SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula

TL;DR

This study leverages SPHEREx near-infrared spectral maps, in combination with Spitzer IRS data, to investigate the aromatic/aliphatic evolution and PAH size across the northwest PDR of NGC7023 (Iris Nebula). It employs two methods—direct integration and spectro-photometric modeling—to measure the 3.3, 3.4, and 11.2 μm PAH bands, enabling spatial maps and ratio diagnostics. By applying PAHdb Monte Carlo fits to the Spitzer PAH spectrum, it derives PAH size indicators $\overline{N_C}$ and $f_{ m small}$, revealing a dense–diffuse demarcation and tentative correlations with the 11.2/3.3 μm ratio; however, decomposing the 3.3 μm complex yields different trends, complicating a straightforward size calibration. The work demonstrates SPHEREx’s potential for large-scale ISM PAH chemistry studies and highlights the need for a broader sample of extended objects to robustly calibrate PAH size tracers and resolve discrepancies between the blended 3.3–3.4 μm complex and its separate sub-features.

Abstract

Observations by the SpectroPhotometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx) are combined with Spitzer spectral map data to study the aromatic, aliphatic, and PAH size evolution across the northwest photo-dissociation region (PDR) of the Iris Nebula (NGC7023). The 3.3-3.4 $μ$m complex (I$_{3.3}$) and 11.2 $μ$m (I$_{11.2}$) PAH band strength are determined through direct integration. In addition, the former is decomposed into a 3.3 (I'$_{3.3}$) and 3.4 $μ$m (I'$_{3.4}$) sub-feature by fitting SPHEREx bandpass-integrated photometry using a modeled, highly sampled, multi-component spectrum. I$_{3.3}$, I$_{11.2}$, I'$_{3.3}$, and I'$_{3.4}$ all peak at the PDR. The NASA Ames PAH IR Spectroscopic Database is used to obtain the average number of carbon atoms ($\overline{\rm N_{C}}$) and small PAH fraction ($f_{\rm small}$) by fitting the isolated PAH component of the Spitzer segment; $70\lesssim\overline{N_{C}}\lesssim76$ and $0.24\lesssim\text{f}_{\rm small}\lesssim0.36$. I'$_{3.4}$/I'$_{3.4}$, I$_{11.2}$/I$_{3.3}$, $\overline{\rm N_{C}}$, and $f_{\rm small}$ all show a demarcation that matches the large-scale morphology of the region. For I'$_{3.3}$ and I'$_{3.4}$ this is reflected by two distinct trends when plotted against each other, one associated with the dense, the other with the diffuse medium; $[N_{\rm H,ali}/N_{\rm H,aro}]_{\rm dense}$ = 0.42$\pm$0.01 and $[N_{\rm H,ali}/N_{\rm H,aro}]_{\rm diffuse}$ = 0.10$\pm$0.01. $\overline{\rm N_{C}}$ and $f_{\rm small}$ are tentatively correlated with I$_{11.2}$/I$_{3.3}$ (R=0.54$\pm$0.05 and -0.45$\pm$0.05, respectively). A wider variety of large(r) extended interstellar medium objects is required to tighten the correlations, turn them into quantitative calibrators for PAH size, and pin down the discrepancy of correlations with I'$_{3.3}$ involved.

Figures (11)

• Figure 1: Composite Hubble Space Telescope ACS image of the northwest photo-dissociation region (PDR) in the reflection nebula NGC7023. Combined H$\alpha$ and infrared I-band data are shown in red, optical V-band data in green, and optical B-band data in blue. The irradiating star HD200775 is located some 43 from the center of the image toward the southeast, just outside the field of view. In orange Spitzer-IRS spectral map pixels are shown. Superimposed are four zones, colored red, green blue, and yellow, that were identified through hierarchical clustering of the Spitzer spectra, see 2018ApJ...858...67B for details. Blue marks the PDR-front, which forms the boundary between the dense medium toward the north from the diffuse medium in the south. Matching SPHEREx spectral extraction apertures are shown in white.
• Figure 2: Extracted SPHEREx and resampled Spitzer segments for the center pixel of the 7$\times$7 constructed spectral cube, see Figure \ref{['fig:hst']}. To highlight the SPHEREx segment, it is multiplied 8$\times$. Uncertainties are indicated with the gray vertical error bars. For the SPHEREx segment, horizontal error bars represent the bandpass of each spectro-photometric data point. IRAC channel 1 (blue) and 4 (green) bandpasses have been indicated as well as their predicted photometry.
• Figure 3: PAH band strength determination through direct integration. A straight baseline (dashed black) is drawn and subtracted to isolate the 3.3--3.4 µ m complex (green) and 11.2 µ m (red) PAH band. Uncertainties are indicated as gray error bars. The derived PAH band strengths and their uncertainty are indicated. See Section \ref{['subsec:direct']} for details.
• Figure 4: Left: Observed versus modeled SPHEREx bandpass-integrated photometry for the central pixel of the 7$\times$7 constructed spectral cube, see Fig. \ref{['fig:hst']}. The dashed-dotted line shows the one-to-one line. Right: Modeled, highly sampled 3.3--3.4 µ m complex (red) overlain atop the observed SPHEREx segment (blue). Also shown are the individual 3.3 (green) and 3.4 µ m (maroon) component as well as the determined baseline (orange). Uncertainties are indicated as gray error bars. Derived band strengths and their uncertainties are given as well. See Section \ref{['subsec:model']} for details.
• Figure 5: Maps of the northwest PDR in NGC7023 in the PAH 3.3--3.4 µ m complex, 3.3, 3.4 and 11.2 µ m bands. Top: 3.3--3.4 µ m complex and 11.2 µ m band strengths determined through direct measurement. See Section \ref{['subsec:direct']} for details. Bottom: 3.3 and 3.4 µ m band strength determined using a multi-component photometric decomposition. See Section \ref{['subsec:model']} for details. The color scales use a 4% cutoff on the high and a 1% cutoff on the low end, indicated by the green and red lines in the color bar, respectively. This suppresses outliers. See Section \ref{['sec:results']} for details.