Distance and extinction to the Milky Way spiral arms along the Galactic centre line of sight

ABSTRACT Emission from the radioactive trace element 26Al has been observed throughout the Milky Way with the COMPTEL and INTEGRAL satellites. In particular, the Doppler shifts measured with INTEGRAL connect 26Al with superbubbles, which may guide 26Al flows off spiral arms in the direction of Galactic rotation. In order to test this paradigm, we have performed galaxy-scale simulations of superbubbles with 26Al injection in a Milky Way-type galaxy. We produce all-sky synthetic γ-ray emission maps of the simulated galaxies. We find that the 1809 keV emission from the radioactive decay of 26Al is highly variable with time and the observer’s position. This allows us to estimate an additional systematic variability of 0.2 dex for a star formation rate derived from 26Al for different times and measurement locations in Milky Way-type galaxies. High-latitude morphological features indicate nearby emission with correspondingly high-integrated γ-ray intensities. We demonstrate that the 26Al scale height from our simulated galaxies depends on the assumed halo gas density. We present the first synthetic 1809 keV longitude-velocity diagrams from 3D hydrodynamic simulations. The line-of-sight velocities for 26Al can be significantly different from the line-of-sight velocities associated with the cold gas. Over time, 26Al velocities consistent with the INTEGRAL observations, within uncertainties, appear at any given longitude, broadly supporting previous suggestions that 26Al injected into expanding superbubbles by massive stars may be responsible for the high velocities found in the INTEGRAL observations. We discuss the effect of systematically varying the location of the superbubbles relative to the spiral arms.

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Can we detect Galactic spiral arms? 3D dust distribution in the Milky Way

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317005695 ◽

2017 ◽

Vol 12 (S330) ◽

pp. 189-192 ◽

Cited By ~ 1

Author(s):

Sara Rezaei Kh. ◽

Coryn A. L. Bailer-Jones ◽

Morgan Fouesneau ◽

Richard Hanson

Keyword(s):

Spatial Correlation ◽

High Precision ◽

Milky Way ◽

Precision Measurements ◽

Line Of Sight ◽

Spiral Arms ◽

Dust Extinction ◽

The Galaxy ◽

Galactic Spiral ◽

Dust Distribution

AbstractWe present a model to map the 3D distribution of dust in the Milky Way. Although dust is just a tiny fraction of what comprises the Galaxy, it plays an important role in various processes. In recent years various maps of dust extinction have been produced, but we still lack a good knowledge of the dust distribution. Our presented approach leverages line-of-sight extinctions towards stars in the Galaxy at measured distances. Since extinction is proportional to the integral of the dust density towards a given star, it is possible to reconstruct the 3D distribution of dust by combining many lines-of-sight in a model accounting for the spatial correlation of the dust. Such a technique can be used to infer the most probable 3D distribution of dust in the Galaxy even in regions which have not been observed. This contribution provides one of the first maps which does not show the “fingers of God” effect. Furthermore, we show that expected high precision measurements of distances and extinctions offer the possibility of mapping the spiral arms in the Galaxy.

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037690 ◽

2020 ◽

Vol 642 ◽

pp. A201 ◽

Cited By ~ 1

Author(s):

S. Reissl ◽

J. M. Stil ◽

E. Chen ◽

R. G. Treß ◽

M. C. Sormani ◽

...

Keyword(s):

Magnetic Field ◽

Interstellar Medium ◽

Faraday Rotation ◽

Milky Way ◽

Line Of Sight ◽

Unit Distance ◽

Spiral Arms ◽

Rotation Measure ◽

The Magnetic Field ◽

Spiral Arm

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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The geometry of the gas surrounding the Central Molecular Zone: on the origin of localized molecular clouds with extreme velocity dispersions

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2054 ◽

2019 ◽

Vol 488 (4) ◽

pp. 4663-4673 ◽

Cited By ~ 7

Author(s):

Mattia C Sormani ◽

Robin G Treß ◽

Simon C O Glover ◽

Ralf S Klessen ◽

Ashley T Barnes ◽

...

