Abstract
Upon an expansion of all of the searches for redshifted H i 21-cm absorption (0.002 1 ≤ z ≤ 5.19), we update recent results regarding the detection of 21-cm in the non-local Universe. ...Specifically, we confirm that photo-ionization of the gas is the mostly likely cause of the low detection rate at high redshift, in addition to finding that at z ≲ 0.1 there may also be a decrease in the detection rate, which we suggest is due to the dilution of the absorption strength by 21-cm emission. By assuming that associated and intervening absorbers have similar cosmological mass densities, we find evidence that the spin temperature of the gas evolves with redshift, consistent with heating by ultraviolet photons. From the near–infrared (λ = 3.4, 4.6 and 12 μm) colours, we see that radio galaxies become more quasar-like in their activity with increasing redshift. We also find that the non-detection of 21-cm absorption at high redshift is not likely to be due to the selection of gas-poor ellipticals, in addition to a strong correlation between the ionizing photon rate and the 3.4 − 4.6 colour, indicating that the UV photons arise from AGN activity. Like previous studies, we find a correlation between the detection of 21-cm absorption and the 4.6 − 12 colour, which is a tracer of star-forming activity. However, this only applies at the lowest redshifts (z ≲ 0.1), the range considered by the other studies.
The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science observation as well as in X-rays with ...eROSITA. The projected separation of the X-ray peaks of the two clusters is ~50′ or ~3.1 Mpc. Here we present an inventory of interesting radio sources in this field around this cluster merger. While the eROSITA observations provide clear indications of a bridge of thermal gas between the clusters, neither ASKAP nor MWA observations show any diffuse radio emission coinciding with the X-ray bridge. We derive an upper limit on the radio emissivity in the bridge region of 〈
J
〉
1 GHz
< 1.2 × 10
−44
W Hz
−1
m
−3
. A non-detection of diffuse radio emission in the X-ray bridge between these two clusters has implications for particle-acceleration mechanisms in cosmological large-scale structure. We also report extended or otherwise noteworthy radio sources in the 30 deg
2
field around Abell 3391-Abell 3395. We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected sizes greater than 1 Mpc. The sky density of field radio galaxies with largest linear sizes of >0.7 Mpc is ≈1.7 deg
−2
, three times higher than previously reported. We find no evidence for a cosmological evolution of the population of Giant Radio Galaxies. Moreover, we find seven candidates for cluster radio relics and radio halos.
The star-forming reservoir in the distant Universe can be detected through H i 21-cm absorption arising from either cool gas associated with a radio source or from within a galaxy intervening the ...sightline to the continuum source. In order to test whether the nature of the absorber can be predicted from the profile shape, we have compiled and analysed all of the known redshifted (z ≥ 0.1) H i 21-cm absorption profiles. Although between individual spectra there is too much variation to assign a typical spectral profile, we confirm that associated absorption profiles are, on average, wider than their intervening counterparts. It is widely hypothesized that this is due to high-velocity nuclear gas feeding the central engine, absent in the more quiescent intervening absorbers. Modelling the column density distribution of the mean associated and intervening spectra, we confirm that the additional low optical depth, wide dispersion component, typical of associated absorbers, arises from gas within the inner parsec. With regard to the potential of predicting the absorber type in the absence of optical spectroscopy, we have implemented machine learning techniques to the 55 associated and 43 intervening spectra, with each of the tested models giving a ≳ 80 per cent accuracy in the prediction of the absorber type. Given the impracticability of follow-up optical spectroscopy of the large number of 21-cm detections expected from the next generation of large radio telescopes, this could provide a powerful new technique with which to determine the nature of the absorbing galaxy.
We present a statistical method for measuring the average H i spin temperature in distant galaxies using the expected detection yields from future wide-field 21 cm absorption surveys. As a ...demonstrative case study, we consider an all-southern-sky simulated survey of 2-h per pointing with the Australian Square Kilometre Array Pathfinder for intervening H i absorbers at intermediate cosmological redshifts between z = 0.4 and 1. For example, if such a survey yielded 1000 absorbers, we would infer a harmonic-mean spin temperature of
$\overline{T}_\mathrm{spin} \sim 100$
K for the population of damped Lyman α absorbers (DLAs) at these redshifts, indicating that more than 50 per cent of the neutral gas in these systems is in a cold neutral medium (CNM). Conversely, a lower yield of only 100 detections would imply
$\overline{T}_\mathrm{spin} \sim 1000$
K and a CNM fraction less than 10 per cent. We propose that this method can be used to provide independent verification of the spin temperature evolution reported in recent 21 cm surveys of known DLAs at high redshift and for measuring the spin temperature at intermediate redshifts below z ≈ 1.7, where the Lyman α line is inaccessible using ground-based observatories. Increasingly more sensitive and larger surveys with the Square Kilometre Array should provide stronger statistical constraints on the average spin temperature. However, these will ultimately be limited by the accuracy to which we can determine the H i column density frequency distribution, the covering factor and the redshift distribution of the background radio source population.
