ABSTRACT
We present results from our analysis of the Hydra I cluster observed in neutral atomic hydrogen (H i) as part of the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY). These ...WALLABY observations cover a 60-square-degree field of view with uniform sensitivity and a spatial resolution of 30 arcsec. We use these wide-field observations to investigate the effect of galaxy environment on H i gas removal and star formation quenching by comparing the properties of cluster, infall, and field galaxies extending up to ∼5R200 from the cluster centre. We find a sharp decrease in the H i-detected fraction of infalling galaxies at a projected distance of ∼1.5R200 from the cluster centre from $\sim 85{{\ \rm per\ cent}}$ to $\sim 35{{\ \rm per\ cent}}$. We see evidence for the environment removing gas from the outskirts of H i-detected cluster and infall galaxies through the decrease in the H i to r-band optical disc diameter ratio. These galaxies lie on the star-forming main sequence, indicating that gas removal is not yet affecting the inner star-forming discs and is limited to the galaxy outskirts. Although we do not detect galaxies undergoing galaxy-wide quenching, we do observe a reduction in recent star formation in the outer disc of cluster galaxies, which is likely due to the smaller gas reservoirs present beyond the optical radius in these galaxies. Stacking of H i non-detections with H i masses below $M_{\rm {HI}}\lesssim 10^{8.4}\, \rm {M}_{\odot }$ will be required to probe the H i of galaxies undergoing quenching at distances ≳60 Mpc with WALLABY.
ABSTRACT
We use high-resolution ASKAP observations of galaxies in the Eridanus supergroup to study their H i, angular momentum, and star formation properties, as part of the WALLABY pre-pilot survey ...efforts. The Eridanus supergroup is composed of three sub-groups in the process of merging to form a cluster. The main focus of this study is the Eridanus (or NGC 1395) sub-group. The baryonic specific angular momentum – baryonic mass (jb−Mb) relation for the Eridanus galaxies is observed to be an unbroken power law of the form $j_{\mathrm{b}} \propto M_{\mathrm{b}}^{0.57 \pm 0.05}$, with a scatter of ∼0.10 ± 0.01 dex, consistent with previous works. We examine the relation between the atomic gas fraction, fatm, and the integrated atomic disc stability parameter q (the fatm−q relation), and find that the Eridanus galaxies deviate significantly from the relation owing to environmental processes such as tidal interactions and ram pressure affecting their H i gas. We find that a majority of the Eridanus galaxies are H i deficient compared to normal star-forming galaxies in the field. We also find that the star formation among the Eridanus galaxies may be suppressed owing to their environment, thus hinting at significant levels of pre-processing within the Eridanus sub-group, even before the galaxies have entered a cluster-like environment.
We present new multiwavelength scaling relations between the neutral hydrogen content (H i) and the stellar properties of nearby galaxies selected from the H i Parkes All-Sky Survey (HIPASS). We use ...these new scaling relations to investigate the environmental dependence of the H i content of galaxies. We find that galaxies in high-density environments tend to have on average less H i than galaxies with the same stellar mass in low-density environments. Our new H i scaling relations allow us to identify individual galaxies, as well as group/cluster environments, that have an ‘anomalous’ H i content. We map the global distribution of H i-deficient and H i-excess galaxies on the sky and compare it to the large-scale structure of galaxies. We find galaxy clusters to be H i deficient, and we identify that the regions surrounding clusters tend to be H i excess. Finally, we demonstrate the potential of using H i scaling relations to predict future H i surveys based on an optical redshift survey. We apply our scaling relations to 16 709 galaxies in the 6dF Galaxy Survey that lie in the HIPASS volume and compare our predictions to the measurements. We find that scaling relations are a good method to estimate the outcome of H i surveys.
PSR B0950+08 is a bright nonrecycled pulsar whose single-pulse fluence variability is reportedly large. Based on observations at two widely separated frequencies, 55 MHz (NenuFAR) and 1.4 GHz ...(Westerbork Synthesis Radio Telescope), we review the properties of these single pulses. We conclude that they are more similar to ordinary pulses of radio emission than to a special kind of short and bright giant pulses, observed from only a handful of pulsars. We argue that a temporal variation of the properties of the interstellar medium along the line of sight to this nearby pulsar, namely the fluctuating size of the decorrelation bandwidth of diffractive scintillation makes an important contribution to the observed single-pulse fluence variability. We further present interesting structures in the low-frequency single-pulse spectra that resemble the “sad trombones” seen in fast radio bursts (FRBs); although for PSR B0950+08 the upward frequency drift is also routinely present. We explain these spectral features with radius-to-frequency mapping, similar to the model developed by Wang et al. (2019, ApJ, 876, L15) for FRBs. Finally, we speculate that μs-scale fluence variability of the general pulsar population remains poorly known, and that its further study may bring important clues about the nature of FRBs.
