MIGHTEE-HI: The direct detection of neutral hydrogen in galaxies at $z>0.25$

TL;DR

The paper demonstrates direct detections of neutral hydrogen in galaxies at $z>0.25$ using interferometric HI spectroscopy from MIGHTEE, targeting DESI redshifts to identify 11 HI galaxies in the range $0.25<z<0.4$ (up to $z=0.3841$). It combines HI measurements with comprehensive SED fits to derive stellar masses and SFRs, placing the hosts on/above the $z\sim0.35$ star-forming main sequence and revealing HI masses larger than local HI-selected counterparts, largely due to the larger survey volume rather than evolution of the HI mass function. By analyzing $W_{50}$ as a proxy for rotation and deriving the baryonic mass with HI plus a helium/molecular gas correction, the study places these galaxies on the local baryonic Tully-Fisher relation with hints of a high-mass flattening, consistent with simulations and possible molecular-mass contributions. The work showcases the feasibility of HI studies beyond the local Universe with MeerKAT and sets the stage for deeper, untargeted HI surveys (and LADUMA/MIGHTEE synergy) to constrain the HI mass function and baryonic scaling relations out to $z\sim0.5$.

Abstract

Atomic hydrogen constitutes the gas reservoir from which molecular gas and star formation in galaxies emerges. However, the weakness of the line means it has been difficult to directly detect in all but the very local Universe. Here we present results from the first search using the MeerKAT International Tiered Extragalactic Exploration (MIGHTEE) Survey for high-redshift ($z>0.25$) H{\sc i} emission from individual galaxies. By searching for 21-cm emission centered on the position and redshift of optically-selected emission-line galaxies we overcome difficulties that hinder untargeted searches. We detect 11 galaxies at $z>0.25$, forming the first sample of $z>0.25$ detections with an interferometer, with the highest redshift detection at $z = 0.3841$. We find they have much larger H{\sc i} masses than their low-redshift H{\sc i}-selected counterparts for a given stellar mass. This can be explained by the much larger cosmological volume probed at these high redshifts, and does not require any evolution of the H{\sc i} mass function. We make the first-ever measurement of the baryonic Tully-Fisher relation (bTFr) with H{\sc i} at $z>0.25$ and find consistency with the local bTFr, but with tentative evidence of a flattening in the relation at these redshifts for higher-mass objects. This may signify evolution, in line with predictions from hydrodynamic simulations, or that the molecular gas mass in these high-mass galaxies could be significant. This study paves the way for future studies of H{\sc i} beyond the local Universe, using both searches targeted at known objects and via pure H{\sc i} selection.

Figures (15)

• Figure 1: Histogram of the redshift distribution of the parent DESI redshift catalogue over the MIGHTEE-H i DR1 area for the redshift range $0.23 < z< 0.5$, corresponding to the MIGHTEE L1 spectral window.
• Figure 2: $80\times 80$ arcsec$^2$ moment-0 maps ( left panels) and the 1-D spectra ( right panels) extracted around the optical redshift, denoted by the vertical dashed line. The solid contour levels shown in the moment-0 maps denote the 2, 3, 4, 5 and 6$\sigma$ levels and dashed contours are the negatives of these. The dashed circles represent the FWHM of the synthesised beam of the MIGHTEE L1 data. The part of the 1-D spectrum coloured dark blue is the spectral range used for measuring the SNR and also that used for creating the moment-0 map. The contour levels in the moment-0 maps and the uncertainty on the H i masses given in the legend of the spectra are determined by measuring the standard deviation in $\sim 500$ spatial and spectral apertures, as described in Sec. \ref{['sec:search']}. The shaded light blue regions in the 1-D spectra show the uncertainty measured in each channel from placing apertures in the 3-D data cube and measuring 16th and 84th percentiles from the distribution of measured fluxes in each channel. The dashed horizontal magenta line denotes the zero flux-density. We note the noise in ID11 is larger than the other objects due to this object lying towards the edge of the MIGHTEE COSMOS coverage.
• Figure 3: Redshift versus H i-mass for the final sample of H i detected galaxies presented in this paper (blue circles). Also shown are the H i-detected galaxies from the MIGHTEE ES data release (pink triangles). The gap between $0.1<z<0.23$ corresponds to the spectral region with significant radio-frequency interference Heywood2024.
• Figure 4: HSC $gri$ images (left panels) and the best fit SEDs to the multi-wavelength photometry using bagpipes for IDs 1--10 (right panels). The dashed circles in the images represent the FWHM of the synthesised beam of the MIGHTEE L1 data. The red circles in the SED panels denote the data used for fitting the SEDS (black line) and the blue points and error bars represent the 50th, 16th and 84th percentiles of the posterior distributions output from bagpipes. We only show the SEDs to $10\mu$m for clarity.
• Figure 5: legacy survey $gri$ image of the two potential galaxies associated with ID11. The large galaxy in the centre of the image is ID11a in Table \ref{['tab:sample']} and the smaller galaxy to the south-east is ID11b. The dashed circle represents the FWHM of the synthesised beam of the MIGHTEE L1 data.