AT2022zod: An Unusual Tidal Disruption Event in an Elliptical Galaxy at Redshift 0.11

Tidal Disruption Events (TDEs) have long been hypothesized as valuable indicators of black holes, offering insight into their demographics and behaviour out to high redshift. TDEs have also enabled the discovery of a few Massive Black Holes (MBHs) with inferred masses of $10^4$–$10^6,M_\odot$, often associated with the nuclei of dwarf galaxies or ultra-compact dwarf galaxies (UCDs). Here we present AT2022zod, an extreme and short-lived optical flare in an elliptical galaxy at z=0.11, residing within 3kpc of the galaxy’s centre. Its luminosity and ~30-day duration make it unlikely to have originated from the host galaxy’s central supermassive black hole (SMBH), which estimate to have a mass around $10^8,M_\odot$. Assuming that the emission mechanism is consistent with known observed TDEs, we find that such a rapidly evolving transient could either be produced by a MBH in the intermediate-mass range or, alternatively, result from the tidal disruption of a star on a non-parabolic orbit around the central SMBH. We suggest that the most plausible origin for AT2022zod is the tidal disruption of a star by a MBH embedded in a UCD. As the Vera C. Rubin Observatory’s Legacy Survey of Space and Time comes online, we propose that AT2022zod serves as an important event for refining search strategies and characterization techniques for intermediate-mass black holes.

💡 Research Summary

The authors present AT2022zod, a short‑lived, high‑luminosity optical flare discovered by the Zwicky Transient Facility (ZTF) on 2022‑10‑18 in the elliptical galaxy SDSS J105602.80+561214.7 at redshift z = 0.11. The transient peaked at g ≈ 19 mag and faded back to baseline within roughly 30 days, a timescale far shorter than the typical months‑to‑years duration of most known tidal disruption events (TDEs). Using a Gaussian rise and exponential decay model, they derive a rise time of ~5 days and an e‑folding decay of ~25 days.

Spectroscopic analysis of the host galaxy shows a quiescent spectrum with very weak emission lines. The stellar velocity dispersion measured from the SDSS spectrum is σ ≈ 158 km s⁻¹, which, via the M–σ relation, implies a central supermassive black hole (SMBH) mass of M• ≈ 1 × 10⁸ M⊙ (95 % credible interval 9.7 × 10⁶–2.6 × 10⁸ M⊙). Such a massive SMBH would typically swallow a star whole rather than produce a luminous, observable flare, making it unlikely that the observed event originated from the central SMBH.

The authors therefore explore two alternative scenarios. (1) The flare is produced by an intermediate‑mass black hole (IMBH) of 10⁴–10⁶ M⊙ embedded in a compact stellar system such as an ultra‑compact dwarf (UCD) or a nuclear star cluster (NSC) that resides within the host galaxy at a projected distance of ~3 kpc from the nucleus. In this regime, the dynamical timescale is short, leading to a rapid rise and decay consistent with the observed light curve. (2) The flare results from a star on a highly eccentric, non‑parabolic orbit being disrupted by the central SMBH. While such an orbit can produce a brief, luminous event, the required orbital parameters are statistically rare.

To contextualize the host, the authors perform full‑spectral fitting with the pyPipe3D pipeline, employing 1271 single‑stellar‑population models from the MILES library. The derived star‑formation history shows an old, passive stellar population; the galaxy lies well below the star‑forming main sequence and is classified as a “retired” system on the WHAN diagram, indicating negligible ongoing star formation or AGN activity. The stellar mass is estimated at M⋆ ≈ 8.5 × 10¹⁰ M⊙, reinforcing the picture of a massive, quiescent elliptical.

Given the lack of alternative explanations such as a supernova or AGN variability, the authors argue that AT2022zod is most plausibly a TDE associated with an IMBH in a UCD or NSC. They highlight the importance of this event as a prototype for future searches with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST). They propose a systematic strategy: (i) automated identification of fast‑rising, fast‑fading transients using machine‑learning classifiers; (ii) cross‑matching with host‑galaxy catalogs to isolate quiescent, massive ellipticals; (iii) rapid multi‑wavelength follow‑up (X‑ray, radio) to confirm TDE signatures; and (iv) statistical modeling to infer the IMBH occupation fraction in UCDs.

In summary, AT2022zod adds a rare, well‑characterized example of a short‑duration TDE that cannot be reconciled with disruption by a ∼10⁸ M⊙ SMBH. Its properties strongly support the presence of a ∼10⁴–10⁶ M⊙ black hole embedded in a compact stellar system, providing a valuable benchmark for the upcoming era of high‑cadence, wide‑field surveys aimed at uncovering the elusive population of intermediate‑mass black holes.