Near- to Mid-Infrared Spectroscopy for the Giant Elliptical Galaxy NGC 1316 (Fornax A) with the AKARI Infrared Camera

Abstract

We present the near-to mid-infrared (∼2.5-13 mm) spectrum of the giant elliptical galaxy NGC 1316 (Fornax A) obtained with the Infrared Camera (IRC) on board AKARI. NGC 1316 is known to have a peculiar polycyclic aromatic hydrocarbon (PAH) emission spectrum with an unusually high ratio of the 11.3 mm to the 7.7 mm emission intensity through the Spitzer IRS observation. NGC 1316 is a 3 Gyr old merger remnant with a dormant nucleus that was rather active 0.1 Gyr ago; the relatively well-known evolutionary history makes the elliptical galaxy an ideal probe for studying long-term effects of past energetic events on the interstellar medium. We have found that there is no significant PAH emission feature at 3.3 mm while we have confirmed the peculiar PAH emission spectrum in the mid-infrared. We have detected a strong absorption feature at 4.3 mm, which may be attributed to solid-state CO 2 in grain mantles. We discuss physical implications for those unusual emission and absorption features.

The globular cluster system of NGC 1316

“…The inner dust features best known from HST images appear white with a color of about C − R = 1.9. They fit well to nonstellar PAH emission (Temi et al 2005;Kaneda et al 2007). Note that the overall symmetry of the dust distribution differs from the spheroid.…”

Organic compounds in galaxies

“…The important question is whether or not AGNs play a key role in making the band ratio low. Kaneda et al (2007) point out that the AGN activity of NGC1316, which shows a low band ratio in its MIR spectrum (Figure 1b), ceased 100 Myr ago and thus its AGN should not make an appreciable effect on the present UIR band spectrum of this galaxy.…”

“…The 3.3 µm band is shown to be absent in NGC1316 based on AKARI IRC observations (Figure 1b), suggesting the dominance of large neutral PAHs (Kaneda et al 2007). While the presence of the UIR bands in elliptical galaxies is well established, the formation and survival of the band carriers in them are not yet clearly understood.…”

“…Recent Spitzer and AKARI observations, however, indicate that the presence of the UIR bands is not uncommon in elliptical galaxies although the appearance of the UIR bands is quite different from those familiar to spiral galaxies (Kaneda et al , 2007. Figure 1 shows some examples of spectra taken with the Infrared Spectrograph (IRS) on board Spitzer (Houck et al 2004) and the Infrared Camera (IRC) on board AKARI , Ohyama et al 2007).…”

“…The 11.2 µm band is clearly seen together with the 17 µm complex, whereas the 6.2, 7.7, and 8.6 µm bands are faint or almost absent (see also Kaneda et al 2008a,b). and (b) NGC1316 with IRC on board AKARI (Kaneda et al 2007). Figure 2.…”