A population of dust-enshrouded objects orbiting the Galactic black hole

Abstract

The central 0.1 parsecs of the Milky Way host a supermassive black hole identified with the position of the radio and infrared source Sagittarius A* (refs. 1,2), a cluster of young, massive stars (the S stars3) and various gaseous features4,5. Recently, two unusual objects have been found to be closely orbiting Sagittarius A*: the so-called G sources, G1 and G2. These objects are unresolved (having a size of the order of 100 astronomical units, except at periapse, where the tidal interaction with the black hole stretches them along the orbit) and they show both thermal dust emission and line emission from ionized gas6,7,8,9,10. G1 and G2 have generated attention because they appear to be tidally interacting with the supermassive Galactic black hole, possibly enhancing its accretion activity. No broad consensus has yet been reached concerning their nature: the G objects show the characteristics of gas and dust clouds but display the dynamical properties of stellar-mass objects. Here we report observations of four additional G objects, all lying within 0.04 parsecs of the black hole and forming a class that is probably unique to this environment. The widely varying orbits derived for the six G objects demonstrate that they were commonly but separately formed.

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Fig. 1: 2006 OSIRIS data-cube visualized with OsrsVol.
Fig. 2: Proper motion and spectrum of the G objects.
Fig. 3: Orbits of the G objects.

Data availability

All data generated or analysed during this study are included in this published article.

Code availability

The orbit fit code is publicly available at https://zenodo.org/record/3305315#.XXmLPC3MzUY.

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Acknowledgements

We thank G. Witzel, R. Schödel and E. Becklin for providing insight and expertise. Support for this work was provided by NSF AAG grant AST-1412615, Jim and Lori Keir, the W. M. Keck Observatory Keck Visiting Scholar programme, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, and Howard and Astrid Preston. A.M.G. acknowledges support from her Lauren B. Leichtman and Arthur E. Levine Endowed Astronomy Chair. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Author information

A.C., R.D.C. and M.R.M. designed the project, and carried out the analysis and the interpretation of the results. A.M.G. supervised the observing, data acquisition and data reduction. T.D. and D.S.C. obtained and reduced the data. A.H., K.K.O’N. and B.N.S. contributed to the analysis and to the manuscript. A.C. wrote the manuscript with support from M.R.M. and R.N.C., and with contributions from all other authors. All authors provided critical feedback and helped gather data.

Correspondence to Anna Ciurlo.

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Extended data figures and tables

Extended Data Fig. 1 Infrared-excess sources and G objects.

Shown are the proper motions of the G sources (from 2006, in blue, to 2018, in yellow) along with the positions of infrared-excess sources7 in orange (data obtained with NACO at the VLT in 2005) and17 in magenta (data obtained with NIRC2 at Keck in 2005). The red trace shows the proper motion of S0-2 (from NIRC2) as reference. The average OSIRIS field is displayed as a dotted rectangle.

Extended Data Fig. 2 New G objects in the L band.

We show the deconvolved L′ images from NIRC2 for several epochs (each year is reported in the top-left corner). The green circles indicate the position of the G objects in the Kn3 OSIRIS band and of Sgr A*.

Extended Data Fig. 3 Spectra of the new G objects showing the Brγ emission line over time.

Top row, G3 (left) and G4 (right); bottom row, G5 (left) and G6 (right). The spectra are extracted epoch by epoch (black). The Gaussian fit of the Brγ emission line (red) is superimposed. There is no significantly detected variation (all values are compatible within 1σ) in the linewidth for any of the objects. The data quality varies and the emission of the objects blends with neighbouring features as it changes radial velocity (RV) and position: this gives sometimes the impression of a broadening of the line which is not real. The emission line at the rest velocity is part of the extended emission present across the field and does not change with time.

Extended Data Fig. 4 Spectra of the G objects showing Brγ and [Fe iii] emission lines.

The Kn3 spectra of G objects G6–G1 are extracted over an aperture of 1.5-pixel radius from the 2006 combined dataset. The dotted lines show the rest-frame velocity of the Brγ and [Fe iii] emission lines. G3, G4, G5 and G6 show both Brγ and [Fe iii] emission moving at the same velocity (Doppler-shifted emission indicated by the arrows), whereas G1 and G2 show only Brγ emission.

Extended Data Fig. 5 Corner plots for orbit fitting of G3, G4, G5 and G6.

See Methods section ‘Orbit fitting’ for details of the parameters displayed.

Extended Data Fig. 6 Unit conversion factors as a function of time.

a, Factor obtained using calibration A stars (single frames in grey triangles, median for each epoch in red dots, dispersion used as error bar). b, Factor obtained using stars in the science field. The dispersion when using stars in the science field is much larger. We use the A stars, for which most of the variation corresponds to hardware changes in the instrument. We use the median value for each instrument period (green solid line) and use the dispersion as an estimate of the uncertainty (green dashed lines). See Methods section ‘Flux calibration’ for details.

Extended Data Table 1 OSIRIS observations used for the orbital fit
Extended Data Table 2 Measured values of positions and radial velocities
Extended Data Table 3 Best fit orbital parameters
Extended Data Table 4 Emission from the G objects

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Ciurlo, A., Campbell, R.D., Morris, M.R. et al. A population of dust-enshrouded objects orbiting the Galactic black hole. Nature 577, 337–340 (2020). https://doi.org/10.1038/s41586-019-1883-y

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