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A dust-parallax distance of 19 megaparsecs to the supermassive black hole in NGC 4151


The active galaxy NGC 4151 has a crucial role as one of only two active galactic nuclei for which black hole mass measurements based on emission line reverberation mapping can be calibrated against other dynamical techniques1,2,3. Unfortunately, effective calibration requires accurate knowledge of the distance to NGC 4151, which is not at present available4. Recently reported distances range from 4 to 29 megaparsecs5,6,7. Strong peculiar motions make a redshift-based distance very uncertain, and the geometry of the galaxy and its nucleus prohibit accurate measurements using other techniques. Here we report a dust-parallax distance to NGC 4151 of megaparsecs. The measurement is based on an adaptation of a geometric method that uses the emission line regions of active galaxies8. Because these regions are too small to be imaged with present technology, we use instead the ratio of the physical and angular sizes of the more extended hot-dust emission9 as determined from time delays10 and infrared interferometry11,12,13,14. This distance leads to an approximately 1.4-fold increase in the dynamical black hole mass, implying a corresponding correction to emission line reverberation masses of black holes if they are calibrated against the two objects with additional dynamical masses.

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Figure 1: Effect of the brightness distribution on the observed sizes and time lags.
Figure 2: Relating time lags and angular sizes to measure the absolute distance to NGC 4151.


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We thank R. Wojtak for discussions on peculiar velocities near the Virgo cluster. S.F.H. acknowledges support from the Marie Curie International Incoming Fellowship within the Seventh European Community Framework Programme (PIIF-GA-2013-623804). The Dark Cosmology Centre is funded by the Danish National Research Foundation. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by JPL, Caltech, under contract with NASA.

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Authors and Affiliations



S.F.H. and D.W. had the idea for the project. S.F.H. collected the data, developed the model and wrote the paper. D.W. assisted in interpreting the results and helped to write the manuscript. M.K. contributed the interferometry data and helped with the data analysis. J.H. contributed to the modelling and interpretation. All authors engaged in discussion and provided comments on the manuscript.

Corresponding author

Correspondence to Sebastian F. Hönig.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Dependence of the measured angular-diameter distance DA on the dust sublimation temperature Tsub.

The red circles represent the distribution of sublimation temperatures in the 1,250 Monte Carlo runs used to model the light curve. Overplotted are dashed contours at 5%, 15%, 33% and 50% peak density. The distribution of DA is consistent with being independent of Tsub, illustrating that the distance determination is insensitive to the detailed dust properties.

Extended Data Figure 2 Dependence of the time lag τin of the inner boundary on the brightness distribution power-law index α.

The black circles represent the fitted τin and α values for each of the 1,250 Monte Carlo representations of the V-band light curves given the inclination and sublimation temperature distributions (68% fitting error levels, mostly smaller than the symbol size).

Extended Data Figure 3 Dependence of the angular size ρin of the inner boundary on the brightness distribution power-law index α.

The black circles represent the distribution of ρin determined from the six interferometric data points for a given α value, sublimation temperature and inclination for each of the 1,250 Monte Carlo representations of the V-band light curve. The cyan error bars represent the additional uncertainties (68% confidence levels) from the combined statistical errors of the observations, the position angle of the emitting disk and the corrections for host and putative accretion disk contributions.

Extended Data Table 1 Constraints on the inclination and position angle of the AGN structure from various observations

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Hönig, S., Watson, D., Kishimoto, M. et al. A dust-parallax distance of 19 megaparsecs to the supermassive black hole in NGC 4151. Nature 515, 528–530 (2014).

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