JWST View of the Supernebula in NGC 5253. I. Overview and Continuum Features

TL;DR

JWST's MIRI-MRS provides subarcsecond mid-infrared view into NGC 5253's central starburst and its pc-scale supernebula, isolating embedded clusters and surrounding HII regions within ~100 pc. The study maps continuum emission, measures solid-state features, and compares mid-infrared to radio luminosities to infer dust heating and photon escape, deriving $\log(Q_0) \approx 52.7$ for the central source. It finds four main MIR continuum sources (D1–D6 plus extended regions) with D1 dominating, PAHs present but weak, and the 9.7 μm silicate feature varying between absorption in D1 and emission in D2, reflecting extreme radiation and low metallicity. The results reveal substantial UV photon leakage (at least $f_{esc} \sim 0.25$) and a porous central region, providing a framework for future JWST studies of line emission and cluster formation in dusty starbursts.

Abstract

We present imaging spectroscopy of the "supernebula" in the nearby dwarf galaxy NGC 5253 with the MIRI-MRS integral field spectrometer of the JWST. NGC 5253 is host to an luminous ($L\sim 10^9~\rm L_\odot$) HII region, powered by a giant young star cluster, a possible local analogue to super star cluster formation at Cosmic Dawn and Noon. In this paper, the first in a series about the mid-infrared line and continuum emission in the center of NGC 5253, we present an overview and continuum spectra. The mid-infrared images reveal four sources of continuum emission from hot dust that we identify as luminous HII regions, which are used to define spectral apertures. The dominant continuum source is the pc-scale supernebula core seen at radio wavelengths. We find that the MIR to radio continuum flux ratio for all regions is identical to that of Galactic HII regions. The 9.7 silicate feature is present and strongest in absorption toward the supernebula. Silicate emission is seen in another HII region. PAH features are present, although weak, particularly in the supernebula; the strongest emission is in an HII region only 15 pc from the supernebula core. PAH features at 6.2$μ$m and 11.3$μ$m are detected in all sources. Comparison of the luminosity implied by the ionization to the observed infrared luminosity suggest that at least 25% ofthe photons are escaping the embedded supernebula core, in spite of its high, $A_V\gtrsim 15$, extinction.

Figures (9)

• Figure 1: Footprints for the JWST MIRI/MRS cubes for the science (left) and background (right) observations. The science footprint is centered on the bright radio "supernebula" near the brightest visual source. Footprints are shown for the four channels: Bands 1-S (4.95$\mu$m; blue), 2-L (near [SIV]10.51; green), 3-L (near [Ne III]15.55; orange) and 4-L (26.0; red). The southeastern corner of the background has continuum and weak line emission beyond 12 in Channels 3 and 4.
• Figure 2: Custom background spectrum, computed from clean regions within the dedicated background. Mean intensity levels correspond to fluxes of $\sim 50$mJy for $\lambda < 20$, rising to $\sim$ 0.5 Jy at 25, for the region corresponding to the footprint at 5. For the 08 apertures used for spectral extraction, background fluxes range from 2-10 mJy from 5-28. Spectral lines were removed by boxcar averaging and the spectrum was loess-smoothed (black line) prior to subtraction from the science cube.
• Figure 3: HST-ACS F555M image (Project 10765, P.I. Zezas) with ALMA 1.3mm continuum (Project 2017.1.00964.S, P.I. Nguyen) contours. The 1.3mm continuum is dominated by free-free emission at this resolution. Cluster 5 calzetti1997 is the brightest visual source, located $\sim 0.25$-0.3 to the southeast of the radio supernebula; Cluster 11 is faint and visible near the center smith2020 Registration of the HST image to ICRS coordinates was done by aligning nearby Gaia stars cohen2018smith2020
• Figure 4: MIRI-MRS continuum images. (Top left): 4.95-5.0. (Top right): 10.6-10.8. (Bottom left): 14.7-15.0. (Bottom right): 23.8-24.3. Contours in this panel are 03 resolution 870 ALMA continuum turner2017consiglio2017.
• Figure 5: JWST 7 continuum contours, with levels of 400, 600, plus 2$^{(n-1)}$x1000 MJy/sr. Peak $1.24 \times 10^5$ MJy/sr. These are overlaid on: (Top left): HST/ACS F814W; (Top right): HST/ACS F656N ("H$\alpha$"); (Bottom left): ALMA 870$\mu$m continuum; (Bottom right): ALMA CO(3-2) integrated intensity.