JWST and Keck Observations of the Off-Nuclear TDE AT 2024tvd: A Massive Nuclear Star Cluster and Minor-Merger Origin for its Black Hole

TL;DR

This study analyzes AT 2024tvd, the first optically selected off-nuclear TDE, using JWST NIRSpec/NIRCam and Keck KCWI to obtain high-resolution spectra and spatially resolved kinematics. A quasi-simultaneous SED from X-ray to infrared is decomposed into three components: the TDE accretion flow around a low-mass MBH ($\\log( M_{\bullet}/M_{⊙}) = 5.50 \\pm 0.04$), a nuclear star cluster with $\\log( M_{NSC}/M_{⊙}) = 7.97^{+0.16}_{-0.26}$, and a warm dust echo with $T_{dust} = 873^{+15}_{-14}$ K and $\\log L_{dust}/({\\rm erg~s^{-1}}) = 40.80^{+0.01}_{-0.01}$. The SED prefers the NSC component, yielding an outer disk radius $\\log(r_{out}/r_g) = 3.82^{+0.12}_{-0.11}$ and an instantaneous accretion rate $\\log(dot{M}/M_{⊙}~{ m yr^{-1}}) = -1.22^{+0.04}_{-0.04}$, with a dust sublimation radius around $R_{sub} \\approx 35$ light-days. Host kinematics show a smooth S0 morphology with ordered bulge rotation and no signs of disturbance at the TDE site; the inferred offset MBH mass is inconsistent with a gravitational recoil scenario, and the data favor a minor-merger origin in which a nucleated satellite hosting a $\\sim 10^{5.5} M_{⊙}$ BH and NSC has merged with the host. The results demonstrate how off-nuclear TDEs can reveal NSC demographics and MBH assembly in merging environments, and illustrate JWST’s power to disentangle accretion, stellar, and dust components in the mid-infrared for time-domain phenomena.

Abstract

We present JWST/NIRSpec and NIRCam observations of the first optically selected off-nuclear tidal disruption event (TDE), AT 2024tvd, along with Keck/KCWI integral field unit spectroscopy. The spectra show broad H and He emission lines that are characteristic of a TDE. Stellar kinematics show smooth host-galaxy morphology and ordered bulge rotation, with no evidence of disturbances in velocity, dispersion, age or metallicity space. We construct the first quasi-simultaneous spectral-energy distribution (SED) from X-rays to infrared for a TDE and decompose it into three components: the TDE accretion flow, an unresolved nuclear star cluster (NSC), and heated dust emission. The accretion component implies a black hole mass of $\log(M_\bullet/M_\odot) = 5.50\pm 0.04$, an instantaneous super-Eddington accretion rate of $\log (\dot{M}/M_{\odot} yr^{-1}) = -1.22 \pm 0.04$, and an outer disk photosphere radius of $\log(r_{out}/r_{g}) = 3.8 \pm 0.1$. The dust emission is well described by a blackbody with $T_{dust} = 873\pm 15$ K and peak luminosity $\log (L_{dust}/erg$ $s^{-1}) = 40.80\pm 0.01$, consistent with a dust echo near the sublimation radius. The SED is best fit when including additional stellar emission above the galaxy background at the TDE location, corresponding to $\log(M_{\star}/M_\odot) = 7.97^{+0.16}_{-0.26}$, which we interpret as a massive NSC or an ultra-compact dwarf galaxy. These results support a minor-merger origin for the MBH responsible for the TDE over scenarios involving gravitational recoil or dynamical ejection from the nucleus.

Figures (9)

• Figure 1: Upper:JWST/NIRCam filter transmission curves. Lower: Infrared spectrum of AT 2024tvd at phase $+199$ days from JWST/NIRSpec, with NIRCam photometry (magenta diamonds) and synthetic photometry derived from the spectrum (blue squares). The observed spectrum is shown in gray, with the black curve displaying the same data rebinned by a factor of 5 for clarity. Gaps correspond to detector chip gaps in the high-resolution gratings. Prominent H and He emission lines are indicated.
• Figure 2: Optical spectrum of AT 2024tvd at phase $+252$ days observed with KCWI. The observed spectrum is shown in gray, while the overlaid black spectrum has been rebinned by a factor of 5 for clarity. Prominent H and He emission lines are marked. The N III Bowen complex is also identified.
• Figure 3: KCWI kinematic maps of the host galaxy of AT 2024tvd and its nearby companion. Flux contours are overlaid, and the inner black box indicates the JWST/NIRSpec field of view.
• Figure 4: JWST/NIRSpec kinematic maps of the host galaxy of AT 2024tvd. Flux contours are overlaid, and the location of the TDE is marked with an "X".
• Figure 5: Upper: SED modeling of AT 2024tvd including the NSC component. The gray curves show the individual model components (accretion disk, nuclear star cluster, and dust emission), while the solid black curve shows the best-fit combined SED. The residuals (logarithm of observed minus calculated) are also shown. In the UV–optical regime, the multicolor blackbody disk follows the characteristic $\lambda^{-4/3}$ slope, whereas at longer wavelengths the finite disk size leads to a steeper infrared decline of $\lambda^{-3}$. Lower: Same as the upper panel but without the NSC component. Note that the residuals are worse, particularly in the UV bands.