More Like this

1. A Massive Black Hole 0.8 kpc from the Host Nucleus Revealed by the Offset Tidal Disruption Event AT2024tvd

   https://doi.org/10.3847/2041-8213/add7de  

   Yao, Yuhan; Chornock, Ryan; Ward, Charlotte; Hammerstein, Erica; Sfaradi, Itai; Margutti, Raffaella; Kelley, Luke Zoltan; Lu, Wenbin; Liu, Chang; Wise, Jacob; et al (May 2025, The Astrophysical Journal Letters)

   Abstract Tidal disruption events (TDEs) that are spatially offset from the nuclei of their host galaxies offer a new probe of massive black hole (MBH) wanderers, binaries, triples, and recoiling MBHs. Here we present AT2024tvd, the first off-nuclear TDE identified through optical sky surveys. High-resolution imaging with the Hubble Space Telescope shows that AT2024tvd is 0 $\stackrel{\text{″}}{.}$914 ± 0 $\stackrel{\text{″}}{.}$010 offset from the apparent center of its host galaxy, corresponding to a projected distance of 0.808 ± 0.009 kpc atz= 0.045. Chandra and Very Large Array observations support the same conclusion for the TDE’s X-ray and radio emission. AT2024tvd exhibits typical properties of nuclear TDEs, including a persistent hot UV/optical component that peaks atL_bb ∼ 6 × 10⁴³erg s⁻¹, broad hydrogen lines in its optical spectra, and delayed brightening of luminous (L_X,peak ∼ 3 × 10⁴³erg s⁻¹), highly variable soft X-ray emission. The MBH mass of AT2024tvd is 10^6±1M_⊙, at least 10 times lower than its host galaxy’s central black hole mass (≳10⁸M_⊙). The MBH in AT2024tvd has two possible origins: a wandering MBH from the lower-mass galaxy in a minor merger during the dynamical friction phase or a recoiling MBH ejected by triple interactions. Combining AT2024tvd with two previously known off-nuclear TDEs discovered in X-rays (3XMM J2150 and EP240222a), which likely involve intermediate-mass black holes in satellite galaxies, we find that the parent galaxies of all three events are very massive (∼10^10.9M_⊙). This result aligns with expectations from cosmological simulations that the number of offset MBHs scales linearly with the host halo mass. 

   more » « less
2. Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ac96ed  

   Chiti, Anirudh; Simon, Joshua D.; Frebel, Anna; Pace, Andrew B.; Ji, Alexander P.; Li, Ting S. (November 2022, The Astrophysical Journal)

   Abstract We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of ${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of $M_{1/2}\left(r_h\right)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V= ${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in line with its orbital parameters. Intriguingly, Grus I has among the lowest central densities ( ${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies. 

   more » « less
3. GW231123: A Binary Black Hole Merger with Total Mass 190–265 M _⊙

   https://doi.org/10.3847/2041-8213/ae0c9c  

   Abac, A G; Abouelfettouh, I; Acernese, F; Ackley, K; Adamcewicz, C; Adhicary, S; Adhikari, D; Adhikari, N; Adhikari, R X; Adkins, V K; et al (October 2025, The Astrophysical Journal Letters)

   Abstract On 2023 November 23, the two LIGO observatories both detected GW231123, a gravitational-wave signal consistent with the merger of two black holes with masses $137_{-18}^{+23}{M}_{\odot }$and $101_{-50}^{+22}{M}_{\odot }$(90% credible intervals), at a luminosity distance of 0.7–4.1 Gpc, a redshift of $0.40_{-0.25}^{+0.27}$, and with a network signal-to-noise ratio of ∼20.7. Both black holes exhibit high spins— $0.90_{-0.19}^{+0.10}$and $0.80_{-0.52}^{+0.20}$, respectively. A massive black hole remnant is supported by an independent ringdown analysis. Some properties of GW231123 are subject to large systematic uncertainties, as indicated by differences in the inferred parameters between signal models. The primary black hole lies within or above the theorized mass gap where black holes between 60–130M_⊙should be rare, due to pair-instability mechanisms, while the secondary spans the gap. The observation of GW231123 therefore suggests the formation of black holes from channels beyond standard stellar collapse and that intermediate-mass black holes of mass ∼200M_⊙form through gravitational-wave-driven mergers. 

