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

2. 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

3. 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

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

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

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

7. 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

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

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

10. 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 !

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

12. 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 ?

13. 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 (!?)

14. 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.

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

16. 1899~ 202x ~ ? Corresponding to ~7% SKA in collecting area !
Mizusawa
Carlo Forte
Corresponding to ~7% SKA in collecting area !

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