arXiv subjects

F. Loi

Publications and source records attributed to F. Loi.

At least 19 recordsLinked to original sources

arXiv2026

Investigating the magnetic field in the inter-cluster filament between Abell 3667 and Abell 3651 with POSSUM

[Abridged abstract] The objective of this study is to measure the magnetic field within the prominent inter-cluster filament recently detected in X-rays by the extended ROentgen Survey with an Imaging Telescope Array (eROSITA). This filament spans over 13 Mpc projected on the sky, connecting the galaxy clusters Abell 3667 and Abell 3651. We employed the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) Rotation Measure (RM) grid to isolate the RM dispersion and median value of background polarised sources induced by the filament's magnetised plasma, and to infer the strength of this magnetic field. The filament region is sampled by 54 background polarised sources. After subtracting the foreground Galactic RM, we detected a marginal residual RM dispersion in the filament region of $6.9\pm3.6$ rad/m$^{2}$, together with a coherent residual RM signal with median $6.3\pm1.3$ rad/m$^{2}$. Assuming simplified single-scale magnetic-field models and adopting informed priors on the thermal electron density distribution derived from the X-ray analysis, we constrained the magnetic field strength to the range 0.1-3.5 $\mu$G within 95$\%$ confidence, with preferred values around 0.2-0.3 $\mu$G depending on the assumed magnetic-field coherence scale. However, we also found that the Galactic foreground RM in this region is highly structured on angular scales comparable to the extent of the filament itself, representing a major source of uncertainty for the RM analysis. Our results provide the first magnetic field constraints based on Faraday rotation measurements in an individual X-ray-detected inter-cluster filament, with field strengths consistent with theoretical expectations for gas in bridges and cluster outskirts. Our analysis also highlights the critical importance of accurately modelling Galactic RM foregrounds for future studies of extra-galactic magnetism with POSSUM and the SKA.

astro-ph.CO↗

arXiv2026

Constraints on the magnetic field in the hadronic scenario for the origin of the radio halo in the A1795 galaxy cluster

The radio halo in the cool-core galaxy cluster, A1795, has recently been proposed to be of hadronic origin, because it extends on megaparsec scales, where no turbulence is expected to be injected into a relaxed cluster. In this paper we aim to constrain the properties of nonthermal protons and the cluster magnetic field strength under the hypothesis of a hadronic origin for the radio halo in the light of the available gamma-ray upper limits for this cluster. We assumed a radial profile and several central magnetic field intensity values, then derived the corresponding properties of the nonthermal protons necessary to reproduce the flux density and surface brightness profile of the radio halo. For the obtained results, we calculated the corresponding gamma-ray emission and compared it with the Fermi-LAT upper limit. We find that, for a magnetic field energy radial profile following that of the thermal gas, central magnetic field intensities $B_0<5\,\mu$G imply a gamma-ray emission at or above the Fermi-LAT upper limit. Therefore, larger values of the magnetic field are necessary in the hadronic scenario. For these values the radial profile of nonthermal protons must slightly decrease with radius, though more slowly than that of the thermal gas. For smaller values of the magnetic field, different mechanisms for producing or accelerating the nonthermal electrons are necessary.

astro-ph.HE↗

arXiv2026

The plethora of diffuse emission in Abell 2034 as revealed by MeerKAT polarization observations

We present MeerKAT observations of the galaxy cluster Abell 2034, a massive (M_500=5.21 10^14 solar masses) nearby cluster in a merging state. Previous observations at 144 MHz have shown that the cluster exhibits a plethora of diffuse emission, with multiple diffuse sources of uncertain classification because of the lack of spectral and polarimetric observations. MeerKAT multi-frequency observations, centered at 816 MHz and 1.28 GHz, together with archival low-frequency LOFAR observations at 144 MHz have allowed us to shed light on the properties of these sources. The polarization properties and spectral index information let us conclude that the cluster hosts one radio relic, a source with a very steep spectrum, previously classified as candidate relic, and filaments of very steep emission around the tailed radio galaxies identified at low frequencies. The presence of a radio halo is confirmed, and its spectrum shows hints for curvature between 144 MHz ad 1.28 GHz. The polarimetric data in the L-band, together with the model of the gas density derived from X-ray observations are used to constrain the magnetic field in the intracluster medium. We assume a radially symmetric magnetic field model, whose strength declines with the cluster gas density as B(r) ~ n_e(r)^0.5, and normalize its strength within R_500. We find that B_500=1 muG best explains the Faraday depth properties of the cluster, though the detection of sources close to the cluster center would be crucial to discriminate among different values. We conclude that the cluster Abell 2034 shows diffuse emission with complex morphologies that do not follow the historical categories of halos and relics. Deep multi-frequency and polarimetric observations are fundamental to understand their origin.