Keyword(s):

Molecular Clouds ◽

Gas Flow ◽

Milky Way ◽

Line Of Sight ◽

Galactic Centre ◽

Molecular Gas ◽

Distinct Population ◽

Data Cubes ◽

Velocity Dispersions

ABSTRACT Observations of molecular gas near the Galactic Centre (|l| < 10°, |b| < 1°) reveal the presence of a distinct population of enigmatic compact clouds that are characterized by extreme velocity dispersions ($\Delta v \gt 100\, {\rm km\, s^{-1}}$). These extended velocity features are very prominent in the data cubes and dominate the kinematics of molecular gas just outside the Central Molecular Zone (CMZ). The prototypical example of such a cloud is Bania Clump 2. We show that similar features are naturally produced in simulations of gas flow in a realistic barred potential. We analyse the structure of the features obtained in the simulations and use this to interpret the observations. We find that the features arise from collisions between material that has been infalling rapidly along the dust lanes of the Milky Way bar and material that belongs to one of the following two categories: (i) material that has ‘overshot’ after falling down the dust lanes on the opposite side; (ii) material which is part of the CMZ. Both types of collisions involve gas with large differences in the line-of-sight velocities, which is what produces the observed extreme velocity dispersions. Examples of both categories can be identified in the observations. If our interpretation is correct, we are directly witnessing (a) collisions of clouds with relative speeds of $\sim 200\, {\rm km\, s^{-1}}$ and (b) the process of accretion of fresh gas onto the CMZ.

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Mapping the tilt of the Milky Way bulge velocity ellipsoids with ARGOS and Gaia DR2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab073 ◽

2021 ◽

Vol 502 (2) ◽

pp. 1740-1752

Author(s):

Iulia T Simion ◽

Juntai Shen ◽

Sergey E Koposov ◽

Melissa Ness ◽

Kenneth Freeman ◽

...

Keyword(s):

Large Scale ◽

Milky Way ◽

Longitudinal Velocity ◽

Major Axis ◽

Specific Model ◽

Likelihood Method ◽

Line Of Sight ◽

Galactic Centre ◽

Stellar Kinematics ◽

Gaia Dr2

ABSTRACT Until the recent advent of Gaia Data Release 2 (DR2) and deep multi-object spectroscopy, it has been difficult to obtain 6D phase space information for large numbers of stars beyond 4 kpc, in particular towards the Galactic Centre, where dust and crowding are significant. We combine line-of-sight velocities from the Abundances and Radial velocity Galactic Origins Survey (ARGOS) with proper motions from Gaia DR2 to obtain a sample of ∼7000 red clump stars with 3D velocities. We perform a large-scale stellar kinematics study of the Milky Way bulge to characterize the bulge velocity ellipsoids in 20 fields. The tilt of the major-axis of the velocity ellipsoid in the radial-longitudinal velocity plane, or vertex deviation, is characteristic of non-axisymmetric systems and a significant tilt is a robust indicator of non-axisymmetry or bar presence. We compare the observations to the predicted kinematics of an N-body boxy-bulge model formed from dynamical instabilities. In the model, the lv values are strongly correlated with the angle (α) between the bulge major-axis and the Sun-Galactic centre line of sight. We use a maximum likelihood method to obtain an independent measurement of α, from bulge stellar kinematics alone, performing a robust error analysis. The most likely value of α given our model is α = (29 ± 3)○, with an additional systematic uncertainty due to comparison with one specific model. In Baade’s window, the metal-rich stars display a larger vertex deviation (lv = −40○) than the metal-poor stars (lv = 10○) but we do not detect significant lv−metallicity trends in the other fields.

