Abstract Whether cluster mergers enhance ram pressure stripping (RPS) and accelerate the subsequent evolution of member galaxies remains an open question. In this paper, we address this question by investigating galaxy populations in the nearby merging cluster Abell 3667 ( z ≃ 0.0553) using spatially resolved data from the Hector Galaxy Survey. We define an RPS sample by combining Hector-selected galaxies with ionised gas disturbances (e.g. asymmetric tails or truncated disks) and supplementary, optically identified jellyfish galaxies lacking Hector data. The majority of the RPS sample (∼ 71 Subscript negative 10 Superscript plus 7 71 − 10 + 7 ${71}_{-10}^{+7}$ %; 20/28) lies within R 200 , where the impact of the merger is greater. Most asymmetric galaxies (∼ 73 Subscript negative 14 Superscript plus 8 73 − 14 + 8 $73_{-14}^{+8}$ %; 11/15), especially those with extreme RPS signatures, are concentrated towards the inner cluster regions ( R ≲ 0.6 R 200 ), distributed along the merger axis between two shock-tracing radio relics. These central asymmetric galaxies present two separate spatial and kinematic concentrations: a population located at the North-West (NW) subcluster and downstream of its radio relic, a region of high-velocity intracluster medium (ICM) bulk motion, with blueshifted line-of-sight velocities, and a primarily redshifted population close to the main cluster (MC), which also shows evidence of a turbulent ICM. Despite their projected spatial association with the MC core and the NW substructure, the velocities of both samples indicate they are not bound to these merging substructures. Tail orientations provide additional insight into their orbital histories: NW tails point away from the cluster centre and are frequently aligned with the merger axis, suggestive of merger-driven stripping, while MC tails show neither pattern clearly. The tails are also broadly westward-pointing, with the MC tails closely tracing due west and the NW tails shifted towards the northwest, pointing to two distinct filamentary accretion events for the NW and MC populations. Taken together, our results indicate enhanced RPS activity in the heart of A3667, primarily driven by infalling galaxies accreted along nearby filaments and interacting with the merger-driven turbulent environment.
Abstract Recent studies by B. Villarroel and colleagues have assembled and analysed datasets of unidentified features measured from digital scans of photographic plates captured by the first-epoch Palomar Observatory Sky Survey (POSS1) in the pre-Sputnik era. These studies have called attention to (i) a purported deficit of features within Earth’s shadow; (ii) the sporadic presence of linear clusters; and (iii) a positive correlation between the timing of feature observations and nuclear tests as well as Unidentified Aerial Phenomena (UAP) sighting reports. These observations were cited as evidence that some fraction of the unidentified features represent glinting artificial objects near Earth. We have examined these claims using two previously published datasets that are closely related to those used in the Villarroel et al. studies. For these datasets, the Villarroel et al. assumption of a spatially uniform-random background distribution of features, essential to the Earth shadow analysis, is shown to be false. After finding the null distribution of feature count deviations from the background, we find no statistically significant deficit in the shadow. We also determine that a third of the features in the reported linear clusters were not confidently distinguished from catalogue stars. We find that the reported correlation between the timing of feature observations and nuclear tests becomes insignificant after properly normalising by the relevant number of observation days and is almost completely determined by the observation schedule of the Palomar telescope. We uncover important inconsistencies in the definitions of the datasets used in these studies, as well as the use of unvalidated datasets containing catalogue stars, scan artefacts, and plate defects. It has not been shown that any of the features in these datasets represent optical transients. We examine the spatial distribution of the plate-derived features, finding an overall gradual increase in number density towards the corners and edges of plates, as well as examples of (i) empty north-south strips that span multiple plates; (ii) clusters and voids having geometric shapes; and (iii) amorphous clusters. We also highlight a circular argument used in these studies, that leverages the results of an inferential analysis to justify conclusions about the origin of the features as well as the validity of the measurements. Finally, we also review the literature concerning historical searches for optical transients in photographic plates corresponding to gamma ray bursts (GRBs); following decades of work, researchers were unable to make a confident identification of a GRB-associated optical transient.
Abstract Broadband Very Long Baseline Interferometry (VLBI) observations spanning 3–14 GHz pose calibration challenges that traditional methods were not designed to handle. Frequency-dependent instrumental responses, radio frequency interference (RFI), and source variability all complicate the estimation of station sensitivity and source flux density. We present a Bayesian hierarchical random-walk model that jointly tracks source flux density and baseline System Equivalent Flux Density (SEFD) from scan-level signal-to-noise ratio (SNR) observations. The model captures temporal changes via random walks, frequency dependence through piecewise spectral indices and band-wise power-law SEFD scaling, and includes RFI and an elevation-dependent SEFD correction fitted from the scan-level data. We applied this model to 25 sessions on the Hobart–Katherine baseline, covering 81 sources over 3–13.5 GHz. Reliable system-temperature ( T sys ) measurements were unavailable across these sessions, so the model uses scan-level SNR together with single-session catalogue anchors for each source, comprising an X-band flux-density value and a catalogue-derived S/X spectral index. It recovers flux density and baseline SEFD jointly across all 25 sessions and the fitted channels, each with full posterior uncertainty, and flags instrumental issues and a persistent 4 GHz RFI peak. Agreement with SKED measurements from sessions not used to construct the priors supports the recovered temporal evolution. The hierarchical approach shares information across sources and channels via partial pooling, reducing calibration uncertainty without relying on small reference source sets. This provides a diagnostic tool for detecting instrumental problems and could support SNR-based schedule optimisation, and may also apply to calibration at next-generation facilities such as SKA-Low and DSA-2000.
