# XMM-LSS Field
The XMM-LSS field is an area of sky surveyed by the XMM-Newton X-ray observatory under the XMM Large-Scale Structure Survey (XMM-LSS), a medium-deep, wide-area X-ray survey designed to use galaxy clusters as tracers of the cosmic web for cosmological studies. The survey was defined to cover 64 square degrees with a sensitivity about 1000 times better than the [ROSAT All-Sky Survey](https://www.edgepedia.org/rosat-all-sky-survey), and the field eventually received roughly 11 deg² of contiguous XMM coverage, making it the largest contiguous deep XMM cluster survey.[1](https://arxiv.org/abs/astro-ph/0202117) • [2](https://doi.org/10.1051/0004-6361/201015182)
| Key fact | Detail |
|---|---|
| Instrument and goal | XMM-Newton survey of galaxy clusters as tracers of large-scale structure, targeting a well-defined statistical sample to redshift unity for cosmological studies[1](https://arxiv.org/abs/astro-ph/0202117) |
| Planned area | 64 square degrees, with sensitivity about 1000 times better than the ROSAT All-Sky Survey[1](https://arxiv.org/abs/astro-ph/0202117) |
| Achieved X-ray coverage | About 11 deg² at a sensitivity of ~10−14 erg s−1 cm−2 (0.5–2 keV) for extended sources, the largest contiguous deep XMM cluster survey[2](https://doi.org/10.1051/0004-6361/201015182) |
| Cluster yield | 52 Class 1 clusters cataloged over the extended 11 deg²; 29 clusters over the initial 5 deg² out to z = 1.05[3](https://doi.org/10.1093/mnras/stu1625) • [4](https://arxiv.org/abs/0709.1950) |
| Point sources | More than 6,000 X-ray point-like sources to a flux of 3 × 10−15 erg s−1 cm−2 (0.5–2.0 keV) over 11.1 square degrees[5](https://heasarc.gsfc.nasa.gov/W3Browse/xmm-newton/xmmlssclas.html) |
| Multi-wavelength setting | Overlaps the CFHTLS D1 deep field, the VIMOS VLT Deep Survey, and the VLA-VIRMOS Deep Field radio survey[6](https://academic.oup.com/mnras/article/372/2/591/972242) |
| Legacy | The field was chosen as one of the two survey areas of the XXL survey, with ~11 deg² covered by 10–20 ks XMM exposures[7](https://doi.org/10.1051/0004-6361/201526766) |
## Survey design and scientific motivation
The survey was motivated by cosmological arguments for a medium-sensitivity, large-area X-ray survey using galaxy clusters as tracers of the cosmic network. Its designers identified two main goals that constrained the survey design: the evolutionary study of the cluster–cluster correlation function and of the cluster number density, extending the cosmological tests attempted with ROSAT cluster samples to two redshift bins between 0 < z < 1 while maintaining the precision of earlier studies.[1](https://arxiv.org/abs/astro-ph/0202117) • [8](https://arxiv.org/abs/astro-ph/0305191)
A defining feature of the design was uniform selection: cluster candidates were detected and classified with canonical criteria applied identically over the survey area, so that the sample carries an explicit selection function (the rules determining which objects a survey can detect). This makes the cluster counts usable for tests of cosmological models and the growth of structure, since the number counts of clusters are sensitive to both.[6](https://academic.oup.com/mnras/article/372/2/591/972242) • [3](https://doi.org/10.1093/mnras/stu1625)
## Location and multi-wavelength coverage
The surveyed region lies at 36° < RA < 37°, −5° < Dec. < −4°. This region includes D1, one of the four deep areas of the Canada–France–Hawaii Telescope Legacy Survey ([CFHTLS](https://www.edgepedia.org/canada-france-hawaii-telescope-legacy-survey)), and also overlaps one of the VIMOS VLT Deep Survey patches and the VLA-VIRMOS Deep Field, which was imaged at 1.4 GHz to the μJy level.[6](https://academic.oup.com/mnras/article/372/2/591/972242) The field is also covered by the PRism MUlti-object Survey (PRIMUS) redshift survey, which used its X-ray and infrared data alongside those of COSMOS, CDFS, and ELAIS-S1 to compare AGN selection techniques.[9](https://doi.org/10.1088/0004-637x/770/1/40)
