Universe. In addition to containing millions of galaxies (e.g., our z=0-3 LFs are based on 4.3 million objects), the combined dataset is also ideal for the study of rare systems such as clusters and proto-clusters of galaxies, active galactic nuclei, Lyman-continuum emitters, Lyman-alpha blobs, and - closer to home - white dwarfs in the Milky Way halo.

Below is a summary of CLAUDS characteristics. For in-depth details please see the CLAUDS overview paper (Sawicki et al. 2019).

Fields and depth

CLAUDS data are distributed over four extensively-studied high-latitude fields: E-COSMOS (centred on the famous COSMOS field), DEEP2-3, ELAIS-N1, and XMM-LSS (including the SXDS). CLAUDS median depth is U=27.1 AB mag over 18.60 deg2. Additionally, selected “ultra-deep” sub-areas in the COSMOS and SXDS fields that total 1.36 deg2 reach a median depth of U=27.7 AB mag.

CLAUDS filters used and approximate locations of the four CLAUDS fields:

  1.     XMM-LSS    u*     02:22:00  -04:43:00

  2.     E-COSMOS   u,u*   10:00:30  +02:12:20            

  3.     ELAIS-N1   u      16:11:00  +55:00:00

  4.     DEEP2-3    u      23:28:00  -00:17:00


CLAUDS depth maps are shown (left), with black circles marking the nominal HSC-SSP Deep/UltraDeep pointings.


CLAUDS uses two CFHT MegaCam filters: u and u*, collectively referred to as U. Transmission curves (including instrument and atmospheric throughputs) are shown at right. The figure also shows the HSC-SSP grizy filters. Note that the u filter is bluer than the u* filter and has a sharper cut-off on the red side than does u*. It is also free of the small (but destructive for some science applications) red leak that is present in the u* filter at ~5000Å.

CLAUDS used the two U filters, u and u*, in part because the u filter was not yet available in the first semester of the CLAUDS observing campaign, and in part because we wanted to make use of extensive archival data available in the XMM-LSS and (central) COSMOS fields.

The E-COSMOS field contains data in both u and u*.  DEEP2-3 and ELAIS-N1 were imaged exclusively with the u filter.  The XMM-LSS field contains only u* imaging.

CLAUDS image data structure

Individual U-band images were pre-processed at CADC and then stacked together onto the track/patch grid used in the HSC-SSP data, including the same pixel scale scale astrometric calibration as those used in the HSC-SSP Data Releases. This arrangement makes is easy to work with data from the two different surveys. We kept u and u* image stacks separate (as is also the case for all derived measurement, such as fluxes, shapes, etc.).

Catalogs, photometric redshifts, and derived quantities

We produced object catalogs from the CLAUDS and HSC-SSP data (uu*+grizy).  We have separate catalogs that were made using two separate object detection and photometry pipelines: SExtractor and HSCpipe. Improved versions of the catalogs are generated periodically as new HSC data become available (CLAUDS data acquisition is already completed - CLAUDS U data are at full depth).

Photometric redshifts were produced using a machine-learning approach (see Sawicki et al. 2019), with the LePhare template-fitting code, and with a hybrid technique that combines the machine-learning photo-z’s with LePhare ones (shown in figure on right - see Moutard et al. (2020) for details). Physical quantities derived from photometry using SED-fitting (i.e., stellar masses etc.) were obtained using the LePhare code.

Improved versions of photometric redshifts and physical quantities will be generated as data improve, and we are in the process of measuring other properties such as parametric and non-parametric galaxy morphologies.

CLAUDS papers and science

CLAUDS-based publications to date: 

  1. The CFHT Large Area U-band Deep Survey (Sawicki et al. 2019) <--- overview paper (READ THIS FIRST)

  2. On the lack of correlation between [O III]/[O II] and Lyman continuum escape fraction (Bassett et al. 2019)

  3. HSC-XD 52: An X-Ray Detected AGN in a Low-mass Galaxy at z~0.56 (Halevi et al. 2019)

  4. UV and U-band luminosity functions from CLAUDS and HSC-SSP - I. Using four million galaxies to simultaneously constrain the very faint and bright regimes to z~3 (Moutard et al. 2020)

  5. Outside the Lyman-break box: detecting Lyman continuum emitters at 3.5<z<5.1 with CLAUDS (Meštrić et al. 2020)

  6. Orphan GRB afterglow searches with the Pan-STARRS1 COSMOS survey (Huang et al. 2020)

  7. Star formation in massive galaxies at redshift z~0.5 (Xu et al. 2020)

  8. A large massive quiescent galaxy sample at z~1.2 (Xu et al. 2020)

  9. A giant central red disk galaxy at redshift z=0.76: challenge to theories of galaxy formation (Xu et al. 2021)

Further papers have been submitted or are in preparation.

CLAUDS data access

CLAUDS data are all in hand and reduced, and we are planning to make a public release soon (probably in late 2021). If you are interested in using the CLAUDS data beforehand, please write to me directly:  marcin.sawicki@smu.ca

If you do use CLAUDS data... 

Please use the following acknowledgement text in your paper:

“These data were obtained and processed as part of the CFHT Large Area U-band Deep Survey (CLAUDS), which is a collaboration between astronomers from Canada, France, and China described in Sawicki et al. (2019, [MNRAS 489, 5202]).  CLAUDS is based on observations obtained with MegaPrime/ MegaCam, a joint project of CFHT and CEA/DAPNIA, at the CFHT which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. CLAUDS uses data obtained in part through the Telescope Access Program (TAP), which has been funded by the National Astronomical Observatories, Chinese Academy of Sciences, and the Special Fund for Astronomy from the Ministry of Finance of China. CLAUDS uses data products from TERAPIX and the Canadian Astronomy Data Centre (CADC) and was carried out using resources from Compute Canada and Canadian Advanced Network For Astrophysical Research (CANFAR).”


CLAUDS:  The CFHT Large Area U-band Deep Survey