Polar Ring Galaxies (PRGs) are a unique class of galaxies characterised by a ring of gas and stars orbiting nearly orthogonal to the main body. This study delves into the evolutionary trajectory of PRGs using the exemplary trio of NGC 3718, NGC 2685, and NGC 4262. We investigate the distinct features of PRGs by analysing their ring and host components to reveal their unique characteristics through Spectral Energy Distribution (SED) fitting. Using CIGALE, we performed SED fitting to independently analyse the ring and host spatially resolved regions, marking the first decomposed SED analysis for PRGs, which examines stellar populations using high-resolution observations from AstroSat UVIT at a resolved scale. The UV-optical surface profiles provide an initial idea that distinct patterns in the galaxies, with differences in FUV and NUV, suggest three distinct stages of ring evolution in the selected galaxies. The study of resolved-scale stellar regions reveals that the ring regions are generally younger than their host galaxies, with the age disparity progressively decreasing along the evolutionary sequence from NGC 3718 to NGC 4262. Star formation rates (SFR) also exhibit a consistent pattern, with higher SFR in the ring of NGC 3718 compared to the others, and a progressive decrease through NGC 2685 and NGC 4262. Finally, the representation of the galaxies in the HI gas fraction versus the NUV–r plane supports the idea that they are in three different evolutionary stages of PRG evolution, with NGC 3718 in the initial stage, NGC 2685 in the intermediate stage, and NGC 4262 representing the final stage. This study concludes that PRGs undergo various evolutionary stages, as evidenced by the observed features in the ring and host components. NGC 3718, NGC 2685, and NGC 4262 represent different stages of this evolution, highlighting the dynamic nature of PRGs and emphasising the importance of studying their evolutionary processes to gain insights into galactic formation and evolution.

The effects of diffraction, reflection and mutual coupling on the spectral smoothness of radio telescopes becomes increasingly important at low frequencies, where the observing wavelength may be significant compared with the antenna or array dimensions. These effects are important for both traditional parabolic antennas, which are prone to the ‘standing wave’ phenomenon caused by interference between direct and scattered wavefronts, and aperture arrays, such as the SKA-Low, MWA, HERA, and LOFAR which have more complicated scattering geometries and added dependence on pointing direction (scan angle). Electromagnetic modelling of these effects is computationally intensive and often only possible at coarse frequency resolution. Therefore, using the example of SKA-Low station configurations, we investigate the feasibility of parameterising scattering matrices, and separating antenna and array contributions to telescope chromaticity. This allows deeper insights into the effect on spectral smoothness and frequency-dependent beam patterns of differing antenna configurations. Even for the complicated SKA-Low element design, band-limited delay-space techniques appear to produce similar results to brute-force electromagnetic models, and allow for faster computation of station beam hypercubes (position, frequency and polarisation-dependent point spread functions) at arbitrary spectral resolution. As such techniques could facilitate improvements in the design of low-frequency spectral-line surveys, we conduct a simulated Cosmic Dawn experiment using different observing techniques and station configurations.

Interactions play a significant role in the formation and evolution of galaxies in the Universe. The galaxy systems, NGC 7252 and NGC 5291 are two nearby interacting systems that are hosting Tidal Dwarf Galaxies (TDGs) and star-forming knots. The present work aims (a) To determine the attenuation-corrected star formation rate (SFR) of the interacting system NGC 7252 (b) To compare the star formation in the NGC 7252 system with that of the NGC 5291 system (c) To explore the relation between surface densities of gas and SFR in these two systems. The study utilises high-resolution FUV and NUV imaging data from the Ultraviolet Imaging Telescope (UVIT) on board AstroSat. Six star-forming regions, including the merger remnant, were identified in the NGC 7252 system. The SFR corrected for attenuation of the knots in the NGC 7252 system is determined using the continuum slope β calculated from the FUV-NUV colour. It has been observed that the attenuation-corrected SFR values of the knots in this system fall within the range of SFR values determined for the NGC 5291 knots. The TDGs in both systems adhere to the same Kennicutt-Schmidt (KS) relation as regular spiral galaxies.

