Godzilla, a monster lurks in the Sunburst galaxy

J. M. Diego, M. Pascale, B. J. Kavanagh, P. Kelly, L. Dai, B. Frye, T. Broadhurst

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


We model the strong lensing effect in the galaxy cluster PSZ1 G311.65-18.48 (z=0.443) with an improved version of the hybrid method WSLAP+. We extend the number of constraints by including the position of critical points, which are combined with the classic positional constraints of the lensed galaxies. We pay special attention to a transient candidate source (Tr) previously discovered in the giant Sunburst arc (z=2.37). Our lens model predicts Tr to be within a fraction of an arcsecond from the critical curve, which has a larger magnification factor than previously found, but still not large enough to explain the observed flux and lack of counterimages. Possible candidate counterimages are discussed that would lower the magnification required to explain Tr, but extreme magnification factors (> 600) are still required, even in that case. The presence of a small mass perturber with a mass comparable to a dwarf galaxy (M108M) near the position of Tr is needed in order to explain the required magnification and morphology of the lensed galaxy. We discuss how the existence of this perturber could potentially be used to constrain models of dark matter. The large apparent brightness and unresolved nature of the magnified object implies a combination of extreme magnification and a very luminous and compact source (ralt;0.4 pc). Possible candidates are discussed, including an hyperluminous star, a small group of stars, or an accretion disk around a relatively small supermassive black hole (SMBH). Based on spectral information and flux requirements, we argue that a luminous blue variable (LBV) star caught during an outburst is the most likely candidate. Owing to the extreme magnification and luminosity of this source, we dub it Godzilla.

Original languageEnglish (US)
Article numberA134
JournalAstronomy and Astrophysics
StatePublished - Sep 1 2022

Bibliographical note

Funding Information:
The authors would like to thank useful comments from Francisco Carrera and the anonymous referee. J.M.D. acknowledges the support of project PGC2018-101814-B-100 (MCIU/AEI/MINECO/FEDER, UE) Ministerio de Ciencia, Investigación y Universidades. This project was funded by the Agencia Estatal de Investigación, Unidad de Excelencia María de Maeztu, ref. MDM-2017-0765. L.D. acknowledges the research grant support from the Alfred P. Sloan Foundation (award number FG-2021-16495). This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are associated with program(s) ID 15101 (P.I H. Dahle), ID 15377 (P.I M. Bayliss), ID 15418 (P.I H. Dahle), ID 15949 (P.I M. Gladders). Some of the results and discussion presented in this work are based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 0103.A-0688(C), 297.A-5012(A).

Publisher Copyright:
© J. M. Diego et al. 2022.


  • Dark matter
  • Gravitational lensing: strong
  • Stars: variables: general


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