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Spiral Structure in the Milky Way: Confronting Observations and Theory

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Spiral Structure in the Milky Way: Confronting Observations and Theory Astronomical News Report on the ESO Workshop Spiral Structure in the Milky Way: Confronting Observations and Theory held at Bahía Inglesa, Copiapó, Chile, 8–11 November 2010 Preben Grosbøl1 Giovanni Carraro1 Yuri Beletsky1 1 ESO The main objectives of the workshop were to review current observational evidence for spiral arms in our Galaxy and confront them with models of spi- ral structure in order to arrive at a con- sistent picture. Of primary importance was to understand just what additional information is required to resolve out- standing issues related to the spiral structure in the Milky Way, especially as new survey instruments (e.g., ALMA, VISTA and VST) are coming online and major space missions like GAIA will be launched in the near future. Figure 1. The participants by the swimming pool at the range 45–408 MHz show four tan- More than 50 years ago, the spiral arms the conference venue, the Hotel Rocas de Bahía in gential points associated with synchro- Bahía Inglesa. of the Milky Way were identified in the tron radiation in spiral arms, consistent distribution of OB-stars, H II-regions and with a four-armed pattern (A. Guzmán). neutral hydrogen. An overview of the Observations early results was presented in the pro- GMCs and the massive stars in the south- ceedings of the IAU Symposium 38 held Recent observational data suggesting a ern Milky Way were identified by com- in 1969, in Basel, Switzerland. Our spiral structure in the Milky Way were bining the new Columbia survey and IRAS knowledge of the evidence for spiral arms reviewed during the first two days of the data (P. Garcia-Fuentes). They follow in the Milky Way and the kinematics in workshop. H I/CO maps of the Galaxy the spiral structure with some scatter and the Solar Neighbourhood has increased have much improved both in sensitivity include several massive GMCs in the significantly over the last few decades. and resolution (presentation by T. Dame; Norma arm. New spectrophotometric Despite this, there is still no consensus on see the workshop web page1 for more distances have significantly improved the the basic parameters of the spiral struc- details). This has led to a better outline of mapping of young stellar clusters in ture in our Galaxy, such as the number of the spiral arms, especially on the far side the Galaxy (A. Moitinho & A. Daminelli). major spiral arms and their location, its of the Galaxy, even though uncertain- There seems to be a lack of giant H II pattern speed(s) and amplitude, and its ties in the rotation curve and the distance regions on the far side of the Milky Way. relation to the central bar. Major new and ambiguity still present issues. The iden- The Perseus arm is well defined by CO future observational facilities (such as tification of a symmetric counterpart to clouds, but is deficient in clusters. Using ALMA, GAIA, LSST, VISTA, VST and the 3-kiloparsec arms was made possible near-infrared (NIR) surveys such as APOGEE) will provide a wealth of data by the high resolution radio data. Accu- GLIMPSE, a global view of the most mas- on the spatial and kinematic distributions rate parallaxes to numerous masers have sive, young stellar clusters was assem- of the material in the Galaxy. Thus, it been determined using very long baseline bled (M. Messineo). Only clusters younger seemed appropriate to perform a census interferometry (VLBI) techniques (talks than 30 Myr display spiral structure in of our current data, confront them with by M. Reid, M. Sato and M. Honma). the Solar Neighbourhood (A. Lokin & theory and models of spiral structure, Since masers are located in Giant Molec- M. Popova). Their ages and relative and thereby map the path towards a con- ular Clouds (GMCs), this has led to much locations suggest a co-rotation radius solidated view of the spiral pattern in the better estimates of the shape of the arms. just outside the Sun. A survey of early- Milky Way. The workshop was held at the Measurements of parallaxes and the type stars in the anti-centre direction small, beautiful seaside resort of Bahía proper motion of the central source in the using Strömgren photometry, which pro- Inglesa in Central Chile over a period of Galaxy provide a new independent esti- vides accurate distances to individual three and a half days. It was a pleasure to mate of the rotational velocity of the Sun. stars, indicates a density enhancement see many students among the 55 par- H I self-absorption observed in the sec- associated with the Perseus arm (M. ticipants (see Figure 1), the number being ond Galactic quadrant (GQ) suggests a Monguío). The IPHAS survey questions limited by the off-season availability of shock in the gas associated with the the existence of a sharp