Properties of Polycyclic Aromatic Hydrocarbons in Local Elliptical Galaxies Revealed by the Infrared Spectrograph onSpitzer

Abstract

We performed mid-infrared spectroscopic observations of 18 local dusty elliptical galaxies by using the Infrared Spectrograph (IRS) on board Spitzer. We have significantly detected polycyclic aromatic hydrocarbon (PAH) features from 14 out of the 18 galaxies, and thus found that the presence of PAHs is not rare but rather common in dusty elliptical galaxies. Most of these galaxies show an unusually weak 7.7 µm emission feature relative to 11.3 µm and 17 µm emission features. A large fraction of the galaxies also exhibit H 2 rotational line and ionic fine-structure line emissions, which have no significant correlation with the PAH emissions. The PAH features are well correlated with the continuum at 35 µm, whereas they are not correlated with the continuum at 6 µm. We conclude that the PAH emission of the elliptical galaxies is mostly of interstellar origin rather than of stellar origin, and that the unusual PAH interband strength ratios are likely to be due to a large fraction of neutral to ionized PAHs.

A star formation study of the ATLAS3Dearly-type galaxies with the AKARI all-sky survey

“…We described the stellar continuum emission with a power-law model. In addition, to describe a silicate feature seen in ETGs around 10 µm (e.g., Bressan et al 2006), we added a Gaussian function to the power-law model, where the width and amplitude of the Gaussian relative to the stellar continuum were determined from the quiescent elliptical galaxy template of Kaneda et al (2008). The Draine & Li (2007) model was used for the PAH emission, with a size distribution and ionised fraction typical of the diffuse ISM of star-forming galaxies (for lack of a better model), while only the amplitude was allowed to vary.…”

“…More recent observations with satellites such as the Infrared Space Observatory (ISO), Spitzer, AKARI and Herschel also detected dust of both circumstellar and interstellar origin in many ETGs (e.g., Temi et al 2003;Kaneda et al 2011;Smith et al 2012). Polycyclic aromatic hydrocarbons (PAHs), that are destroyed by X-rays much more easily than dust, are also detected in some ETGs (e.g., Kaneda et al 2005Kaneda et al , 2008Rampazzo et al 2013). As PAHs are considered a star formation tracer, their presence in ETGs again suggests that star formation is ongoing.…”

Large-scale distributions of mid- and far-infrared emission from the center to the halo of M 82 revealed with AKARI

“…The C-C stretching modes at 6.2 and 7.7 μm are predominantly emitted by PAH cations, while the C-H outof-plane mode at 11.3 μm arises mainly from neutral PAHs (Allamandola et al 1989;Draine & Li 2007). Neutral PAHs emit significantly less in the 6−8 μm emission (Joblin et al 1994;Kaneda et al 2008a). The size effect on the PAH 11.3 μm/7.7 μm ratio is relatively small (Draine & Li 2007), although, in general, smaller PAHs can emit features at shorter wavelengths.…”

TheSpitzerdiscovery of a galaxy with infrared emission solely due to AGN activity

“…We have further investigated the possible origins of the PAHs from a post starburst in the host red elliptical by comparing with NGC 708. This source exhibits the strong 11.3 μm feature on a weak continuum (Kaneda et al 2008). We have scaled the observed mid-IR IRS spectrum to the overall IRAS photometry.…”