A Parallax-Based Distance Estimator for Spiral Arm Sources

Abstract

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 H I surveys. A source's 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 Way's spiral structure as seen from the northern hemisphere.

The prevalence of star formation as a function of Galactocentric radius

“…By separating the Hi-GAL sources into the northern and southern Galactic subsamples, some of the spiral arm features in the RGC distributions are recovered. In Figures 2b and 2c, we show the average positions of the spiral arms within the longitude limits according to the most recent spiral arm models from the BeSSeL project (Reid et al 2016), reflecting the RGC-dependent arm widths (Reid et al 2014). In the top panel of Figure 2c, there is a clear peak in the total source distribution at RGC ∼6.6 kpc which likely corresponds to the Centaurus arm, and the peak at 4.5 kpc corresponds to the Norma arm in our longitude range.…”

“…Such factors may include the availability of dense molecular gas (Roman-Duval et al 2016), turbulent pressure (Wolfire et al 2003), local magnetic-field strength (Heiles & Troland 2005), or the presence of a triggering agent such as a wind-or radiation-driven bubble (Bertoldi 1989;Bisbas et al 2009). Reid et al 2014;Reid et al 2016). The Milky Way has either two or four arms (depending on the choice of tracer; Robitaille et al 2012) and exhibits spatial offsets between tracers of cold molecular gas and those of active star formation (e.g.…”

A New 95 GHz Methanol Maser Catalog. I. Data

“…Column 10: The distance for each maser, mainly estimated from the Galactic model of Reid et al (2014), with the exception of some sources with parallax distance measurements (Hirota et al 2008a(Hirota et al , 2008bReid et al 2014;Xu et al 2016) marked by P. Sources marked with a superscript G, D, S, or E have distances determined by Green & McClure-Griffiths (2011), Schlingman et al (2011), or Ellsworth-Bowers et al (2015, respectively. The remaining sources are assumed to be associated with star formation regions within spiral arms and distances have been determined through the Bayesian model of Reid et al (2016). These sources are indicated with a superscript R. For those sources where the Bayesian model identifies it is unlikely to be located in a spiral arm (probability <0.8), the distance is given by their referenced paper (marked by O).…”

“…For some sources (particularly those with low LSR velocities located close to the direction of the galactic center or anti-center), rotation models do not give a reliable distance estimate. For some of these we have used the distances assigned in the original papers, for the remainder we have assumed that they are within spiral arms, and estimated a distance from the Bayesian Distance Calculator (marked by R; Reid et al 2016). Note that the Bayesian Distance calculation shows that the probability of the masers being located in a spiral arm is greater than 0.8; for those sources where the Bayesian model identifies the probability of the masers being located in a spiral arm is less than 0.8, the distance is given by their referenced paper (marked by O).…”

Cygnus X-3: Its Little Friend’s Counterpart, the Distance to Cygnus X-3, and Outflows/Jets

“…(2006) have noted that parts of the Perseus arm (which is in the direction of the LF) have shown anomalous motions that may lead to larger errors in the estimated kinematic distances. To address this we used a parallax-based distance estimator tool 6 (Reid et al 2016) that uses a Bayesian approach to account for spiral arm signatures, kinematic distance estimates, Galactic latitude, and measured parallaxes to determine source distances and their associated probabilities. We ran this tool using a velocity of −47.5 km s −1 for the LF and arrive at a distance of 6.08±0.64 kpc with a 62% probability (see Figure 3).…”