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

The Astrophysical Journal (Impact Factor: 5.99). 09/2011; 744(1). DOI: 10.1088/0004-637X/744/1/68
Source: arXiv


We present the results of near- to mid-infrared slit spectroscopic
observations (2.55--13.4 um) 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 um 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
HII regions. In Group A, the band ratios of I(3.3)/I(11.3), I(6.2)/I(11.3),
I(7.7)/$I(11.3) and $I(8.6)/$I(11.3) 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 um 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)/I(11.3) v.s. $I(7.7)/$I(11.3) as an efficient diagnostic tool
to infer the physical conditions of the interstellar medium.

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Available from: Hideki Umehata, Sep 02, 2014
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    • "We find that the results do not depend on T * for T * =20,000–50,000 K because of the Lynman cut-off. See Mori et al. (2012) "
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    ABSTRACT: We present the results of the near-infrared (NIR) to mid-infrared (MIR) slit spectroscopic observations of the diffuse emission toward nine positions in the nearby irregular galaxy Large Magellanic Cloud (LMC) with the Infrared Camera (IRC) on board AKARI. The unique characteristic of AKARI/IRC provides a great opportunity to analyze variations in the unidentified infrared (UIR) bands based on continuous spectra from 2.5 to of the same slit area. The observed variation of suggests destruction of small-sized UIR band carriers, polycyclic aromatic hydrocarbons (PAHs) in harsh environments. This result demonstrates that the UIR band provides us powerful information on the excitation conditions and/or the size distribution of PAHs, which is of importance for understanding the evolutionary process of hydrocarbon grains in the Universe. It also suggests a new diagnostic diagram of two band ratios, such as versus , for the interstellar radiation conditions. We discuss on the applicability of the diagnostic diagram to other astronomical objects, comparing the LMC results with those observed in other galaxies such as NGC 6946, NGC 1313, and M51.
    The 2nd AKARI Conference; 11/2012
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    The Astrophysical Journal 09/2011; 744(2). DOI:10.1088/0004-637X/744/2/160 · 5.99 Impact Factor
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    The Astrophysical Journal Letters 04/2012; 751(1). DOI:10.1088/2041-8205/751/1/L18 · 5.34 Impact Factor
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