astro-ph.CO↗

arXiv2026

First Sardinia Radio Telescope detection of the Sunyaev-Zel'dovich effect at 18.6 GHz

Galaxy clusters imprint a distinctive signature on the cosmic microwave background through the thermal Sunyaev-Zel'dovich (SZ) effect, which enables to study the intracluster plasma distribution and makes them powerful cosmological probes. We present the first Sardinia Radio Telescope (SRT) detection of the SZ effect in the galaxy cluster MACS J1752+4440 at 18.6 GHz, with a resolution of 0.9'. We detected a decrement in brightness toward the cluster centre, which we attributed to the thermal SZ effect. We modelled the signal using a spherically symmetric $\beta$ model for the electron density distribution and we employed a Bayesian retrieval to estimate the core radius, central electron density, and $\beta$ parameter of the cluster. We found values consistent with expectations for a galaxy cluster of the mass of MACS J1752+4440: a core radius of ($160 \pm 30$) kpc, a central electron density of ($2.5^{+0.7}_{-0.5} \cdot 10^{-3}$) cm$^{-3}$, and $\beta$=$0.6\pm0.1$. The mean Compton-$y$ parameter within a radius of 3.5' is $(2.6 \pm 0.3) \cdot 10^{-5}$, higher than the value reported by Planck, which is coherent considering the different resolution of the instruments and the modelling adopted. This work demonstrates the potential of the SRT to detect the onset of the SZ decrement at low frequencies, providing higher angular resolution than current all-sky surveys and enabling an improved reconstruction of the SZ decrement profile and the plasma distribution in the intracluster medium.

astro-ph.CO↗

arXiv2026

High significance detection at 4.8 GHz of the radio halo in the Coma galaxy cluster with the Sardinia Radio Telescope

We present the results of observations of the radio halo in the Coma galaxy cluster at 4.8 GHz performed with the Sardinia Radio Telescope. The radio halo in this cluster is detected for the first time at this frequency with a statistical significance higher than $3\sigma$. After the removal of the Radio Frequency Interference and of the discrete sources contribution, and after the correction for the Sunyaev-Zel'dovich effect, we estimate a flux density of $61\pm11$ mJy, higher than the value previously reported in literature at this frequency. By using the value we obtained, it is possible to estimate an integrated spectral index between 4.8 and 6.6 GHz of $\alpha\sim1.17$, where $F(\nu)\propto \nu^{-\alpha}$, indicating a possible higher-frequency slowdown of the spectral steepening observed between 1.4 and 4.8 GHz. Such a spectral behavior is compatible with turbulent re-acceleration if the seed electrons have a spectrum extending up to high energies, as in the case of continuous injection by hadronic interactions or dark matter annihilation. We also report the detection at 4.8 GHz of a polarized spot inside the halo, without an evident counterpart, already detected at 6.6 GHz.

astro-ph.CO↗

arXiv2025

Constraints on dark matter annihilation and turbulent reacceleration set by high-frequency observations of the radio halo in the Coma cluster

