IoW_20170816 - Gaia

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/DPCI/CU8/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones

[Published: 16/08/2017]