Abstract We present the deepest Murchison Widefield Array (MWA) survey to date, targeting the GAMA9 and GAMA23 survey fields at 215.68 MHz. With a bandwidth of 30.72 MHz, the two images cover tilde 1 250 degree squared ∼ 1 250 deg 2 ${\sim}1\,250\text{ deg}^2$ for the GAMA9 field and tilde 850 degree squared ∼ 850 deg 2 ${\sim}850\text{ deg}^2$ for the GAMA23 field, with an angular resolution of tilde 60 double prime ∼ 60 ″ ${\sim}60''$ . The GAMA9 image was formed from 1 282 2-min snapshot observations (42.7 h) and the GAMA23 image from 1 701 observations (56.7 h) achieving depths of 0.923 plus or minus 0.003 0.923 ± 0.003 $0.923 \pm 0.003$ and 0.331 plus or minus 0.001 0.331 ± 0.001 $0.331 \pm 0.001$ mJy/beam at the field centres, respectively. We processed the data with an updated version of the GLEAM-X pipeline tuned for deep surveys and make this MWA Deep Imaging Pipeline code public. Updates to the pipeline include automation of the observation downloading, calibration, and imaging along with optimisation of the CLEAN ing methods and parameters. Above 5 sigma 5 σ $5\sigma$ , we catalogue 53 419 and 80 407 radio components in the GAMA9 and GAMA23 fields, respectively. We find that the completeness corrected source counts for both fields are in good agreement down to 25–50 mJy with those from LOFAR and earlier MWA surveys. We demonstrate the utility of this survey by highlighting a few sources of particular interest and describing future science possible with this survey.
Abstract The relationship between a galaxy’s specific angular momentum j and its mass M , parameterised by j proportional to upper M Superscript alpha j ∝ M α $j \propto M^\alpha$ and known as the Fall relation, has emerged as a fundamental scaling relation reflecting key physical and morphological properties of galaxies. This relation has been well studied for galaxies with masses above 10 Superscript 9 10 9 $10^{9}$ M Subscript circled dot ⊙ $_{\odot}$ . However, whether or not it holds for the low-mass dwarf galaxies, especially given their varied morphologies, remains uncertain. Here we use H alpha α $\alpha$ observations of 49 star-forming dwarf galaxies from the SH alpha α $\alpha$ DE survey, as well as 20 high-mass ‘control’ galaxies, to investigate the stellar j Subscript asterisk j ∗ $j_{*}$ – upper M Subscript asterisk M ∗ $M_{*}$ relation down to masses below 10 Superscript 6 10 6 $10^{6}$ M Subscript circled dot ⊙ $_{\odot}$ . We find that the star-forming dwarf galaxies follow the same j Subscript asterisk j ∗ $j_{*}$ – upper M Subscript asterisk M ∗ $M_{*}$ relation as high-mass disc-like galaxies, with alpha equals 0.53 plus or minus 0.4 α = 0.53 ± 0.4 $\alpha = 0.53 \pm{0.4}$ , demonstrating that the relation holds across 5 orders of magnitude in mass. We then select a matching sample from the IllustrisTNG cosmological simulation and create mock observations resembling the SH alpha α $\alpha$ DE survey. We find that the simulated dwarf galaxy population follows the extrapolated j Subscript asterisk j ∗ $j_{*}$ – upper M Subscript asterisk M ∗ $M_{*}$ relation with a flattening and significant scatter towards lower j Subscript asterisk j ∗ $j_{*}$ values. Following the evolution of these galaxies, we find that dwarf galaxies experience a gradual loss in j Subscript asterisk j ∗ $j_{*}$ with time, while high-mass galaxies experience a sudden jump in j Subscript asterisk j ∗ $j_{*}$ before settling into a stable state. This dynamical evolution leads to a redshift dependence in alpha α $\alpha$ , with alpha equals 0.45 α = 0.45 $\alpha = 0.45$ at z equals 2 z = 2 $z = 2$ and 0.55 at z equals 0 z = 0 $z = 0$ . Despite the apparent simplicity in the present-day j Subscript asterisk j ∗ $j_{*}$ – upper M Subscript asterisk M ∗ $M_{*}$ relation over a wide mass range, the evolution of j leading to this relation is complex and dynamic.
