MIGHTEE/COSMOS-3D: The discovery of three spectroscopically confirmed radio-selected star-forming galaxies at z=4.9-5.6

TL;DR

This work leverages deep MIGHTEE 1.3 GHz radio data in COSMOS, JWST COSMOS-Web WFSS spectroscopy, and extensive multi-wavelength photometry to identify and spectroscopically confirm three high-redshift radio sources powered by star formation at $z=4.9$--$5.6$. The sources have radio luminosities around a few ×$10^{24}$ W Hz$^{-1}$ and show star-formation–driven morphologies with SFRs ranging from roughly 100 to 1800 $M_yr^{-1}$, broadly consistent across UV/optical, Hα, and radio tracers. Their steep radio spectra are consistent with inverse-Compton losses against the CMB at high redshift, which can bias radio-only SFRs high if not accounted for. The results demonstrate the feasibility of detecting and characterizing star-formation-dominated radio emitters at $z>4.5$, providing a dust-unbiased view of early star formation and informing expectations from the evolving radio luminosity function.

Abstract

Radio observations offer a dust-independent probe of star formation and active galactic nucleus (AGN) activity, but sufficiently deep data are required to access the crossover luminosity between these processes at high redshift ($z>4.5$). We present three spectroscopically confirmed high-redshift radio sources (HzRSs) detected at 1.3 GHz at $z=4.9$-$5.6$, with radio luminosities spanning $L_{\rm 1.3 \, GHz}\approx2$-$5\times10^{24} \, \rm W \, Hz^{-1}$. These sources were first identified as high-redshift candidates through spectral energy distribution (SED) fitting of archival Hubble, JWST NIRCam+MIRI, and ground-based photometry, and then spectroscopically confirmed via the $\rm H\,α$ emission line using wide-field slitless spectroscopy from JWST COSMOS-3D. The star formation rates (SFRs) measured from SED fitting, the $\rm H\,α$ flux, and the 1.3 GHz luminosity, span $\sim100$-$1800\, M_{\odot} \, \rm yr^{-1}$, demonstrating broad agreement between these SFR tracers. We find that these three sources lie either on or $0.5$-1.0 dex above the star-forming main sequence at $z=4$-6 and have undergone a recent burst of star formation. The sources have extended rest-UV/optical morphologies with no evidence for a dominant point source component, indicating that an AGN is unlikely to dominate their rest-UV and optical emission. Two of the sources have complex, multi-component rest-frame UV/optical morphologies, suggesting that their starbursts may be triggered by merging activity. These HzRSs open up a new window towards probing radio emission powered by star formation alone at $z> 4.5$, representing a remarkable opportunity to begin tracing star formation, independent of dust, in the early Universe.

Figures (5)

• Figure 1: The radio luminosities of the spectroscopically confirmed HzRSs presented in this work, shown by the gold stars. We show the parent sample which overlaps with COSMOS-3D, determined from crossmatching MIGHTEE radio sources to sources with a photometric redshift $z_{\rm phot}>4.5$, by the smaller red stars. The coloured triangles represent AGN (blue) and SFGs (orange) from the MIGHTEE Early Science data Whittam22. The black points represent samples from the literature, compiled from Bornancini07Bryant09Jarvis09Saxena18_search, scaled to 1.3 GHz assuming a spectral index of $\alpha = 0.7$. The green square shows the highest-redshift confirmed radio loud AGN discovered by Endsley22.
• Figure 2: RGB images of the HzRSs presented in this work. The images are $3\times3$ arcsec, and are generated using F444W (red), F277W (green), and F150W (blue). A 5 kpc scale bar is shown for reference. We note that MGT J09594$+$02233 and MGT J10000$+$02225 are composed of multiple components, whereas MGT J10003$+$01573 shows a disk-like morphology with a possible bulge.
• Figure 3: SED fitting (top panels) and JWST grism spectroscopy (bottom panels) of the three HzRSs presented in this work. For each source, the top panel shows the best-fit SED from BAGPIPES when we fix the redshift to $z_{\rm spec}$. The black diamonds indicate the photometry from Hubble F814W, JWST NIRCam, and JWST MIRI. The grey points indicate the ground-based photometry from Subaru HSC and VISTA. Non-detections are replaced with arrows indicating $2\,\sigma$ upper limits. Open blue circles indicate the model photometry. The inset panel shows the redshift probability distribution, $P(z)$, when we place a flat prior on the redshift instead of fixing it to $z_{\rm spec}$. The purple dashed lines in the bottom panels indicate the position of detected emission lines matching the photometric redshift. We also mark the corresponding spectroscopic redshift on the $P(z)$ panel. We find excellent agreement between the photometric redshifts derived from using flat priors and the spectroscopic redshifts measured from the grism spectra for all three sources.
• Figure 4: JWST NIRCam postage stamp cutouts of the HzRSs presented in this work. For each source, the first four stamps are $3\times3$ arcsec. The final postage stamp cutout is $15\times15$ arcsec. In this final cutout the MIGHTEE continuum contours are overlaid. Contours are drawn at the 90th, 95th, 97.5th, and 99th percentiles, and at the maximum, of the radio cutout pixel values. The JWST images saturate at $2\,\sigma$ below and $8\,\sigma$ above the noise level. We show a 5 kpc scale bar in the bottom left, and the JWST PSF FWHM and MIGHTEE beam size in the bottom right. Although the radio contours for MGT J09594$+$02233 appear offset from the NIR position, the offset is $1$ arcsec, which is within the uncertainty of the radio centroid at the signal-to-noise ratio of the 1.3 GHz detection.
• Figure 5: A comparison of the SFRs and stellar masses of the sources presented in this work against the star-forming main sequence at $z\simeq4-6$, compiled from Khusanova21Clarke24DiCesare25 and shown by the dashed, solid, and dotted lines respectively. The shaded regions indicate the intrinsic scatter. SFRs derived from the $\mathrm{H}\,\alpha$ flux, SED fitting, and 1.3 GHz radio continuum (assuming Chabrier and Salpeter IMFs) are shown as red circles, blue squares, purple upward triangles, and yellow downward triangles, respectively. Each source is labelled by its abridged ID.