Detection of unidentified infrared bands in a Halpha filament in the dwarf galaxy NGC1569 with AKARI

TL;DR

This study detects unidentified infrared bands in a Halpha filament of NGC1569 using AKARI, revealing specific UIR features, their relative strengths, and possible origins related to galactic outflows and grain fragmentation.

Contribution

First infrared spectroscopic detection of UIR bands in a galaxy filament, analyzing their characteristics and proposing a new formation mechanism involving grain fragmentation in shocks.

Findings

UIR bands detected at 6.2, 7.7, and 11.3 microns in the filament.

The 11.3 micron band is relatively stronger than in the galaxy disk.

UIR carrier destruction timescale is much shorter than the outflow timescale.

Abstract

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.