Dr Rebecca Bowler

Abstract:

In the local universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H i), radio surveys to probe the cosmic evolution of H i in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H i survey, we report the first untargeted detection of an OHM at z > 0.5, LADUMA J033046.20-275518.1 (nicknamed "Nkalakatha"). The host system, WISEA J033046.26-275518.3, is an infrared-luminous radio galaxy whose optical redshift z ≈ 0.52 confirms the MeerKAT emission-line detection as OH at a redshift z OH = 0.5225 ± 0.0001 rather than H i at lower redshift. The detected spectral line has 18.4σ peak significance, a width of 459 ± 59 km s-1, and an integrated luminosity of (6.31 ± 0.18 [statistical] ± 0.31 [systematic]) × 103 L ⊙, placing it among the most luminous OHMs known. The galaxy's far-infrared luminosity L FIR = (1.576 ±0.013) × 1012 L ⊙ marks it as an ultraluminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step toward a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.

Abstract:

In this paper we present photometric redshifts for 2.7 million galaxies in the XMM-LSS and COSMOS fields, both with rich optical and near-infrared data from VISTA and HyperSuprimeCam. Both template fitting (using galaxy and Active Galactic Nuclei templates within LePhare) and machine learning (using GPz) methods are run on the aperture photometry of sources selected in the Ks-band. The resulting predictions are then combined using a Hierarchical Bayesian model, to produce consensus photometric redshift point estimates and probability distribution functions that outperform each method individually. Our point estimates have a root mean square error of ∼0.08 − 0.09, and an outlier fraction of ∼3 − 4 percent when compared to spectroscopic redshifts. We also compare our results to the COSMOS2020 photometric redshifts, which contains fewer sources, but had access to a larger number of bands and greater wavelength coverage, finding that comparable photo-z quality can be achieved (for bright and intermediate luminosity sources where a direct comparison can be made). Our resulting redshifts represent the most accurate set of photometric redshifts (for a catalogue this large) for these deep multi-square degree multi-wavelength fields to date.

Abstract:

We study the 3D axis of rotation (3D spin) of 77 Hi galaxies from the MIGHTEE-Hi Early Science observations, and its relation to the filaments of the cosmic web. For this Hi-selected sample, the alignment between the spin axis and the closest filament (|cos ψ|) is higher for galaxies closer to the filaments, with 〈|cos ψ|〉 = 0.66 ± 0.04 for galaxies <5 Mpc from their closest filament compared to 〈|cos ψ|〉 = 0.37 ± 0.08 for galaxies at 5 < d < 10 Mpc. We find that galaxies with a low Hi-to-stellar mass ratio (log10(MHi/M∗) < 0.11) are more aligned with their closest filaments, with 〈|cos ψ|〉 = 0.58 ± 0.04; whilst galaxies with (log10(MHi/M∗) > 0.11) tend to be mis-aligned, with 〈|cos ψ|〉 = 0.44 ± 0.04. We find tentative evidence that the spin axis of Hi-selected galaxies tend to be aligned with associated filaments (d < 10 Mpc), but this depends on the gas fractions. Galaxies that have accumulated more stellar mass compared to their gas mass tend towards stronger alignment. Our results suggest that those galaxies that have accrued high gas fraction with respect to their stellar mass may have had their spin axis alignment with the filament disrupted by a recent gas-rich merger, whereas the spin vector for those galaxies in which the neutral gas has not been strongly replenished through a recent merger tend to orientate towards alignment with the filament. We also investigate the spin transition between galaxies with a high Hi content and a low Hi content at a threshold of MHI ≈ 109.5 M⊙ found in simulations; however, we find no evidence for such a transition with the current data.