Obscuration in active galactic nuclei (AGN) provides valuable insights into the nature of the material surrounding the central engine. Comptonthick AGN (CTAGN), characterised by a column density of NH ≥ 1.5 × 1024 cm−2, are heavily obscured by substantial amounts of dust and gas. While X-ray observations are primarily used to determine this column density, our understanding of obscuration properties in the sub-mm regime, particularly for CTAGN, remains limited. In this study, we analyse archival data from the Atacama Large Millimetre/sub-millimetre Array (ALMA) for both CTAGN and non-CTAGN sources, as identified by the 70-month catalogue of the all-sky hard X-ray Swift/Burst Alert Monitor survey and other X-ray surveys. Integrated intensity maps (moment 0) of CO(3–2) emission reveal a concentrated distribution of dense gas around the nucleus. Utilising a constant CO-to-H2 conversion factor, XCO = 2.2 × 1020 cm−2 (K km s−1)−1, we find that the derived molecular hydrogen column densities, NH2, are generally lower than the total hydrogen column densities, NH, obtained from X-ray observations. However, the NH2 values derived in this work are slightly higher than those reported in previous studies due to the adoption of a higher CO-to-H2 conversion factor. This discrepancy between NH and NH2 is consistent with prior findings that X-ray-derived column densities are typically higher, except in the case of non-CTAGN, where NH2 can exceed NH. Statistical analysis using Kendall and Spearman tests reveals a positive monotonic relationship between NH and NH2, although the correlation is not statistically significant. This suggests a complex interplay of factors influencing these properties. The optically thick nature of CO in dense regions may contribute to the observed discrepancies. Our results highlight the importance of adopting an accurate CO-to-H2 conversion factor to derive reliable column densities, which could serve as an alternative method for identifying CTAGN. Further investigations with more comprehensive data sets and refined methodologies are needed to better understand the relationship between sub-millimetre and X-ray properties in AGNs.

AstroSat observed transient neutron star low-mass X-ray binary XTE J1701-462 for a total duration of ∼ 135 ks during its 2022 outburst. We report the results of a detailed spectral and timing analysis carried out using this data. The source traced a complete ‘Z’ shaped structure in the hardness intensity diagram (HID). The source exhibited an extended horizontal branch and a short-dipping flaring branch in the HID. The spectra of the source were fitted with different approaches. We find that most suitable spectral model comprises emission from a standard multi-color accretion disk (diskbb in XSPEC) and Comptonized radiation from a hot central corona, described by Comptb model of XSPEC. The observed disk component is cool, having a temperature in the range of ∼ 0.28 – 0.42 keV and truncated far (∼ 250 - 1600 km) from the compact object. The Compton corona has an optical depth in the range of ∼ 3.4 – 5.1 and a temperature in the range of 3.3 – 4.5 keV. The disk and corona flux as well as truncation radius vary significantly along the HID. The temperature kTin depends on both luminosity and inner disk radius and hence shows marginal variation as compared to the truncation radius. We discuss possible scenarios to explain the relationship between the spectral evolution and motion of the source along the HID. The timing analysis revealed horizontal branch oscillations (HBOs) in the frequency range ∼ 34 – 40 Hz. The frequency and rms strength of HBO vary systematically as the source moves along the horizontal branch (HB). The observed correlation of the HBO properties with the position on the HB is similar to that previously reported in this source using RXTE data during the 2006 outburst of the source. The source also showed normal branch oscillations (NBOs) with frequency ∼ 6.7 Hz in the middle and the lower normal branch. The energy-dependent study of the HBO properties suggests that the HBO is stronger in the higher energy band. We also observed very-low frequency noise (VLFN) and band-limited noise (BLN) components in the power density spectra. The break frequency of BLN component was found to be tightly correlated with the HBO frequency. We discuss possible models to explain the origin and nature of the observed features in the PDS.

The cosmic 21 cm signal serves as a crucial probe for studying the evolutionary history of the Universe. However, detecting the 21 cm signal poses significant challenges due to its extremely faint nature. To mitigate the interference from the Earth’s radio frequency interference (RFI), the ground and the ionospheric effects, the Discovering the Sky at the Longest Wavelength (DSL) project will deploy a constellation of satellites in Lunar orbit, with its high-frequency daughter satellite tasked with detecting the global 21 cm signal from cosmic dawn and reionization era (CD/EoR).We intend to employ the Vari-Zeroth-Order Polynomial (VZOP) for foreground fitting and subtracting. We have studied the effect of thermal noise, thermal radiation from the Moon, the Lunar reflection, anisotropic frequency-dependent beam, inaccurate antenna beam pattern, and RFI contamination. We discovered that the RFI contamination can significantly affect the fitting process and thus prevent us from detecting the signal. Therefore, experimenting on the far side of the moon is crucial. We also discovered that using VZOP together with DSL, after 1080 orbits around the Moon, which takes about 103 days, we can successfully detect the CD/EoR 21 cm signal.

