(abridged) Non-ionizing stellar continua are a source of photons for continuum pumping in the hydrogen Lyman transitions. In the environments where these transitions are optically thick, deexcitation occurs through higher series lines, so that the fl
ux in these lines has a fluorescent contribution in addition to recombination; in particular, Balmer emissivities are systematically enhanced above case B. The effectiveness of such mechanism in HII regions and the adequacy of photoionization models as a tool to study it are the two main focuses of this work. We find that photoionization models of H II regions illuminated by low-resolution population synthesis models significantly overpredict the fluorescent contribution to the Balmer lines. Conversely, photoionization models in which the non-ionizing part of the continuum is omitted or is not transferred underpredict the fluorescent contribution to the Balmer lines, producing a bias of similar amplitude in the opposite direction. In this paper, we carry out realistic estimations of the fluorescent Balmer intensity and discuss the variations to be expected as the simulated observational setup and the stellar populations parameters are varied. In all the cases explored, we find that fluorescent excitation provides a significant contribution. We also show that differential fluorescent enhancement may produce line-of-sight differences in the Balmer decrement, mimicking interstellar extinction. Fluorescent excitation emerges from our study as a small but important mechanism for the enhancement of Balmer lines, which should be taken into account in the abundance analysis of photoionized regions, particularly in the case of high-precision applications such as the determination of primordial helium.
This work aims to provide a theoretical formulation of Surface Brightness Fluctuations (SBF) in the framework of probabilistic synthesis models, and to distinguish between the different distributions involved in the SBF definition. RESULTS: We propos
e three definitions of SBF: (i) stellar population SBF, which can be computed from synthesis models and provide an intrinsic metric of fit for stellar population studies; (ii) theoretical SBF, which include the stellar population SBF plus an additional term that takes into account the distribution of the number of stars per resolution element psi(N); theoretical SBF coincide with Tonry & Schneider (1998) definition in the very particular case that psi(N) is assumed to be a Poisson distribution. However, the Poisson contribution to theoretical SBF is around 0.1% of the contribution due to the stellar population SBF, so there is no justification to include any reference to Poisson statistics in the SBF definition; (iii) observational SBF, which are those obtained in observations that are distributed around the theoretical SBF. Finally, we show alternative ways to compute SBF and extend the application of stellar population SBF to defining a metric of fitting for standard stellar population studies. CONCLUSIONS: We demostrate that SBF are observational evidence of a probabilistic paradigm in population synthesis, where integrated luminosities have an intrinsic distributed nature, and they rule out the commonly assumed deterministic paradigm of stellar population modeling.
In general, synthesis models provide the mean value of the distribution of possible integrated luminosities, this distribution (and not only its mean value) being the actual description of the integrated luminosity. Therefore, to obtain the closest m
odel to an observation only provides confi- dence about the precision of such a fit, but not information about the accuracy of the result. In this contribution we show how to overcome this drawback and we propose the use of the theoretical mean-averaged dispersion that can be produced by synthesis models as a metric of fitting to infer accurate physical parameters of observed systems.
