Aims. In this paper we study the influence of the ionizing cluster mass on the emission line spectrum of H ii regions in order to determine the influence of low mass clusters on the integrated emission line spectra of galaxies. Methods. For this purpose, we present a grid of photoionization models that covers metallicities from Z = 0.001 to Z = 0.040, ages from 0.1 to 10 Ma (with a time step of 0.1 Ma), and cluster initial masses from 1 to 10 7 M . The stellar masses follow a Salpeter initial mass function (IMF) in an instantaneous burst mode of star formation. We obtain power-law scale-relations between emission-line luminosities and ionizing cluster masses from the grids and we evaluate the dependences on the ionizing cluster mass for some line luminosities, equivalent widths and line ratios. Results. Power-law scale-relations are shown to be useful tools to obtain robust diagnostics, as examples: (a) Hα/Hβ ratio varies from the usually assumed value of 2.86, these variations imply the existence of a lower limit to the attainable precision in extinction star formation rate indicator for ages older than ∼4.5 Ma. We also show that the ionizing cluster mass dependence explains why empirical calibrations produce more reliable diagnostics of some emission lines than photoionization models grids. Finally, we show preliminary results about the contribution of low mass clusters (M < 10 4 M ) to the integrated emission line spectra of galaxies, which can be as high as 80% for some relevant lines.

Aims. We study how IMF sampling affects the ionizing flux and emission line spectra of low mass stellar clusters. Methods. We performed 2 × 10 6 Monte Carlo simulations of zero-age solar-metallicity stellar clusters covering the 20−10 6 M mass range. We study the distribution of cluster stellar masses, M clus , ionizing fluxes, Q(H 0 ), and effective temperatures, T clus eff . We compute photoionization models that broadly describe the results of the simulations and compare them with photoionization grids.Results. Our main results are: (a) a large number of low mass clusters (80% for M clus = 100 M ) are unable to form an H ii region. (b) There are a few overluminous stellar clusters that form H ii regions. These overluminous clusters preserve statistically the mean value of Q(H 0 ) obtained by synthesis models, but the mean value cannot be used as a description of particular clusters. (c) The ionizing continuum of clusters with M clus < ∼ 10 4 M is more accurately described by an individual star with self-consistent effective temperature (T * eff ) and Q(H 0 ) than by the ensemble of stars (or a cluster T clus eff ) produced by synthesis models. (d) Photoionization grids of stellar clusters cannot be used to derive the global properties of low mass clusters. Conclusions. Although variations in the upper mass limit, m up , of the IMF would reproduce the effects of IMF sampling, we find that an ad hoc law that relates m up to M clus in the modeling of stellar clusters is useless, since: (a) it does not cover the whole range of possible cases; and (b) the modeling of stellar clusters with an IMF is motivated by the need to derive the global properties of the cluster: however, in clusters affected by sampling effects we have no access to global information of the cluster but only particular information about a few individual stars.

Abstract. The combination of stellar population synthesis models and photoionization models allows a better understanding of the spectral features of HII regions and HII galaxies. In this work we show that sampling effects in the initial mass function (IMF) are very important in the low cluster mass case. To this aim, we compute photoionization models ionized by realistic clusters made up of various combinations of individual stars and clusters made up with a synthesis model. We discuss the differences in the position on diagnostic diagrams and their implications.

Abstract. The combination of stellar population synthesis models and photoionization models allows a better understanding of the spectral features of HII regions and HII galaxies. In this work we show that sampling effects in the initial mass function (IMF) are very important in the low cluster mass case. To this aim, we compute photoionization models ionized by realistic clusters made up of various combinations of individual stars and clusters made up with a synthesis model. We discuss the differences in the position on diagnostic diagrams and their implications.