Rubin-LSST

Abstract:

We present optical and near-infrared (NIR) observations of the Type Ib supernova (SN) 2024rbc. Emission from the first CO overtone, resting on a dust continuum at 2.3−2.4 μm, was observed at 62 days postexplosion. The CO band heads are not resolved, and the emission appears broad, lacking sharp spectral features. This is the first observation of CO in the ejecta of a Type Ib SN reported in literature. The spectra of SN 2024rbc exhibit strong He I lines and numerous neutral and ionized metal lines. Comparing the spectral evolution of SN 2024rbc to other Type Ib, Ic, and IIb SNe indicates it is a Type Ib SN. We compare the velocities of key optical lines to examine the evolution of the ejecta. Additionally, fitting SN light-curve models of helium star progenitors computed with the STELLA code to photometric observations indicates a 56Ni mass of 0.07 M⊙ and an ejecta mass of 1.7 M⊙. Fitting an LTE model to the CO overtone implies a CO mass of 5.2 × 10−4 M⊙, a CO temperature of 4040 K, and a CO velocity width of 5905 km s−1. We also fitted a modified blackbody model to the dust continuum, deriving a dust temperature of 910 K and a dust mass of 1.3 × 10−3 M⊙. The CO provides direct evidence for the onset of dust formation, and the observed dust continuum likely originates from newly formed dust in the ejecta. However, robust dust mass estimates require mid-infrared (MIR) observations.

Abstract:

Subpixel centroiding is widely used to improve the spatial resolution of hybrid pixel detectors for electron imaging by estimating the true interaction position within an entry pixel. Existing centroiding strategies are typically optimised using localisation accuracy. However, observations show that improved localisation does not necessarily translate into improved modulation transfer function (MTF) or reliable virtual-pixel rebinning. This work proposes a unified interpretation of these observations based on the ambiguity of the centroid reconstruction problem. We show that observable-based centroid estimators solve an intrinsically non-unique inverse problem using deterministic reconstruction rules, which can introduce entry-phase-dependent reconstruction bias that governs the spatial distribution of reconstructed subpixel coordinates. This framework explains why localisation accuracy alone is insufficient to predict imaging performance and identifies approximate intra-pixel translational symmetry as a prerequisite for faithful virtual-pixel imaging. The proposed interpretation is evaluated using simulated 200 keV and 300 keV electron data together with measured 200 keV data acquired with a Timepix4 detector. Comparisons of charge-weighted, timing-based, and morphology-dependent centroiding strategies demonstrate that estimators with similar localisation performance can exhibit markedly different MTFs and rebinned flat-field behaviour. The results suggest that future centroid optimisation should consider subpixel phase bias and rebinnability alongside localisation accuracy.

Abstract:

NGC 1399, the bright central galaxy of the Fornax cluster, contains an extensive population of globular clusters (GCs) extending beyond 200 kpc into its outer halo, approximately 6.5 effective radii (Re). We conducted dynamical mass modelling of NGC 1399 out to 5 Re (∼150 kpc) using the most comprehensive radial velocity catalogue of GCs to date, which includes data from the Fornax cluster VLT Spectroscopic Survey (FVSS) and previous velocity measurements from the literature. Applying spherical Jeans equations, we performed dispersion-kurtosis modelling of NGC 1399’s GC kinematics to derive its mass profile and the orbital anisotropy of its GCs. We also investigated the impact of including intra-cluster GCs in the mass modelling, which were selected based on spatial segregation and a velocity σ-clipping method. Our findings indicate that both cusp-like (NFW) and core-like (Burkert) halos can reproduce the observed kinematics of GCs. Including the intra-cluster GCs in the mass modelling yields broadly consistent mass profiles for the two halo types, with the full GC sample giving a virial mass of log Mvir = 13.81 ± 0.09 M⊙ for the NFW halo, slightly higher than the value inferred for the Burkert halo. Regardless of the dark matter halo profile adopted, we observe that the intra-cluster GCs exhibit radial anisotropy in the outskirts, especially the blue, metal-poor GCs. These results suggest that GCs in NGC 1399’s outer halo are influenced by an additional halo component associated with the galaxy cluster potential. The radial anisotropy observed in these outer-halo GCs provides strong dynamical evidence that they were accreted from external sources, which is consistent with prior photometric and spectroscopic studies of GCs. This study highlights the importance of including intra-cluster GCs in dynamical mass modelling to fully account for the contribution of dark matter to the cluster gravitational potential.

Abstract:

Context. Measuring the expansion rate of the Universe is a central challenge in cosmology, particularly due to the persistent statistically significant discrepancy between the measurements of the Hubble constant, H 0 , obtained using early- and late-time probes, commonly known as the Hubble tension. Independent and robust methods are therefore essential to validate existing measurements and assess potential systematic effects. Aims. We employed the cosmic chronometer (CC) approach, which uses the differential age evolution of quiescent galaxies to measure the Hubble parameter H ( z ) without assuming an underlying cosmological model. In contrast to previous CC studies, we restricted our analysis to relics – ultra-compact massive galaxies (UCMGs) hosting the oldest stellar populations – thereby minimising uncertainties related to star formation history and merger history. Methods. We selected a sample of 189 relic galaxies in the redshift range 0.07 ≤ z ≤ 0.22 from the E-INSPIRE UCMG catalogue. Using the D n 4000 spectral index of relic galaxies and its redshift evolution, combined with MILES stellar population synthesis models, we derived the differential age relation required to infer H ( z ). We accounted not only for metallicity effects but also for the impact of α -element enhancement, which has not been explicitly propagated into the systematic uncertainty budget of the D n 4000 CC method. Results. We obtain an independent measurement of H ( z = 0.15) = 85 ± 53 km s −1 Mpc −1 . Although the total uncertainty remains large and dominated by statistical limitations, the systematic component is 13% (8.7% when α enrichment is not propagated), among the tightest estimates in current CC studies. We find that α enrichment plays a major role in the systematic error budget, particularly in the high-metallicity regime, and must be properly accounted for in future analyses. Conclusions. We demonstrate the proof of concept that relic galaxies provide a promising pathway to reduce systematic uncertainties in the CC method. The statistical uncertainties, which dominate the error budget, can be significantly reduced by the increase of data volume expected during the next years with Euclid , the Vera C. Rubin Observatory, DESI, and 4MOST.