Image of the Week

Preliminary View of THE Gaia sky in colour

Figure 1: The map shows a source density distribution in the sky in Galactic coordinates. Each HEALPix of level 7 is colour coded depending on the density of sources within the pixel. HEALPix of level 7 implies a pixel size of approximately 750 square arcmin or 0.21 square degree. A selection of sources with G<17 was used for this plot.


In September 2016 Gaia released its first results along with the most precise, all-sky astrometric map ever produced. But Gaia is just getting started. There is much more to come!

The photometric catalogue part of the first Gaia data release contained a measurement of the average flux in the G band for over 1.1 billion sources. Even though no colour information was released at that time (in agreement with the release plan), many scientists have already made effective use of the single G-band photometry in a number of publications. Often this required cross-matching the Gaia catalogue with other photometric catalogues to acquire photometry in some additional bands, with the effect of reducing the number of usable sources and possibly introducing some inconsistencies due to the different origins of the photometry.

Figure 2: The map shows the (BP-RP) colour distribution in the sky in Galactic coordinates. Each HEALPix of level 7 is colour coded depending on the median colour of sources within the pixel. HEALPix of level 7 implies a pixel size of approximately 750 square arcmin or 0.21 square degree. A selection of sources with G<17 was used for this plot.

The second data release is planned for April 2018 and will present a significant advance in all photometry-based investigations by providing photometry in 3 bands (the G broad-band and the integrated BP and RP bands) for a large number of sources (the exact number is still to be defined and will depend on the DPAC validation results) calibrated to a consistent and homogeneous photometric system. A full description of the 3 passbands will be provided together with photometric zero-points linking the internal photometric system to the absolute one. Using these three bands, the DPAC is estimating stellar effective temperatures and line-of-sight extinctions for many stars brighter than about G=17, and where good parallaxes are available, their luminosities and radii too. The aim is to publish these in the second data release, with parameters based on the full BP/RP spectra following in the third release.

One of the major challenges in the Gaia photometric processing is caused by the large number of different instrument configurations that can be activated during the acquisition of an observation. This results in effectively different instruments that need to be calibrated to a homogeneous system. Today there is no external catalogue available offering the accuracy and amount of data required to calibrate such a complex instrument. For this reason, the definition of a reference system homogeneous over the entire set of instrument configurations must rely on the Gaia data itself. External data is only used to link the internal photometric reference system to the absolute one. For more details on the photometric calibration algorithms refer to Carrasco et al. (A&A 595, A7, 2016 and A&A 601, C1, 2017).

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. This is why no colours were released in Gaia DR1 when there had been no iteration yet between the astrometric and photometric processing. For Gaia DR2, the photometric processing will have used high accuracy astrometric data from Gaia itself and therefore will produce much improved colours. However no special treatment of crowded or contaminated spectra will be in place yet. This will affect the accuracy of the colour information in crowded areas and in regions around very bright sources. The extent of this will be assessed by the DPAC validation activities and documented.

As an appetizer to the second Gaia data release, we present here a coloured all-sky view of the stars in our Galaxy and its neighbours, based on the first two years of observations from ESA’s Gaia satellite, taken from July 2014 to May 2016.

Figure 1 shows the sky density map (number of sources per pixel) and the colour distribution in the sky can be found in Figure 2. These plots are based on preliminary results and on a random selection of sources brighter than magnitude 17 in the G-band. The pixelization scheme adopted here is HEALPix level 7 (implying a pixel size of approximately 750 square arcmin or 0.21 square degree). Each HEALPix pixel in the colour sky map is colour coded according to the median colour (BP-RP) of all sources falling in the corresponding area in the sky. These images were released in coordination with a story on ESA Science & Technology.

The colour sky distribution offers a taste of the extraordinary photometric catalogue that will become available in April 2018. High-extinction star-forming regions close to the Galactic plane show up with lower density in the density map, stand out in the colour map thanks to the reddening effects. Streams and stellar associations are also clearly visible in both density and colour maps. These are just a few of the features that can be researched with such a catalogue.

The plots on this page have been generated using TOPCAT on a restricted sample of the preliminary photometric catalogue. We wish to thank M. Taylor for developing such a fantastic tool!

Credits: ESA/Gaia/DPAC/CU5/CU8/DPCI/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones

[Published: 16/08/2017]


Image of the Week Archive

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
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
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
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
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
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
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
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.