Hintze Centre for Astrophysical Surveys

Abstract:

We present the Virtual Research Assistant (VRA) of the ATLAS sky survey, which performs preliminary eyeballing on our clean transient data stream. The VRA uses histogram-based gradient-boosted decision tree classifiers trained on real data to score incoming alerts on two axes: “Real” and “Galactic.” The alerts are then ranked using a geometric distance such that the most “real” and “extragalactic” receive high scores; the scores are updated when new lightcurve data is obtained on subsequent visits. To assess the quality of the training we use the recall at rank K, which is more informative to our science goal than general metrics (e.g., accuracy, F1-scores). We also establish benchmarks for our metric based on the pre-VRA eyeballing strategy, to ensure our models provide notable improvements before being added to the ATLAS pipeline. Then, policies are defined on the ranked list to select the most promising alerts for humans to eyeball and to automatically remove bogus alerts. In production the VRA method has resulted in a reduction in eyeballing workload by 85% with a loss of follow-up opportunity <0.08%. It also allows us to automatically trigger follow-up observations with the Lesedi telescope, paving the way toward automated methods that will be required in the era of LSST. Finally, this is a demonstration that feature-based methods remain extremely relevant in our field, being trainable on only a few thousand samples and highly interpretable; they also offer a direct way to inject expertise into models through feature engineering.

Abstract:

Radio continuum emission provides a unique opportunity to study star formation unbiased by dust obscuration. However, if radio observations are to be used to accurately trace star formation to high redshifts, it is crucial that the physical processes that affect the radio emission from star-forming galaxies are well understood. While inverse Compton (IC) losses from the cosmic microwave background (CMB) are negligible in the local universe, the rapid increase in the strength of the CMB energy density with redshift [] means that this effect becomes increasingly important at . Using a sample of high-redshift () Lyman-break galaxies selected in the rest-frame ultraviolet (UV), we have stacked radio observations from the MIGHTEE survey to estimate their 1.4-GHz flux densities. We find that for a given rest-frame UV magnitude, the 1.4-GHz flux density and luminosity decrease with redshift. We compare these results to the theoretical predicted effect of energy losses due to IC scattering off the CMB, and find that the observed decrease is consistent with this explanation. We discuss other possible causes for the observed decrease in radio flux density with redshift at a given UV magnitude, such as a top-heavy initial mass function at high redshift or an evolution of the dust properties, but suggest that IC scattering is the most compelling explanation.

Abstract:

We present a dynamical and chemical study of the centre of a massive early-type strong-lens galaxy ESO 286−G022 (SNL-1). Analysing new data obtained through the adaptive-optics-assisted Narrow-Field Mode of Very Large Telescope/Multi-Unit Spectroscopic Explorer, we aim to measure the mass distribution and internal properties of SNL-1 at resolution. In particular, we aim to address the tension in the reported initial mass function (IMF) measurements of SNL-1 between strong-lens/dynamical and spectral-fitting techniques. We fit a triaxial orbital dynamical model to the measured stellar kinematics, including constraining the mass of the (resolved) central supermassive black hole. The dynamical model is consistent with the mass-to-light ratio expected for a Kroupa-like IMF. We also employ a highly flexible spectral-fitting technique, which instead favours a Salpeter-like IMF (low-mass slope ) over the same spatial region. To conclude, we discuss possible origins of this discrepancy, both intrinsic and technical.

Abstract:

We discuss the results of the spectroscopic and photometric monitoring of the type IIn supernova (SN) 2023ldh. Survey archive data show that the SN progenitor experienced erratic variability in the years before exploding. Beginning May 2023, the source showed a general slow luminosity rise that lasted for over four months, with some superposed luminosity fluctuations. In analogy to SN 2009ip , we call this brightening ‘Event A’. During Event A, SN 2023ldh reached a maximum absolute magnitude of M r = −15.52 ± 0.24 mag. The light curves then decreased by about 1 mag in all filters for about two weeks reaching a relative minimum, which was followed by a steep brightening (Event B) to an absolute peak magnitude of M r = −18.53 ± 0.23 mag, replicating the evolution of SN 2009ip and similar to that of type IIn SNe. The three spectra of SN 2023ldh obtained during Event A show multi-component P Cygni profiles of H I and Fe II lines. During the rise to the Event B peak, the spectrum shows a blue continuum dominated by Balmer lines in emission with Lorentzian profiles, with a full width at half maximum velocity of about 650 km s −1 . Later, in the post-peak phase, the spectrum reddens, and broader wings appear in the H α line profile. Metal lines with P Cygni profiles and velocities of about 2000 km s −1 are clearly visible. Beginning around three months past maximum and until very late phases, the Ca II lines become among the most prominent features, while H α is dominated by an intermediate-width component with a boxy profile. Although SN 2023ldh mimics the evolution of other SN 2009ip -like transients, it is slightly more luminous and has a slower photometric evolution. The surprisingly homogeneous observational properties of SN 2009ip -like events may indicate similar explosion scenarios and similar progenitor parameters.