We study the impact of the recent observation of the radio halo in the Coma galaxy cluster at 6.6 GHz with the Sardinia Radio Telescope on models based on turbulent reacceleration of electrons produced in dark matter annihilation processes. Observing at that frequency it is possible to obtain information on the electrons spectrum at energies where the effect of turbulent reacceleration becomes sub-dominant with respect to energy losses, and therefore to obtain information on the properties of seed electrons. Under the assumption that dark matter particles are neutralino-like particles annihilating at a rate close to the maximum allowed by Fermi-LAT upper limits in dwarf galaxies, we obtain some constraints on the intensity of the reacceleration and on the value of the neutralino mass. In particular, models with mass of the order of 10 GeV are generally disfavored, because they produce a high-frequency radio spectrum that can not reproduce the possible flattening observed between 5 and 6.6 GHz; on the other hand, models with mass of the order of 500 GeV, in order to reproduce the observed spectrum at frequencies below 100 MHz, require a reacceleration phase longer than $10^9$ yr, which would require more than one event responsible of the generation of turbulence in the cluster. The resulting optimal mass values are in the range 100--200 GeV, with a preference for the quark annihilation channel.

astro-ph.CO↗

arXiv2025

Constraining the magnetic field in the galaxy cluster Abell 2142 using MeerKAT L-band polarisation data

Magnetic fields permeate the Universe, including galaxy clusters, and affect the thermodynamical properties of the intra-cluster medium (ICM). Cosmological simulations predict that seed magnetic fields are amplified up to the $\mu$G-level in the ICM, but the magnetic field strength and structure have been studied in only a few clusters. Abell 2142 is a local massive cluster that shows evidence of a post-merger dynamical state. In this work, we aim to constrain the magnetic field intensity, radial profile and power spectrum within its ICM, providing key insights into the nature of this non-thermal component in galaxy clusters. We present MeerKAT observations of Abell 2142 in the L-band (872-1712 MHz), imaged in polarisation for the first time with this purpose. We derive the Rotation Measure (RM) from the radio galaxies' polarised emission by applying the RM synthesis technique and analyse both the RM and fractional polarisation ($\mathrm{F_p}$). To investigate the magnetic field distribution within the ICM, we compare our results with mock RM maps generated from 3D simulations of the cluster. We find that the RM dispersion, $\sigma_{\mathrm{RM}}$, decreases with projected radius, whereas the $\mathrm{F_p}$ increases. Both trends suggest that the magnetic field intensity decreases at larger distances from the cluster center, in agreement with studies on other clusters. Assuming that the magnetic field energy density scales with the gas thermal energy ($B \propto n_e^{0.5}$), a magnetic field with a power spectrum ranging from scales between 7 and 470 kpc, with peak at $\sim 140$ kpc and mean central strength of $9.5 \pm 1.0 \ \mu$G, provides the best fit to our data. The high central magnetic field lies at the upper end of the range observed in other systems and supports the possibility of an hadronic contribution to the diffuse radio emission previously detected at the cluster center.

astro-ph.CO↗

arXiv2025

Relighting the fire in Hickson Compact Group (HCG) 15: magnetised fossil plasma revealed by the SKA Pathfinders & Precursors

In the context of the life cycle and evolution of active galactic nuclei (AGN), the environment plays an important role. In particular, the over-dense environments of galaxy groups, where dynamical interactions and bulk motions have significant impact, offer an excellent but under-explored window into the life cycles of AGN and the processes that shape the evolution of relativistic plasma. Pilot Survey observations with the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey recovered diffuse emission associated with the nearby (z = 0.0228) galaxy group HCG15, which was revealed to be strongly linearly polarised. We study the properties of this emission in unprecedented detail to settle open questions about its nature and its relation to the group-member galaxies. We perform a multi-frequency spectropolarimetric study of HCG15 incorporating our ASKAP EMU observations as well as new data from MeerKAT, LOFAR, the GMRT, and the Karl G. Jansky Very Large Array (VLA), plus X-ray data from XMM-Newton and optical spectra from the Himalayan Chandra Telescope (HCT). Our study confirms that the diffuse structure represents remnant emission from historic AGN activity, likely associated with HCG15-D, some 80-86 Myr ago (based on ageing analysis). We detect significant highly linearly-polarised emission from a diffuse 'ridge'-like structure with a highly ordered magnetic field. Our analysis suggests that this emission is generated by draping of magnetic field lines in the intra-group medium (IGrM), although further exploration with simulations would aid our understanding. We confirm that HCG15-C is a group-member galaxy. Finally, we report the detection of thermal emission associated with a background cluster at redshift z ~ 0.87 projected onto the IGrM of HCG15, which matches the position and redshift of the recent SZ detection of ACT-CL J0207.8+0209.

