A new deep infrared survey has produced a more continuous view of the crowded central regions of the Large and Small Magellanic Clouds, with about one million detected sources in the LMC and 920,000 in the SMC. The observations were taken across roughly 40 epochs in each of two filters, giving the dataset more continuous spatial coverage than the VMC survey.
The study is presented as an arXiv preprint describing a dataset. Its aim was to resolve stars fainter than the red clump and support studies of motion, changing brightness, star formation and structure in the inner Clouds.
A long campaign in crowded fields
The programme used 160 hours of VISTA observations collected from December 2020 to January 2023. It covered one approximately 1.77-square-degree tile in each galaxy, observing in the near-infrared J and Ks bands. The repeated visits give the central fields a time dimension for both deeper source detection and measurements involving motion or changing brightness.
The standard VISTA processing route corrected detector effects and the sky background, combined jittered exposures and pawprints, built source catalogues, and applied position and brightness calibration. A separate PSF-photometry analysis, a way of measuring sources in crowded images, was run in each band for individual epochs, half-tiles and stacked images. The nearest sources were then matched between bands within one arcsecond.
Across the catalogue, the researchers report about 2.5 million detections. Eight in 10 were seen in both filters. Of those two-filter detections, about two-thirds had star-like profiles and one-third had galaxy-like profiles, while 0.3% were classified as noise or saturated objects. The profile labels are not definitive: galaxy-like profiles can represent blends or background galaxies.
The counts are quality-filtered rather than a complete census. The catalogue retained detections with a ppErrBits quality value below 256, a rule excluding detector 16, underexposed areas, stacked-frame edges, saturation and troublesome calibration. Absolute completeness, meaning the share of real sources that the process recovers, has not yet been measured and will require artificial-star tests.
Sharper images, and an early motion result
The reported image widths were lower than in VMC in both bands: 0.79 plus or minus 0.08 arcseconds in J and 0.75 plus or minus 0.07 arcseconds in Ks, compared with 0.99 plus or minus 0.11 and 0.93 plus or minus 0.10 arcseconds in VMC. The quoted limiting sensitivities were close between the two surveys, at 19.53 plus or minus 0.16 and 18.68 plus or minus 0.10 magnitudes for this work, against 19.80 plus or minus 0.40 and 18.72 plus or minus 0.28 for VMC. The supplied analysis reports the comparison but does not give a formal test of its statistical significance.
A preliminary combined-band test suggests that the new observations could sharpen proper-motion measurements, which track a star's apparent movement across the sky. The reported precision was about 2.5 times better than in previous studies. Selecting sources detected in at least 70% of epochs added stars about half a magnitude fainter; the VMC-only sample was limited to sources with Ks magnitudes below 18.5.
The time coverage may also help with variable stars. The paper projects well-sampled J and Ks light curves for about 4,000 RR Lyrae stars in the LMC and 500 in the SMC, along with roughly 1,800 delta Scuti and SX Phoenicis stars. Preliminary pulsator results show small scatter around period-luminosity relations, but the broader variable-star analysis is still to come.
A possible split in the SMC's red-clump stars
One early application examined the SMC's red-clump feature in the catalogue's brightness distribution. Across the central 1.5 square degrees, the preliminary analysis suggests that the red clump has two components rather than one. In the reported fit, the two peaks differed by 0.13 plus or minus 0.01 magnitudes, and a two-Gaussian model had 64% higher fit quality than a one-Gaussian model.
That result is an indication from one central region, not a finished explanation of the SMC's structure. The authors also point to the catalogue's potential for central star-formation-history work, reporting reduced photometric-uncertainty and sharpness distributions around the main-sequence turn-off. Final measurements of the star-formation history and other structural properties remain future applications.
Useful now, with important pieces still in progress
The VDFS-based CASU and WFAU products are publicly available through VSA and ESO SAF. The separate PSF-photometry catalogues were not released at this stage because validation was still under way; they are planned for release with their first respective publications.
For now, the dataset is best understood as a foundation for follow-up studies. Crowding and blending remain important, the source totals depend on the quality selections, and several of the most interesting results are preliminary or projected. The central questions are how complete the catalogues are, what final motions and variable-star measurements will show, and whether dedicated processing can improve studies of star formation and semi-resolved clusters.
Paper data and sources
Original title: Deep VISTA observations of the Magellanic Clouds (VMCDeep)
Authors: M. -R. L. Cioni, A. O. Omkumar, N. J. G. Cross et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-28
DOI: Not available
Original paper · Full text