1. The Megamaser Cosmology Project
Jim Braatz
NRAO
Co-I’s: Mark Reid Lincoln Greenhill Fred Lo
Jim Condon Christian Henkel
Ingyin Zaw Avanti Tilak Lei Hao
Cheng-Yu Kuo Liz Humphreys

2. The Impact of an H0 Prior on CMB Models
H0 = 72 ± 3r ± 7s km s-1 Mpc-1
Freedman et al. 2001
H0 = 62 ± 1r ± 5s km s-1 Mpc-1
Sandage et al. 2006
WMAP Likelihood Surfaces
Independent measurement of H0 is required to constrain the cosmological model.

3. Water masers in AGN Accretion Disks
H2O emission at 22 GHz
N(H2) ~ cm-3
T ~ K
UGC 3789: Braatz & Gugliucci 2008

4. Measuring Distances to H2O Megamasers
NGC 4258
NGC 4258 is the prototype for the maser distance method
BUT this galaxy is too close to determine H0 directly
Nearby maser galaxies like NGC 4258 can be used to recalibrate the Cepheid scale  Vr
D = r/
2Vr
2
a = Vr2/r
D = Vr2/a
Herrnstein et al. (1999)
D = 7.2  0.5 Mpc

5. The Megamaser Cosmology Project
In the framework of cosmology set by analysis of CMB anisotropies, a precise measurement of H0 will provide a strong constraint on the equation of state parameter, w, of dark energy.
The MCP is an NRAO “Large Project” with the goal, in about 5 years, of determining H0 to 3% by measuring angular diameter distances to about 10 galaxies in the Hubble flow.
The Process:
Survey with the GBT to identify maser disk galaxies
Image the sub-pc disks with the High Sensitivity Array (VLBA+GBT+EB)
Measure accelerations in the disk with GBT monitoring
Model the maser disk dynamics and determine distance to the host galaxy

6. Progress in Megamaser Surveys
112 Extragalactic maser galaxies are now known
About 30% have h.v. components > 250 km s-1
About 10% are disk masers and are candidates for high precision distance measurements
Primary sample for new surveys: Type 2 AGNs from SDSS

7. GBT Spectra of Maser Disks

8. UGC 3789 MBH ~ 1.0 x 107 Msun from acceleration analysis
Braatz and Gugliucci, 2008

9. UGC 3789

10. UGC 3789
Reid et al, in prep.

11. UGC 3789 Preliminary Properties of UGC 3789 Maser Disk
R ~ pc ( mas)
V ~ km/s
Mbh ~ 1.2 x 107 Msun
a ~ 3.6 km s-1 yr-1 (mean value)
D ~ 51 Mpc (15%)
H0 ~ 65 km s-1 Mpc-1
Braatz et al, in prep.

12. NGC 6323

13. Backup Slides

15. The Extragalactic Distance Scale
Structures
Largest
NGC 4258
UGC 3789
NGC 3393
NGC 1194
NGC 6323
IC 2560
Mrk 1419
Cepheids
Direct Measurement of H0
0 Mpc
100 Mpc
200 Mpc
One method covers all scales out to the size of largest structures
Maser distances can be used to calibrate other distance methods e.g. Cepheids, SN Ia, Tully-Fisher

16. The CMB and Cosmology
“The single most important complement to the CMB for measuring the DE equation of state at z ~ 0.5 is a determination of the Hubble constant to better than a few percent.”
Hu 2005
The CMB on its own does not measure many fundamental parameters; instead there are degemeracies. E.g. Ωmh2
Independent measurement of H0 is necessary
Spergel et al., (WMAP 3-yr)

17. The Accretion Disk in NGC 4258
Warp in the disk is fully characterized
Accelerations measured in both systemic and h.v. features
Blue features absorbed by gas in the disk
Disk characterization reduces systematic error in distance determination
Alternative to LMC to anchor the extragalactic distance scale
Humphreys et al. 2008
(CfA group)

18. A GBT Gallery of Extragalactic H2O masers
Braatz and Gugliucci, 2008, ApJ, 678, 96

19. Measuring Distances to H2O Megamasers
The Prototype: NGC 4258  Vr
D = r/
a = Vr2/r
D = Vr2/a
2Vr
2
Herrnstein et al. (1999)
D = 7.2  0.5 Mpc
Herrnstein et al. 1999

20. NGC 6323

21. Disk Masers in the Era of VSOP-2
Precision measurement of disk geometry
Substructure: striation and spiral structure
What fraction of the disk do masers trace? (skin structure)
Accurate characterization of the disk is critical to minimize systematic errors
Constraints on blended systemic features
Elliptical orbits?
Distinguish phase effects from maser clump effects
Proper motion measurements? (~ 3 µas yr-1 for UGC 3789)
Sensitivity (and flex scheduling) is essential (Baselines to GBT, EVLA, EB)