astro-ph.GA↗

arXiv2024

Observations of the galaxy cluster CL 0217+70 and its surrounding region at 1.4 GHz with the Sardinia Radio Telescope

We present the results of observations performed with the Sardinia Radio Telescope (SRT) at 1.3-1.8 GHz of the galaxy cluster CL 0217+70 and a $3^\circ \times 3^\circ$ region around it. We combine the SRT data with archival Very Large Array (VLA) data to obtain images having the VLA angular resolution, but sensitive up to largest scales. The SRT+VLA combination allows us to derive a cluster radio halo flux density higher by $\sim14\%$ compared to the VLA-only data, although consistent within $1\sigma$. We derive a spectral index map between 140 MHz and 1.4 GHz, finding an extended region with spectral index $\alpha\sim0.6$ on the external part of the south-eastern candidate relic, questioning the real nature of this relic. Moreover, we detect an extended emission outside the cluster in the south-eastern area, having an angular extension of $\sim50$ arcmin on the longer side, which would correspond to $\sim10$ Mpc at the cluster distance; the emissivity that this region would have if located at the cluster distance is in line with the one estimated in candidate filaments of the cosmic web; however, the peculiar orientation of this region, not pointed towards the cluster, and the low Galactic latitude of this cluster suggest that its origin can be due to a foreground emission originating in our Galaxy.

astro-ph.CO↗

arXiv2024

A Deep Dive into the NGC 741 Galaxy Group: Insights into a Spectacular Head-Tail Radio Galaxy from VLA, MeerKAT, uGMRT and LOFAR

We present deep, wideband multifrequency radio observations (144 MHz$-$8 GHz) of the remarkable galaxy group NGC 741, which yield crucial insights into the interaction between the infalling head-tail radio galaxy (NGC 742) and the main group. Our new data provide an unprecedentedly detailed view of the NGC 741-742 system, including the shock cone, disrupted jets from NGC 742, the long ($\sim$ 255 kpc) braided southern radio tail, and eastern lobe-like structure, and reveal, for the first time, complex radio filaments throughout the tail and lobe, and a likely vortex ring behind the shock cone. The cone traces the bow shock caused by the supersonic ($\mathcal{M}\sim2$) interaction between the head-tail radio galaxy NGC 742 and the intragroup medium (IGrM) while the ring may have been formed by interaction between the NGC 742 shock and a previously existing lobe associated with NGC 741. This interaction plausibly compressed and re-accelerated the radio plasma. We estimate that shock-heating by NGC 742 has likely injected $\sim$2-5$\times$10$^{57}$ erg of thermal energy into the central 10 kpc cooling region of the IGrM, potentially affecting the cooling and feedback cycle of NGC 741. A comparison with Chandra X-ray images shows that some of the previously detected thermal filaments align with radio edges, suggesting compression of the IGrM as the relativistic plasma of the NGC 742 tail interacts with the surrounding medium. Our results highlight that multi-frequency observations are key to disentangling the complex, intertwined origins of the variety of radio features seen in the galaxy group NGC 741, and the need for simulations to reproduce all the detected features.

astro-ph.GA↗

arXiv2024

Probing the Magnetised Gas Distribution in Galaxy Groups and the Cosmic Web with POSSUM Faraday Rotation Measures

We present initial results from the Polarisation Sky Survey of the Universe's Magnetism (POSSUM), analysing 22,817 Faraday Rotation Measures (RMs) with median uncertainties of 1.2 rad m^-2 across 1,520 square degrees to study magnetised gas associated with 55 nearby galaxy groups (z less than 0.025) with halo masses between 10^12.5 and 10^14.0 M_sun. We identify two distinct gas phases: the Intragroup Medium (IGrM) within 0-2 splashback radii and the Warm-Hot Intergalactic Medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e., Galactic foreground RM-subtracted) RMs by 6.9 +/- 1.8 rad m^-2 and 4.2 +/- 1.2 rad m^-2, respectively. Estimated magnetic field strengths are several microGauss within the IGrM and 0.1-1 microGauss in the WHIM. We estimate the plasma beta in both phases and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that "missing baryons" in the WHIM likely extend beyond the gravitational radii of group-mass halos to Mpc scales, consistent with large-scale, outflow-driven "magnetised bubbles" seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetised thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel'dovich effect methods, and when combined with Fast Radio Burst Dispersion Measures, data from the full POSSUM survey, comprising approximately a million RMs, will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetised universe.

