Kinematics with Gaia DR2: the force of a dwarf

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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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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The escape speed curve of the Galaxy obtained from Gaia DR2 implies a heavy Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833748 ◽

2018 ◽

Vol 616 ◽

pp. L9 ◽

Cited By ~ 41

Author(s):

G. Monari ◽

B. Famaey ◽

I. Carrillo ◽

T. Piffl ◽

M. Steinmetz ◽

...

Keyword(s):

Mass Concentration ◽

Milky Way ◽

Galactic Centre ◽

High Concentration ◽

Circular Velocity ◽

Halo Stars ◽

Dark Matter Mass ◽

The Galaxy ◽

Escape Speed ◽

Gaia Dr2

We measure the escape speed curve of the Milky Way based on the analysis of the velocity distribution of ~2850 counter-rotating halo stars from the Gaia Data Release 2. The distances were estimated through the StarHorse code, and only stars with distance errors smaller than 10% were used in the study. The escape speed curve is measured at Galactocentric radii ranging from ~5 kpc to ~10.5 kpc. The local Galactic escape at the Sun’s position is estimated to be ve(r⊙) = 580 ± 63 km s−1, and it rises towards the Galactic centre. Defined as the minimum speed required to reach three virial radii, our estimate of the escape speed as a function of radius implies for a Navarro–Frenk–White profile and local circular velocity of 240 km s−1 a dark matter mass M200 = 1.28−0.50+0.68 × 1012 M⊙ and a high concentration c200 = 11.09−1.79+2.94. Assuming the mass-concentration relation of ΛCDM, we obtain M200 = 1.55−0.51+0.64 × 1012 M⊙ and c200 = 7.93−0.27+0.33 for a local circular velocity of 228 km s−1.

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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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Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3297 ◽

2020 ◽

Vol 500 (1) ◽

pp. 986-997

Author(s):

Kuan-Wei Huang ◽

Sergey E Koposov

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Galactic Centre ◽

Imaging Data ◽

Credible Intervals ◽

Specific Frequency ◽

Number Counts ◽

Gaia Dr2

ABSTRACT We report the result of searching for globular clusters (GCs) around 55 Milky Way (MW) satellite dwarf galaxies within the distance of 450 kpc from the Galactic Centre except for the Large and Small Magellanic Clouds and the Sagittarius dwarf. For each dwarf, we analyse the stellar distribution of sources in Gaia DR2, selected by magnitude, proper motion, and source morphology. Using the kernel density estimation of stellar number counts, we identify 11 possible GC candidates. Cross-matched with existing imaging data, all 11 objects are known either GCs or galaxies and only Fornax GC 1–6 among them are associated with the targeted dwarf galaxy. Using simulated GCs, we calculate the GC detection limit $M_{\rm V}^{\rm lim}$ that spans the range from $M_{\rm V}^{\rm lim}\sim -7$ for distant dwarfs to $M_{\rm V}^{\rm lim}\sim 0$ for nearby systems. Assuming a Gaussian GC luminosity function, we compute that the completeness of the GC search is above 90 per cent for most dwarf galaxies. We construct the 90 per cent credible intervals/upper limits on the GC specific frequency SN of the MW dwarf galaxies: 12 < SN < 47 for Fornax, SN < 20 for the dwarfs with −12 < MV < −10, SN < 30 for the dwarfs with −10 < MV < −7, and SN < 90 for the dwarfs with MV > −7. Based on SN, we derive the probability of galaxies hosting GCs given their luminosity, finding that the probability of galaxies fainter than MV = −9 to host GCs is lower than 0.1.

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Simultaneous calibration of spectro-photometric distances and the Gaia DR2 parallax zero-point offset with deep learning

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz2245 ◽

2019 ◽

Vol 489 (2) ◽

pp. 2079-2096 ◽

Cited By ~ 40

Author(s):

Henry W Leung ◽

Jo Bovy

Keyword(s):

Milky Way ◽

Zero Point ◽

Galactic Centre ◽

Dynamical Structure ◽

The Sun ◽

Large Area ◽

Data Set ◽

The Mean ◽

Gaia Dr2 ◽

Flexible Model

ABSTRACT Gaia measures the five astrometric parameters for stars in the Milky Way, but only four of them (positions and proper motion, but not distance) are well measured beyond a few kpc from the Sun. Modern spectroscopic surveys such as APOGEE cover a large area of the Milky Way disc and we can use the relation between spectra and luminosity to determine distances to stars beyond Gaia’s parallax reach. Here, we design a deep neural network trained on stars in common between Gaia and APOGEE that determines spectro-photometric distances to APOGEE stars, while including a flexible model to calibrate parallax zero-point biases in Gaia DR2. We determine the zero-point offset to be $-52.3 \pm 2.0\, \mu \mathrm{as}$ when modelling it as a global constant, but also train a multivariate zero-point offset model that depends on G, GBP − GRP colour, and Teff and that can be applied to all ≈58 million stars in Gaia DR2 within APOGEE’s colour–magnitude range and within APOGEE’s sky footprint. Our spectro-photometric distances are more precise than Gaia at distances ${\gtrsim} 2\, \mathrm{kpc}$ from the Sun. We release a catalogue of spectro-photometric distances for the entire APOGEE DR14 data set which covers Galactocentric radii $2\, \mathrm{kpc} \lesssim R \lesssim 19\, \mathrm{kpc}$; ${\approx} 150\, 000$ stars have ${\lt} 10{{\ \rm per\ cent}}$ uncertainty, making this a powerful sample to study the chemo-dynamical structure of the disc. We use this sample to map the mean [Fe/H] and 15 abundance ratios [X/Fe] from the Galactic Centre to the edge of the disc. Among many interesting trends, we find that the bulge and bar region at $R \lesssim 5\, \mathrm{kpc}$ clearly stands out in [Fe/H] and most abundance ratios.

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

Astronomy and Astrophysics ◽

10.1051/0004-6361/202040073 ◽

2021 ◽

Author(s):

F. Nogueras-Lara ◽

R. Schödel ◽

N. Neumayer

Keyword(s):

Milky Way ◽

Line Of Sight ◽

Galactic Centre ◽

Spiral Arms ◽

Centre Line

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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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