Shedding light on the physics of the Universe
A medium-band array turns a single visit into a low-resolution spectrum for every source in the field. That capability starts with kilonovae and reaches across the whole of time-domain and extragalactic astronomy.
Why medium bands
The primary mission of 7DT is the rapid identification of electromagnetic counterparts to gravitational-wave events. A typical kilonova peaks near an absolute magnitude of −15 to −17 and fades by roughly 0.5 mag per day, inside a localization region of hundreds to thousands of square degrees — where even a single 100 deg² patch is expected to hold of order a hundred unrelated transients over a seven-day window. Finding one demands wide-area coverage, fast response, and spectral information good enough to reject the contaminants, which is precisely what a medium-band array delivers.
The same data serve a much broader program. Low-resolution spectral mapping of the southern sky supports photometric redshift catalogs, resolved stellar populations in nearby galaxies, active galactic nuclei, variable stars, and the time-domain study of solar-system bodies. Several of the science verification observations taken during commissioning have since matured into refereed or near-complete analyzes.
Seven science themes
Multi-messenger Astronomy
The mission 7DT was built for. A kilonova peaks near absolute magnitude −15 to −17 and fades by roughly half a magnitude per day, inside a gravitational-wave localization of hundreds to thousands of square degrees — where even a single 100 deg² patch is expected to hold of order a hundred unrelated transients over a seven-day window. 7DT answers with wide-field tiling, sub-minute response to an alert, and enough spectral information in a single visit to reject the contaminants without follow-up spectroscopy.
02Transients
Medium-band spectral sampling gives direct color and continuum information for a transient from one epoch of imaging. A hybrid classification framework built on 7DT spectral energy distributions — an unsupervised anomaly detector coupled to a supervised multiclass classifier — reaches macro F1 ≈ 0.80 across eight common transient types and recovers more than 90 percent of optically detectable kilonovae, including AT2017gfo, without ever being trained on one.
03Galaxy Formation & Evolution
Medium-band mapping over 1.25 square degrees is IFU-like data at survey scale. Star-forming clumps are identified through Hα emission in stellar-continuum-subtracted images, and pilot studies indicate that pixel-based SED fitting of 7DT and SPHEREx data can recover spatially resolved stellar populations approaching the quality of high-resolution IFU spectroscopy — for galaxies that would otherwise demand dedicated campaigns on much larger telescopes. Both studies are as yet unpublished (Shim et al., submitted; Lee et al., in prep).
04Cosmology & Photometric Redshifts
At R = 30–70 across 0.4–0.9 µm, 7DT straddles the boundary between broadband photometry and low-resolution spectroscopy, capturing the 4000 Å break and prominent emission lines at a resolution that lifts much of the color–redshift degeneracy inherent to broadband surveys. Forecasts give σ_NMAD = 0.003–0.007 at 19 < m625 < 22 for the five-year stacked WTS, with strong gains when combined with SPHEREx all-sky data.
05Active Galactic Nuclei
Repeated medium-band visits measure how an AGN spectrum changes, not merely how its brightness does. The 10–14 day cadence of WTS over five years is matched to reverberation mapping through long-term spectral variability, while the nightly cadence of IMS reaches variability on timescales of a single night.
06Galactic Science
Two-epoch 7DT photometry with sixteen medium bands across 400–825 nm identified 110 variable young stellar objects in the central region of Orion A — 14 percent of 769 candidates — including seven extreme cases varying by more than 0.5 mag. The wavelength dependence of the variability distinguishes extinction-like, gray and spot-like mechanisms on day timescales, which otherwise requires rapid filter cycling or simultaneous multi-band instrumentation.
07Solar System Objects
Medium-band imaging of the main-belt asteroids 13 Egeria and 10 Hygiea targets the 0.7 µm absorption feature characteristic of Ch-type bodies, a tracer of their thermal history. Time-series observations of the third interstellar object, 3I/ATLAS, followed the emergence of extended CN emission as it fell inside 3 au — behavior resembling that of 2I/Borisov at comparable heliocentric distance.
What commissioning observed
Science verification ran alongside instrument commissioning from first light, covering the full range of intended use cases.
- 01
Main-belt asteroids 13 Egeria and 10 Hygiea, medium-band imaging of the 0.7 µm hydration feature
- 02
Orion Molecular Cloud 2/3 at two epochs on a one-day cadence, and Serpens Main at three epochs
- 03
Nineteen galaxies from the PHANGS sample, compared pixel-by-pixel against PHANGS-MUSE IFU data
- 04
The galaxy cluster PSZ G227.44−31.24 and the COSMOS field, for photometric redshift validation to z ≈ 0.5
- 05
The X-ray transient EP240219a and the gravitational-wave event S240422ed, as target-of-opportunity tests