This work presents visual morphological and dynamical classifications for 637 spatially resolved galaxies, most of which are at intermediate redshift (z ∼ 0.3), in the Middle-Ages Galaxy Properties with Integral field spectroscopy (MAGPI) Survey. For each galaxy, we obtain a minimum of 11 independent visual classifications by knowledgeable classifiers. We use an extension of the standard Dawid-Skene Bayesian model introducing classifier-specific confidence parameters and galaxy-specific difficulty parameters to quantify classifier confidence and infer reliable statistical confidence estimates. Selecting sub-samples of 86 bright (r < 20 mag) high-confidence (> 0.98) morphological classifications at redshifts (0.2 ≤ z ≤ 0.4), we confirm the full range of morphological types is represented in MAGPI as intended in the survey design. Similarly, with a sub-sample of 82 bright high-confidence stellar kinematic classifications, we find that the rotating and non-rotating galaxies seen at low redshift are already in place at intermediate redshifts. We do not find evidence that the kinematic morphology-density relation seen at z ∼ 0 is established at z ∼ 0.3. We suggest that galaxies without obvious stellar rotation are dynamically pre-processed sometime before z ∼ 0.3 within lower mass groups before joining denser environments.

Current and future surveys rely on machine learning classification to obtain large and complete samples of transients. Many of these algorithms are restricted by training samples that contain a limited number of spectroscopically confirmed events. Here, we present the first real-time application of Active Learning to optimise spectroscopic follow-up with the goal of improving training sets of early type Ia supernovae (SNe Ia) classifiers.

Using a photometric classifier for early SN Ia, we apply an Active Learning strategy for follow-up optimisation using the real-time FINK broker processing of the ZTF public stream. We perform follow-up observations at the ANU 2.3m telescope in Australia and obtain 92 spectroscopic classified events that are incorporated in our training set.

We show that our follow-up strategy yields a training set that, with 25% less spectra, improves classification metrics when compared to publicly reported spectra. Our strategy selects in average fainter events and, not only supernovae types, but also microlensing events and flaring stars which are usually not incorporated on training sets.

Our results confirm the effectiveness of active learning strategies to construct optimal training samples for astronomical classifiers. With the Rubin Observatory LSST soon online, we propose improvements to obtain earlier candidates and optimise follow-up. This work paves the way to the deployment of real-time AL follow-up strategies in the era of large surveys.

We present a chemo-dynamical study conducted with 2dF + $$+$$ AAOmega of \sim 6000 $$\sim 6000$$ Gaia DR3 non-variable candidate metal-poor stars that lie in the direction of the Galactic plane. Our spectral analysis reveals 15 new extremely metal-poor (EMP) stars, with the lowest metallicity at \left[ {{\rm{Fe/H}}} \right] = - 4.0 \pm 0.2 $$\left[ {{\rm{Fe/H}}} \right] = - 4.0 \pm 0.2$$ dex. Two of the EMP stars are also carbon enhanced, with the largest enhancement of \left[ {{\rm{C/Fe}}} \right] = 1.3 \pm 0.1 $$\left[ {{\rm{C/Fe}}} \right] = 1.3 \pm 0.1$$ occurring in a dwarf. Using our \left[ {{\rm{C/Fe}}} \right] $$\left[ {{\rm{C/Fe}}} \right]$$ results, we demonstrate that the number of carbon-depleted stars decreases with lower metallicities, and the fraction of carbon-enhanced stars increases, in agreement with previous studies.

