用AKARI探测矮星系NGC1569中的H α灯丝中未识别的红外波段
Detection of unidentified infrared bands in a Halpha filament in the dwarf galaxy NGC1569 with AKARI
摘要：
我们用AKARI上的红外相机（IRC）对NGC 1569进行了6个红外波段（3.2，4.1，7，11，15和24微米）的观测。还用IRC进行了Halpha灯丝的近-中红外（2- 13微米）光谱分析。与Halpha灯丝相关的延伸结构在7微米处显得明亮，表明灯丝在UIR带发射处是明亮的。IRC的后续光谱观测证实了灯丝中存在6.2、7.7和11.3微米的发射。相对于斯皮策望远镜上的红外线摄谱仪观测到的星系盘，暗条光谱显示出相对于7.7微米波段的11.3微米UIR波段发射。近红外光谱还表明在灯丝中存在2.5- 5微米的过量连续发射。H α暗条被认为是由NGC 1569盘中恒星形成活动产生的星系外流形成的。据估计，外流中UIR带载体的破坏时间尺度（约1.3 × 10^3年）比外流的时间尺度（约530万年）短得多。因此，带状载流子不太可能在外流环境中存活。或者，我们认为，在细丝中的带载流子可能产生的冲击，产生的H α发射的大碳颗粒的碎片。近红外过量连续发射不能单独由自由-自由发射来解释，可能需要热尘埃的贡献，尽管从HI复合线估计的自由-自由发射强度具有很大的不确定性。
英文摘要：
We performed observations of NGC1569 for 6 infrared bands (3.2, 4.1, 7, 11, 15, and 24 micron) with the Infrared Camera (IRC) onboard AKARI. Near- to mid-infrared (2--13 micron) spectroscopy of a Halpha filament was also carried out with the IRC. The extended structure associated with a Halpha filament appears bright at 7 micron, suggesting that the filament is bright at the UIR band emission. Follow-up spectroscopic observations with the IRC confirm the presence of 6.2, 7.7, and 11.3 micron emission in the filament. The filament spectrum exhibits strong 11.3 micron UIR band emission relative to the 7.7 micron band compared to the galaxy disk observed with the Infrared Spectrograph on Spitzer. The near-infrared spectrum also suggests the presence of excess continuum emission in 2.5--5 micron in the filament. The Halpha filament is thought to have been formed by the galactic outflow originating from the star-formation activity in the disk of NGC1569. The destruction timescale of the UIR band carriers in the outflow is estimated to be much shorter (~ 1.3 x 10^3 yr) than the timescale of the outflow (~ 5.3 Myr). Thus it is unlikely that the band carriers survive the outflow environment. Alternatively, we suggest that the band carriers in the filaments may be produced by the fragmentation of large carbonaceous grains in shocks, which produces the Halpha emission. The NIR excess continuum emission cannot be accounted for by free-free emission alone and a hot dust contribution may be needed, although the free-free emission intensity estimated from HI recombination lines has a large uncertainty.
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