The role of spiral arms in Milky Way star formation

Ragan, S. E.; Moore, T. J. T.; Eden, David; Hoare, M. G.; Urquhart, J. S.; Elia, D.; Molinari, S.

doi:10.1093/mnras/sty1672

Cited by 25 publications

(23 citation statements)

References 82 publications

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“…It was noted in Roberts (1969) and Sormani et al (2017) that the gas may shock in front of or even behind the minimum of the potential well. A similar explanation for an observed offset between gas content and star formation tracers in the Milky Way is discussed in Ragan et al (2018) and the references therein in greater detail. The exact position of the shock strongly depends on the local parameters such as the sound speed.…”

Section: The Milky Way From An Outside Perspectivesupporting

confidence: 55%

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Reißl

Stil

Chen

et al. 2020

A&A

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Context. The Faraday rotation measure (RM) is often used to study the magnetic field strength and orientation within the ionized medium of the Milky Way. Recent observations indicate an RM magnitude in the spiral arms that exceeds the commonly assumed range. This raises the question of how and under what conditions spiral arms create such strong Faraday rotation. Aims. We investigate the effect of spiral arms on Galactic Faraday rotation through shock compression of the interstellar medium. It has recently been suggested that the Sagittarius spiral arm creates a strong peak in Faraday rotation where the line of sight is tangent to the arm, and that enhanced Faraday rotation follows along side lines which intersect the arm. Here our aim is to understand the physical conditions that may give rise to this effect and the role of viewing geometry. Methods. We apply a magnetohydrodynamic simulation of the multi-phase interstellar medium in a Milky Way-type spiral galaxy disk in combination with radiative transfer in order to evaluate different tracers of spiral arm structures. For observers embedded in the disk, dust intensity, synchrotron emission, and the kinematics of molecular gas observations are derived to identify which spiral arm tangents are observable. Faraday rotation measures are calculated through the disk and evaluated in the context of different observer positions. The observer’s perspectives are related to the parameters of the local bubbles surrounding the observer and their contribution to the total Faraday rotation measure along the line of sight. Results. We reproduce a scattering of tangent points for the different tracers of about 6° per spiral arm similar to the Milky Way. For the RM, the model shows that compression of the interstellar medium and associated amplification of the magnetic field in spiral arms enhances Faraday rotation by a few hundred rad m−2 in addition to the mean contribution of the disk. The arm–interarm contrast in Faraday rotation per unit distance along the line of sight is approximately ~10 in the inner Galaxy, fading to ~2 in the outer Galaxy in tandem with the waning contrast of other tracers of spiral arms. We identify a shark fin pattern in the RM Milky Way observations and in the synthetic data that is characteristic for a galaxy with spiral arms.

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Section: The Milky Way From An Outside Perspectivesupporting

confidence: 55%

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Reißl

Stil

Chen

et al. 2020

A&A

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“…10), we qualitatively observe that spiral arm features seem to be associated with 13 CO components with larger linewidths. In general, we can see increased median σ v values within Galactocentric distances 6 kpc; as already speculated in Riener et al (2020), these increasing σ v values towards the inner Galaxy could be due to the presence of the Galactic bar and the observed overdensity of star-forming regions (Anderson et al 2017;Ragan et al 2018), but could also partly result from our inability to correctly decompose strongly blended emission lines. We can further see an increase of the median σ v values with heliocentric distance; for emission lines with v LSR < 20 km s −1 this is partly due to our use of the size-linewidth prior (Sect.…”

Section: Face-on View Of the 13 Co Emissionsupporting

confidence: 77%

“…Urquhart et al 2018), recent studies have found no significant impact of Galactic structure on the clump or star formation efficiency of dense clumps (e.g. Moore et al 2012;Eden et al 2013Eden et al , 2015Ragan et al 2016Ragan et al , 2018, or the physical properties of filaments (Schisano et al 2020) and molecular clumps (Rigby et al 2019). However, the last study reported differences in the linewidths between clumps located in interarm and spiral arm structures.…”

