SPITZER IRS spectra of Virgo early-type galaxies: detection of stellar silicate

emission

Laura Silva (INAF-TS)

Alessandro Bressan (INAF-PD), Pasquale Panuzzo (INAF-PD), Lucio Buson (INAF-PD), Marcel Clemens (INAF-PD) GianLuigi Granato (INAF-PD), Roberto

Rampazzo (INAF-PD), Jose-Ramon Valdes (IANOE), Olga Vega (IANOE) AGB dusty envelopes as a tool to disentangle age and

metallicity effects in the SEDs of E galaxies

MIR observations: 10m feature and SF signatures

Bressan, Granato, Silva 1998, A&A, 332, 135Bressan et al. 2006, ApJL, accepted (astro-ph/0602014)Panuzzo et al. 2006, in preparation

The brightest stars in intermediate age (100 Myr to a few Gyrs) stellarpopulations are AGB stars

AGB stars are embedded within dusty envelopes originated by very strong stellar winds (10-7-10-4 Mo/yr)

We included the effect of AGB dusty envelopes in SSP:

Spherical symmetry and vexp independent of r =(M,R,L,Z).

Radiative transfer of the photospheric SED through the envelope computed with the Granato & Danese 94 code.

(Bressan, Granato, Silva 1998)

Modeling intermediate and old stellar populations: the effect of AGB dusty envelopes

SSP with AGBs

Age & Metallicity degeneracy hampers reconstructing the history of baryonassembly in the early universe, recorded in the fossil stellar populations (early typegalaxies)

Old & Metal Poor = Young & Metal Rich

NO Dusty AGBs Lot of Dusty AGBs

OPT:

MIR:

Age & m

etallicity

Age

Metallicity

10 m bump: 13 (76%)

N4339 N4365 N4371 N4377 N4382 N4442 N4473 N4474 N4551 N4564 N4570 N4621 N4660

Signs of MIR activity: 4 (24%)

N4486 (M87), N4636, N4435 , N4550

Systematic spectroscopic study with Spitzer IRS of 17 Early type galaxies along CM relation in Virgo

Cluster (PID 3419 cycle 1 - PI A. Bressan )

+ 50 hours in cycle 2 with IRS Blue Peak Up: fainter virgo and bright Coma…

13 galaxies (76%)with a broad and spatially extended 10m emission feature

10Gyr SSP models withZ=0.008, 0.02, 0.05

SSP with 5Gyr Z=0.008

Dashed: mean outflowAGB starDotted: C-rich star U Cam

M giant sed model + circumstellar envelope model

Old Stellar PopulationsMIR emission from dusty (silicates) AGB Stars

(Bressan, Granato & Silva 98)

Degeneracy in the Optical

Degeneracy in the MIR

MIR +

Optical NIRRemoves degeneracy

4 galaxies (24%) with MIR signatures of activity

(M87)

NGC 4435S0 Liner-HII (Ho et al 97) GRASIL fit (Silva et al 98, Vega et al 05)

MODELPost starburst 108 yrCurrent SFR 0.14 MŸ/yr<SFR> 2.8 MŸ/yrFormed stars 3 108 MŸ

Mgas 2.5 108 MŸ

Fmol = 70% 1=43

NGC7331 Smith et al 05

Summarizing:• 17 E-type galaxies in Virgo observed with SL1 SL2 LL2

• Signature of AGB stars well detected by Spitzer IRS • In the majority (76%) of galaxies no other features• The bump is in very good agreement with model predictions• OPT and NIR follow-up is underway to break the age-Z degeneracy

• 2 galaxies show emission lines but no PAHsNGC4486 (M87) NGC 4636 (E/S0 Liner )

• 2 galaxies show PAHs (NGC 4550-SB0 Liner, NGC 4435-SB0 Liner HII)

• N4435 MIR SED remarkably similar to N7331 (Spiral) - Model suggests a post-starburst with residual SFR