Our dynamical analysis reveals that the fraction of prograde and retrograde disk stars, defined as {z_{{\rm{max}}}} \lt 3 $${z_{{\rm{max}}}} \lt 3$$ kpc, with {J_\phi }/{J_{{\rm{tot}}}} \gt 0.75 $${J_\phi }/{J_{{\rm{tot}}}} \gt 0.75$$ and {J_\phi }/{J_{{\rm{tot}}}} \lt - 0.75 $${J_\phi }/{J_{{\rm{tot}}}} \lt - 0.75$$ respectively, changes as metallicities decrease. Disk stars on retrograde orbits make up \sim 10 $$\sim 10$$ % of all the stars in our sample with metallicities below - 2.1 $$- 2.1$$ dex. Interestingly, the portion of retrograde disk stars compared with the number of kinematically classified halo stars is approximately constant at 4.6 $$4.6$$ % for all metallicities below - 1.5 $$- 1.5$$ dex. We also see that {J_\phi } $${J_\phi }$$ increases from 380 \pm 50 $$380 \pm 50$$ to 1320 \pm 90 $$1320 \pm 90$$ km s{^{ - 1}} $${^{ - 1}}$$ kpc across metallicity range - 1.5 $$- 1.5$$ to - 1.1 $$- 1.1$$ , consistent with the spin-up of the Galactic disk. Over the metallicity range - 3.0 \lt \left[ {{\rm{Fe/H}}} \right] \lt - 2.0 $$- 3.0 \lt \left[ {{\rm{Fe/H}}} \right] \lt - 2.0$$ , the slopes of the metallicity distribution functions for the prograde and retrograde disk stars are similar and comparable to that for the halo population. However, detailed chemical analyses based on high resolution spectra are needed to distinguish the accreted versus in-situ contributions. Finally, we show that our spectroscopic parameters reveal serious systematics in the metallicities published in recent studies that apply various machine learning techniques to Gaia XP spectra.

We have conducted a widefield, wideband, snapshot survey using the Australian SKA Pathfinder (ASKAP) referred to as the Rapid ASKAP Continuum Survey (RACS). RACS covers ≈ 90% of the sky, with multiple observing epochs in three frequency bands sampling the ASKAP frequency range of 700–1 800 MHz. This paper describes the third major epoch at 1 655.5 MHz, RACS-high, and the subsequent imaging and catalogue data release. The RACS-high observations at 1 655.5MHz are otherwise similar to the previously released RACS-mid (at 1367.5 MHz), and were calibrated and imaged with minimal changes. From the 1 493 images covering the sky up to declination ≈ +48°, we present a catalogue of 2 677 509 radio sources. The catalogue is constructed from images with a median root-mean-square noise of ≈ 195 μJy PSF−1 (point-spread function) and a median angular resolution of 11″. 8 × 8″. 1. The overall reliability of the catalogue is estimated to be 99.18 %, and we find a decrease in reliability as angular resolution improves. We estimate the brightness scale to be accurate to 10 %, and the astrometric accuracy to be within ≈ 0″. 6 in right ascension and ≈ 0″. 7 in declination after correction of a systematic declination-dependent offset. All data products from RACS-high, including calibrated visibility datasets, images from individual observations, full-sensitivity mosaics, and the all-sky catalogue are available at the CSIRO ASKAP Science Data Archive.

Since August 2014, a monitoring survey at a frequency of 111 MHz has been conducted on the Large Phased Array (LPA) radio telescope of the P.N. Lebedev Physical Institute (LPI). We report the discovery of a bright pulse having a dispersion measure (DM) equal to 134.4 ± 2 pc cm–3, a peak flux density (Sp) equal to 20 ± 4 Jy and a half-width (We) equal to 211 ± 6 ms. The excessive DM of the pulse, after taking into account the MilkyWay contribution, is 114 pc cm–3 that indicates its extragalactic origin. Such value of DM corresponds to the luminosity distance 713 Mpc. The above parameters make the pulse to be a reliable candidate to the fast radio burst (FRB) event, and then it is the second FRB detected at such a large λ ∼ 2.7 m wavelength and the first one among non-repeating FRBs. The normalized luminosity Lν of the event, which we have designated as FRB 20190203, estimated under assumption that the whole excessive DM is determined by the intergalactic environment toward the host galaxy, is equal to ≃ 1034 erg s–1Hz–1. In addition to the study of radio data we analyzed data from the quasi-simultaneous observations of the sky in the high energy (≥ 80 keV) band by the omnidirectional detector SPI/ACS aboard the INTEGRAL orbital observatory (in order to look for a possible gamma-ray counterpart of FRB 20190203). We did not detect any transient events exceeding the background at a statistically significant level. In the INTEGRAL archive, the FRB 20190203 localization region has been observed many times with with a total exposure of ∼ 73.2 days. We have analyzed the data but were unable to find any reliable short gamma-ray bursts from the FRB 20190203 position. Finally we note that the observed properties of FRB 20190203 can be reproduced well in the framework of a maser synchrotron model operating in the far reverse shock (at a distance of ∼ 1015 cm) of a magnetar. However, triggering the burst requires a high conversion efficiency (at the level of 1%) of the shock wave energy into the radio emission.