P. J. Diamond

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

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. 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 difficult 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. 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 determined—7.2 ± 0.3u2009Mpc—is the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.

A STARBURST REVEALED—LUMINOUS RADIO SUPERNOVAE IN THE NUCLEI OF ARP 220

We report 18 cm VLBI continuum imaging observations of Arp 220, the prototype luminous infrared galaxy (log Lfir = 12.11 L☉). In previous work, we showed that Arp 220 has compact, high-Tb nuclear radio emission 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 3×8 mas angular resolution, showing approximately a dozen unresolved sources, S18 cm = 0.2-1.2 mJy, within a 02 × 04 (75×150 pc) region centered on the NW nucleus of this merging system. At 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 Tb > 106 K. No other 18 cm emission is detected on scales from 3 to 100 mas (1-30 pc). We interpret these compact radio sources as luminous radio supernovae of the class in which RSN 1986J is a prototype. This interpretation is consistent with a simple starburst model for the infrared luminosity of Arp 220 that has a star formation rate of 50-100 M☉ yr-1 and a luminous supernova rate, νsn = 1.75-3.5 yr-1. In this model prescription, virtually all supernova explosions in Arp 220 must result in luminous RSNe, comparable to the most luminous RSNe observed. We discuss possible mechanisms for the origin of very luminous RSNe in luminous infrared galaxies and suggest that it is likely due to the dense, compact starburst environment. Although our observations do not rule out the presence of an AGN that may contribute to the infrared luminosity in Arp 220, it is not necessary to appeal to AGN activity to account for the overall radio/infrared characteristics of Arp 220.

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92 + 0.48, in the 12.2 GHz (2 0-3 -1 E) transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz (5 1-6 0 A +) and the 12.2 GHz (2 0-3 -1 E) transition from a number of star formation regions and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

Imaging the Magnetic Field in the Atmosphere of TX Camelopardalis

Recent advances in very long baseline interferometry (VLBI) allow full interferometric imaging, in both linear and circular polarization, of the 43 GHz SiO maser emission in the immediate circumstellar environment of late-type stars. The SiO masers act as unique probes of the magnetic field in the near circumstellar environment and provide information on the stellar field morphology of distant stars at unprecedented angular resolution. This technique allows direct imaging of the magnetic field in the extended atmospheres of distant stars. We report on such observations of the ν = 1, J = 1-0 SiO maser emission toward the Mira variable TX Cam, finding a mean magnetic field strength of B sec θ ~ 5-10 G, where θ is the angle between the line of sight and the magnetic field, as well as evidence for an orderly field topology over significant portions of the maser shell. The implied field geometry and implications for possible mass-loss mechanisms are discussed.

Discovery of a Subparsec Jet 4000 Radii Away from the Central Schwarzschild Engine of NGC 4258

We report the VLBI detection of compact continuum emission associated with the maser disk in NGC 4258. The strongest emission is located about 0.5 mas (0.015 pc) north of the dynamical center of the disk. It has an average flux density of about 3 mJy and varies by ~100% on timescales of weeks. We postulate that we have detected nonthermal synchrotron emission in the base of the northern jet, which is seen on parsec-to-kiloparsec scales in radio to X-ray emission. We also report a detection of emission from the southern jet which may be attenuated by thermal absorption in a layer of ionized gas above the molecular disk. The average flux density of the maser emission in the systemic velocity range is correlated with the flux density of the northern continuum emission. Together with the geometry of the disk, these data suggest that the masers amplify the southern continuum emission and that the southern and northern jet emission are correlated in strength. We discuss the results in the context of jet emission models and properties of other compact radio continuum sources.

The Water Masers in the Elliptical Galaxy NGC 1052

Very luminous water masers have been detected toward only two elliptical galaxies; the emission lines are relatively broad (≈ 90 km s−1) and smooth, unlike the narrow maser spikes seen in other active galaxies. We have made the first VLBI observations of the water masers in one of these galaxies, NGC 1052. We find that the masers lie along, rather than perpendicular to, the jet. The scale of the maser emission is about 400 μas (7000 AU). The masers could be associated with the radio jet, and we show that the jet is energetically capable of powering the observed maser emission by driving slow, nondissociative shocks into circumnuclear, dense molecular clouds. Alternatively, the masers could represent the amplification of the radio continuum emission of the jet by foreground molecular clouds. Further VLBI observations should be able to distinguish between these two possibilities.

Symmetric Parsec-Scale OH Megamaser Structures in Arp 220

The parsec-scale OH megamaser emission in the luminous IR galaxy Arp 220 has been imaged in detail using a global VLBI array. Four major emission regions are revealed in the 1667 MHz line, each with complex spatial and velocity structure showing intriguing symmetries. These emission regions have no associated continuum emission to stringent limits, and the brighter components have a maser amplification ratio exceeding 800. No compact emission is detected in the 1665 MHz line. The compact maser emission, with high amplification and unmeasurably small 1665/1667 line ratio, appears to be the result of saturated masers in physically compact masing clouds. The diffuse emission, on the other hand, appears to fit the traditional OH megamaser model of a low-gain masing screen on scales of hundreds of parsecs. Infrared pumping is indicated for the diffuse emission, but collisional pumping is probably important for the compact components. The compact components may trace shock fronts in the dense nuclear environment and may be related to active galactic nucleus activity.

Deceleration in the Expansion of SN 1993J

A rarity among supernova, SN 1993J in M81 can be studied with high spatial resolution. Its radio power and distance permit VLBI observations to monitor the expansion of its angular structure. This radio structure was previously revealed to be shell-like and to be undergoing a self-similar expansion at a constant rate. From VLBI observations at wavelengths of 3.6 and 6 cm in the period 6-42 months after explosion, we have discovered that the expansion is decelerating. Our measurement of this deceleration yields estimates of the density profiles of the supernova ejecta and circumstellar material in standard supernova explosion models.

VLBA Continuum Observations of NGC 4258: Constraints onan Advection-dominated Accretion Flow

We report a 3 sigma upper limit of 220 microJanskys on any 22-GHz continuum emission coincident with the central engine in NGC 4258. If NGC 4258 is powered by an advection-dominated accretion flow, this radio upper limit implies that the inner advection-dominated flow cannot extend significantly beyond about 100 Schwarzschild radii.

R Aquarii: First detection of circumstellar SiO maser proper motions

We have made the first detection of circumstellar SiO maser proper motions in the envelope of a late-type star. Using the Very Long Baseline Array (VLBA), we have obtained observations at four epochs of the 43 GHz, v = 1, J = 1-0 SiO maser emission toward the Mira variable in the symbiotic binary R Aqr. The maser emission has a ringlike structure approximately 31 mas across with a slight elongation in the north-south direction. We find that the emission changes significantly over a timescale of about 1-2 months with almost no similarity in structure for timescales 6 months. Our observations show that over a 98 day period the masers have an average inward proper motion of about 1 mas. This contraction of the ring implies an infall velocity of about 4 km s-1 for the SiO masers in the circumstellar envelope.