A direct observation of the spiral structure of the Galaxy is hindered by our position in the middle of the Galactic plane. We propose a method based on the analysis of the birthplaces of high Galactic latitude runaway stars to map the spiral arms and determine their dynamics. As a proof of concept, the method is applied to a local sample of early-type stars and a sample of runaways stars to obtain estimates of the pattern speed and the spiral arm's phase angle. We also estimate the performance of this method once the data gathered by Gaia, in particular for runaway stars observed on the other side of the Galaxy, is available.
Wolf-Rayet stars are advanced evolutionary stages of massive stars. Despite their large mass-loss rates and high wind velocities, none of them display a bow shock, although a fraction of them are classified as runaway. Our 2.5-D numerical simulations of circumstellar matter around a 60Mo runaway star show that the fast Wolf-Rayet stellar wind is released into a wind-blown cavity filled with various shocks and discontinuities generated throughout the precedent evolutionary phases. The resulting fast-wind slow-wind interaction leads to the formation of spherical shells of swept-up dusty material…
Runaway stars ejected from the Galactic disk populate the halo of the Milky Way. To predict the spatial and kinematic properties of runaways, we inject stars into a Galactic potential, compute their trajectories through the Galaxy, and derive simulated catalogs for comparison with observations. Runaways have a flattened spatial distribution, with higher velocity stars at Galactic latitudes less than 30 degrees. Due to their shorter stellar lifetimes, massive runaway stars are more concentrated towards the disk than low mass runaways. Bound (unbound) runaways that reach the halo probably origin…
We report on the presence of 6.7-GHz methanol masers, known tracers of high-mass star formation, in the 3-kpc arms of the inner Galaxy. We present 49 detections from the Methanol Multibeam Survey, the largest Galactic plane survey for 6.7-GHz methanol masers, which coincide in longitude, latitude and velocity with the recently discovered far-side 3-kpc arm and the well known near-side 3-kpc arm. The presence of these masers is significant evidence for high-mass star formation actively occurring in both 3-kpc arms.
Since the discovery of hypervelocity stars in 2005, it has been widely believed that only the disruption of a binary system by a supermassive black hole at the Galactic center (GC), that is, the so-called Hills mechanism, is capable of accelerating stars to beyond the Galactic escape velocity. In the meantime, however, driven by the Gaia space mission, there is mounting evidence that many of the most extreme high-velocity early-type stars at high Galactic latitudes do originate in the Galactic disk and not in the GC. Moreover, the ejection velocities of these extreme disk-runaway stars exceed …
In this paper we analyze a sample of metal-rich (>-0.8 dex) main sequence stars in the extended solar neighborhood, investigating kinematic outliers from the background population. The data, which are taken from the Sloan Digital Sky Survey, are kinematically profiled as a function of distance from the Galactic plane using full six dimensional phase space information. Each star is examined in the context of these kinematic profiles and likelihoods are assigned to quantify whether a star matches the underlying profile. Since some of these stars are likely to have been ejected from the disc, we …
We report the results of our systematic survey for Galactic 6.7 GHz Class II CH$_3$OH maser emission toward a sample of young stellar objects. The survey was conducted with the Shanghai Tianma Radio Telescope (TMRT). The sample consists of 3348 sources selected from the all-sky \emph{Wide-Field Infrared Survey Explorer (WISE)} point source catalog. We have discussed the selection criteria in detail and the detection results of those at high Galactic latitudes (i.e. $|b|>$ 2$^\circ$) in a previous paper (paper I). Here, we present the results from the survey of those at low Galactic latitudes, …
We present gas and stellar kinematics of a high-resolution zoom-in cosmological chemodynamical simulation, which fortuitously captures the formation and evolution of a star-forming barred spiral galaxy, from redshift $z\sim3$ to $z\sim2$ at the peak of the cosmic star formation rate. The galaxy disc grows by accreting gas and substructures from the environment. The spiral pattern becomes fully organised when the gas settles from a thick (with vertical dispersion $σ_{v} >$ 50 km/s) to a thin ($σ_{v} \sim 25$ km/s) disc component in less than 1 Gyr. Our simulated disc galaxy also has a central X…
We present detailed analysis of the transient X-ray source 2XMMi J003833.3+402133 detected by XMM-Newton in January 2008 during a survey of M 31. The X-ray spectrum is well fitted by either a steep power law plus a blackbody model or a double blackbody model. Prior observations with XMM-Newton, Chandra, Swift and ROSAT spanning 1991 to 2007, as well as an additional Swift observation in 2011, all failed to detect this source. No counterpart was detected in deep optical imaging with the Canada France Hawaii Telescope down to a 3sigma lower limit of g = 26.5 mag. This source has previously been …
The discovery of new clear windows in the Galactic plane using the VVV near-IR extinction maps allows the study of the structure of the Milky Way (MW) disk. The ultimate goal of this work is to map the spiral arms in the far side of the MW, which is a relatively unexplored region of our Galaxy, using red clump (RC) giants as distance indicators. We search for near-IR clear windows located at low Galactic latitudes ($|b|< 1$ deg) in the MW disk using the VVV near-IR extinction maps. We have identified two new windows named VVV WIN 1607-5258 and VVV WIN 1475-5877, respectively, that complement t…
