Image of the Week

the Galaxy in your preferred colours

Figure 1. All-sky map (Hammer projection) in the JWST NIRCam photometric system. The plot shows the median of the color F070W - F090W in each HEALPix (level=7), as measured from the Gaia XP spectra using synthetic photometry. Image credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: R. Sordo, F. De Angeli, M. Riello.

The full 3rd Data Release of Gaia will provide to the community an impressive amount of ~220 million XP spectra, both internally and externally calibrated (De Angeli et al 2022, Montegriffo et al 2022). Spectra for all types of sources will be represented, as long as visible at the limiting magnitude: the database of spectra will be unprecedented in the combination of both precision in the flux calibration and variety. From the Gaia XP spectra, a source classification and astrophysical parameter determination will be presented too (Creevey et al 2022).

In the paper Gaia Collaboration, Montegriffo et al 2022, “Gaia Data Release 3: The Galaxy in your preferred colours. Synthetic photometry from Gaia low-resolution spectra”, published at the time of the Data Release 3, the potential of the Gaia low resolution spectra is exploited further, by doing what astronomers are used to do since the dawn of astrophysics: photometry.

Why do we need new photometry?

Even if the integrated Gaia photometry is exquisite and accurate at the milli-mag level, and has already allowed the community to explore the whole sky in an unprecedented way, it has some drawbacks. The Gaia G, G_BP and G_RP passbands are very large and their diagnostic capability is not tailored to specific scientific problems, as it is usually the case for photometric surveys.

In fact, astronomers looked at the sky for a century, devising photometric systems specifically designed for diagnostic purposes, or to characterize unknown sources. Photometry is quicker and allows us to go further in distance than spectroscopy, the latter providing a wealth of more information, but at a cost. With the Gaia spectra, we can overcome these limitations in the Gaia photometric system, and really go further, taking advantage of some key properties of the Gaia spectral database.

First, the spectra collected by Gaia are calibrated in flux units with a precision difficult to find in literature except for very limited samples. Second, those spectra are distributed in the whole sky, being Gaia an all-sky space-based survey. Third, there is no pre-selection on observed targets, so that all source types are represented, and their Gaia XP spectra are calibrated in the same way.

The externally calibrated XP spectra can be coupled to a set of (optical) passbands: the resulting synthetic photometry can reach the typical accuracy of 1-2% with a precision rivalling the best available photometry from literature in the same system. Using an existing set of reliable photometric standard stars as "second level calibrators" also the accuracy can be pushed to the millimag level, in some cases.

How precise is this photometry?

The key to precision and accuracy is in the excellent quality of the calibration of the spectra, which is one of the most difficult tasks in astronomy. The process is detailed in its complexity in De Angeli et al 2022 and Montegriffo et al 2022. Albeit minute systematic differences remain, as addressed in Montegriffo et al 2022, the agreement of the Gaia BP/RP spectra with other, well calibrated sets of observed spectra from literature is very high. The Data Processing and Analysis Consortium (DPAC) offers to the community a tool, GaiaXPy, that, among other functionalities, allows every user to perform synthetic photometry on the large set of Gaia XP spectra published in Gaia DR3, by plugging in any existing (or desired!) photometric system covering the optical wavelength range.

The paper Gaia Collaboration, Montegriffo et al 2022 explains the theoretical foundation of the method that allows GaiaXPy to easily, quickly and reliably compute synthetic photometry from Gaia BP/RP spectra: an apparently simple procedure that is indeed quite tricky to perform correctly! The paper presents also a thorough assessment of the quality of these measures for a set of well-known photometric systems, showing detailed comparisons with the best available samples of their reference photometric measures.

Finally, a catalogue of these measures (Gaia Synthetic Photometric Catalogue, GSPC) in a set of well-known photometric systems is published as part of the Gaia DR3, for the ~220 million sources having their Gaia XP mean spectra in Gaia DR3. This catalogue can be queried through the Gaia archive. The GSPC can be used to provide a reference for any optical photometric measure, and for the calibration/validation of photometric surveys.

From the GSPC, the detailed colour-magnitude diagram shown in Figure 2 was constructed, by combining measures in the Johnson-Kron-Cousins B and I passbands for a sample of nearly 4 million stars situated in the Galactic Caps (|b| > 50deg). Well-known features, associated with specific evolutionary states of stars, can be recognized thanks to the high accuracy of the photometric measures.

Figure 2. Colour-magnitude diagram generated by combining the Johnson-Kron-Cousins (B-I) colour and the absolute magnitude in the I band, as measured from the Gaia parallax and the Gaia XP spectra using synthetic photometry. The main loci of the stellar evolutions are marked (MS: Main Sequence; RGB: Red Giant Branch; TRGB: Tip of the RGB; AGB: Asymptotic Giant Branch HB: Horizontal Branch; EHB: Extreme HB; WD: White Dwarfs). Image credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: M. Bellazzini.

The sky in your preferred colours

Synthetic photometry from XP spectra allows us to generate sky maps using our preferred colour computed from optical passbands. We divide the sky in (HEALPix, level 7) pixels, and we take for each pixel the median colour of all associated stars within. Using GSPC measures, in Figure 1 we are able to take a glimpse on the JWST world. Even if the first real measurements are yet to come, by implementing in GaiaXPy its optical passbands F090W and F070W and simulating its forthcoming measures we can already see our sky through the eyes of the JWST NIRCam instrument.​​​​

References

• De Angeli et al 2022, “Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data”
• Montegriffo et al 2022 “Gaia Data Release 3: External calibration of BP/RP low-resolution spectroscopic data”
• Gaia collaboration, Montegriffo et al 2022 “Gaia Data Release 3: The Galaxy in your preferred colours: synthetic photometry from Gaia low-resolution spectra”
• Creevey et al 2022, “Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I - methods and content overview”

AcknowledgEments

The data used to prepare this material are presented in the paper Gaia collaboration, Montegriffo et al 2022 “Gaia Data Release 3: The Galaxy in your preferred colours: synthetic photometry from Gaia low-resolution spectra”

Credits: ESA/Gaia/DPAC, R. Sordo, M. Bellazzini, F. De Angeli, M. Riello, A. Vallenari, T. Roegiers

[Published: 23/05/2022]