Cosmology

Abstract:

We present the first measurement of HI mass of star-forming galaxies in different large scale structure environments from a blind survey at z ∼ 0.37. In particular, we carry out a spectral line stacking analysis considering 2875 spectra of colour-selected star-forming galaxies undetected in HI at 0.23 < z < 0.49 in the COSMOS field, extracted from the MIGHTEE-HI Early Science datacubes, acquired with the MeerKAT radio telescope. We stack galaxies belonging to different subsamples depending on three different definitions of large scale structure environment: local galaxy overdensity, position inside the host dark matter halo (central, satellite, or isolated), and cosmic web type (field, filament, or knot). We first stack the full star-forming galaxy sample and find a robust HI detection yielding an average galaxy HI mass of MHI = (8.12 ± 0.75) × 109 M⊙ at ∼11.8σ. Next, we investigate the different subsamples finding a negligible difference in MHI as a function of the galaxy overdensity. We report an HI excess compared to the full sample in satellite galaxies (MHI = (11.31 ± 1.22) × 109, at ∼10.2σ) and in filaments (MHI = (11.62 ± 0.90) × 109. Conversely, we report non-detections for the central and knot galaxies subsamples, which appear to be HI-deficient. We find the same qualitative results also when stacking in units of HI fraction (fHI). We conclude that the HI amount in star-forming galaxies at the studied redshifts correlates with the large scale structure environment.

Abstract:

Local and low-redshift (z<3) galaxies are known to broadly follow a bimodal distribution: actively star forming galaxies with relatively stable star-formation rates, and passive systems. These two populations are connected by galaxies in relatively slow transition. In contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems ^1-4: these galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence -so-called mini-quenching events ^5,6. However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation ^7,8. Early quenched galaxies have been identified out to redshift z < 5 [e.g. ^9-12], and these are all found to be massive (M_⋆ > 10¹⁰ M_⊙) and relatively old. Here we report a (mini-)quenched galaxy at z=7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue (UV =0.16 ± 0.03 mag), but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4-6×10⁸ M_⊙) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching.