Spitzer IRS spectra of Virgo Early-Type Galaxies: Detection of Stellar Silicate Emission

Abstract

We present high signal to noise ratio Spitzer Infrared Spectrograph observations of 17 Virgo early-type galaxies. The galaxies were selected from those that define the colour-magnitude relation of the cluster, with the aim of detecting the silicate emission of their dusty, masslosing evolved stars. To flux calibrate these extended sources we have devised a new procedure that allows us to obtain the intrinsic spectral energy distribution and to disentangle resolved and unresolved emission within the same object. We have found that thirteen objects of the sample (76%) are passively evolving galaxies with a pronounced broad silicate feature which is spatially extended and likely of stellar origin, in agreement with model predictions. The other 4 objects (24%) are characterized by different levels of activity. In NGC 4486 (M 87) the line emission and the broad silicate emission are evidently unresolved and, given also the typical shape of the continuum, they likely originate in the nuclear torus. NGC 4636 shows emission lines superimposed on extended (i.e. stellar) silicate emission, thus pushing the percentage of galaxies with silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central unresolved region. A more detailed analysis of our sample, with updated models, will be presented in a forthcoming paper.

Nearby early-type galaxies with ionized gas

“…The spectra also clearly show the presence of a less pronounced bump at around 18 μm, likely arising from the same silicate circumstellar dust as predicted by Bressan et al (1998) and as shown in the SSP SED. An hint of the presence of this bump can be found in the spectra presented in Bressan et al (2006), although it is only clear in the present spectra that cover a larger spectral window. Finally we note the presence of a dip at 8 μm probably due to photospheric SiO absorption bands (e.g.…”

“…The data reduction used for these data was first presented in Bressan et al (2006), but was optimized later, as described below.…”

“…We can, as a first step, separate the spectra into "passive" and "active" (see Bressan et al 2006), where "passive" spectra are those that do not show any emission lines or PAH emission features, while "active" refers to objects that show one or the other, or both.…”

“…This spectral window is particularly rich in information on the ISM, excitation mechanisms and galaxy evolution. Bressan et al (2006) showed that the MIR spectra of the majority of the massive ETGs in clusters show neither Polycyclic Aromatic Hydrocarbons (PAHs) features nor emission lines, and exhibit only the broad silicate emission feature around 10 μm arising from circumstellar dust around oxygen-rich AGB stars, superimposed on a stellar photospheric continuum. ETGs with such MIR spectra, with either un-excited, or likely absent (see Clemens et al 2010), ISM, represent the class of passively evolving ETGs and can be considered the fossil record of galaxy evolution.…”

Nearby early-type galaxies with ionized gas

“…Appendix and Tables 2 and 3 are only available in electronic form at http://www.aanda.org for a review); and colder components detected in the mid and far infrared, CO and HI (for reviews see Sadler et al 2002;Temi et al 2004;Bressan et al 2006;Sage & Welch 2006;Morganti et al 2006;Helmboldt 2007;Oosterloo et al 2007;Panuzzo et al 2007). The hot gas component dominates the ISM in E/S0 galaxies, and only comparatively small quantities are detected in the warm and cold phases (Bregman et al 1992).…”