astro-ph.GA↗

arXiv2024

A 'MeerKAT-meets-LOFAR' study of the complex multi-component (mini-)halo in the extreme sloshing cluster Abell 2142

Clusters of galaxies are turbulent environments, whether merging systems with a turbulent intracluster medium (ICM) or relaxed systems sloshing within the potential well. In many such clusters, diffuse radio sources associated with the ICM are found: radio haloes and mini-haloes. Abell 2142 is a rich cluster undergoing extreme core sloshing, generating four cold fronts and a complex multi-component radio halo. Recent work revealed three halo components which span 2.4 Mpc. Particle acceleration on such scales is poorly understood, and requires high-quality multi-frequency data to understand. We use new deep MeerKAT L-band (1283 MHz) observations, combined with LOFAR HBA (143 MHz) data and X-ray data from XMM-Newton and Chandra to study the spectrum of the halo and the connection between the thermal and non-thermal components of the ICM. We detect the third halo component for the first time at 1283 MHz and confirm its ultra-steep spectrum nature, recovering $\alpha_{\rm H3, total} = -1.68 \pm 0.10$. All components follow power-law spectra which steepen toward the cluster outskirts. We profile the halo along three directions, finding evidence of asymmetry and spectral steepening perpendicular to the main axis of the cluster. Our thermal/non-thermal investigation shows sub-linear correlations that are steeper at 1283 MHz than 143 MHz, and we find different connections in different components of the halo. We find both a moderate anti-correlation (H1, the core) and positive correlation (H2, the ridge) between radio spectral index and X-ray temperature. Our results are broadly consistent with an interpretation of inhomogeneous turbulent (re-)acceleration. However, the anti-correlation between radio spectral index and X- ray temperature in the cluster core is challenging to explain; the presence of three cold fronts and a generally lower temperature may provide the foundations of an explanation.

astro-ph.CO↗

arXiv2024

Sardinia Radio Telescope observations of the Coma Cluster

We present deep total intensity and polarization observations of the Coma cluster at 1.4 and 6.6 GHz performed with the Sardinia Radio Telescope. By combining the single-dish 1.4 GHz data with archival Very Large Array observations we obtain new images of the central radio halo and of the peripheral radio relic where we properly recover the brightness from the large scale structures. At 6.6 GHz we detect both the relic and the central part of the halo in total intensity and polarization. These are the highest frequency images available to date for these radio sources in this galaxy cluster. In the halo, we find a localized spot of polarized signal, with fractional polarization of about 45%. The polarized emission possibly extends along the north-east side of the diffuse emission. The relic is highly polarized, up to 55%, as usually found for these sources. We confirm the halo spectrum is curved, in agreement with previous single-dish results. The spectral index is alpha=1.48 +/- 0.07 at a reference frequency of 1 GHz and varies from alpha ~1.1, at 0.1 GHz, up to alpha ~ 1.8, at 10 GHz. We compare the Coma radio halo surface brightness profile at 1.4 GHz (central brightness and e-folding radius) with the same properties of the other halos, and we find that it has one of the lowest emissivities observed so far. Reanalyzing the relic's spectrum in the light of the new data, we obtain a refined radio Mach number of M=2.9 +/- 0.1.

astro-ph.CO↗

arXiv2024

Evolutionary Map of the Universe (EMU): a pilot search for diffuse, non-thermal radio emission in galaxy clusters with the Australian SKA Pathfinder

