Maser Astrometry with VLBI and Galactic Structure Mareki Honma Mizusawa VLBI Observatory, NAOJ Milky Way & VERA Ogasawara station

Back to 115 years ago … Mizusawa Carlo Forte International Latitude Observatory (ILO), founded in 1899 ILO Mizusawa: now Mizusawa VLBI Observatory ILO Carlo Forte: now Cagliari Astron. Observatory both working on radio/VLBI

Galaxy-scale maser astrometry Parallax Sun Galaxy Center GC distance D=　8 kpc, π=　125 μas requires 10-μas accuracy Galaxy scale astrometry : frontier in 21st century (e.g. GAIA) Relative VLBI astrometry certainly can measure this δx ~ θ sep c δt / B δx ~ 10 μas for reasonable parameters

Arrays VERA, VLBA, EVN. All in the Northern hemisphere. LBA, only one in the South

Astrometry examples Chibueze+(2014) NGC6334I(N) D ~ 1.27 kpc Min+(2014) R Aqr D ~ 218 pc

EVN astrometry Rygl+(2012), astrometry of masers in Cygnus X complex

Galactic structure Reid+(2009): Early results based on 18 sources R0 = 8.4 +/- 0.6 kpc, Θ0 = 247 +/- 16 km/s 489 citations as of Oct 2014, good demonstration of impacts of VLBI astrometry in this field

Galactic structure II: Honma+(2012) 52 sources from VERA/VLBA/EVN R0=8.05 +/- 0.45 kpc, Θ0=238 +/- 14 km/s

III. Reid+(2014) 103 sources R0 = 8.34+/-0.16 kpc Θ0 = 240 +/- 8 km/s

Galactic constants Reid+ (2009) Honma+ (2012) (2014) Nsrc 18 52 103 R0 (kpc) 8.4 (+/- 0.6) 8.05 (+/- 0.45) 8.34 (+/- 0.16) Θ0　(km/s) 247 (+/- 16) 238 (+/- 14) 240 (+/- 8) Galactic constants are already measured at a few percent level !

Most updated data as of Oct 2014 114 sources from VLBA/VERA/EVN

Tracing spiral arms Non-circular systematic motion has been detected Slow-rotation along the Perseus arm Useful for testing the spiral arm scenario --- density wave or material wave ?

Broader impacts beyond astronomy Direct Dark Matter search Galactic rotation is the key to know the flux of dark matter particles on Earth. Gravitational wave Until recently the largest uncertainty in the Hulse-Taylor pulsar was Galactic acceleration. Mass extinction on the Earth !!?? There is a claim that Sun’s spiral-arm passage could have caused the global climate change (!?)

Concluding remarks VLBI can certainly do 10-uas level astrometry. Parallax and proper motions are obtained for more than 100 source. VLBI astrometry readily provides excellent results on the Galactic structure and beyond.

East Asian VLBI KVN 21m x3 SHAO 65m Array extension to East Asia regions Better sensitivity, better imaging capability -> better astrometry SHAO 65m

1899~ 202x ~ ? Corresponding to ~7% SKA in collecting area ! Mizusawa Carlo Forte Corresponding to ~7% SKA in collecting area !

Thank you for attention Thank you for attention! And let’s keep collaboration for next 100 years !