arXiv subjects

M. Murgia

Publications and source records attributed to M. Murgia.

At least 37 records · Page 2Linked to original sources

arXiv2023

Radio multifrequency observations of galaxy clusters. The Abell 399$-$401 pair

Galaxy clusters are assembled via merging of smaller structures, in a process that generates shocks and turbulence in the intra cluster medium and produces radio diffuse emission in the form of halos and relics. The cluster pair A399-A401 represents a special case: both clusters host a radio halo. Recent Low Frequency Array (LOFAR) observations at 140 MHz revealed the presence of a radio bridge connecting the two clusters along with two relic candidates. These relics include one South of A399 and the other in between the two clusters, in proximity of a shock front detected in X-ray observations. In this paper we present observations of the A399-A401 cluster pair at 1.7, 1.4, 1.2 GHz and 346 MHz from the Westerbork Synthesis Radio Telescope (WSRT). We detect the radio halo in the A399 cluster at 346 MHz, extending up to $\sim 650$ kpc and with a $125 \pm 6$ mJy flux density. Its spectral index between 140 MHz and 346 MHz is $α= 1.75 \pm 0.14$. The two candidate relics are also seen at 346 MHz and we determine their spectral indices to be $α= 1.10 \pm 0.14$ and $α= 1.46 \pm 0.14$. The low surface brightness bridge connecting the two clusters is below the noise level at 346 MHz, therefore we constrain the bridge average spectral index to be steep, i.e. $α> 1.5$ at 95% confidence level. This result favours the scenario where dynamically-induced turbulence is a viable mechanism to reaccelerate a population of mildly relativistic particles and amplify magnetic fields on scales of a few Mpcs. Key words: galaxies: clusters: general - galaxies: clusters: individual: Abell 399 - radio continuum: general

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arXiv2023

Young but fading radio sources: searching for remnants among compact steep-spectrum radio sources

The incidence of young but fading radio sources provides important information on the life cycle of radio emission in radio-loud active galactic nuclei. Despite its importance for constraining the models of radio source evolution, there are no systematic studies of remnants in complete samples of young radio sources. We report results on the study of 18 compact steep-spectrum (CSS) radio sources, selected from the statistically complete B3-VLA CSS sample, characterized by a steep optically-thin spectrum (alpha > 1.0) and no core detection in earlier studies. Our deep multi-frequency Very Large Array (VLA), pc-scale Very Long Baseline Array (VLBA), and eMERLIN observations allowed us to locate the core component in 10 objects. In 3 CSS sources there is no clear evidence of present-time active regions, suggesting they are likely in a remnant phase. Among sources with core detection, we find 3 objects that have no clear active regions (hotspots) at the edges of the radio structure, suggesting that the radio emission may have just restarted. Our results support a power-law distribution of the source ages, although the poor statistics prevents us from setting solid constraints on the percentage of remnants and restarted sources in sub-populations of radio sources.

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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.

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arXiv2023

The CARMENES search for exoplanets around M dwarfs. A long-period planet around GJ 1151 measured with CARMENES and HARPS-N data

Detecting a planetary companion in a short-period orbit through radio emission from the interaction with its host star is a new prospect in exoplanet science. Recently, a tantalising signal was found close to the low-mass stellar system GJ 1151 using LOFAR observations. We studied spectroscopic time-series data of GJ 1151 in order to search for planetary companions, investigate possible signatures of stellar magnetic activity, and to find possible explanations for the radio signal. We used the combined radial velocities measured from spectra acquired with the CARMENES, HARPS-N, and HPF instruments, extracted activity indices from those spectra in order to mitigate the impact of stellar magnetic activity on the data, and performed a detailed analysis of Gaia astrometry and all available photometric time series coming from the MEarth and ASAS-SN surveys. We found a M$>$10.6 M$_{\oplus}$ companion to GJ 1151 in a 390d orbit at a separation of 0.57 au. Evidence for a second modulation is also present; this could be due to long-term magnetic variability or a second (substellar) companion. The star shows episodes of elevated magnetic activity, one of which could be linked to the observed LOFAR radio emission. We show that it is highly unlikely that the detected GJ 1151 b, or any additional outer companion is the source of the detected signal. We cannot firmly rule out the suggested explanation of an undetected short-period planet that could be related to the radio emission, as we establish an upper limit of 1.2 M$_{\oplus}$ for the minimum mass.