## Cluster samples
**Early samples.** An initial sample of twelve galaxy groups and clusters was selected as extended X-ray sources from a 3.5 deg² XMM image mosaic above a flux limit of 8 × 10−15 erg s−1 cm−2 in the 0.5–2 keV band; optical spectroscopy placed each source at 0.29 < z < 0.56, and the sample showed little or no evidence for X-ray luminosity evolution at a given X-ray temperature compared to lower-redshift group and cluster samples.[10](https://arxiv.org/abs/astro-ph/0508003) The first catalog over the CFHTLS D1 area contained 13 spectroscopically confirmed, X-ray-selected clusters over 0.8 deg² to a redshift of unity, described at the time as the highest-density cluster sample to date; cluster bolometric luminosities ranged from 0.03 to 5 × 1044 erg s−1, and slightly more clusters with 0.5–2.0 keV fluxes above 2 × 10−14 erg s−1 cm−2 were detected than expected from deep ROSAT surveys.[6](https://academic.oup.com/mnras/article/372/2/591/972242)
**Larger samples.** A Class 1 sample of 29 galaxy clusters was later compiled over the initial 5 deg², reaching z = 1.05.[4](https://arxiv.org/abs/0709.1950) Over the extended 11 deg², the Class 1 sample grew to 52 clusters with measured X-ray properties.[3](https://doi.org/10.1093/mnras/stu1625) The survey's full expectations were more ambitious: some 600 new galaxy clusters out to z ~ 1.5–2 and more than 10,000 active galactic nuclei out to z ~ 4.[11](https://cesam.lam.fr/xmm-lss/)
## Cosmological results and limitations
The redshift distribution of the Class 1 clusters over the extended 11 deg² showed a deficit of sources compared to cosmological expectations, whether [Wilkinson Microwave Anisotropy Probe](https://www.edgepedia.org/wilkinson-microwave-anisotropy-probe)-9 or Planck-2013 cosmic microwave background parameters were assumed. The authors pointed out that a systematically evolving, unresolved central component in clusters and groups, from AGN contamination or a cool core, can affect the classification of sources as extended and may partly account for the observed redshift distribution.[3](https://doi.org/10.1093/mnras/stu1625)
## Legacy
The XMM-LSS field became the foundation for later work. The XXL survey, the largest program on [XMM-Newton](https://www.edgepedia.org/xmm-newton), selected the XMM-LSS field as one of its two survey areas precisely because about 11 deg² already had 10–20 ks XMM exposures, supplemented by the XMM-Subaru Deep Survey.[7](https://doi.org/10.1051/0004-6361/201526766) The field's data products, including the catalog of more than 6,000 X-ray point sources with optical counterparts and redshifts, remain archived at NASA's HEASARC as the XMMLSSCLAS database.[5](https://heasarc.gsfc.nasa.gov/W3Browse/xmm-newton/xmmlssclas.html)
## References
1. [M. Pierre, I. Valtchanov, A. Refregier (2002). The XMM Large Scale Structure Survey: Scientific Motivation and First Observations. arXiv.](https://arxiv.org/abs/astro-ph/0202117)
2. [XMM-LSS survey: optical assessment and properties of different X-ray selected cluster classes. Astronomy & Astrophysics.](https://doi.org/10.1051/0004-6361/201015182)
3. [XMM-LSS survey: the Class 1 cluster sample over the extended 11 deg2 and its spatial distribution. Monthly Notices of the Royal Astronomical Society.](https://doi.org/10.1093/mnras/stu1625)
4. [The XMM-LSS survey: the Class 1 cluster sample over the initial 5 square degrees and its cosmological modelling. arXiv.](https://arxiv.org/abs/0709.1950)
5. [XMMLSSCLAS – XMM-Newton Large-Scale Structure Optical Counterparts and Redshifts, NASA HEASARC.](https://heasarc.gsfc.nasa.gov/W3Browse/xmm-newton/xmmlssclas.html)
6. [XMM Large-Scale Structure survey: a well-controlled X-ray cluster sample over the D1 CFHTLS area. Monthly Notices of the Royal Astronomical Society.](https://academic.oup.com/mnras/article/372/2/591/972242)
7. [The XXL Survey – I. Scientific motivations, XMM-Newton observing plan, follow-up observations and simulation programme. Astronomy & Astrophysics.](https://doi.org/10.1051/0004-6361/201526766)
8. [M. Pierre et al. (2003). The XMM-LSS survey. Survey design and first results. arXiv.](https://arxiv.org/abs/astro-ph/0305191)
9. [PRIMUS: Infrared and X-ray AGN Selection Techniques at 0.2 < z < 1.2. The Astrophysical Journal.](https://doi.org/10.1088/0004-637x/770/1/40)
10. [J. P. Willis et al. (2005). The XMM Large-Scale Structure Survey: An initial sample of galaxy groups and clusters to a redshift z<0.6. arXiv.](https://arxiv.org/abs/astro-ph/0508003)
11. [XMM-LSS Survey home page, LAM CESAM.](https://cesam.lam.fr/xmm-lss/)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Galaxies and large-scale structure › Galaxy groups, clusters, and large-scale structure › Redshift surveys and galaxy mapping*
*Initially written Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license