Anomaly Explorer

What is an anomaly? A spectral autoencoder is a neural network trained to compress and reconstruct normal DESI spectra (stars, galaxies, quasars). When it encounters a spectrum that doesn’t match any learned pattern, the reconstruction is poor — producing a high residual. Objects with total residual (anomaly score) above 5.0 are flagged. These are spectra the model literally “doesn’t know what to do with.”

What the score means: The anomaly score is the sum of reconstruction errors across DESI’s three spectrograph arms. Higher = more unusual. The score tiers are:

Table Columns

Score

Total reconstruction error across all three spectrograph arms (B + R + Z). Higher = more anomalous.

RA

Right Ascension (degrees, 0–360). East-west position on the sky in the ICRS coordinate system.

Dec

Declination (degrees, -90 to +90). North-south position on the sky.

Band

Which spectrograph arm has the largest residual: B (blue, 3600–5800Å), R (red, 5760–7620Å), or Z (near-IR, 7520–9824Å).

rB

Reconstruction error in the B (blue) arm. High rB = anomalous blue-end features (e.g. unusual emission lines, UV excess).

rR

Reconstruction error in the R (red) arm. High rR = anomalous mid-optical features (e.g. unusual continuum, absorption).

rZ

Reconstruction error in the Z (near-infrared) arm. High rZ = anomalous near-IR features (e.g. high-redshift emission shifted into IR).

TID

DESI TARGETID — unique identifier for this object in the DESI DR1 catalog.

Astronomy Terms

AGN

Active Galactic Nucleus — a supermassive black hole at a galaxy’s center actively accreting matter, producing bright emission across the spectrum.

QSO

Quasi-Stellar Object (Quasar) — an extremely luminous AGN, often at high redshift (z > 1). Key tracer for large-scale structure measurements.

Near-IR

Near-Infrared — wavelengths just beyond visible red light (~7000–10000Å in the Z-band). High-redshift features shift into this range.

High-z

High redshift — objects at great cosmological distances (z > 1.5), seen as they were billions of years ago.

BAL

Broad Absorption Line — a QSO showing wide absorption troughs from high-velocity outflows. Rare (~10% of QSOs) and often missed by pipelines.

PSF

Point Spread Function — the image of a point source (star or distant QSO). “PSF morphology” means it looks like a point, not an extended galaxy.

REX

Round Exponential — a Legacy Survey morphology classification for a small, round, slightly extended source.

SER

Sérsic profile — a Legacy Survey classification for galaxies fit with a Sérsic surface brightness profile.

SIMBAD

Set of Identifications, Measurements and Bibliography for Astronomical Data — the most comprehensive database of known astronomical objects (CDS, Strasbourg).

NED

NASA/IPAC Extragalactic Database — a database focused on extragalactic objects (galaxies, QSOs, clusters).

f_NL

The amplitude of primordial non-Gaussianity — a key parameter for distinguishing between the Big Bounce and inflation.

How do we know these are previously unidentified? We cross-match anomaly positions against multiple astronomical databases. An object NOT found in any of these catalogs is a strong candidate for being genuinely new.

Current status: 6 major databases cross-matched, representing over 3 billion cataloged objects. SIMBAD: 0.2% matched. NED: 12.7%. AllWISE: 1.5%. Milliquas: 0%. Gaia: 0.6% (1 star). SDSS: API down, retry pending. The top 1,000 anomalies are overwhelmingly absent from all major astronomical catalogs — these are genuinely uncataloged objects.

Prior work: Autoencoder anomaly detection on DESI was pioneered by Liang et al. (2023) on ~250K EDR spectra and Nicolaou et al. (2026, MNRAS, 46 co-authors) on ~208K EDR spectra. This catalog extends their approach by ~90x in scale to the full DR1 release. Both teams must be cited in any publication using this catalog.