SPHEREx Wide-Field Infrared Spectral Mapping of Interstellar Ices and Polycyclic Aromatic Hydrocarbons

Abstract

We present some of the first infrared spectral maps acquired by SPHEREx. These maps, which to our knowledge are the largest of their type ever compiled in the near-infrared, reveal multiple strong lines due to interstellar ices and polycyclic aromatic hydrocarbons (PAHs) throughout the Cygnus X and North American Nebula regions. The maps emphasize the strongest features arising from the 3 $μ$m H$_2$O, 4.27 $μ$m CO$_2$, and 4.67 $μ$m CO lines and the 3.28 $μ$m PAH feature, all of which are detected over large areas with complex and filamentary spatial distributions. The ice absorption maps of H$_2$O and CO$_2$ in particular broadly trace dense, cold, and well-shielded regions across Cygnus X, consistent with the established picture of efficient ice formation in dense molecular clouds. The interstellar ice features are also detected abundantly in diffuse absorption over wide areas. The relative strength of the H$_2$O and CO$_2$ features varies among different lines of sight, indicating possible differences in local physical conditions or chemical variations. The 3.28 $μ$m PAH emission correlates with the emission from the 7.7 and 11.2 $μ$m features, but shows small differences that may trace the grain size distribution and variations in the ambient UV field. SPHEREx all-sky spectral imaging, of which only a small fraction is showcased in this work, will support numerous science investigations including the structure of the Galaxy, the physics of the interstellar medium, and the chemistry of stars.

Figures (13)

• Figure 1: A three-color image of the CygX region constructed from SPHEREx mosaics. In this image, blue is 1.1 - 1.22 , green is 2.8 - 3.20 (containing the H$_2$O ice absorption feature), and red is 4.75 - 5.1 . Continuum emission from warm dust extends across the image as evidenced by the extended orange clouds. Dark lanes are seen where the dust and H$_2$O ice absorption are the strongest. Red sources appear in these dark lanes where there are background sources or embedded young stars shining through the dust. Blue stars are seen in regions with lower extinction or are foreground objects.
• Figure 2: An illustration of the banding correction described in §\ref{['sec:bandcorr']}. Top: an image of a 2$\times$2 region near the center of the continuum-subtracted PAH mosaic, constructed using wavelengths in the range 3.24 - 3.34 . The dark bands appear at the wavelengths furthest from the peak of the PAH line. Bottom: the mosaic after applying the banding correction.
• Figure 3: Ice peak optical depth maps in H$_2$O ice (top) and CO$_2$ ice (bottom) for the same region in CygX shown in Figure \ref{['fig:colorcyg']}. The optical depth is shown using a color scale with white corresponding to $\tau=0.75.$ The distributions of the two ice species is similar, but there are some differences in relative absorption depths as shown in Figure \ref{['fig:icezoom']}. Some residual "banding" or striping is visible in these images, see the discussion in §\ref{['sec:banding']}.
• Figure 4: Three selected regions of H$_2$O ice and CO$_2$ ice absorption line images from Figure \ref{['fig:iceabs']}, along with the corresponding section of the PAH line image. The ice absorption images show the peak optical depth in the feature in inverse grayscale, ranging from 0 to 1.1. The PAH images show lower flux regions as black and higher flux as white. The top images zoom in on a region near the center of the CygX mosaic shown in Figure \ref{['fig:colorcyg']}, and includes DR 15 which is the bright H2 region near the bottom of the PAH image. The middle images include the dark cloud LDN 896. The bottom row shows the dark cloud complex that is slightly to the right of center in Figure \ref{['fig:colorcyg']}, cataloged as the 2MASS dark clouds Dobashi 2362 and 2367 2011Dobashi, and known from Spitzer/IRAC imaging as being composed of a network of filamentary dark clouds 2020Pari
• Figure 5: Spectra and optical depths of sample sources in the CygX dark clouds obtained with aperture photometry using the L2 images. The Galactic coordinates ($l, b$) are shown in the lower right corner of each panel. The plots on the left show the photometry along with the continuum fit for wavelengths $>$2.5 plotted in red. The plots on the right show the calculated optical depths for the spectra on the left after modeling and removing the broad H$_2$O ice feature at 4.6 . The dashed line connects the data points and is not a fit to the absorption depth. Note that the relative strengths of the features vary among the different lines of sight throughout the clouds.