truncation of the accommodation. Perseus arm. New all-sky radio maps in stellar disc (S. Sale). The Messenger 143 – March 2011 47 Astronomical News Grosbøl P. et al., Report on the ESO Workshop “Spiral Structure in the Milky Way” The general spiral structure displayed ble angular speed of the pattern. There by young objects and gas agrees was wide agreement that gas and well with, and is best fitted by, a four- young objects show a four-armed struc- armed pattern (D. Russeil). Kinematic ture with pitch angles in the range of distance estimates are not always 12–14 degrees. The major issue is whether reliable due to possible systematic per- the older stellar component has only turbations by a density wave. NIR star a two-armed structure (Perseus and Scu- counts using 2MASS and GLIMPSE can tum–Centaurus) or also four arms. In give structural information when the the former case, the two minor gas arms underlying stellar population has features (Sagittarius and Norma) would either in its magnitude distribution like the be generated by secondary compressions red clump (R. Benjamin & P. Polido). In or driven by the bar. It would be possi- integrated longitude–magnitude dia- ble to distinguish these options by more grams, structures in the bar and bulge accurate mapping of the arms (e.g., by can be seen as tangential points of spiral VLBI parallaxes of masers) and detailed arms and the truncation of the disc. analysis of the stellar density–velocity The distribution of CS emission sources field in a region of a few kiloparsecs in the Galactic Plane shows a squared around the Sun. Whereas new surveys feature which resembles the orbital shape (e.g., VISTA and GAIA) will provide accu- near the 4:1 resonance (J. Lépine). Spiral rate mapping of the stellar component, and bar perturbations in external galax- radial velocities may still be an issue. ies and possible implications for the Galaxy were also discussed (M. Arnaboldi The current data are consistent with the & M. Dumke). Figure 2. The conference poster, showing the major observed spiral pattern being associ- features of the Milky Way spiral structure. ated with a density wave. The life span of such waves in the Galaxy is not easy Theory to evaluate, as only very indirect means dispersion and surface density (V. can be used, such as radial mixing The third day of the workshop was Korchagin). The number of arms and the and changes of velocity dispersion with dedicated to theoretical models and their pattern speed of the growing modes the mean age of the stellar populations. comparison with observational data. depend on these functions and suggest Most estimates of the pattern speed of Magnetohydrodynamic simulations in a a four-armed pattern for a standard model the spiral arms show values significantly realistic Galactic potential show that of the Milky Way. lower than that of the bar, suggesting the response to a two-armed spiral per- that the disc of our Galaxy has at least turbation in the stars can yield additional, two components with different pattern slightly tighter, arms of compressed gas Future surveys speeds. While star formation and shocks between the stellar arms (M. A. Martos in the gas of the Perseus arm second & G. Gómez). The Galactic bar may drive Finally, the last half-day was dedicated to GQ indicate co-rotation for the spiral to the spiral pattern and induce additional future surveys. Several new surveys be outside this radius, the kinematics of gaseous arms, which will look like are being prepared and will improve our local young clusters place it much closer a four-armed pattern (O. Gerhard). New knowledge of the stellar disc by orders to the Solar radius. analytic models of the Milky Way were of magnitude. VISTA surveys in the NIR presented and used to analyse the will allow a much better view towards the importance of ordered and chaotic stellar centre of the Galaxy while GAIA will Acknowledgements orbits (B. Pichardo & S. Villegas). The assemble both spatial and kinematic data We express our deep gratitude to Paulina Jirón, velocity ellipsoid can be used to test dif- for more than a billion stars (V. Ivanov, J. Hernan Fernandez, Mariía Eugenia Gómez and the ferent potential models with spirals and Alonso-Garcia & J. de Bruijne). Surveys entire Local Organising Committee for a very suc- bars (D. Chakrabarty & B. Famaey). like APOGEE using SDSS-II data are cessful meeting. Although it is non-trivial to define a best improving the kinematic data significantly fit to such distributions, they indicate (S. Majewski). Links that the spiral has a significantly lower pattern speed than that of the Galactic 1 Conference web page: http://www.eso.org/sci/ bar. Radial mixing can be estimated from Discussions meetings/MW2010/index.html pencil beam surveys and used to place constraints on possible spiral modes in During the last two days of the workshop, the Galaxy and their lifetime (I.
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