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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...

Compiling data from literature and the Atacama Large Millimeter/submillimeter Array archive, we show enhanced HCN(4-3)/HCO + (4-3) and/or HCN(4-3)/CS(7-6) integrated intensity ratios in circumnuclear molecular gas around active galactic nuclei (AGNs) compared to those in starburst (SB) galaxies (submillimeter HCNenhancement). The number of sample galaxies is significantly increased from our previous work. We expect that this feature could potentially be an extinction-free energy diagnostic tool of nuclear regions of galaxies. Non-LTE radiative transfer modelings of the above molecular emission lines involving both collisional and radiative excitation, as well as a photon trapping effect, were conducted to investigate the cause of the high line ratios in AGNs. As a result, we found that enhanced abundance ratios of HCNtoHCO + and HCNtoCS in AGNs as compared to SB galaxies by a factor of a few to even 10 are a plausible explanation for the submillimeter HCNenhancement. However, a counterargument of a systematically higher gas density in AGNs than in SB galaxies can also be a plausible scenario. Although we cannot fully distinguishthese two scenarios at this moment owing to an insufficient amount of multi-transition, multi-species data, the former scenario is indicative of abnormal chemical composition in AGNs. Regarding the actual mechanism to realize the composition, we suggest that it is difficult with conventional gas-phase X-ray-dominated region ionization models to reproduce the observed high line ratios. We might have to take into account other mechanisms such as neutral-neutral reactions that are efficiently activated in high-temperature environments and/or mechanically heated regions to further understand the high line ratios in AGNs.

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Context. The nearby Sy 1 galaxy NGC 1097 represents an ideal laboratory for exploring the molecular chemistry in the surroundings of an active galactic nucleus (AGN). Aims. Exploring the distribution of different molecular species allows us to understand the physical processes affecting the interstellar medium both in the AGN vicinity and in the outer star forming molecular ring. Methods. We carried out 3 mm ALMA observations that include seven different molecular species, namely HCN, HCO + , CCH, CS, HNCO, SiO, HC 3 N, and SO, as well as the 13 C isotopologues of the first two. Spectra were extracted from selected positions and all species were imaged over the central 2 kpc (∼30 ) of the galaxy at a resolution of ∼2.2 × 1.5 (150 pc × 100 pc). Results. HCO + and CS appear to be slightly enhanced in the star forming ring. CCH shows the largest variations across NGC 1097 and is suggested to be a good tracer of both obscured and early stage star formation. HNCO, SiO, and HC 3 N are significantly enhanced in the inner circumnuclear disk surrounding the AGN. Conclusions. Differences in the molecular abundances are observed between the star forming ring and the inner circumnuclear disk. We conclude that the HCN/HCO + and HCN/CS differences observed between AGN-dominated and starburst (SB) galaxies are not due to a HCN enhancement due to X-rays, but rather this enhancement is produced by shocked material at distances of 200 pc from the AGN. Additionally, we claim that lower HCN/CS is a combination of a small underabundance of CS in AGNs, together with excitation effects, where a high density gas component (∼10 6 cm −3 ) may be more prominent in SB galaxies. However, the most promising are the differences found among the dense gas tracers that, at our modest spatial resolution, seem to outline the physical structure of the molecular disk around the AGN. In this picture, HNCO probes the well-shielded gas in the disk, surrounding the dense material moderately exposed to the X-ray radiation traced by HC 3 N. Finally SiO might be the innermost molecule in the disk structure.