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Spatial and velocity offsets of Galactic masers from the centres of spiral arms

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2199 ◽

2019 ◽

Vol 489 (2) ◽

pp. 2819-2829 ◽

Cited By ~ 4

Author(s):

Jacques P Vallée

Keyword(s):

Star Formation ◽

Pitch Angle ◽

Milky Way ◽

Galactic Centre ◽

Spiral Arms ◽

Corotation Radius ◽

Pattern Speed ◽

Spiral Model ◽

Spiral Arm ◽

Co Gas

ABSTRACT Some theories about the spiral arms of galaxies predict an offset between different tracers of star formation. Our goal in this paper is to find such an offset between the observed locations of radio masers and the locations of the arms, using a recent four-arm model fitted to the CO 1–0 gas. Our method is to compare a recent global four-arm spiral model (as fitted to the arms’ tangents in the observed broad CO 1–0 gas) with the recent results for the trigonometric distances of radio masers, for the main arms (Cygnus–Norma, Perseus, Sagittarius–Carina, Scutum and Norma). Our results indicate that most radio masers are near the inner edge of each spiral arm (towards the Galactic Centre). These masers are offset from the model arm (where the broad CO 1–0 molecular region resides), by 0.34 ± 0.06 kpc inward. In radial velocity space, the median offset between masers and the CO-fitted model is around 10 ± 1 km s–1. Based on the fact that the masers are observed here to be radially inward of the broad CO gas in the Cygnus arm at 15 kpc along the Galactic meridian, the corotation radius of the Milky Way disc is >15 kpc distant from the Galactic Centre and the density wave’s angular pattern speed is <15 km s–1 kpc–1. The pitch angle of the arm should be measured using many arm tracers, and located on both sides of the Galactic meridian, to ensure better precision and to avoid a bias pertinent to a single tracer.

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Kinematics with Gaia DR2: the force of a dwarf

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2343 ◽

2019 ◽

Vol 490 (1) ◽

pp. 797-812 ◽

Cited By ~ 18

Author(s):

I Carrillo ◽

I Minchev ◽

M Steinmetz ◽

G Monari ◽

C F P Laporte ◽

...

Keyword(s):

Milky Way ◽

Line Of Sight ◽

Galactic Centre ◽

Time Intervals ◽

Opposite Trend ◽

Bayesian Inference Method ◽

Velocity Information ◽

Bending Modes ◽

Short Time ◽

Gaia Dr2

ABSTRACT We use Gaia DR2 astrometric and line-of-sight velocity information combined with two sets of distances obtained with a Bayesian inference method to study the 3D velocity distribution in the Milky Way disc. We search for variations in all Galactocentric cylindrical velocity components (Vϕ, VR, and Vz) with Galactic radius, azimuth, and distance from the disc mid-plane. We confirm recent work showing that bulk vertical motions in the R–z plane are consistent with a combination of breathing and bending modes. In the x–y plane, we show that, although the amplitudes change, the structure produced by these modes is mostly invariant as a function of distance from the plane. Comparing to two different Galactic disc models, we demonstrate that the observed patterns can drastically change in short time intervals, showing the complexity of understanding the origin of vertical perturbations. A strong radial VR gradient was identified in the inner disc, transitioning smoothly from 16 km s−1 kpc−1 at an azimuth of 30° < ϕ < 45° ahead of the Sun-Galactic centre line to −16 km s−1 kpc−1 at an azimuth of −45° < ϕ < −30° lagging the solar azimuth. We use a simulation with no significant recent mergers to show that exactly the opposite trend is expected from a barred potential, but overestimated distances can flip this trend to match the data. Alternatively, using an N-body simulation of the Sagittarius dwarf–Milky Way interaction, we demonstrate that a major recent perturbation is necessary to reproduce the observations. Such an impact may have strongly perturbed the existing bar or even triggered its formation in the last 1–2 Gyr.

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An eclectic view of our Milky Way galaxy

Canadian Journal of Physics ◽

10.1139/cjp-2013-0429 ◽

2014 ◽

Vol 92 (9) ◽

pp. 959-963 ◽

Cited By ~ 3

Author(s):

David G. Turner

Keyword(s):

