2. MODEL COMPONENTS

The ingredients determining the observed stellar part of a galaxy SED are the spectra of individual stars, prescriptions for superposing them, plus effects that cannot be accounted for from first principles. This section addresses the latest developments related to spectral libraries and the construction of realistic synthetic populations from evolution models and a stellar initial mass function (IMF). I will also comment on rarely discussed, but potentially very important, morphological effects.

2.1. Stellar Spectra

Galaxies are made of stars. Stellar spectra are the basic building blocks of a galaxy SED. A stellar spectral library can be either empirical or theoretical ([34]). In the past, models were often limited by the amount of available computer time, lack of atomic data, or the complexity of the physics. On the other hand, dust reddening and the restrictions imposed by the available spectral windows often necessitate a theoretical approach. Recently, there has been a shift in preference away from empirical to theoretical libraries. The main reasons are the reliability of the latest generation of model atmospheres and the need to cover the full parameter space, which would otherwise be observationally inaccessible. Nearby stars have the Galactic chemical evolutionary history imprinted, and their spectra are therefore sometimes badly suited for comparisons with those of, e.g., elliptical ([47]) or Lyman-break galaxies ([40]).

STELIB is a new empirical stellar library compiled by [28]. STELIB consists of a homogeneous library of several hundred stellar spectra in the visible range (3200 to 9500 Å) at intermediate spectral resolution (~ 3 Å). This library includes stars of various spectral types and luminosity classes, spanning a broad range in metallicity. The spectral resolution, wavelength and spectral type coverage of this library represent a substantial improvement over previous libraries used in population synthesis models. It is implemented in the latest version of GISSEL ([2]).

Current theoretical efforts are reflected in the synthetic library of [37] who computed ~ 1600 high-resolution stellar spectra, with a sampling of 0.3 Å and covering the wavelength range from 3000 to 7000 Å. The library was computed with the latest improvements in stellar atmospheres, incorporating non-LTE, line-blanketed TLUSTY models ([25]) for hot, massive stars and spherical, line-blanketed Phoenix models accounting for tri-atomic molecules ([16]) for cool stars. The grid covers the full Hertzsprung-Russell diagram (HRD) for a wide range of chemical abundances. The replacement of the traditional Kurucz ([22]) atmospheres at both the hot and cool end is significant. For instance, important diagnostics such as the helium lines are strongly affected by non-LTE effects in O stars. LTE models predict too weak equivalent widths. An example of the corresponding improvement at low temperature (T_eff) is shown in Fig. 2. Phoenix models self-consistently account for the overall SED because of their extensive molecular line list. These molecular transitions are missing in the widely used Kurucz models. [33] provided an empirical correction to the Kurucz models. This correction becomes unnecessary with the new fully blanketed models.

Figure 2. Comparison of the fluxes predicted by the models of [22], [33], and [16]. The latter are based on Phoenix atmospheres and predict the overall shape in a self-consistent manner. Upper panel: supergiants; lower panel: dwarfs ([37]).

The ultraviolet (UV) part of the spectrum is emitted by hot stars. The strong winds in hot stars require modeling with spherically extended, expanding non-LTE atmospheres. Above the Lyman limit, these models can account for the observed strong stellar-wind lines related to highly ionized metals such as, e.g., Si IV 1400 or C IV 1550 ([31]). The wavelength range below 912 Å is of greater relevance to the topic of this conference: it is the source of the radiative heating of the interstellar gas. The dramatic effects of non-LTE and sphericity effects are demonstrated in the newly released model atmosphere set of [46]. In Fig. 3, these models ("WM-basic") are contrasted with classical, static LTE atmospheres ("Kurucz") and with CoStar models, which are similar to WM-basic models, except for the exclusion of line-blanketing. WM-basic is considered to have the best physical ingredients and is the atmosphere of choice for photoionization modeling. All models agree rather well in the wavelength range where the H0 ionizing radiation is emitted ( > 504 Å); the number of photons predicted to be emitted in the hydrogen Lyman continuum is a robust quantity which has changed by less than 0.2 dex since the classical work of [42]. The behavior of the ionized helium continuum below 228 Å is in sharp contrast to that of hydrogen. Now, wind effects drastically alter the spectrum, depending on blanketing and wind density. For solar chemical composition (top panel), high wind densities lead to He⁺ recombination and a marked drop of the flux. Lower composition leads to weaker winds, and He⁺ remains ionized. In this case, a vast flux excess over Kurucz models is predicted, yet blanketing still lowers the output below that of the CoStar models.

Figure 3. Comparison of the emergent fluxes for an early O supergiant from a WM-basic (solid), a CoStar (dashed), and a Kurucz model (dotted) at solar metallicity and 0.2 ZY ([46]).