We report parallaxes and proper motions of three water maser sources in high-mass star-forming regions in the Outer Spiral Arm of the Milky Way. The observations were conducted with the Very Long Baseline Array as part of Bar and Spiral Structure Leg
acy Survey and double the number of such measurements in the literature. The Outer Arm has a pitch angle of 14.9 +/- 2.7 deg and a Galactocentric distance of 14.1 +/- 0.6 kpc toward the Galactic anticenter. The average motion of these sources toward the Galactic center is 10.7 +/- 2.1 km/s and we see no sign of a significant fall in the rotation curve out to 15 kpc from the Galactic center. The three-dimensional locations of these star-forming regions are consistent with a Galactic warp of several hundred parsecs from the plane.
Using the Very Long Baseline Array, we have measured a trigonometric parallax for the micro quasar GRS 1915+105, which contains a black hole and a K-giant companion. This yields a direct distance estimate of 8.6 (+2.0,-1.6) kpc and a revised estimate
for the mass of the black hole of 12.4 (+2.0,-1.8) Msun. GRS 1915+105 is at about the same distance as some HII regions and water masers associated with high-mass star formation in the Sagittarius spiral arm of the Galaxy. The absolute proper motion of GRS 1915+105 is -3.19 +/- 0.03 mas/y and -6.24 +/- 0.05 mas/y toward the east and north, respectively, which corresponds to a modest peculiar speed of 22 +/-24 km/s at the parallax distance, suggesting that the binary did not receive a large velocity kick when the black hole formed. On one observational epoch, GRS 1915+105 displayed superluminal motion along the direction of its approaching jet. Considering previous observations of jet motions, the jet in GRS 1915+105 can be modeled with a jet inclination to the line of sight of 60 +/- 5 deg and a variable flow speed between 0.65c and 0.81c, which possibly indicates deceleration of the jet at distances from the black hole >2000 AU. Finally, using our measurements of distance and estimates of black hole mass and inclination, we provisionally confirm our earlier result that the black hole is spinning very rapidly.
We report trigonometric parallaxes and proper motions of water masers for 12 massive star forming regions in the Perseus spiral arm of the Milky Way as part of the Bar and Spiral Structure Legacy (BeSSeL) Survey. Combining our results with 14 paralla
x measurements in the literature, we estimate a pitch angle of 9.9 +/- 1.5 degrees for a section of the Perseus arm. The three-dimensional Galactic peculiar motions of these sources indicate that on average they are moving toward the Galactic center and slower than the Galactic rotation.
We report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way as part of the BeSSeL Survey with the VLBA. Combining these results with eight others from the literature, we
investigated the structure and kinematics of the arm between Galactocentric azimuth around -2 and 65 deg. We found that the spiral pitch angle is 7.3 +- 1.5 deg; the arms half-width, defined as the rms deviation from the fitted spiral, is around 0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 +- 0.2 kpc from the Sun. Unlike for adjacent spiral arms, we found no evidence for significant peculiar motions of sources in the Sagittarius arm opposite to Galactic rotation.
The Hubble constant Ho describes not only the expansion of local space at redshift z ~ 0, but is also a fundamental parameter determining the evolution of the universe. Recent measurements of Ho anchored on Cepheid observations have reached a precisi
on of several percent. However, this problem is so important that confirmation from several methods is needed to better constrain Ho and, with it, dark energy and the curvature of space. A particularly direct method involves the determination of distances to local galaxies far enough to be part of the Hubble flow through water vapor (H2O) masers orbiting nuclear supermassive black holes. The goal of this article is to describe the relevance of Ho with respect to fundamental cosmological questions and to summarize recent progress of the the `Megamaser Cosmology Project (MCP) related to the Hubble constant.
We present the results of a survey of radio continuum sources near the Galactic plane using the Very Long Baseline Array (VLBA). Our observations are designed to identify compact extragalactic sources of milliarcsecond size that can be used for paral
lax measurements in the Bar and Spiral Structure Legacy Survey. We selected point sources from the NVSS and CORNISH catalogs with flux densities above 30 mJy and within $1.5degr$ of known maser targets. Of the 1529 sources observed, 199 were detected. For sources detected on 3 or more baselines, we determined accurate positions and evaluated their quality as potential calibrators. Most of the 1330 sources that were not detected with the VLBA are probably of extragalactic origin.
We are using the VLBA and the Japanese VERA project to measure trigonometric parallaxes and proper motions of masers found in high-mass star-forming regions across the Milky Way. Early results from 18 sources locate several spiral arms. The Perseus s
piral arm has a pitch angle of 16 +/- 3 degrees, which favors four rather than two spiral arms for the Galaxy. Combining positions, distances, proper motions, and radial velocities yields complete 3-dimensional kinematic information. We find that star forming regions on average are orbiting the Galaxy ~15 km/s slower than expected for circular orbits. By fitting the measurements to a model of the Galaxy, we estimate the distance to the Galactic center R_o = 8.4 +/- 0.6 kpc and a circular rotation speed Theta_o = 254 +/- 16 km/s. The ratio Theta_o/R_o can be determined to higher accuracy than either parameter individually, and we find it to be 30.3 +/- 0.9 km/s/kpc, in good agreement with the angular rotation rate determined from the proper motion of Sgr A*. The data favor a rotation curve for the Galaxy that is nearly flat or slightly rising with Galactocentric distance. Kinematic distances are generally too large, sometimes by factors greater than two; they can be brought into better agreement with the trigonometric parallaxes by increasing Theta_o/R_o from the IAU recommended value of 25.9 km/s/kpc to a value near 30 km/s/kpc. We offer a revised prescription for calculating kinematic distances and their uncertainties, as well as a new approach for defining Galactic coordinates. Finally, our estimates of Theta_o and To/R_o, when coupled with direct estimates of R_o, provide evidence that the rotation curve of the Milky Way is similar to that of the Andromeda galaxy, suggesting that the dark matter halos of these two dominant Local Group galaxy are comparably massive.
We report trigonometric parallaxes for the massive star-forming regions G23.01-0.41 and G23.44-0.18, corresponding to distances of 4.59 [+0.38/-0.33] kpc and 5.88 [+1.37/-0.93] kpc, respectively. The distance to G23.01-0.41 is smaller than its near k
inematic distance assuming a standard model of the Milky Way and less than half of its far kinematic distance, which has usually been assumed. This places it in the Crux-Scutum spiral arm. The distance to G23.44-0.18 is close to its near kinematic distance and most likely places it in the Norma spiral arm near the end of the Galactic bar. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find large deviations from circular Galactic orbits for these sources: both sources show peculiar motions of 20 to 30 km/s counter to Galactic rotation and toward the Galactic center. These large peculiar motions might be the result of gravitational perturbations from the Galactic bar.
We report trigonometric parallaxes for the high-mass star forming regions G35.20-0.74 and G35.20-1.74, corresponding to distances of 2.19 (+0.24 -0.20) kpc and 3.27 (+0.56 -0.42) kpc, respectively. The distances to both sources are close to their nea
r kinematic distances and place them in the Carina-Sagittarius spiral arm. Combining the distances and proper motions with observed radial velocities gives the locations and full space motions of the star forming regions. Assuming a standard model of the Galaxy, G35.20-0.74 and G35.20-1.74 have peculiar motions of ~13 km/s and ~16 km/s counter to Galactic rotation and ~9 km/s toward the North Galactic Pole.
We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and pla
ces it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.
