Stellar population diagnostics in the nearby Universe
The light coming from galaxies is composed by the superposition of the fluxes emitted by their constituent stars or stellar populations. Depending of this stellar content, galaxies are usually differentiated in star forming galaxies, which show a characteristic “blue” colour as result of a recent and strong star formation episode, and in red/passive galaxies, with more evolved stars and low gas content in which the star formation is mainly quenched. The determination of the type of stars that populate galaxies, as well as the epoch when they were formed, provides valuable information about the process of galaxy formation and the assembly of their inner structures, being still today a great challenge for the scientific community.
The J-PLUS photometric system does not only cover a wide spectral range through 12 bands, it also contains narrow and intermediate bands in strategical spectral ranges that are very sensitive to stellar population features in the local Universe (see Fig.1), e.g. the filter F660 covers the Balmer's serie Hα which is a good indicator of the ongoing star formation rate in galaxies with nebular emissions. In addition, the narrow/intermediate bands F395, F410, and F430 sample the 4000Å-break with great detail being determinant to stablish good photometric redshifts, and therefore stellar masses, and at the same time providing good age predictions, since this feature is representative and sensitive to age (equivalent to spectroscopic indices such as Dn4000). The narrow band F378 is settled on the emission line [OII], which is sensitive to the metallicity in ionized nebular clouds. Regarding F515, it is centred in the Mgb and covers the line Fe5270, both estimators of the stellar continuum metallicity. For emission line galaxies, F515 can be also used to estimate the [OIII] luminosity.
There is a clear relation between the colours of a galaxy and the kind of stellar populations that reign on it. A detailed analysis of the spectral energy distribution (SED) can provide vital information about the stellar components in a galaxy (Cid-Fernandes et al. 2005, Mathis et al. 2006, Koleva et al. 2008, Walcher et al. 2011, Díaz-García et al. 2015). For this purpose, we make use of the Multi-Filter FITting code for stellar population diagnostics (MUFFIT), which has been largely designed to deal with large scale multi-filter surveys. MUFFIT is a SED-fitting code based in a χ² test weighted with errors, that is able to detect those bands that may be affected by strong emission lines (specially troublesome for intermediate and narrow bands) to remove them during the fitting process. In order to set constraints on the stellar population parameters, e.g. age and metallicity, and thanks to the state-of-the-art synthesis models (such as Bruzual & Charlot 2003; Maraston et al. 2005; MIUSCAT, Vazdekis et al. 2012), which also include single stellar populations (SSP), MUFFIT compares the SED of each galaxy with a mixture of two SSPs (an old component and a young one that can be representative of a burst or the tail of a extended star formation history). In addition, to provide a statistical reinforce, a Monte Carlo approach is carried out taking the photometric uncertainties in each band, a necessary step to properly develop a study of the stellar population parameters, and to face in the proper way the undesirable degeneracies among parameters.
The combination of all J-PLUS bands is key to disentangle the stars that populate each galaxy in our local Universe. For the most local ones, we expect to obtain and unprecedented sample of galaxies to visualize their structural assembly step-by-step, through a 2D spatially sampling of galaxies developed by our team and MUFFIT. We will provide 2D stellar population diagnostics spatially resolved, studying among others the age and metallicity gradients. For the non-spacially resolved galaxies, we will explore their stellar content by the analysis of their integrated populations. In Fig.2, we present an example of spectral fitting using MUFFIT and MIUSCAT SSP models. This fitting illustrates a hypothetical J-PLUS observation of M32, by the convolution of its spectrum (Santos et al. 2002) with the J-PLUS photometric system. From the results, we find out that M32 presents significant hints of a composite population of 2.5 ± 1 Gyr and another older of 11±3.5 Gyr, both in the order of solar metallicities [Fe/H] = 0.10±0.15 dex, and low extinction values A_V=0.2±0.1. This outcome is consistent with recent similar studies, e.g. Coelho et al. (2009) and Monachesi et al. (2012), showing the potential of J-PLUS when we use the adequate techniques, as MUFFIT, for stellar population diagnostics and photometric data sets. Given the wide area that will cover this survey (~8500 deg²), J-PLUS will provide unique results on the stellar population topic, rather in its nearby part where the photometric system design is optimal to face 2D studies.