SED OF NORMAL GALAXIES

1. SED OF NORMAL GALAXIES Josefa Masegosa *, IsabelMárquez*, Brigitte Rocca-Volmerange * *, Michel Fioc * * & Damien Leborgne*** *Instituto de Astrofísica de Andalucía (C.S.I.C.), Granada (SPAIN) **Institut d’Astrophysique de Paris (IAP), Paris (FRANCE) ***CEA, Saclay (FRANCE) INTRODUCTION With the aim to get a first insight on the reliability of PEGASE.3 to investigate SED evolution of galaxies, we have studied a large sample of spiral galaxies avoiding systems with Starburst or Nuclear Activity. We have selected a large sample of normal galaxies according to the following IR criteria : s60/100 < 0.45 s12/25 > 0.60 (Helou et al 1987, Masegosa & Márquez 2005) With this definition we granted the exclusion of Starburst and/or AGN systems. All the galaxies in the IRAS Faint Source Catalog (FSC) with accurate fluxes in the 4 IRAS bands (quality flag 1) were selected. This results in a sample of 15 Sa, 26 Sb, 77 Sc and 9 Sd galaxies. Their spectral energy distribution (SED) from far-UV till far-IR is studied by using the data provided by NED. Sa Sb Sc Sd Fig. 1. s12/s25 and s60/s100 ratios are presented for the four morphological types Sa, Sb, Sc and Sd. The filled histograms correspond to the galaxies with a reliable SED fitting. Fig. 2. SED fitting (full line) to the observed UV-optical to far IR magnitudes (dots connected by straight lines). Examples for the four morphological types are presented. In the bottom right box of each pannel the main output parameters are shown. • PEGASE.3 • PEGASE.3 is an improved version of the spectral evolution code by Fioc & Rocca-Volmerange (1997). It takes into account the effect of dust, which absorbs and scatters optical and UV photons that re-emit in the IR and submm range. For a given evolutionary scenario (star formation law, initial mass function, infall rate, etc.), PEGASE consistently computes the following quantities: • Metallicity of the ISM and of newly formed stars • Amount and composition of the dust • Attenuation of the stellar radiation by dust using a radiative transfer code • Grain re-emission in the IR, taking into account the stochastic fluctuations of temperature • SED and colors of a galaxy from the far-UV to the far IR-submm range. SED ANALYSIS We collected data on optical UBV, near IR JHK integrated magnitudes for all the galaxies and performed the SED fitting for each individual galaxy. By using a template of the same morphological type than each target galaxy, we found that a good fit, parametrized by both χ2 and visual inspection, can be obtained for 60% of the Sb and Sc galaxies and 30% of the Sa. For the remaining galaxies no good fits were found due to different reasons: ● Sa appear to be a rather heterogeneous family with only 30% of them with a well defined optical morphology. For the remaining 70%, large departures from Sa morphology are found: external rings, interacting galaxies (entering into the IRAS beam), distorted morphologies, etc. Then the use of an Sa template for them is a simplistic aproximation. ● Sb and Sc are better defined, with smaller dispersion in the empirical SED which translate into a large percentage of well fitted galaxies (60%). For the 40%, not well fitted cases, we have found good fits in the medium to far IR range at the price of getting large departures at UV-optical frequencies. A closer inspection of these galaxies has shown that most of them seem to be barred galaxies, and thus an extra component needs to be included. ● Sd show a rather good fitting by using Sd template only for 3 galaxies. For the others 6 an Sc template better reproduces the observed SED, reflecting a poor classification among the Sd family. Examples of good spectral fittings for each morphological type are shown in Figure 2. An average SED for each morphological type is shown in Figure 3. Galaxies with poor fitting have been excluded get these averaged SEDs. Fig. 4. Far IR luminosities (8-1000 μm) for the four morphological subclases coded as in Fig. 1. Fig. 3. Average SED for the four morphological types, normalized to K magnitude. AVERAGE PROPERTIES OF NORMAL SPIRAL GALAXIES Our color selection produces a sample of normal galaxies dominated by Sc and Sb galaxies, and with Sa and Sd spirals less well represented. A range in luminosity between 109 and 1011 Lo is found. There is a trend for spiral galaxies of later types to be brighter in the Far Infrared (see Fig. 4). The scatter is reduced when considering luminosity ratios, both for LFIR/LB and LFIR/LK (Fig. 5), excepting for the Sa, that cover the largest range in LFIR/LB. The galaxies with reliable SED fitting (full histograms) constitute a more homogeneus family for each type, with smaller scatter. Specific star formation rates (SF per unit mass), provided by LFIR/LK , are the smallest for Sa, increasing to Sb and Sc. A similar conclusion is reached by Dale et al. (2006), whereas their trend is less clear mainly due to the inclusion in their sample of star forming and/or active galaxies. • CONCLUSIONS AND WORK IN PROGRESS • PEGASE.3 provides a rather good representation for the SED of normal galaxies, selected by FIR colors. • It produces reliable fits for well defined Sa and for a large percentage of Sb and Sc. • This percentage is higher when only unbarred galaxies are considered. Rather poor fittings are obtained for Sd due to missclassification problems based on NED morphologies. • Specific star formation efficiencies increase for later types, i.e., the star formation per unit mass is higher for Sc than Sb, and for Sb than for Sa. • In this work, fixed fitting parameters as galactic extinction and inclination have been used for this first evaluation of the reliability of PEGASE.3. SED fitting will be improved by fine tunning of the input parameters. • The subsequent work will consist on the analysis of the SED for Starburst and/or AGN systems including these components in PEGASE.3 models. Fig. 5. The actual SF efficiency and SSFR can be estimated through LFIR/LB and LFIR/LK ratios. In the figure we show both for the four morphological types coded as in Fig. 1.