# 19.2 The Galaxy in your preferred colours – Synthetic photometry from Gaia low-resolution spectra

Gaia DR3 provides for the first time flux-calibrated low-resolution spectrophotometry for $\simeq 220$ million sources, in the wavelength range 330 nm$\ \leq\lambda\leq\$1050 nm (BP/RP spectra). Synthetic photometry, directly tied to a flux in physical units, can be obtained from these spectra for any passband fully enclosed in this wavelength range, provided that its width is larger that the local Line Spread Function.

Gaia Collaboration et al. (2022h) describes how synthetic photometry can be obtained from BP/RP spectra, illustrating the performances that can be achieved under a range of different conditions, e.g., passband width and wavelength range, as well as the limits and the problems affecting it. Existing top-quality photometry can be reproduced within a few per cent over a wide range of magnitudes and colour, for wide and medium bands, and up to millimag accuracy when synthetic photometry is standardised with respect to these external sources. Some examples of the potential scientific application are presented, including, e.g., the detection of multiple populations in globular clusters, the estimate of metallicity extended to the very metal-poor regime, and the classification of white dwarfs. A catalogue providing standardised photometry for $\simeq 2.2\times 10^{8}$ sources in several wide bands of frequently used photometric systems is provided (Gaia Synthetic Photometry Catalogue; GSPC; accessible from the table synthetic_photometry_gspc in the Gaia ESA Archive) as well as a catalogue of $\simeq 10^{5}$ white dwarfs with DA/non-DA classification obtained with a random forest algorithm (Gaia Synthetic Photometry Catalogue for White Dwarfs; GSPC-WD; provided as a stand-alone table).

The GSPC includes standardised magnitudes, fluxes, and error on fluxes for:

• $U_{\rm JKC}$,$B_{\rm JKC}$,$V_{\rm JKC}$,$R_{\rm JKC}$,$I_{\rm JKC}$,

• $u_{\rm SDSS}$,$g_{\rm SDSS}$,$r_{\rm SDSS}$,$i_{\rm SDSS}$,$z_{\rm SDSS}$,

• $y_{\rm PS1}$,

• $F606W_{\rm ACS/WFC}$, and $F814W_{\rm ACS/WFC}$.

In addition to the BP/RP synthetic photometry listed above, the GSPC contains:

• Gaia DR3 source_id, allowing a direct cross match with other catalogues in the Gaia ESA Archive, by means of ADQL join queries.

• The quality parameter c_star ($C^{\star}$; Riello et al. 2021), that can be useful to select the sources with the most reliable photometry.

• A flag for each passband (${\tt Xflag}$, where X = Ujkc,Bjkc,...), which has a value of 1 if the ($G_{\rm BP}$-$G_{\rm RP}$) colour and $G$ magnitude of the considered star is within the ranges where standardisation and validation have been performed. In pactice, the X magnitude of a source having ${\tt Xflag}=0$ should be considered as an extrapolation of the adopted standardisation.

To keep only high-quality measures, we adopted a unique criterion based on the signal-to-noise ratio for all the magnitudes in all the systems. A given source has valid photometry in the passband ${\tt X}$ only if

 ${\tt XFlux/XFluxError>30.0}$

that is, the signal-to-noise ratio in that passband is larger than 30. Additional information and recommendations for proper use can be found in Sect. 6.2 and Sect. 7 of the paper, respectively.