Section: Introductionmentioning

confidence: 95%

Autonomous Gaussian decomposition of the Galactic Ring Survey

Riener

Kainulainen

Henshaw

et al. 2020

A&A

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Knowledge about the distribution of CO emission in the Milky Way is essential to understanding the impact of the Galactic environment on the formation and evolution of structures in the interstellar medium. However, our current insight as to the fraction of CO in the spiral arm and interarm regions is still limited by large uncertainties in assumed rotation curve models or distance determination techniques. In this work we use the Bayesian approach from Reid et al. (2016, ApJ, 823, 77; 2019, ApJ, 885, 131), which is based on our most precise knowledge at present about the structure and kinematics of the Milky Way, to obtain the current best assessment of the Galactic distribution of 13CO from the Galactic Ring Survey. We performed two different distance estimates that either included (Run A) or excluded (Run B) a model for Galactic features, such as spiral arms or spurs. We also included a prior for the solution of the kinematic distance ambiguity that was determined from a compilation of literature distances and an assumed size-linewidth relationship. Even though the two distance runs show strong differences due to the prior for Galactic features for Run A and larger uncertainties due to kinematic distances in Run B, the majority of their distance results are consistent with each other within the uncertainties. We find that the fraction of 13CO emission associated with spiral arm features ranges from 76 to 84% between the two distance runs. The vertical distribution of the gas is concentrated around the Galactic midplane, showing full-width at half-maximum values of ~75 pc. We do not find any significant difference between gas emission properties associated with spiral arm and interarm features. In particular, the distribution of velocity dispersion values of gas emission in spurs and spiral arms is very similar. We detect a trend of higher velocity dispersion values with increasing heliocentric distance, which we, however, attribute to beam averaging effects caused by differences in spatial resolution. We argue that the true distribution of the gas emission is likely more similar to a combination of the two distance results discussed, and we highlight the importance of using complementary distance estimations to safeguard against the pitfalls of any single approach. We conclude that the methodology presented in this work is a promising way to determine distances to gas emission features in Galactic plane surveys.

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“…Data is binned into equally populated bins, rather than equal sized ones. See Ragan et al (2018) for further details. Shaded regions denote those outside the survey area.…”

Section: Locations Of Star Formationmentioning

confidence: 99%

Young stars as tracers of a barred-spiral Milky Way

Pettitt

Ragan

Smith

2019

Monthly Notices of the Royal Astronomical Society

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Identifying the structure of our Galaxy has always been fraught with difficulties, and while modern surveys continue to make progress building a map of the Milky Way, there is still much to understand. The arm and bar features are important drivers in shaping the interstellar medium, but their exact nature and influence still require attention. We present results of smoothed particle hydrodynamic simulations of gas in the Milky Way including star formation, stellar feedback, and ISM cooling, when exposed to different arm and bar features, with the aim of better understanding how well newly formed stars trace out the underlying structure of the Galaxy. The bar is given a faster pattern speed than the arms, resulting in a complex, time-dependent morphology and star formation. Inter-arm branches and spurs are easily influenced by the bar, especially in the two-armed spiral models where there is a wide region of resonance overlap in the disc. As the bar over-takes the spiral arms it induces small boosts in star formation and enhances spiral features, which occur at regularly spaced beat-like intervals. The locations of star formation events are similar to those seen in observational data, and do not show a perfect 1:1 correspondence with the underlying spiral potential, though arm tangencies are generally well traced by young stars. Stellar velocity fields from the newly formed stars are compared to data from Gaia DR2, showing that the spiral and bar features can reproduce many of the nonaxisymmetric features seen in the data. A simple analytical model is used to show many of these feature are a natural response of gas to rigidly rotating spiral and bar potentials.

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The role of spiral arms in Milky Way star formation

Cited by 25 publications

References 82 publications

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Autonomous Gaussian decomposition of the Galactic Ring Survey

Young stars as tracers of a barred-spiral Milky Way

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