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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...

We report 18 cm VLBI continuum imaging observations of Arp 220, the prototype luminous infrared galaxy (log). In previous work, we showed that Arp 220 has compact, high-nuclear radio emission L ϭ 12.11 L T fir , b that might be interpreted as a dust-enshrouded active galactic nucleus (AGN) radio core, or, alternately, as multiple, very luminous radio supernovae from a very active nuclear starburst. In this work, we present a new 18 cm VLBI image, with mas angular resolution, showing approximately a dozen unresolved sources, 3 # 8 S ϭ 18 cm mJy, within a 0Љ .2 # 0Љ .4 (pc) region centered on the NW nucleus of this merging system. At 0.2-1.2 75 # 150 least two additional sources are detected in the SE nucleus. These point sources account for about 3% of the total 18 cm radio emission associated with Arp 220 and for all the estimated radio flux density with K.

We have used a Very Long Baseline Interferometry (VLBI) array at 18cm wavelength to image the nucleus of the luminous IR galaxy Arp 220 at ~1 pc linear resolution, and with very high sensitivity. The resulting map has an rms of 5.5 microJy/beam, and careful image analysis results in 49 confirmed point sources ranging in flux density from 1.2 mJy down to ~60 microJy. Comparison with high sensitivity data from 12 months earlier reveals at least four new sources. The favored interpretation of these sources is that they are radio supernovae, and if all new supernovae are detectable at this sensitivity, a resulting estimate of the supernova rate in the Arp 220 system is 4 +/- 2 per year. The implied star formation rate is sufficient to power the entire observed far-infrared luminosity of the galaxy. The two nuclei of Arp 220 exhibit striking similarities in their radio properties, though the western nucleus is more compact, and appears to be ~3 times more luminous than the eastern nucleus. There are also some puzzling differences, and differential free-free absorption, synchrotron aging and expansion losses may all be playing a role. Comparison with the nearby starburst galaxy M82 supports the hypothesis that the activity in Arp 220 is essentially a scaled-up version of that in M82.Comment: 24 pages, 3 figures, 1 table. Accepted for publication in Ap.