Clusters of galaxies have been found to host Mpc-scale diffuse, non-thermal radio emission in the form of central radio halos and peripheral relics. Turbulence and shock-related processes in the intra-cluster medium are generally considered responsible for the emission, though details of these processes are still not clear. The low surface brightness makes detection of the emission a challenge, but with recent surveys with high-sensitivity radio telescopes we are beginning to build large samples of these sources. The Evolutionary Map of the Universe (EMU) is a Southern Sky survey being performed by the Australian SKA Pathfinder (ASKAP) over the next few years and is well-suited to detect and characterise such emission. To assess prospects of the full survey, we have performed a pilot search of diffuse sources in 71 clusters from the Planck Sunyaev-Zeldovich (SZ) cluster catalogue (PSZ2) found in archival ASKAP observations. After re-imaging the archival data and performing both (u,v)-plane and image-plane angular scale filtering, we detect 21 radio halos (12 for the first time, excluding an additional six candidates), 11 relics (in seven clusters, and six for the first time, excluding a further five candidate relics), along with 12 other, unclassified diffuse radio sources. From these detections, we predict the full EMU survey will uncover up to ~254 radio halos and ~85 radio relics in the 858 PSZ2 clusters that will be covered by EMU. The percentage of clusters found to host diffuse emission in this work is similar to the number reported in recent cluster surveys with the LOw Frequency ARray (LOFAR) Two-metre Sky Survey (Botteon, et al. 2022a, A&A, 660, A78), suggesting EMU will complement similar searches being performed in the Northern Sky and provide us with statistically significant samples of halos and relics at the completion of the full survey.

astro-ph.CO↗

arXiv2023

A MeerKAT-meets-LOFAR study of Abell 1413: a moderately disturbed non-cool-core cluster hosting a $\sim 500$ kpc 'mini'-halo

Many relaxed cool-core clusters host diffuse radio emission on scales of hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for generating them, as well as their connection with central active galactic nuclei, is elusive and many questions related to their physical properties and origins remain unanswered. This paper presents new radio observations of the galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a moderately-disturbed non-cool-core cluster. Our study reveals an asymmetric mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported at similar frequencies. The spectral index is flatter than previously reported, with an integrated value of $\alpha = -1.01 \pm 0.06$, shows significant spatial variation, and a tentative radial steepening. We studied the point-to-point X-ray/radio surface brightness correlation to investigate the thermal/non-thermal connection: our results show a strong connection between these components, with a super-linear slope of $b = 1.63 \pm 0.10$ at 1283 MHz and $b = 1.20 \pm 0.12$ at 145 MHz. We also explore the X-ray surface brightness/radio spectral index correlation, finding a slope of $b = 0.59 \pm 0.11$. Both investigations support the evidence of spectral steepening. Finally, in the context of understanding the particle acceleration mechanism, we present a simple theoretical model which demonstrates that hybrid scenarios - secondary electrons (re-)accelerated by turbulence - reproduce a super-linear correlation slope.

astro-ph.GA↗

arXiv2023

New filamentary remnant radio emission and duty cycle constraints in the radio galaxy NGC 6086

Radio galaxies are a subclass of active galactic nuclei in which accretion onto the supermassive black hole releases energy via relativistic jets. The jets are not constantly active throughout the life of the host galaxy and alternate between active and quiescent phases. Remnant radio galaxies are detected during a quiescent phase and define a class of unique sources to constrain the AGN duty cycle. We present, a spatially resolved radio analysis of the radio galaxy associated with NGC 6086 and constraints on the spectral age of the diffuse emission to investigate the duty cycle and evolution of the source. We use three new low-frequency, high-sensitivity observations, performed with the Low Frequency Array at 144 MHz and with the upgraded Giant Metrewave Radio Telescope at 400 MHz and 675 MHz. To these, we add two Very Large Array archival observations at 1400 and 4700 MHz. In the new observations, we detect a second pair of larger lobes and three regions with a filamentary morphology. We analyse the spectral index trend in the inner remnant lobes and see systematic steeper values at the lower frequencies compared to the GHz ones. Steeper spectral indices are found in the newly detected outer lobes (up to 2.1), as expected if they trace a previous phase of activity of the AGN. However, the differences between the spectra suggest different dynamical evolution within the intragroup medium during their expansion and/or different magnetic field values. We place constraints on the age of the inner and outer lobes and derive the duty cycle of the source. This results in a total active time of $\sim$39%. The filamentary structures have a steep spectral index ($\sim$1) without any spectral index trend and only one of them shows a steepening in the spectrum. Their origin is not yet clear, but they may have formed due to the compression of the plasma or due to magnetic field substructures.

