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Abstract:

The improved sensitivity of interferometric facilities to the 21 cm line of atomic hydrogen (H i) enables studies of its properties in galaxies beyond the local Universe. In this work, we perform a 21 cm line spectral stacking analysis combining the MeerKAT International GigaHertz Tiered Extragalactic Exploration and COSMOS H i Large Extra-galactic Survey surveys in the COSMOS field to derive a robust H i–stellar mass relation at z ≈ 0.36. In particular, by stacking thousands of star-forming galaxies subdivided into stellar mass bins, we optimize the signal-to-noise ratio of targets and derive mean H i masses in the different stellar mass intervals for the investigated galaxy population. We combine spectra from the two surveys, estimate H i masses, and derive the scaling relation log10MHI=(0.32±0.04)log10M⋆+(6.65±0.36) . Our findings indicate that galaxies at z ≈ 0.36 are H i richer than those at z ≈ 0 but H i poorer than those at z ≈ 1, with a slope consistent across redshift, suggesting that stellar mass does not significantly affect H i exchange mechanisms. We also observe a slower growth rate H i relative to the molecular gas, supporting the idea that the accretion of cold gas is slower than the rate of consumption of molecular gas to form stars. This study contributes to understanding the role of atomic gas in galaxy evolution and sets the stage for future development of the field in the upcoming Square Kilometre Array era.

Abstract:

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) polycyclic aromatic hydrocarbon (PAH) features in the central regions (∼0.26″ at 6 μm; ∼50–440 pc depending on the source) of local luminous IR galaxies. In this work, we examine the effect of nuclear obscuration on the PAH features of deeply obscured nuclei, predominantly found in local luminous IR galaxies, and we compare these nuclei with “normal” star-forming regions. We extend previous work to include shorter wavelength PAH ratios now available with the NIRSpec+MIRI/MRS spectral range. We introduce a new diagnostic diagram for selecting deeply obscured nuclei based on the 3.3 and 6.2 μm PAH features and/or mid-IR continuum ratios at ∼3 and 5 μm. We find that the PAH equivalent width ratio of the brightest PAH features at shorter wavelengths (at 3.3 and 6.2 μm) is impacted by nuclear obscuration. Although the sample of luminous IR galaxies used in this analysis is relatively small, we find that sources exhibiting a high silicate absorption feature cluster tightly in a specific region of the diagram, whereas star-forming regions experiencing lower extinction levels occupy a different area in the diagram. This demonstrates the potential of this technique to identify buried nuclei. To leverage the excellent sensitivity of the MIRI imager on board JWST, we extend our method of identifying deeply obscured nuclei at higher redshifts using a selection of MIRI filters. Specifically, the combination of various MIRI JWST filters enables the identification of buried sources beyond the local Universe and up to z ∼ 3, where other commonly used obscuration tracers such as the 9.7 μm silicate band, are out of the spectral range of MRS. Our results pave the way for identifying distant deeply obscured nuclei with JWST.