EAVN Astrometry toward the Extreme Outer Galaxy: Kinematic distance with the proper motion of G034.84-00.95

We aim to reveal the structure and kinematics of the Outer-Scutum-Centaurus (OSC) arm located on the far side of the Milky Way through very long baseline interferometry (VLBI) astrometry using KaVA, which is composed of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We report the proper motion of a 22 GHz H$_{2}$O maser source, which is associated with the star-forming region G034.84$-$00.95, to be ($\mu_{\alpha} \rm{cos}\delta$, $\mu_{\delta}$) = ($-$1.61$\pm$0.18, $-$4.29$\pm$0.16) mas yr$^{-1}$ in equatorial coordinates (J2000). We estimate the 2D kinematic distance to the source to be 18.6$\pm$1.0 kpc, which is derived from the variance-weighted average of kinematic distances with LSR velocity and the Galactic-longitude component of the measured proper motion. Our result places the source in the OSC arm and implies that G034.84$-$00.95 is moving away from the Galactic plane with a vertical velocity of $-$38$\pm$16 km s$^{-1}$. Since the H I supershell GS033+06$-$49 is located at a kinematic distance roughly equal to that of G034.84$-$00.95, it is expected that gas circulation occurs between the outer Galactic disk around G034.84$-$00.95 with a Galactocentric distance of 12.8$^{+1.0}_{-0.9}$ kpc and halo. We evaluate possible origins of the fast vertical motion of G034.84$-$00.95, which are (1) supernova explosions and (2) cloud collisions with the Galactic disk. However, neither of the possibilities are matched with the results of VLBI astrometry as well as spatial distributions of H II regions and H I gas.Comment: Accepted for publication in PASJ. 14 figures; 8 table

High precision astrometry through phase-referencingVLBI at 22 GHz

Feasibility of the high-precision astrometry of Galactic water maser sources was studied on the basis of fast-switching phase-referencing VLBI observations at 22GHz held with VLBA (Very Long Baseline Array) of NRAO (Na- tional Radio Astronomy Observatory) as the first step towards investigation of the structure and dynamics of the Galaxy.Phase-referencing VLBI is a technique of observation which provides us relative position of two closely spaced astronomical radio sources with high accuracy. The technique can be applied to high-precision astrometry of Galactic maser sources using extragalactic continuum sources as reference points, yielding trigonometric parallax distances and proper motions of the maser sources throughout the Galaxy. The information obtained will be widely used in many fields of modern astronomy including, in particular, the study of the dynamics and structure of the Galaxy.In the present observations, all ten antennas of VLBA were switched between a Galactic water maser source and an adjacent extragalactic source with a duty cycle of 40 seconds for efficient compensation of degrading effects of the atmospheric phase fluctuations. The observations of the same pair of sources were repeated twice with a month interval for detection of proper motions of the maser sources with respect to the extragalactic reference sources. Two pairs of maser and extragalactic sources were observed in this way. The observed data were reduced using the NRAO AIPS(Astronomical Image Processing System) package.Results of observations of strong maser sources in the Galactic star forming region W3(OH) and an adjacent fairly strong extragalactic continuum source, ICRF0241+622, clearly showed that the atmospheric phase fluctuation is really well compensated though the two sources are separated by a finite angular distance of 2.17 degree. In fact, the large atmospheric phase fluctuations of several hundreds degrees observed in the fringe phases of the two sources were mostly compensated in the phase-referencing calibration with the AIPS. After the calibration, residual error due to the atmospheric fluctuation in the resultant difference of the fringe phases averaged for a switching cycle (40 second) was about 10 degrees. The 10 degrees phase error corresponds to the positional accuracy of about 30 micro-arcseconds for a projected baseline of 2000 km, which is much better than HIPPARCOS\u27s accuracy.On the other hand, results of observations of another pair of Galactic and extragalactic sources : masers in Galactic star forming region IRAS21008+4700 and an adjacent extragalactic continuum source, ICRF2100+468, with angular separation of only 0.18 degree, showed about 400 micro-arcsecond displacement of the masers relative to ICRF2100+468 during a month. The displacement was almost parallel to the Galactic plane. The distance of IRAS21008+4700 is estimated to be 7.3 kpc based on the simple flat rotation model of the Galaxy. The amount and direction of the observed displacement is just consistent with the predicted proper motion of the object due mainly to the Galactic rotation and, to much lesser extent, to the Solar motion and annual parallax. Thus we conclude that we detected for the first time the proper motion due to the Galactic rotation of a distant object of several kilo-parsecs apart during only a month interval.The proper motion was detected also for the W3(OH) and ICRF0241+622 pair. In this case, the direction of the proper motion was not parallel to the Galactic plane as expected from the proximity of W3(OH) to the Sun compared with IRAS21008+4700. In fact, at the estimated distance of W3(OH), 2.3 - 3.3 kpc , the most dominating systematic displacement comes from the annual trigonometric parallax. The predicted proper motion due to the combined effects of the annual parallax, Solar motion and Galactic rotation is fairly consistent with the observed motion. In other words, the phase-referencing VLBI observations are well capable of detecting the annual trigonometric parallaxes of Galactic sources a few kpc away.The above results firmly show a feasibility of the phase-referencing VLBI in astrometry with high accuracy of the order of 10 micro-arcseconds. This implies that directly measured distances and proper motions of the maser sources will be available throughout the Galaxy with the phase-referencing VLBI