Image of the Week

Globular Cluster colour-magnitude diagrams

 

  NGC 104 (47 Tuc) NGC 5139 (Omega Cen)  
   
   
  NGC 6397

NGC 6809 (M55)

 

Globular cluster fields are among the densest regions in the sky and therefore provide a reliable indication of the limits of the Gaia performances both in terms of astrometry and photometry. Their colour-magnitude diagrams are important instruments in the calibration of fundamental stellar parameters and in the testing of stellar evolution models.

The pictures on this page show a few examples of the first colour-magnitude diagrams of Galactic globular clusters: NGC 104 (47 Tuc), NGC 5139 (Omega Cen), NGC 6397 and NGC 6809 (M55). The photometric data at this stage is still very preliminary. The photometry is only internally calibrated and calibrations have not converged yet to a consistent internal system. This photometric data is not fully indicative of the quality of the data that will be contained in the first Gaia data release in the summer 2016. This data was produced during an internal test campaign and, due to the iterative nature of the processing and the addition of more recent data, the quality of the photometry included in the first data release is expected to be higher1.

Even with all these caveats in mind, the colour-magnitude diagrams on this page already show well-defined and narrow sequences, with populated red giant and horizontal branches typical of these low-metallicity and old clusters. The scientific requirements for the Gaia mission require that stars can generally be resolved in fields with densities up to 0.25 star per square arcsec. Although this seems to imply that a significant fraction of the Galactic globular clusters might not be observable in their core regions, thanks to the different scanning directions for different observations, the central cores are not totally lost and a large fraction of known Galactic globular clusters appears to be well populated in the Gaia photometric data.

Furthermore Gaia's astrometric data will enable us to vastly improve these colour-magnitude diagrams by separating the field stars from the cluster members kinematically. Astrometric measurements of the accuracy expected from the Gaia final catalogue will also allow us to study the internal dynamics of these clusters as well as to derive accurate distances and space motions.

(1) Note that the first Gaia data release (Gaia-DR1) announced for Summer 2016 will not contain colour information, which is derived from the data collected by the two Gaia spectro-photometers. The release of colours is planned for Summer 2017 (Gaia-DR2). The calibration of the low-resolution spectra, in particular the dispersion and geometric calibrations, as well as the correct treatment of crowded or contaminated spectra, require a good knowledge of the source positions, at an accuracy that only Gaia itself can provide. These plots are based on preliminary colour results required internally for the calibration of the G-band photometry. The production of the Gaia-DR2 will start later this year and will take advantage of improved accuracy in the Gaia data (thanks to the iterative nature of the cyclic processing) and the experience accumulated so far with these preliminary calibrations.

Credits: ESA/Gaia/DPAC/CU5/F. De Angeli, D.W. Evans, M. Riello (University of Cambridge)

[Published: 28/01/2016]

 

Image of the Week Archive

2017
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.