T. M. Dame

The Milky Way in Molecular Clouds: A New Complete CO Survey

New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth ( °) sample the entire Galactic plane over a strip 4°-10° wide in latitude, and beamwidth or ° sample nearly all large local clouds at higher latitudes. Compared with the previous composite CO survey of Dame et al. (1987), the new survey has 16 times more spectra, up to 3.4 times higher angular resolution, and up to 10 times higher sensitivity per unit solid angle. Each of the component surveys was integrated individually using clipping or moment masking to produce composite spatial and longitude-velocity maps of the Galaxy that display nearly all of the statistically significant emission in each survey but little noise. The composite maps provide detailed information on individual molecular clouds, suggest relationships between clouds and regions widely separated on the sky, and clearly display the main structural features of the molecular Galaxy. In addition, since the gas, dust, and Population I objects associated with molecular clouds contribute to the Galactic emission in every major wavelength band, the precise kinematic information provided by the present survey will form the foundation for many large-scale Galactic studies. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| 5°), X shows little systematic variation with latitude from a mean value of (1.8 ± 0.3) × 1020 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

TRIGONOMETRIC PARALLAXES OF HIGH MASS STAR FORMING REGIONS: THE STRUCTURE AND KINEMATICS OF THE MILKY WAY

Over 100 trigonometric parallaxes and proper motions for masers associated with young, high- mass stars have been measured with the Bar and Spiral Structure Legacy Survey, a Very Long Baseline Array key science project, the European VLBI Network, and the Japanese VLBI Exploration of Radio Astrometry project. These measurements provide strong evidence for the existence of spiral arms in the MilkyWay, accurately locating many arm segments and yielding spiral pitch angles ranging from about 7 degrees to 20 degrees. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the MilkyWay with the three- dimensional position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, R-0, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, Theta(0), to be 240 +/- 8 km s(-1), and a rotation curve that is nearly flat ( i. e., a slope of -0.2 +/- 0.4 km s(-1) kpc(-1)) between Galactocentric radii of approximate to 5 and 16 kpc. Assuming a universal spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44 +/- 0.16 kpc. With this large data set, the parameters R-0 and Theta(0) are no longer highly correlated and are relatively insensitive to different forms of the rotation curve. If one adopts a theoretically motivated prior that high- mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, V-circle dot = 14.6 +/- 5.0 km s(-1). While Theta(0) and V-circle dot are significantly correlated, the sum of these parameters is well constrained, Theta(0) + V circle dot = 255.2 +/- 5.1 km s(-1), as is the angular speed of the Sun in its orbit about the Galactic center, ( Theta(0) + V-circle dot)/R-0 = 30.57 +/- 0.43 km s(-1) kpc(-1). These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity.

ON THE NATURE OF THE LOCAL SPIRAL ARM OF THE MILKY WAY

Trigonometric parallax measurements of nine water masers associated with the Local Arm of the Milky Way were carried out as part of the BeSSeL Survey using the Very Long Baseline Array. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and three-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local Arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (> 5 kpc), and shallow pitch angle (similar to 10 degrees) suggest that it is more like the adjacent Perseus and Sagittarius Arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local Arm to be closer to the Perseus than to the Sagittarius Arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near zero toward both the Galactic center and north Galactic pole, and counter rotation of similar to 5 km s(-1) were observed, indicating that the Local Arm has similar kinematic properties as found for other major spiral arms.

The Bar and Spiral Structure Legacy (BeSSeL) survey: Mapping the Milky Way with VLBI astrometry†

Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our galaxy and to determine fundamental parameters such as the rotation velocity (Theta(0)) and curve and the distance to the Galactic center (R-0). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R0 and Theta(0) need a substantial revision. In particular the combination of 8.5 kpc and 220 km s(-1) can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr A* gives best values of R-0 = 8.3 +/- 0.23 kpc and Theta(0) = 239 or 246 +/- 7 km s(-1), for Solar motions of V-circle dot = 12.23 and 5.25 km s(-1), respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) survey

PARALLAXES FOR W49N AND G048.60+0.02: DISTANT STAR FORMING REGIONS IN THE PERSEUS SPIRAL ARM

We report trigonometric parallax measurements of 22 GHz H2O masers in two massive star-forming regions from Very Long Baseline Array observations as part of the Bar and Spiral Structure Legacy Survey. The distances of 11.11(-0.69)(+0.79) kpc to W49N (G043.16+0.01) and 10.75(-0.55)(+0.61) kpc to G048.60+0.02 locate them in a distant section of the Perseus arm near the solar circle in the first Galactic quadrant. This allows us to locate accurately the inner portion of the Perseus arm for the first time. Combining the present results with sources measured in the outer portion of the arm in the second and third quadrants yields a global pitch angle of 9 degrees.5 +/- 1 degrees.3 for the Perseus arm. We have found almost no H2O maser sources in the Perseus arm for 50 degrees < l < 80 degrees, suggesting that this approximate to 6 kpc section of the arm has little massive star formation activity.