Milky Way ◽

Density Wave ◽

Star Clusters ◽

Newtonian Potential ◽

Galactic Centre ◽

Galactic Rotation ◽

Velocity Mapping ◽

Open Star Clusters ◽

Milky Way Galaxy ◽

Spiral Arms

The nature of our Milky Way galaxy is re-examined from an eclectic point of view. Evidence for a central bar, for example, is not reflected in the distribution of RR Lyrae variables in the central bulge (Majaess. Acta Astron. 60, 55 (2010); Dékány et al. arXiv:1309.5933. 2013), and it is not clear if either a 2-armed or 4-armed spiral pattern is appropriate for the spiral arms. Radial velocity mapping of the Galaxy using radio H I, H II, or CO observations is compromised by the assumptions adopted for simple Galactic rotation. The Sun’s local standard of rest (LSR) velocity is ∼14 km s−1 rather than 20 km s−1, the local circular velocity is 251 ± 9 km s−1 rather than 220 km s−1, and young groups of stars exhibit a 10–20 km s−1 “kick” relative to what is expected from Galactic rotation. By implication, the same may be true for star-forming gas clouds affected by the Galaxy’s spiral density wave, raising concerns about their use for mapping spiral arms. Proper motion data in conjunction with the newly-estimated velocity components for the Sun’s motion imply a distance to the Galactic centre of R0 = 8.34 ± 0.27 kpc, consistent with recent estimates, which average 8.24 ± 0.09 kpc. A cosinusoidal Galactic potential is not ruled out by observations of open star clusters. The planetary nebula cluster Bica 6, for example, has a near-escape orbit for a Newtonian potential, but a near-normal orbit in a cosinusoidal potential field. The nearby cluster Collinder 464 also displays unusually large tidal effects consistent with those expected for a cosinusoidal potential. A standard Newtonian version of the Virial theorem for star clusters yields very reasonable masses (∼3 × 1011 M⊙ and ∼4 × 1011 M⊙) for the Milky Way and M31 subgroups of the Local Group, respectively. A cosinusoidal relation should yield identical results.

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Variation of the amount of hydrogen along the galactic ridge

Symposium - International Astronomical Union ◽

10.1017/s0074180900100889 ◽

1967 ◽

Vol 31 ◽

pp. 171-172

Author(s):

Th. Schmidt-Kaler

Keyword(s):

Surface Brightness ◽

Milky Way ◽

Neutral Hydrogen ◽

Line Of Sight ◽

Optical Surface ◽

Hydrogen Density ◽

Continuous Radiation ◽

Galactic Longitude

The integralNHof neutral-hydrogen density along the line of sight is determined from the Kootwijk and Sydney surveys. The run ofNHwith galactic longitude agrees well with that of thermal continuous radiation and that of the optical surface brightness of the Milky Way.

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2D kinematics of massive stars near the Galactic Centre

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3329 ◽

2020 ◽

Vol 500 (3) ◽

pp. 3213-3239

Author(s):

Mattia Libralato ◽

Daniel J Lennon ◽

Andrea Bellini ◽

Roeland van der Marel ◽

Simon J Clark ◽

...

Keyword(s):

Moving Objects ◽

Hubble Space Telescope ◽

Massive Stars ◽

Line Of Sight ◽

Galactic Centre ◽

Harsh Environment ◽

Proper Motions ◽

Sgr A ◽

Better Than ◽

Rule Out

ABSTRACT The presence of massive stars (MSs) in the region close to the Galactic Centre (GC) poses several questions about their origin. The harsh environment of the GC favours specific formation scenarios, each of which should imprint characteristic kinematic features on the MSs. We present a 2D kinematic analysis of MSs in a GC region surrounding Sgr A* based on high-precision proper motions obtained with the Hubble Space Telescope. Thanks to a careful data reduction, well-measured bright stars in our proper-motion catalogues have errors better than 0.5 mas yr−1. We discuss the absolute motion of the MSs in the field and their motion relative to Sgr A*, the Arches, and the Quintuplet. For the majority of the MSs, we rule out any distance further than 3–4 kpc from Sgr A* using only kinematic arguments. If their membership to the GC is confirmed, most of the isolated MSs are likely not associated with either the Arches or Quintuplet clusters or Sgr A*. Only a few MSs have proper motions, suggesting that they are likely members of the Arches cluster, in agreement with previous spectroscopic results. Line-of-sight radial velocities and distances are required to shed further light on the origin of most of these massive objects. We also present an analysis of other fast-moving objects in the GC region, finding no clear excess of high-velocity escaping stars. We make our astro-photometric catalogues publicly available.

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