The SPHEREx Ices Investigation: An Overview

Abstract

SPHEREx is a NASA mission designed to perform an all-sky spectroscopic survey in the 0.75 - 5 $μ$m wavelength range. Its primary science objectives are to investigate: (1) inflationary cosmology, (2) the history of galaxy formation, and (3) the abundance of molecular ices - critical for prebiotic chemistry - found on the surfaces of interstellar dust grains within planet-forming regions. This paper focuses on the third theme, the SPHEREx Ices investigation, for which SPHEREx is conducting a spectroscopic survey of nearly ten million preselected sources throughout the Milky Way and Magellanic Clouds to characterize their ice absorption features. By selecting targets based on infrared color, spatial isolation, and brightness, the Ices Investigation secures high-signal-to-noise spectra across a broad range of astrophysical environments that are relatively free of spectral contamination. Rather than attempting to decompose each spectrum into its individual ice components, the Ices Investigation prioritizes accurate measurements of the integrated optical depths of key molecular ice absorption features. This approach enables statistically powerful correlation studies between ice abundances and environmental parameters - including extinction, temperature, gas composition, radiation field strength, cosmic ray flux, and star formation activity. The data pipeline developed for this purpose incorporates machine learning for continuum estimation, drawing on both SPHEREx and ancillary datasets. Ultimately, the expansive spectral archive produced by SPHEREx, combined with targeted follow-up from facilities like JWST, will transform our understanding of Galactic ice formation, evolution, abundance and their inheritance into planetary systems and prebiotic inventories.

Figures (6)

• Figure 1: Schematic illustration of the transition from gas-phase water to water ice. At the surface of a molecular cloud, far-ultraviolet (FUV) radiation efficiently photodissociates water molecules -- regardless of whether they form in the gas or are produced on dust grains and subsequently photodesorbed. Deeper into the cloud, the FUV field is attenuated but still present; in this partially shielded zone, photodesorbed water molecules survive longer, leading to an increased abundance of gas-phase water. Toward the cloud interior, the FUV field is almost fully suppressed, photodesorption becomes inefficient, and with grains remaining cold ($\scriptsize{\raisebox{-2pt}{$\stackrel{ <}{\sim}$}}\normalsize$ 25 K), thermal desorption is negligible. Under these shielded, low-temperature conditions, water predominantly exists as ice on grain surfaces, and the abundance of water ice greatly exceeds that of gas-phase water.
• Figure 2: ( Left) Predicted abundances of gas-phase H$_2$O (solid), H$_2$O ice (dot–dashed), and O$_2$ (dotted) as a function of $A_V$ for a cloud with $n_\mathrm{H} = 10^4$ cm$^{-3}$, exposed to FUV field strengths, $G_0$, of 1, 100, and 1000 times the Solar neighborhood value. Increasing $G_0$ shifts the abundance profiles to larger $A_V$. While the freeze-out depth depends on $G_0$, the total H$_2$O ice column is unchanged. The enhanced O$_2$ peak at $G_0 = 1000$ results from thermal desorption of atomic O from warm grains, which suppresses H$_2$O ice formation and leaves more O in the gas phase. ( Right) Dependence on gas density for $n = 10^3$, $10^4$, and $10^5$ cm$^{-3}$ at $G_0 = 100$. Both the threshold $A_V$ for ice formation and the $A_V$ of the H$_2$O and O$_2$ peaks increase with $G_0/n$, though the peak abundances are largely independent of density. Arrows indicate $\log n_\mathrm{H}$. Hollenbach2009.
• Figure 3: ISO Short Wavelength Spectrometer (SWS) spectrum of the Galactic source, W33A, an extremely embedded high-mass young stellar object Gibb2004. This source exhibits strong ice absorption features due to H$_2$O, CH$_3$OH, CO$_2$, XCN, CO, and weaker $^{13}$CO$_2$ and OCS features. Also shown are the SPHEREx Bands 4, 5, and 6, along with their respective spectral resolving powers, $\lambda/\Delta\lambda$ (see Table 1).
• Figure 4: ( Top) Distribution of the more than 9.9 million pre‑selected SPHEREx ice targets in the Milky Way and Magellanic Clouds, color‑coded by estimated foreground dust extinction ($A_V$ in magnitudes; Ashby2023). ( Middle left) SPHEREx greyscale mosaic of the Cygnus region at 0.98 $\mu$m; the yellow rectangle indicates the area expanded to the right for H$_2$O‑ice and CO$_2$‑ice. ( Middle right) Percent flux decrement, 100 $\times$ (continuum-flux)/continuum, at 3.0 $\mu$m (H$_2$O‑ice) and 4.28 $\mu$m (CO$_2$‑ice). ( Lower left) Greyscale mosaic of the H$_2$O‑ice with SPLICES targets overlaid, using the same foreground‑extinction color scale as in the top panel. ( Lower right) SPHEREx spectra for three of the 4,280 SPLICES targets within this region. The small dip in the spectra at 2.4 $\mu$m is an artifact caused by the instrument dichroic and will be automatically removed in future data processing.
• Figure 5: The distribution of line-of-sight dust visual extinctions toward SPLICES targets in $A_V$ terms, extrapolated roughly from the following relation: $A_K = 0.918\,(H - [4.5] - 0.08)$Majewski2011. Here the WISE $W2$ magnitude has been substituted for the IRAC 4.5 $\mu$m magnitude because it is available for all SPLICES targets. $A_K/Av = 0.112$Rieke1985 was assumed in order to derive the $A_V$ estimates. In cases where the objects were detected in the 2MASS $Ks$ band but not in $H$, a similar relation was used to determine $A_V$ from the $Ks-$[4.5] color. More than 83,000 SPLICES targets have an estimated line-of-sight $A_V$ extinction greater than 30 mag. These $A_V$ values are highly uncertain and should be regarded only as rough indicators of the total foreground dust extinction -- i.e., that SPLICES targets are either intrinsically red or likely to exhibit molecular absorption features in the SPHEREx bands. Detailed modeling and spectral typing will yield significantly more accurate $A_V$ estimates (see Section 5.1).