Dr Rebecca Bowler

Abstract:

In this paper, we combine the Early Science radio continuum data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey, with optical and near-infrared data and release the cross-matched catalogues. The radio data used in this work covers 0.86 deg² of the COSMOS field, reaches a thermal noise of 1.7 μJy beam⁻¹ and contains 6102 radio components. We visually inspect and cross-match the radio sample with optical and near-infrared data from the Hyper Suprime-Cam (HSC) and UltraVISTA surveys. This allows the properties of active galactic nuclei and star-forming populations of galaxies to be probed out to z ≈ 5. Additionally, we use the likelihood ratio method to automatically cross-match the radio and optical catalogues and compare this to the visually cross-matched catalogue. We find that 94 per cent of our radio source catalogue can be matched with this method, with a reliability of 95 per cent. We proceed to show that visual classification will still remain an essential process for the cross-matching of complex and extended radio sources. In the near future, the MIGHTEE survey will be expanded in area to cover a total of ∼20 deg²; thus the combination of automated and visual identification will be critical. We compare the redshift distribution of SFG and AGN to the SKADS and T-RECS simulations and find more AGN than predicted at z ∼ 1.

Abstract:

We study the M_HI−M_⋆ relation over the last billion years using the MIGHTEE-H i sample. We first model the upper envelope of the M_HI−M_⋆ relation with a Bayesian technique applied to a total number of 249 H i-selected galaxies, without binning the datasets, while taking account of the intrinsic scatter. We fit the envelope with both linear and non-linear models, and find that the non-linear model is preferred over the linear one with a measured transition stellar mass of log10 (M_⋆M_⊙) = 9.15±0.87, beyond which the slope flattens. This finding supports the view that the lack of H i gas is ultimately responsible for the decreasing star formation rate observed in the massive main-sequence galaxies. For spirals alone, which are biased towards the massive galaxies in our sample, the slope beyond the transition mass is shallower than for the full sample, indicative of distinct gas processes ongoing for the spirals/high-mass galaxies from other types with lower stellar masses. We then create mock catalogues for the MIGHTEE-H i detections and non-detections with two main galaxy populations of late- and early-type galaxies to measure the underlying M_HI−M_⋆ relation. We find that the turnover in this relation persists whether considering the two galaxy populations as a whole or separately. We note that an underlying linear relation could mimic this turnover in the observed scaling relation, but a model with a turnover is strongly preferred. Measurements on the logarithmic average of H i masses against the stellar mass are provided as a benchmark for future studies.

Abstract:

We have conducted a search for z ≃ 7 Lyman-break galaxies over 8.2 deg² of near-infrared imaging from the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey in the XMM–Newton-Large Scale Structure (XMM-LSS) and the Extended Chandra Deep Field-South (ECDF-S) fields. Candidate galaxies were selected from a full photometric redshift analysis down to a Y + J depth of 25.3 (5σ), utilizing deep auxiliary optical and Spitzer/Infrared Array Camera (IRAC) data to remove brown dwarf and red interloper galaxy contaminants. Our final sample consists of 28 candidate galaxies at 6.5 ≤ z ≤ 7.5 with −23.5 ≤ M_UV ≤ −21.6. We derive stellar masses of 9.1 ≤ log₁₀(M_⋆/M_⊙) ≤ 10.9 for the sample, suggesting that these candidates represent some of the most massive galaxies known at this epoch. We measure the rest-frame ultraviolet (UV) luminosity function (LF) at z ≃ 7, confirming previous findings of a gradual decline in number density at the bright end (M_UV < −22) that is well described by a double power law (DPL). We show that quasar contamination in this magnitude range is expected to be minimal, in contrast to conclusions from recent pure-parallel Hubble studies. Our results are up to a factor of 10 lower than previous determinations from optical-only ground-based studies at M_UV ≲ −23. We find that the inclusion of YJHK_s photometry is vital for removing brown dwarf contaminants, and z ≃ 7 samples based on red optical data alone could be highly contaminated (≳50 per cent). In comparison with other robust z > 5 samples, our results further support little evolution in the very bright end of the rest-frame UV LF from z = 5–10, potentially signalling a lack of mass quenching and/or dust obscuration in the most massive galaxies in the first Gyr.