MID INFRARED COLORS OF EARLY TYPE GALAXIES

ALESSANDRO BRESSAN, HERVÉ AUSSEL and GIAN LUIGI GRANATOOsservatorio Astronomico di Padova, Padova, IT

GIULIA RODIGHIERODipartimento di Astronomia, Padova, IT

PASQUALE PANUZZO and LAURA SILVASISSA, Trieste, IT

Abstract. We compare the mid infrared colors of early type galaxies with isochrones that suitablyaccount for the effects of dust in the circumstellar envelopes of AGB stars. We show that thesestars affect significantly the integrated colors in the ISO bands. The excess at 10µm observed insome early type galaxies combined with the optical spectrum, is compatible with a 100% old metalrich population. Limits on the additional young components are: age 0.2 Gyr, mass=0.5% and E(B-V)≥0.5 or age 3 Gyr, mass 10% and E(B-V)≥0.2.

Keywords: AGB stars: dust – galaxies: stellar content – infrared: galaxies

1. Isochrones in the Mid Infrared

Bressan, Granato and Silva (1998) have suggested that the presence of dust en-shrouded AGB stars should significantly affect the infrared colors of simple stellarpopulations (SSP). With the advent of ISO observations of the galactic bulge andnearby elliptical galaxies it has become possible to quantify these predictions. Fig-ure 1 shows the HR diagram of Bulge stars in the field of NGC 5266 (Glass etal., 1999) superimposed to isochrones by Bressan et al., 1998. The new isochronesrest on a quite elaborate picture by means of which it is possible to relate a dustyenvelope of a given optical depth with the basic parameters of the star along theisochrone. Key ingredients are the empirical relation connecting the mass-loss rateand the pulsational period of the star, and the relation between the dust to gas ratio(δ) and the luminosity and escape velocity of the star. The isochrones in Figure 1nicely reproduce the observed slope of the AGB. Assuming for instance a constantδ (eg proportional to the metallicity of the system) would steepen the theoreticalAGB in a unacceptable way. We compare the mid-infrared integrated colors ofisochrones with observations of early type galaxies (Madden, Vigroux, 1999) inFigure 2. The age of the isochrones is decreasing from right to left (shown inGyr), and the metal content is Z = 0.008 (dashed line) and Z = 0.02 (solid line). Asilicate mixture (Bressan et al., 1998) has been adopted. By contrast the asteriskin the upper right, indicates the region occupied by isochrones of different age andmetallicity computed without accounting for dusty envelopes around AGB stars.

Astrophysics and Space Science 277 (Suppl.): 251–254, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

252 A. BRESSAN ET AL.

Figure 1. The HR diagram of Baade’s Window toward NGC 6522 in the mid infrared (from theISOGAL survey, Glass et al., 1999) compared with our isochrones of solar metallicity.

As already anticipated in Bressan et al. (1998), effects of dusty envelopes appearin the integrated spectrum below a few µm, and dominate around 10-15µm. Thisexplains the almost horizontal displacement of the integrated colors. The age is thedominant parameter affecting the mid-infrared colors while the metallicity plays asecondary role. Interstellar dust is also affecting the location of early type galaxiesin this diagram as it is evident from the location of the galaxies below the curveof youth of SSPs. The four open stars in Figure 2 show the location of old earlytype galaxy models with residual star formation activity and different dust content,computed with GRASIL (see e.g. Silva et al., 1998). Interstellar dust reprocessing,in particular the effects of PAH molecules, operates unfortunately in the samedirection of age, rendering the dating method presented here more difficult.

2. The Nature of the 10 µm Excess

The analysis of ISO colors revealed that some ‘old’ ellipticals show an excessaround 10 µm which cannot be interpreted as due to current star formation becauseof the lack of the corresponding PAH emission feature at 7.7 µm (Bregman et al.,1998). Impey et al. (1986) found that in M32 this excess is distributed as the stellarlight and attributed it to the dusty envelopes of AGB stars. In order to understandthe nature of the 10µm excess we have analyzed the ISO-CVF data of NGC 1399and NGC 1404. Figure 3 compares the infrared data of NGC 1399 with some suit-able models and, for completeness also depicts the shape of the optical spectrumof a typical giant elliptical (NGC 4649). The effect of dusty AGB stars is visible in

EARLY TYPE GALAXIES 253

Figure 2. Mid infrared colors of SSPs and model galaxies compared with the observed location of asample of early type galaxies (NGC labels) by Madden and Vigroux 1999. Filled dots are isochronesof increasing age, as indicated in the figure from left to right. The dashed line is for Z = 0.008 andthe solid line is for Z = 0.02. The asterisk represents the location of isochrones of different ageand metallicity computed without dust enshrouded AGB stars. Open stars refer to galaxy modelscomputed with GRASIL.

the lower panel. An old population with solar metallicity is able to reproduce fairlywell both the optical and the mid infrared spectral energy distribution (SED) ofthis galaxy. A young component is still marginally admitted but it must be heavilyextinguished: age 0.2 Gyr, mass=0.5% and E(B-V)≥0.5 or age 3 Gyr, mass 10%and E(B-V)≥0.2. While this young population can give rise to the feature at 10µm, independently from the metallicity of the old component, the optical SEDindicates a preference for a solar composition. The 10 µm excess in NGC 1404 isless pronounced. Both galaxies show an UV excess (Burstein et al., 1988) whichagain is less pronounced in NGC 1404 than in NGC 1399. Bressan et al., 1994interpreted this UV excess with the presence of an old super metal rich populationwhose fraction is larger in NGC 1399 than in NGC 1404. Consistently we believethat the larger 10 µm excess observed in NGC 1399 is due to a larger fraction ofmetal rich stars. From this point of view the presence of a young population in thecore of these galaxies seems less favorite.

Acknowledgements

We acknowledge the TMR grant ERBFMRX-CT96-0086.

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Figure 3. The spectral energy distribution of a giant elliptical is compared with the SSP models.Upper panel, continuous line: old with solar composition; short-dashed line: old metal poor plus0.5% young (0.2 Gyr, E(B-V) = 0.5) solar; long-dashed line: old solar plus 10% young (3 Gyr,E(B-V) = 0.2) solar. The old component is extingushed with E(B-V) = 0.05. In the lower panel thesame models are compared with the CVF data (18" × 18") of NGC 1399. The dot dashed line there,refers to an old SSP without dusty AGB envelopes.

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