The Square Kilometre Array (SKA)

Abstract:

Type Ia supernova (SN Ia) standardisation parameters exhibit evidence for systematic variation across the host galaxy star-formation rate–stellar mass (SFR−M⋆) plane, motivating the incorporation of galaxy SFR information in cosmological inference. SFRs are commonly estimated via spectral energy distribution (SED) fitting with far-infrared (FIR) measurements to account for dust-obscured star formation. Such FIR coverage will, however, be limited for upcoming time-domain surveys such as the Rubin Observatory Legacy Survey of Space and Time (LSST), necessitating the use of alternative SFR tracers. Here, we reconstruct the SFR–M⋆ plane using 1.4 GHz diagnostics, to test the consistency of host classifications against FIR-constrained SED-based estimates. Within this plane, SN Ia host galaxies are divided into three regions: Region 1 (low-mass), Region 2 (high-mass star-forming) and Region 3 (high-mass passive). We find that ∼84 per cent of SN hosts retain identical region assignments when using radio versus FIR-constrained SED-derived SFRs. Measuring SN Ia nuisance parameters (α, β, M) within each subregion, we find consistent values between the two SFR–M⋆ plane reconstructions, indicating limited sensitivity to SFR estimator choice, with the largest deviations in Region 3 at ∼1.1σ. Across the three 1.4 GHz SFR–M⋆ subregions, we confirm the region-dependent variation in SN Ia standardisation parameters–particularly β–reported in our earlier SED-based analysis. With near-complete radio coverage of the LSST footprint anticipated from current and forthcoming radio continuum surveys (e.g., Square Kilometre Array), radio SFR calibrations will become an increasingly useful and scalable approach to host galaxy classification, supporting the construction of robust SN Ia subsamples for precision cosmology.

Abstract:

Stellar halos are the only major stellar component of disk galaxies that lack systematic observational characterization, yet they encode critical information about galaxy merger histories. We present the first systematic census of stellar halos in a large, flux-limited sample of 169 high-inclination central galaxies with stellar masses 7.3≤logM⋆/M⊙≤11.0 and redshift z < 0.1, using Hyper Suprime-Cam Subaru Strategic Survey Deep optical images. Stellar halos are detected in 93 galaxies, primarily through their low isophotal ellipticities in the outskirts, improving upon conventional methods of stellar halo identification. The halo detection rate reaches ∼50% at logM⋆/M⊙>9.9 and ≳70% for Milky Way (MW)-mass galaxies. We derive halo surface brightness profiles, colors, and masses, finding that stellar halos generally follow power-law radial profiles. Higher-mass galaxies, on average, exhibit smaller power-law indices and larger halo mass fractions, indicating more extended halos and more active merger histories. A significant stellar halo color–mass correlation, driven mainly by the mass–metallicity relation, suggests dominance by a few massive accretion events. MW-mass galaxies have a median stellar halo fraction of 10% ± 5%. Among nearby galaxies with halo measurements within 25 Mpc, two-thirds (including the MW) lie below the mean stellar halo fraction–galaxy mass relation. Overall, the nearby galaxies show a median halo deficit of ∼0.3 dex, implying unusually quiescent merger histories. We show that this deficit follows a broader trend in which typical halo fractions increase with heliocentric distance, tracking the gradual rise in matter density toward the cosmic average by z ≲ 0.07.

Abstract:

Context. Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; > 100 GeV) γ -ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole (SMBH; R S = 10 −6 pc for M SMBH = 10 7 M ⊙ ) to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data. Aims. The closest composite Seyfert–starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems. Methods. Data obtained in H.E.S.S. observations have been analyzed to search for VHE γ -ray counterparts to the GeV γ -ray signals detected with Fermi -LAT and for potential spectral components in the VHE range. Results. No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE γ -ray fluxes were applied to constrain theoretical models involving different spectral components.

Abstract:

Relativistic jets from supermassive black holes in active galactic nuclei are amongst the most powerful phenomena in the universe. Similar jets from stellar-mass black holes offer a chance to study the phenomena on accessible observation time scales. However, such comparative studies across black hole masses and time scales remain hampered by the long-standing perception that stellar-mass black hole jets are in a less relativistic regime. Here, we show the detection of two distinct, relativistic jet ejections from the Galactic black hole X-ray binary 4U 1543-47 during a single outburst, with radio interferometry monitoring observations. Our measurements reveal a likely Lorentz factor of approximately 8 and a minimum of 4.6 at launch with 95% confidence, demonstrating that stellar-mass black holes in X-ray binaries can launch jets as relativistic as those seen in active galactic nuclei.

Abstract:

Abstract The interplay between atomic gas and the star-formation history of a galaxy are intrinsically linked, and we need to decouple these dependencies to understand their role in galaxy formation and evolution. In this paper, we analyse the star formation histories (SFHs) of 203 galaxies from the MIGHTEE-Hi Survey Early Science Release data, crossmatched to with multi-wavelength photometry across the COSMOS and XMM-LSS fields. We focus on the relationships between Hi properties and star formation, with a sample which primarily traces gas-rich, star-forming systems at low redshift, extending to low stellar masses and probing regimes that are difficult to access with optically-selected samples. A strong correlation emerges between a galaxy’s Hi-to-stellar mass ratio and the time of formation, alongside an inverse correlation between stellar mass and time of formation, regardless of the inferred SFH. Additionally, galaxies with lower stellar masses and higher Hi-to-stellar mass ratios exhibit longer gas depletion times compared to more massive galaxies, which appear to have depleted their gas and formed stars more efficiently. This suggests that smaller, gas-rich galaxies have higher depletion times due to shallower potential wells and less efficient star formation. Within this Hi-selected sample, the efficiency of star formation is regulated primarily by stellar mass and gas fraction, with low-mass galaxies retaining extended atomic reservoirs due to inefficient conversion of Hi into stars.