Abridged: We have used the well-selected sample of ~1750 embedded, young, massive stars identified by the RMS survey to investigate the Galactic distribution of recent massive star formation. We describe the various methods used to assign distances extracted from the literature, and solve the distance ambiguities towards ~200 sources located within the Solar circle using archival HI data. These distances are used to calculate bolometric luminosities and estimate the survey completeness (~2x10^4 lsun). In total, we calculate the distance and luminosity of ~1650 sources, one third of which are a…
We present a map of the spiral structure of the Galaxy, as traced by molecular CS emission associated with IRAS sources which are believed to be compact HII regions. The CS line velocities are used to determine the kinematic distances of the sources, in order to investigate their distribution in the galactic plane. This allows us to use 870 objects to trace the arms, a number larger than that of previous studies based on classical HII regions. The distance ambiguity of the kinematic distances, when it exists, is solved by different procedures, including the latitude distribution and an analysi…
We report four trigonometric parallaxes for high-mass star-forming regions within 4 kpc of the Galactic center. These measurements were made with the VLBA as part of the BeSSeL Survey. By associating these sources kinematically with large-scale features in CO and HI 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-bar's major axis near its leading edge…
Accurate distances of the Galactic spiral arms from dust-scattered X-ray emission of gamma-ray bursts
Scuola Universitaria Superiore IUSS Pavia（帕维亚高等大学）；Department of Physics, University of Trento（特伦托大学物理系）；Istituto Nazionale di Astrofisica, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano（国家天体物理研究所，米兰空间天体物理学与宇宙物理学研究所）；INAF - Osservatorio Astrofisico di Arcetri（国家天体物理研究所-阿尔切特里天文台）；Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris（巴黎高等师范学院物理实验室，ENS，PSL大学，CNRS，索邦大学，巴黎大学）；Ruđer Bošković Institute（鲁德·博什科维奇研究所）；University of Zagreb Faculty of Electrical Engineering and Computing（萨格勒布大学电气工程和计算学院）
We present a large, homogeneous catalogue of molecular clouds within 4 kpc from the Sun at low Galactic latitudes ($|b|$ $<$ 10\degr) with unprecedented accurate distance determinations. Based on the three-dimensional dust reddening map and estimates of colour excesses and distances of over 32 million stars presented in Chen et al, we have identified 567 dust/molecular clouds with a hierarchical structure identification method and obtained their distance estimates by a dust model fitting algorithm. The typical distance uncertainty is less than 5 per cent. As far as we know, this is the first l…
Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand the Galaxy's morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Center in our model that ar…
Hypervelocity stars (HVSs) represent a unique population of stars in the Galaxy reflecting properties of the whole Galactic potential. Determining their origin is of fundamental importance to constrain the shape and mass of the dark halo. The leading scenario for the ejection of HVSs is an encounter with the supermassive black hole in the Galactic Centre. However, new proper motions from the \textit{Gaia} mission indicate that only the fastest HVSs can be traced back to the Galactic centre and the remaining stars originate in the disc or halo. In this paper, we study HVSs generated by encounte…
We present the discovery of five new unbound hypervelocity stars (HVSs) in the outer Milky Way halo. Using a conservative estimate of Galactic escape velocity, our targeted spectroscopic survey has now identified 16 unbound HVSs as well as a comparable number of HVSs ejected on bound trajectories. A Galactic center origin for the HVSs is supported by their unbound velocities, the observed number of unbound stars, their stellar nature, their ejection time distribution, and their Galactic latitude and longitude distribution. Other proposed origins for the unbound HVSs, such as runaway ejections …
The magnetic structure in the Galactic disk, the Galactic center and the Galactic halo can be delineated more clearly than ever before. In the Galactic disk, the magnetic structure has been revealed by starlight polarization within 2 or 3 kpc of the Solar vicinity, by the distribution of the Zeeman splitting of OH masers in two or three nearby spiral arms, and by pulsar dispersion measures and rotation measures in nearly half of the disk. The polarized thermal dust emission of clouds at infrared, mm and submm wavelengths and the diffuse synchrotron emission are also related to the large-scale …
Using a sample of 213,713 stars from the Radial Velocity Experiment (RAVE) survey, limited to a distance of 2 kpc from the Sun and to |z|<1 kpc, we report the detection of a velocity gradient of disc stars in the fourth quadrant, directed radially from the Galactic centre. In the direction of the Galactic centre, we apply a simple method independent of stellar proper motions and of Galactic parameters to assess the existence of this gradient in the RAVE data. This velocity gradient corresponds to |K+C| < 3 km/s/kpc, where K and C are the Oort constants measuring the local divergence and radial…
By positional matching to the catalogue of Galactic Ring Survey molecular clouds, we have derived distances to 793 Bolocam Galactic Plane Survey (BGPS) sources out of a possible 806 located within the region defined by Galactic longitudes l = 28.5 degr to 31.5 degr and latitudes |b| < 1 degr. This section of the Galactic Plane contains several major features of Galactic structure at different distances, mainly mid-arm sections of the Perseus and Sagittarius spiral arms and the tangent of the Scutum-Centarus arm, which is coincident with the end of the Galactic Long Bar. By utilising the catalo…
We present the physical and evolutionary properties of prestellar and protostellar clumps in the Herschel Infrared GALactic plane survey (Hi-GAL) in two large areas centered in the Galactic plane and covering the tips of the long Galactic bar at the intersection with the spiral arms. The areas fall in the longitude ranges 19 < l < 33 and 340 < l < 350, while latitude is -1 < b < 1. Newly formed high mass stars and prestellar objects are identified and their properties derived and compared. A study is also presented on five giant molecular complexes at the further edge of the bar. The star-form…
