Measuring the Hubble constant from reverberating accretion discs in active galaxies

Abstract

The standard paradigm of active galactic nuclei (AGN) postulates that their luminosity, L ~ 10³⁹⁻⁴⁸erg s⁻¹, derives from the accretion of gas onto a supermassive black hole, mass M ~ 10⁶⁻⁹M☉, at the centre of a host galaxy. Echo or reverberation mapping affords a method of relating flux variations at different wavelengths to determine the nature of the flux emitting regions, with μ-arcsecond resolution. The results of an intensive two-month campaign of ground based spectrophotometric monitoring of the Seyfert 1 galaxy NGC 7469, with a temporal resolution of ≤1 day, are presented. Application of echo mapping techniques reveal the virial mass of the central source to be MNGC 7469 ~10⁶⁻⁷ M☉, and a compact broad Balmer line emitting region ~ 5 light days from the central source. Together, this evidence suggests the existence of a supermassive black hole in NGC 7469. Further, evidence for significant wavelength- dependent continuum time delays is presented, with optical continuum variations lagging those at UV wavelengths by about 1-2 days. The wavelength-dependent time delays, (λ), are consistent with the predicted T ∝ λ ⁴/ ³ relationship for an irradiated blackbody accretion disc with temperature structure T(R) ∝ R⁻³/⁴ and hence may represent the indirect detection of an accretion disc structure in NGC 7469. It is shown that wavelength-dependent time delays test the standard black-hole accretion disc paradigm of AGN, by measuring T(R) of the gaseous material surrounding the purported black hole. Moreover, a new direct method is presented that combines observed time delays and the spectral energy distribution of an AGN to derive a redshift-independent luminosity distance; assuming the observed time delays are indeed due to a classical accretion disc structure. The luminosity distance permits an estimate of the Hubble constant, H₀-the expansion rate of the Universe. The first application of the method yields H₀(cos i/0.7)¹/² = 38 ± 7km s⁻¹ Mp ⁻¹. A more accurate determination of H₀ requires either an independent accurate determination of the disc inclination i or statistical average of a moderate sample of active galaxies. This method permits determination of redshift-independent luminosity distances to AGNs, thereby, giving a new route to H₀, and by extension to fainter objects at z ~ 1, q₀.