   more » « less
4. Investigating the Star Formation Rates of Active Galactic Nucleus Hosts Relative to the Star-forming Main Sequence

   https://doi.org/10.3847/1538-4357/ad2177  

   Cristello, Nathan; Zou, Fan; Brandt, W. N.; Chen, Chien-Ting J.; Leja, Joel; Ni, Qingling; Yang, Guang (February 2024, The Astrophysical Journal)

   Abstract A fundamental question in galaxy and black hole evolution remains how galaxies and their supermassive black holes have evolved together over cosmic time. Specifically, it is still unclear how the position of X-ray active galactic nucleus (AGN) host galaxies with respect to the star-forming main sequence (MS) may change with the X-ray luminosity (L_X) of the AGN or the stellar mass (M_⋆) of the host galaxy. We use data from the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) to probe this issue. XMM-SERVS is covered by the largest medium-depth X-ray survey (with superb supporting multiwavelength data) and thus contains the largest sample to date for study. To ensure consistency, we locally derive the MS from a large reference galaxy sample. In our analysis, we demonstrate that the turnover of the galaxy MS does not allow reliable conclusions to be drawn for high-mass AGNs, and we establish a robust safe regime where the results do not depend upon the choice of MS definition. Under this framework, our results indicate that less massive AGN host galaxies ( $\log M_{\star }\sim 9.5–10.5M_{\odot }$) generally possess enhanced star formation rates compared to their normal-galaxy counterparts while the more massive AGN host galaxies ( $\log M_{\star }\sim 10.5–11.5M_{\odot }$) lie on or below the star-forming MS. Further, we propose an empirical model for how the placement of an AGN with respect to the MS (SFR_norm) evolves as a function of bothM_⋆andL_X. 

   more » « less
5. A Panchromatic Study of the X-Ray Binary Population in NGC 300 on Subgalactic Scales

   https://doi.org/10.3847/1538-4357/ad46d9  

   Binder, Breanna_A; Williams, Rosalie; Payne, Jacob; Eracleous, Michael; Belles, Alexander; Williams, Benjamin_F (July 2024, The Astrophysical Journal)

   Abstract The population-wide properties and demographics of extragalactic X-ray binaries (XRBs) correlate with the star formation rates (SFRs), stellar masses (M_⋆), and environmental factors (such as metallicity,Z) of their host galaxy. Although there is evidence that XRB scaling relations (L_X/SFR for high-mass XRBs (HMXBs) andL_X/M_⋆for low-mass XRBs) may depend on metallicity and stellar age across large samples of XRB-hosting galaxies, disentangling the effects of metallicity and stellar age from stochastic effects, particularly on subgalactic scales, remains a challenge. We use archival X-ray through IR observations of the nearby galaxy NGC 300 to self-consistently model the broadband spectral energy distribution and examine radial trends in its XRB population. We measure a current (<100 Myr) SFR of 0.18 ± 0.08 M_⊙yr⁻¹and stellar massM_⋆= $\left({2.15}_{-0.14}^{+0.26}\right)\times {10}^9$M_⊙. Although we measure a metallicity gradient and radially resolved star formation histories that are consistent with the literature, there is a clear excess in the number of X-ray sources below ∼10³⁷erg s⁻¹that are likely a mix of variable XRBs and additional background active galactic nuclei. When we compare the subgalacticL_X/SFR ratios as a function ofZto the galaxy-integratedL_X-SFR-Zrelationships from the literature, we find that only the regions hosting the youngest (≲30 Myr) HMXBs agree with predictions, hinting at time evolution of theL_X–SFR–Zrelationship. 

   more » « less