Observations of the Near- To Mid-Infrared Unidentified Emission Bands in the Interstellar Medium of the Large Magellanic Cloud

Abstract

We present the results of near-to mid-infrared slit spectroscopic observations (2.55-13.4 µm) of the diffuse emission toward nine positions in the Large Magellanic Cloud with the Infrared Camera (IRC) on board AKARI. The target positions are selected to cover a wide range of the intensity of the incident radiation field. The unidentified infrared bands at 3.3, 6.2, 7.7, 8.6 and 11.3 µm are detected toward all the targets, and ionized gas signatures: hydrogen recombination lines and ionic forbidden lines toward three of them. We classify the targets into two groups: those without the ionized gas signatures (Group A) and those with the ionized signatures (Group B). Group A includes molecular clouds and photo-dissociation regions, whereas Group B consists of H II regions.In Group A, the band ratios of I 3.3 µm /I 11.3 µm , I 6.2 µm /I 11.3 µm , I 7.7 µm /I 11.3 µm and I 8.6 µm /I 11.3 µm show positive correlation with the IRAS and AKARI colors, but those of Group B do not follow the correlation. We discuss the results in terms of the polycyclic aromatic hydrocarbon (PAH) model and attribute the difference to the destruction of small PAHs and an increase in the recombination due to the high electron density in Group B. In the present study, the 3.3 µm band provides crucial information on the size distribution and/or the excitation conditions of PAHs and plays a key role in the distinction of Group A from B. The results suggest the possibility of the diagram of I 3.3 µm /I 11.3 µm v.s. I 7.7 µm /I 11.3 µm as an efficient diagnostic tool to infer the physical conditions of the interstellar medium.Subject headings: dust, extinction -Magellanic Clouds -galaxies: ISM -infrared: galaxies -infrared: ISMRecent laboratory experiments and quantum chemical calculations suggest that the properties of the UIR bands (e.g., shapes, center wavelengths, interband ratios, etc.) reflect the chemical and physical properties of PAHs (e.g., molecular structure, size distribution, ionization state, temperature, etc.), which may be altered in interstellar and circumstellar environments (Tielens 2008). Therefore the UIR bands have a great potential to be used as efficient diagnostic tools to infer the physical condition of the ISM even in remote galaxies.

Theoretical study of deuteronated PAHs as carriers for IR emission features in the ISM

“…(Onaka et al 1996;Mattila et al 1996;Verstraete et al 1996;Moutou et al 1999;Hony et al 2001;Verstraete et al 2001;Peeters et al 2002;Abergel et al 2002;Acke & van den Ancker 2004;Sakon et al 2004). These features are also observed in external galaxies of varying metallicity (Genzel et al 1998;Mattila et al 1999;Helou et al 2000;Lu et al 2003;Regan et al 2004;Brandl et al 2006;Armus et al 2007;Onaka et al 2008;Mori et al 2012). Apart from strong UIR band emission, there are weak broad features distributed in the emission plateaux in the ∼ 3 − 20 µm range (Tielens 2008).…”

INVESTIGATION OF PAHs IN GALACTIC PLANETARY NEBULAE WITH THE AKARI/IRC AND THE SPITZER/IRS

“…We calculate the UIR band ratios with different effective temperatures (T ) and ionization fractions (f ion ) based on a PAH model for molecular clouds and photodissociation regions (Case I in Mori et al, 2012) to investigate the difference between the PAHs in PNe and in interstellar fields. The results of the models are shown by the gray lines in the upper panel.…”

“…The most probable candidates are polycyclic aromatic hydrocarbons (PAHs) (Duley & Williams, 1981). The UIR band ratios can be good diagnostic tools to investigate the PAH properties and the local environments (e.g., Berné et al, 2007;Mori et al, 2012). We investigate the ionization fraction and the size distribution of PAHs in PNe by means of the variation in the UIR band ratios.…”

Near-Infrared Spectroscopy of the Diffuse Galactic Emission

“…A trend is seen that the aliphatic to aromatic ratio decreases towards the ionized region. This implies a preferential destruction of aliphatic bonds relative to aromatic bonds in the ionized gas as described in Joblin et al (1996) or that the aliphatic component resides mostly in small band carriers, which are quickly destroyed in the ionized gas (Mori et al 2012).…”

“…In addition, it contains emission features of hydrocarbon dust around 3 μm, which allow us to investigate aromatic and aliphatic components (Boulanger et al 2011;Kwok & Zhang 2011;Jones 2012). The 3.3 μm emission band, which is commonly attributed to polycyclic aromatic hydrocarbons (PAHs), is useful to study the size distribution and the processing of the band carriers in the ISM (e.g., Mori et al 2012). Deuterated hydrocarbon features are also expected to be present in 4.4-4.7 μm (e.g., Peeters et al 2004.…”