ABSTRACT
We present SiO J = 2–1 maps of the Sgr B2 molecular cloud, which show shocked gas with a turbulent substructure comprising at least three cavities at velocities of $10,40\, \rm km\, s^{-1}$ ...and an arc at velocities of $-20,10\, \rm km\, s^{-1}$. The spatial anticorrelation of shocked gas at low and high velocities, and the presence of bridging features in position-velocity diagrams suggest that these structures formed in a cloud–cloud collision. Some of the known compact H ii regions spatially overlap with sites of strong SiO emission at velocities of $40,85\, \rm km\, s^{-1}$, and are between or along the edges of SiO gas features at $100,120\, \rm km\, s^{-1}$, suggesting that the stars responsible for ionizing the compact H ii regions formed in compressed gas due to this collision. We find gas densities and kinetic temperatures of the order of $n_{\rm H_2}\sim 10^5\, \rm cm^{-3}$ and $\sim 30\, \rm K$, respectively, towards three positions of Sgr B2. The average values of the SiO relative abundances, integrated line intensities, and line widths are ∼10−9, $\sim 11\, \rm K\, km\, s^{-1}$, and $\sim 31\, \rm km\, s^{-1}$, respectively. These values agree with those obtained with chemical models that mimic grain sputtering by C-type shocks. A comparison of our observations with hydrodynamical simulations shows that a cloud–cloud collision that took place $\lesssim 0.5\, \rm Myr$ ago can explain the density distribution with a mean column density of $\bar{N}_{\rm H_2}\gtrsim 5\times 10^{22}\, \rm cm^{-2}$, and the morphology and kinematics of shocked gas in different velocity channels. Colliding clouds are efficient at producing internal shocks with velocities $\sim 5\!-\!50\, \rm km\, s^{-1}$. High-velocity shocks are produced during the early stages of the collision and can readily ignite star formation, while moderate- and low-velocity shocks are important over longer time-scales and can explain the widespread SiO emission in Sgr B2.
Abstract
We report on the commensal ASKAP detection of a fast radio burst (FRB), FRB 20211127I, and the detection of neutral hydrogen (H
i
) emission in the FRB host galaxy, WALLABY J131913–185018 ...(hereafter W13–18). This collaboration between the CRAFT and WALLABY survey teams marks the fifth, and most distant, FRB host galaxy detected in H
i
, not including the Milky Way. We find that W13–18 has an H
i
mass of
M
HI
= 6.5 × 10
9
M
⊙
, an H
i
-to-stellar mass ratio of 2.17, and coincides with a continuum radio source of flux density at 1.4 GHz of 1.3 mJy. The H
i
global spectrum of W13–18 appears to be asymmetric, albeit the H
i
observation has a low signal-to-noise ratio (S/N), and the galaxy itself appears modestly undisturbed. These properties are compared to the early literature of H
i
emission detected in other FRB hosts to date, where either the H
i
global spectra were strongly asymmetric, or there were clearly disrupted H
i
intensity map distributions. W13–18 lacks a sufficient S/N to determine whether it is significantly less asymmetric in its H
i
distribution than previous examples of FRB host galaxies. However, there are no strong signs of a major interaction in the optical image of the host galaxy that would stimulate a burst of star formation and hence the production of putative FRB progenitors related to massive stars and their compact remnants.
Observations show that spiral galaxies in galaxy clusters tend to have on average less neutral hydrogen (H i) than galaxies of the same type and size in the field. There is accumulating evidence that ...such H i-deficient galaxies are also relatively frequent in galaxy groups. An important question is that which mechanisms are responsible for the gas deficiency in galaxy groups. To gain a better understanding of how environment affects the gas content of galaxies, we identified a sample of six H i-deficient galaxies from the H i Parkes All Sky Survey (HIPASS) using H i-optical scaling relations. One of the galaxies is located in the outskirts of the Fornax cluster, four are in loose galaxy groups and one is in a galaxy triplet. We present new high-resolution H i observations with the Australia Telescope Compact Array (ATCA) of these galaxies. We discuss the possible cause of H i-deficiency in the sample based on H i observations and various multi-wavelength data. We find that the galaxies have truncated H i discs, lopsided gas distribution and some show asymmetries in their stellar discs. We conclude that both ram-pressure stripping and tidal interactions are important gas removal mechanisms in low-density environments.
Abstract
This study uses H
i
image data from the Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) pilot survey with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, ...covering the Hydra cluster out to 2.5
r
200
. We present the projected phase–space distribution of H
i
-detected galaxies in Hydra, and identify that nearly two-thirds of the galaxies within
1.25
r
200
may be in the early stages of ram pressure stripping. More than half of these may be only weakly stripped, with the ratio of strippable H
i
(i.e., where the galactic restoring force is lower than the ram pressure in the disk) mass fraction (over total H
i
mass) distributed uniformly below 90%. Consequently, the H
i
mass is expected to decrease by only a few 0.1 dex after the currently strippable portion of H
i
in these systems has been stripped. A more detailed look at the subset of galaxies that are spatially resolved by WALLABY observations shows that, while it typically takes less than 200 Myr for ram pressure stripping to remove the currently strippable portion of H
i
, it may take more than 600 Myr to significantly change the total H
i
mass. Our results provide new clues to understanding the different rates of H
i
depletion and star formation quenching in cluster galaxies.
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