Abstract
The James Webb Space Telescope (JWST) will deliver an unprecedented quantity of high-quality spectral data over the 0.6-28 μm range. It will combine sensitivity, spectral resolution, and spatial resolution. Specific tools are required to provide efficient scientific analysis of such large data sets. Our aim is to illustrate the potential of unsupervised learning methods to get insights into chemical variations in the populations that carry the aromatic infrared bands (AIBs), more specifically polycyclic aromatic hydrocarbon (PAH) species and carbonaceous very small grains (VSGs). We present a method based on linear fitting and blind signal separation for extracting representative spectra for a spectral data set. The method is fast and robust, which ensures its applicability to JWST spectral cubes. We tested this method on a sample of ISO-SWS data, which resemble most closely the JWST spectra in terms of spectral resolution and coverage. Four representative spectra were extracted. Their main characteristics appear consistent with previous studies with populations dominated by cationic PAHs, neutral PAHs, evaporating VSGs, and large ionized PAHs, known as the PAH x population. In addition, the 3 μm range, which is considered here for the first time in a blind signal separation (BSS) method, reveals the presence of aliphatics connected to neutral PAHs. Each representative spectrum is found to carry second-order spectral signatures (e.g., small bands), which are connected with the underlying chemical diversity of populations. However, the precise attribution of theses signatures remains limited by the combined small size and heterogeneity of the sample of astronomical spectra available in this study. The upcoming JWST data will allow us to overcome this limitation. The large data sets of hyperspectral images provided by JWST analysed with the proposed method, which is fast and robust, will open promising perspectives for our understanding of the chemical evolution of the AIB carriers.
Highlights
The aromatic infrared bands (AIBs) have held the attention of a large community of astronomers since their discovery in the 1970s (Gillett et al 1973)
At first named unidentified infrared bands (UIBs), they obtained the current denomination of AIBs after Leger & Puget (1984) and Allamandola et al (1985) proposed that they can be emitted by large carbonaceous molecules, namely polycyclic aromatic hydrocarbons (PAHs)
The first conclusion that can be drawn from this figure is that the extracted spectra using Maximum Angle Signal Separation (MASS)-negative matrix factorization (NMF) applied to the ISO data are qualitatively in good agreement with those of Pilleri et al (2012), and in particular we recognize the four chemical populations presented in this earlier study and ascribed to cationic PAHs (PAH+), neutral PAHs (PAH0), evaporating very small grains, and PAHx
Summary
The aromatic infrared bands (AIBs) have held the attention of a large community of astronomers since their discovery in the 1970s (Gillett et al 1973). At first named unidentified infrared bands (UIBs), they obtained the current denomination of AIBs after Leger & Puget (1984) and Allamandola et al (1985) proposed that they can be emitted by large carbonaceous molecules, namely polycyclic aromatic hydrocarbons (PAHs). The strongest AIBs, at 3.30, 6.20, 7.70, 8.60, 11.30, and 12.70 μm correspond to the cooling of the molecule through vibrational stretching and bending modes of its C–H and C–C bonds To study this emission the community has counted on several space observatories: the Infrared Astronomical Satellite (IRAS, Neugebauer et al 1984), the Infrared Space Observatory (ISO, Kessler et al 1996), the Spitzer Space Telescope (Spitzer, Werner et al 2004), and AKARI (Murakami et al 2007). The upcoming James Webb Space Telescope (JWST, Gardner et al 2006) has inspired a lot of hope as its capabilities will surpass all former space observatories
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