ABSTRACT
We present new low-frequency (ν = 88–216 MHz) observations of the complex merging galaxy cluster Abell 3266. These new observations are taken with the Murchison Widefield Array (MWA) in its ...Phase II ‘extended’, long-baseline configuration, offering the highest resolution low-frequency view of the cluster to date. We report on the detection of four steep spectrum (α ≲ −1 for Sν ∝ να) extended radio sources within the cluster. We confirm the detection of a ∼570 kpc radio relic to the south of the cluster, and a possible bridge of emission connecting the relic to the cluster core. We also detect two new ultra-steep-spectrum (α ≲ −1.7) fossil plasma sources to the north and west of the cluster centre without associated compact radio emission. A previously detected radio galaxy in the cluster is also found to have a spectrally steepening tail with steep-spectrum components highlighted by the MWA. The timeline of the merger and the overall mass of the system are consistent with clusters hosting giant radio haloes, yet we do not detect a radio halo in Abell 3266. After simulating a range of radio haloes at 216 MHz, we place upper limits on the radio luminosity corresponding to ∼7.2 × 1024 W Hz−1 at 150 MHz assuming the expected 500 kpc radius – up to a factor of 5 lower than expected from scaling relations.
ABSTRACT
Abell 3266 is a massive and complex merging galaxy cluster that exhibits significant substructure. We present new, highly sensitive radio continuum observations of Abell 3266 performed with ...the Australian Square Kilometre Array Pathfinder (0.8–1.1 GHz) and the Australia Telescope Compact Array (1.1–3.1 GHz). These deep observations provide new insights into recently reported diffuse non-thermal phenomena associated with the intracluster medium, including a ‘wrong-way’ relic, a fossil plasma source, and an as-yet unclassified central diffuse ridge, which we reveal comprises the brightest part of a large-scale radio halo detected here for the first time. The ‘wrong-way’ relic is highly atypical of its kind: it exhibits many classical signatures of a shock-related radio relic, while at the same time exhibiting strong spectral steepening. While radio relics are generally consistent with a quasi-stationary shock scenario, the ‘wrong-way’ relic is not. We study the spectral properties of the fossil plasma source; it exhibits an ultrasteep and highly curved radio spectrum, indicating an extremely aged electron population. The larger scale radio halo fills much of the cluster centre, and presents a strong connection between the thermal and non-thermal components of the intracluster medium, along with evidence of substructure. Whether the central diffuse ridge is simply a brighter component of the halo, or a mini-halo, remains an open question. Finally, we study the morphological and spectral properties of the multiple complex radio galaxies in this cluster in unprecedented detail, tracing their evolutionary history.
Low-mass galaxy cluster systems and groups will play an essential role in upcoming cosmological studies, such as those to be carried out with eROSITA. Though the effects of active galactic nuclei ...(AGNs) and merging processes are of special importance to quantify biases like selection effects or deviations from hydrostatic equilibrium, they are poorly understood on the galaxy-group scale. We present an analysis of recent deep Chandra and XMM-Newton integrations of NGC 741 that provides an excellent example of a group with multiple concurrent phenomena: both an old central radio galaxy and a spectacular infalling head-tail source, strongly bent jets, a 100-kpc radio trail, intriguing narrow X-ray filaments, and gas-sloshing features. Supported principally by X-ray and radio continuum data, we address the merging history of the group, the nature of the X-ray filaments, the extent of gas-stripping from NGC 742, the character of cavities in the group, and the roles of the central AGN and infalling galaxy in heating the intra-group medium.
Using the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP BETA), we have carried out the first z = 0-1 survey for Hi and OH absorption towards the ...gravitationally lensed quasars PKS B1830-211 and MG J0414+0534. Although we detected all previously reported intervening systems towards PKS B1830-211, in the case of MG J0414+0534, three systems were not found, indicating that the original identifications may have been confused with radio frequency interference. Given the sensitivity of our data, we find that our detection yield is consistent with the expected frequency of intervening Hi systems estimated from previous surveys for 21-cm emission in nearby galaxies and z ~ 3 damped Lyman a absorbers. We find spectral variability in the z = 0.886 face-on spiral galaxy towards PKS B1830-211 from observations undertaken with the Westerbork Synthesis Radio Telescope in 1997/1998 and ASKAP BETA in 2014/2015. The Hi equivalent width varies by a few per cent over approximately yearly time-scales. This long-term spectral variability is correlated between the north-east and south-west images of the core, and with the total flux density of the source, implying that it is observationally coupled to intrinsic changes in the quasar. The absence of any detectable variability in the ratio of Hi associated with the two core images is in stark contrast to the behaviour previously seen in the molecular lines. We therefore infer that coherent opaque Hi structures in this galaxy are larger than the parsec-scale molecular clouds found at mm-wavelengths.
It has been reported that there is a deficit of stellar heated dust, as evident from the lack of far-infrared (FIR) emission, in sources within the Herschel-SPIRE sample with X-ray luminosities ...exceeding a critical value of LX ∼ 1037 W. Such a scenario would be consistent with the suppression of star formation by the AGN, required by current theoretical models. Since absorption of the 21 cm transition of neutral hydrogen (H I), which traces the star-forming reservoir, also exhibits a critical value in the ultraviolet band (above ionising photon rates of Q ≈ 3 × 1056 s−1), we test the SPIRE sample for the incidence of the detection of 250 μm emission with Q. The highest value at which FIR emission is detected above the SPIRE confusion limit is Q = 8.9 × 1057 s−1, which is ≈30 times that for the H I, with no critical value apparent. Since complete ionisation of the neutral atomic gas is expected at Q ≳ 3 × 1056 s−1, this may suggest that much of the FIR must arise from heating of the dust by the AGN. However, integrating the ionising photon rate of each star over the initial mass function, we cannot rule out that the high observed ionising photon rates are due to a population of hot, massive stars.
PDF