Trigonometric parallaxes to star-forming regions within 4 kpc of the galactic center

We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the Very Long Baseline Array as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and H I longitude-velocity diagrams, we begin to outline some major features of the inner Milky Way: the Connecting arm, the near and far 3 kpc arms, and the Norma arm. The Connecting arm in the first Galactic quadrant lies closer to the Galactic center than the far 3 kpc arm and is offset by the long-bars major axis near its leading edge, supporting the presence of an inner Lindblad resonance. Assuming the 3 kpc arms are a continuous physical structure, the relative Galactocentric distance of its near and far sides suggests highly elliptical streamlines of gas around the bar(s) and a bar corotation radius, r {sub CR} ≳ 3.6 kpc. At a Galactic longitude near 10° and a heliocentric distance of about 5 kpc, the near 3 kpc arm and the Norma arm intersect on a face-on view of our Galaxy, while passing at different Galactic latitudes. We provide an accurate distance measurement to the W 31morexa0» star-forming complex of 4.95{sub −0.43}{sup +0.51} kpc from the Sun, which associates it with a bright CO feature belonging to the near 3 kpc arm.«xa0less

The Distance of the Soft Gamma Repeater SGR 1806–20

We present 12CO (J = 1-0) observations in the direction of the soft gamma repeater SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and we discuss in this paper the implications of these observations for the distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The distance of SGR 1806-20 is estimated to be 14.5 ± 1.4 kpc and this soft gamma repeater is very likely associated with one of the brightest H II regions in the Galaxy, W31. The large size of G10.0-0.3 (25 × 38 pc) for a young supernova remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that G10.0-0.3 could be expanding in the very low density region produced by the wind of the blue star.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VII. G9.62+0.20 AND THE EXPANDING 3 kpc ARM

We report a trigonometric parallax of 12 GHz methanol masers associated with the massive star-forming region G9.62+0.20, corresponding to a distance of 5.2(-0.6)(+0.6) kpc. With an LSR velocity of about 2 km s(-1), the regions kinematic distances of 0.5 and 16 kpc differ greatly from the distance derived here. Our measurement of the peculiar motion of the star-forming region shows a very large deviation from a circular Galactic orbit: 41 km s(-1) radially outward from the Galactic center and 60 km s(-1) counter to Galactic rotation. The combination of its radial velocity and distance places G9.62+0.20 in the inner region of the Galaxy close to the expanding near 3 kpc arm, where the bulge/bar potential has strong gravitational influence. We also map the distribution of 12 GHz methanol masers, locate them with respect to a hypercompact H II region, and compare our data with the periodic flare phenomenon reported previously for this source.

The Distance to the Soft Gamma Repeater SGR 1627-41.

We report millimeter observations of the line of sight to the recently discovered soft gamma repeater SGR 1627-41, which has been tentatively associated with the supernova remnant (SNR) G337.0-0.1. Among the eight molecular clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is probably interacting with one of the most massive giant molecular clouds (GMCs) in the Galaxy, at a distance of 11 kpc from the Sun. Based on the high extinction to the persistent X-ray counterpart of SGR 1627-41, we present evidence for an association of this new soft gamma repeater (SGR) with the SNR G337.0-0.1; they both appear to be located on the near side of the GMC. This is the second SGR located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a neutron star with a high transverse velocity ( approximately 1000 km s-1) escaping the young ( approximately 5000 yr) SNR G337.0-0.1.

A PARALLAX-BASED DISTANCE ESTIMATOR FOR SPIRAL ARM SOURCES

The spiral arms of the Milky Way are being accurately located for the first time via trigonometric parallaxes of massive star forming regions with the BeSSeL Survey, using the Very Long Baseline Array and the European VLBI Network, and with the Japanese VERA project. Here we describe a computer program that leverages these results to significantly improve the accuracy and reliability of distance estimates to other sources that are known to follow spiral structure. Using a Bayesian approach, sources are assigned to arms based on their (l,b,v) coordinates with respect to arm signatures seen in CO and HI surveys. A sources kinematic distance, displacement from the plane, and proximity to individual parallax sources are also considered in generating a full distance probability density function. Using this program to estimate distances to large numbers of star forming regions, we generate a realistic visualization of the Milky Ways spiral structure as seen from the northern hemisphere.