astro-ph.GA↗

arXiv2023

The MeerKAT Fornax Survey II. The rapid removal of HI from dwarf galaxies in the Fornax cluster

We present MeerKAT Fornax Survey atomic hydrogen (HI) observations of the dwarf galaxies located in the central ~2.5 x 4 deg$^2$ of the Fornax galaxy cluster. The HI images presented in this work have a $3\sigma$ column density sensitivity between 2.7 and 50 x 10$^{18}$ cm$^{-2}$ over 25 km s$^{-1}$ for spatial resolution between 4 and 1 kpc. We are able to detect an impressive MHI = 5 x 10$^{5}$ Msun 3$\sigma$ point source with a line width of 50 km s$^{-1}$ at a distance of 20 Mpc. We detect HI in 17 out of the 304 dwarfs in our field -- 14 out of the 36 late type dwarfs (LTDs), and 3 of the 268 early type dwarfs (ETDs). The HI-detected LTDs have likely just joined the cluster and are on their first infall as they are located at large clustocentric radii, with comparable MHI and mean stellar surface brightness at fixed luminosity as blue, star-forming LTDs in the field. The HI-detected ETDs have likely been in the cluster longer than the LTDs and acquired their HI through a recent merger or accretion from nearby HI. Eight of the HI-detected LTDs host irregular or asymmetric HI emission and disturbed or lopsided stellar emission. There are two clear cases of ram-pressure shaping the HI, with the LTDs displaying compressed HI on the side closest to the cluster centre and a one-sided, starless tail pointing away from the cluster centre. The HI-detected dwarfs avoid the most massive potentials, consistent with massive galaxies playing an active role in the removal of HI. We create a simple toy model to quantify the timescale of HI stripping in the cluster. We find that a MHI = 10$^{8}$ Msun dwarf will be stripped in ~ 240 Myr. The model is consistent with our observations, where low mass LTDs are directly stripped of their HI from a single encounter and more massive LTDs can harbour a disturbed HI morphology due to longer times or multiple encounters being required to fully strip their HI.

astro-ph.GA↗

arXiv2023

The MeerKAT Fornax Survey -- I. Survey description and first evidence of ram pressure in the Fornax galaxy cluster

The MeerKAT Fornax Survey maps the distribution and kinematics of atomic neutral hydrogen gas (HI) in the nearby Fornax galaxy cluster using the MeerKAT telescope. The 12 deg^2 survey footprint covers the central region of the cluster out to ~ Rvir and stretches out to ~ 2 Rvir towards south west to include the NGC 1316 galaxy group. The HI column density sensitivity (3 sigma over 25 km/s) ranges from 5e+19/cm^2 at a resolution of ~ 10" (~ 1 kpc at the 20 Mpc distance of Fornax) down to ~ 1e+18/cm^2 at ~ 1' (~ 6 kpc), and slightly below this level at the lowest resolution of ~ 100" (~ 10 kpc). The HI mass sensitivity (3 sigma over 50 km/s) is 6e+5 Msun. The HI velocity resolution is 1.4 km/s. In this paper we describe the survey design and HI data processing, and we present a sample of six galaxies with long, one-sided, star-less HI tails (of which only one was previously known) radially oriented within the cluster and with measurable internal velocity gradients. We argue that the joint properties of the HI tails represent the first unambiguous evidence of ram pressure shaping the distribution of HI in the Fornax cluster. The disturbed optical morphology of all host galaxies supports the idea that the tails consist of HI initially pulled out of the galaxies' stellar body by tidal forces. Ram pressure was then able to further displace the weakly bound HI and give the tails their present direction, length and velocity gradient.

astro-ph.GA↗