The Interstellar Medium in I Zw 18 seen with JWST/MIRI: II. Warm Molecular Hydrogen and Warm Dust

TL;DR

JWST/MIRI observations of I Zw 18, a nearby extremely metal-poor dwarf, reveal detectable warm H$_2$ emission across eleven 120 pc apertures, with 7 rotational transitions enabling LTE-based population-diagram modeling. The H$_2$ gas is dense ($n_H\sim10^5\ \mathrm{cm^{-3}}$) and exposed to a strong UV field ($G_0\sim10^3$), yet coexists with highly ionized gas emitting [O IV] and [Ne V], indicating multiple phases in pressure-equilibrium within ~120 pc scales. Allowing the H$_2$ ortho/para ratio to vary yields regions with $OPR>3$, consistent with selective photodissociation in propagating fronts and self-shielding effects; in other regions $OPR\lesssim3$, highlighting spatially varying PDR conditions. A warm dust continuum is detected with a notable broad emission feature near $14\mu$m, which is tentatively attributed to Al$_2$O$_3$-based dust, while PAH emission is largely absent, underscoring the minimal and distinct dust chemistry in this metal-poor ISM. These results demonstrate that warm, dense H$_2$ can persist in extremely metal-poor environments and provide empirical constraints on H$_2$ formation/destruction and dust processing in early-like galaxies.

Abstract

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of IZw18, a nearby dwarf galaxy with a metallicity of $\sim 3$% Solar. Here, we investigate warm molecular hydrogen, H2, observed in spectra extracted in $\sim 120$ pc apertures centered on eleven regions of interest. We detect 7 H2 rotational lines, some of which are among the weakest ever measured. The H2 population diagrams are fit with local-thermodynamic-equilibrium models and models of photodissociation regions. We also fit the ortho-/para-H2 ratios (OPRs); in three of the six regions for which it was possible to fit the OPR, we find values significantly greater than 3, the maximum value for local thermodynamic equilibrium. To our knowledge, although predicted theoretically, this is the first time that OPR significantly $> 3$ has been measured in interstellar gas. We find that OPR tends to increase with decreasing H2 column density, consistent with the expected effects of self-shielding in advancing photodissociation fronts. The population diagrams are consistent with H nucleon densities of $\sim 10^5$ cm$^{-3}$, and an interstellar radiation field scaling factor, G0, of $\sim 10^3$. This warm, dense H2 gas co-exists with the same highly ionized gas that emits [OIV] and [NeV]. Emission from T $\geq 50$K dust is detected, including an as-yet unidentified dust emission feature near 14 $μ$m; possible identification as Al$_2$O$_3$ is discussed. The continuum emission from several regions requires that a considerable fraction of the refractory elements be incorporated in dust. Despite stacking spectra in the SE where H2 is found, no significant emission from polycyclic aromatic hydrocarbons is detected.

Figures (14)

• Figure 1: 17$^{\prime\prime}$$\times$17$^{\prime\prime}$ RGB image of I Zw 18 showing the apertures of interest overlaid in the right panel. As in the legend, red is the MIRI F1130W background-subtracted image, green the F560W background-subtracted image, and blue is the HST F606W image astrometrically corrected to Gaia. In the right panel, the circles illustrate the apertures (0$.\!\!^{\prime\prime}$65 radius, $\sim 120$ pc diameter) for spectral extraction; the rectangles correspond to the MRS/IFU FoVs of the two separate pointings in the four channels. The smallest FoV, Channel 1, is illustrated by a transparent shading; the two pointings overlap allowing the construction of a complete spectral map even at the shortest wavelengths. The MIRI 14 $\mu$m-continuum sources are apparent as red knots in the SE, and the faint knot in the NW associated with VLA-NW-A. The horizontal line in the lower right corner corresponds to 100 pc.
• Figure 2: One-dimensional spectra extracted from the convolved cubes within the 0$.\!\!^{\prime\prime}$65-radius apertures shown in Fig. \ref{['fig:apertures']}. The vertical axis for flux density is in units of $10^{-17}$ W m$^{-2}$$\mu$m$^{-1}$, and the horizontal wavelength axis in $\mu$m; the horizontal axis has been restricted to show only the relevant H$_2$ lines. The gray curves show the spectra, while the heavy colored lines show the smoothed continua; the zero level for each spectrum is given as a horizontal dotted colored line. For better visibility of the spectra, the spectra are offset by small increments (in units of $10^{-17}$ W m$^{-2}$$\mu$m$^{-1}$) as denoted in the figure. The vertical dotted lines correspond to the detected transitions given in Tables \ref{['tab:nwflux']} and \ref{['tab:seflux']}; to avoid overcrowding, not all lines are labeled. The spectra for VLA-NW-B and VLA-NW-C are missing the short-wavelength MIRI channels because part of the aperture falls outside the MIRI FoV at those wavelengths.
• Figure 3: Molecular hydrogen population diagrams for the apertures with 5 or more H$_2$ detections at a $3\sigma$ level or greater, together with the best-fit models with $T_u$ = 2000 K. Fits with OPR = 3 letting $T_l$ and $n$ vary are shown as blue curves, according to the MCMC sampler spread; the analogous fits letting OPR, $T_l$ and $n$ vary are shown as red curves (see Sect. \ref{['sec:opr']}). The total H$_2$ column density shown in the legends is given by Eq. \ref{['eqn:ntot']}.
• Figure 4: Corner plots for the ($T_l$, $n$) fits of the H$_2$ population diagrams with $T_u$ = 2000 K. For each region, the upper and right panels show the PDF, and the lower left panel the co-dependence of the two parameters. It can be seen that $T_l$ and $n$ are correlated, with an extension toward flatness particularly pronounced in JWST-SE-1 and VLA-NW-A.
• Figure 5: Corner plots for the ($T_l$, $n$, OPR) fits of the H$_2$ population diagrams with $T_u$ = 2000 K. As in Fig. \ref{['fig:corner']}, for each region, the top and right-most panels show the PDF, and the lower corner panels the co-dependence of the three parameters. As in Fig. \ref{['fig:corner']}, $T_l$ and $n$ are correlated, but here the OPR also is correlated with $T_l$ and $n$, with a tendency toward flatness (lower left sub-panels) in JWST-SE-3 and SE; these are also the regions where OPR $<\,3$.