Image of the Week

A signature of the rotation of the galactic Bar in the outer milky way

   
 

Distribution of stars with positive Galactocentric cylindrical radial velocity from the cross-matched TGAS and LAMOST catalogues, displaying their azimuthal rotation velocity as a function of Galactocentric cylindrical radius. Stars with parallax accuracy better than 20% were selected. The bin size is 20 pc in radius, and 2 km s-1 in velocity, and the units of the colour bar indicate the number of stars per bin. The three curves indicate the theoretical position of the gap between the high and low-velocity modes for three different shapes of the Galaxy rotation curve, and for a fast pattern speed of the bar of 54 km s-1 kpc-1. These theoretical curves correspond to the observed gap.

 
 

Our Milky Way galaxy is a barred galaxy, meaning that it harbours an elongated stellar structure in its central parts. The actual structure of the Milky Way bar is, however, still highly controversial.

Old estimates of the length of the bar favoured a relatively short bar rotating at a fast pace, but recent investigations of the inner Galaxy have rather tended to favour a long bar extending from the Galactic centre to more than half-way to the Sun. Such a long bar must rotate more slowly than a short one.

The effect of the rotation of the Galactic bar can actually affect the motion of stars in the vicinity of the Sun in the Galaxy, due to a mecahnism known as a "resonance". A resonance with the bar occurs when the frequency of radial oscillations (how close or far from the Galactic center a star gets along its orbit) matches the rotation frequency of the star with respect to the bar. Resonating stars feel a very regular gravitational pull from the bar that modifies their orbit.

Using data from the TGAS/Gaia DR 1 catalogue on parallaxes and proper motions, and combining them with data from a ground-based spectroscopic survey based in China (LAMOST) a group of astronomers from France, Sweden, UK, and Canada have uncovered the signature of the resonance with the rotation of the bar in the velocities of stars located in the outer parts of the Galaxy with respect to the Sun.

This signature is shown in the image above, which indicates that the azimuthal rotation velocity of stars has a bimodal distribution, with a gap separating the high-velocity mode from the low-velocity mode. The position of this gap as a function of radius is a direct indication of the pattern speed of the bar, and the authors could conclude that the bar is indeed rotating fast, and must thus in principle be shorter than the recent investigations of the inner Galaxy indicate.

An article on this was just accepted at the Monthly Notices for the Royal Astronomical Society (MNRAS) and is also available through arXiv.

This research was performed by Giacomo Monari of the University of Strasbourg funded through a CNES grant, and who recently moved to the Oskar Klein Centre for Cosmoparticle Physics), Daisuke Kawata from Mullard Space Science Laboratory, Jason Hunt from the Dunlap institute for Astronomy and Astrophysics, and Benoit Famaey, CNRS researcher at the University of Strasbourg.

Credits: Giacomo Monari & Benoit Famaey (CNES, Observatoire astronomique de Strasbourg, Stockholm University) and Daisuke Kawata & Jason Hunt (MSSL, University of Toronto)

[Published: 25/11/2016]

 

Image of the Week Archive

2017
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.