A geometric distance to the galaxy NGC4258 from orbital motions in a nuclear gas disk

Abstract

The accurate measurement of extragalactic distances is a central challenge of modern astronomy, being required for any realistic description of the age, geometry and fate of the Universe. The measurement of relative extragalactic distances has become fairly routine, but estimates of absolute distances are rare 1 . In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but such techniques have hitherto been dif®cult to apply to other galaxies. As a result, uncertainties in the expansion rate and age of the Universe are dominated by uncertainties in the absolute calibration of the extragalactic distance ladder 2 . Here we report a geometric distance to the galaxy NGC4258, which we infer from the direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy. The distance so deter-minedÐ7:2 6 0:3 MpcÐis the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.NGC4258 is one of 22 nearby active galactic nuclei (AGN) known to possess nuclear water masers (the microwave equivalent of lasers). The enormous surface brightnesses ( ) 10 12 K), relatively small sizes ( ( 10 14 cm) and narrow linewidths (a few km s -1 ) of these masers make them ideal probes of the structure and dynamics of the molecular gas in which they residue. Very-long-baseline interferometry (VLBI) observations of the NGC4258 maser have provided the ®rst direct images of an AGN accretion disk, revealing a thin, subparsec-scale, differentially rotating warped disk in the nucleus of this relatively weak Seyfert 2 AGN 3±6 . Two distinct populations of masers exist in NGC4258. The ®rst are the highvelocity masers. These masers amplify their own spontaneous emission and are offset 61,000 km s -1 and 4.7±8.0 mas (0.16± 0.28 pc for a distance of 7.2 Mpc) on either side of the disk centre. The keplerian rotation curve traced by these masers requires a central binding mass (M), presumably in the form of a supermassive black hole, of 3:9 6 0:1 3 10 7 D=7:2 Mpcsin i s =sin 82 2 2 Figure 1 The NGC4258 water maser. The upper panel shows the best-®tting warped-disk model superposed on actual maser positions as measured by the VLBA of the NRAO, with top as North. The ®lled square marks the centre of the disk, as determined from a global disk-®tting analysis 8 . The ®lled triangles show the positions of the high-velocity masers, so called because they occur at frequencies corresponding to Doppler shifts of ,61,000 km s -1 with respect to the galaxy systemic velocity of ,470 km s -1 . This is apparent in the VLBA total power spectrum (lower panel). The inset shows line-of-sight (LOS) velocity versus impact parameter for the best-®tting keplerian disk, with the maser data superposed. The high-velocity masers trace a keplerian curve to better than 1%. Monitoring of these features indicates that they drift by less than ,1 km s -1 yr -1 (refs 14±16) and requires that they lie within 5±1...

Observational evidence for massive black holes in the centers of active galaxies

“…The high-velocity features show no proper motions with respect to a fixed-velocity component in the systemic range (Herrnstein 1996). The systemic features show proper motions of about 32 µas yr −1 (Herrnstein et al 1999). …”

“…The distance to the maser of 7.2 ± 0.3 Mpc was determined from analysis of the proper motions and accelerations of the systemic features (Herrnstein et al 1999). Fifteen features were tracked over a period of two years to an accuracy of 0.5-10 µas in relative position and 0.4 km s −1 in velocity.…”

Observational probes of cosmic acceleration

“…The recent determination of H 0 = 73.8 ± 2.4 km s −1 Mpc −1 by Riess et al (2011) yields a 1σ uncertainty of only 3.3%, including all identified sources of systematic uncertainty and calibration error. One important change in this analysis is a shift to Cepheid calibration based on the maser distances to NGC 4258 (Herrnstein et al, 1999;Humphreys et al, 2008Humphreys et al, , 2013 and on parallaxes to Galactic Cepheids measured with Hipparcos (van Leeuwen et al, 2007) and with the HST fine-guidance sensors (Benedict et al, 2007). These calibrations circumvent the statistical and systematic uncertainties in the LMC distance, and they directly calibrate the P − L relation in the metallicity range typical of calibrator galaxies, albeit with a sample of only ∼ 10 stars reaching an error-on-the-mean of 2.8% in the case of Milky Way parallaxes.…”

Masers in AGN environments

“…In principal, two geometric distance estimates may be obtained for each maser galaxy, one via analysis of centripetal acceleration, and one via analysis of proper motion (Herrnstein et al 1999). Both observables are greatest for masers close to the systemic velocity.…”