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arXiv2022

MeerKAT's View of the Bullet Cluster 1E 0657-55.8

The Bullet cluster (1E 0657-55.8) is a massive merging system at redshift $z$=0.296, known to host a powerful radio halo and a relic. Here we present high fidelity MeerKAT L-band (0.9-1.7 GHz) observations of the Bullet cluster, in which we trace a larger extent of both the radio halo and relic in comparison to previous studies. The size of the recovered radio halo is 1.6 Mpc $\times$ 1.3 Mpc and the largest linear size of the relic is ~988 kpc. We detect a new decrement feature on the southern outskirts of the halo emission, where a region appears to have a lower surface brightness in comparison to its surroundings. The larger extension on the outskirts of the halo is faint, which suggests lower relativistic electron density or a weaker magnetic field. An in-band spectral index map of the radio halo reveals a hint of radial steepening towards the edges of the diffuse source, likely due to synchrotron electron ageing. The integrated spectral index of the radio halo is 1.1$\pm$0.2. We perform a radio-X-ray surface brightness point-to-point analysis, which reveals a linear correlation for the radio halo. This indicates that the radio halo emission is produced by primary re-acceleration mechanisms. Finally, we derive a radio Mach number of M_R= 4.6$\pm$0.9 for the relic shock region, which is higher than the Mach number inferred by earlier analyses based on X-ray data. Discrepancies between radio and X-ray Mach numbers have been observed for multiple systems, with studies suggesting that this is due to various factors, including relic orientation.

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arXiv2022

The galaxy group NGC 507: newly detected AGN remnant plasma transported by sloshing

Jets from active galactic nuclei (AGN) are known to recurrently enrich their surrounding medium with mildly-relativistic particles and magnetic fields. Here, we present a detailed multi-frequency analysis of the nearby (z=0.01646) galaxy group NGC 507. In particular, we present new high-sensitivity and high spatial resolution radio images in the frequency range 144-675 MHz obtained using LOFAR and uGMRT observations. These reveal the presence of previously undetected diffuse radio emission with complex, filamentary morphology, likely related to a previous outburst of the central galaxy. Based on spectral ageing considerations, we derived that the plasma was first injected by the AGN 240-380 Myr ago and is now cooling. Our analysis of deep archival XMM-Newton data confirms that the system is dynamically disturbed, as previously suggested. We detect two discontinuities in the X-ray surface brightness distribution (in East and South direction) tracing a spiral pattern, which we interpret as cold fronts produced by sloshing motions. The remarkable spatial coincidence observed between the newly-detected arc-like radio filament and the southern concave X-ray discontinuity strongly suggests that the remnant plasma has been displaced by the sloshing motions on large scales. Overall, NGC 507 represents one of the clearest examples known to date in which a direct interaction between old AGN remnant plasma and the external medium is observed in a galaxy group. Our results are consistent with simulations, which suggest that filamentary emission can be created by the cluster/group weather disrupting AGN lobes and spreading their relativistic content into the surrounding medium.

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arXiv2022

High angular resolution Sunyaev Zel'dovich observations: the case of MISTRAL

The MIllimeter Sardinia radio Telescope Receiver based on Array of Lumped elements kids, MISTRAL, is a millimetric ($\simeq 90GHz$) multipixel camera being built for the Sardinia Radio Telescope. It is going to be a facility instrument and will sample the sky with 12 arcsec angular resolution, 4 arcmin field of view, through 408 Kinetic Inductance Detectors (KIDs). The construction and the beginning of commissioning is planned to be in 2022. MISTRAL will allow the scientific community to propose a wide variety of scientific cases including protoplanetary discs study, star forming regions, galaxies radial profiles, and high angular resolution measurements of the Sunyaev Zel'dovich (SZ) effect with the investigation of the morphology of galaxy cluster and the search for the Cosmic Web.

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arXiv2022

Searching for Anomalous Microwave Emission in nearby galaxies. K-band observations with the Sardinia Radio Telescope

We observed four nearby spiral galaxies (NGC 3627, NGC 4254, NGC 4736 and NGC 5055) in the K band with the 64-m Sardinia Radio Telescope, with the aim of detecting the Anomalous Microwave Emission (AME), a radiation component presumably due to spinning dust grains, observed so far in the Milky Way and in a handful of other galaxies only (most notably, M 31). We mapped the galaxies at 18.6 and 24.6 GHz and studied their global photometry together with other radio-continuum data from the literature, in order to find AME as emission in excess of the synchrotron and thermal components. We only find upper limits for AME. These non-detections, and other upper limits in the literature, are nevertheless consistent with the average AME emissivity from the few detections: it is $ε^\mathrm{AME}_{\mathrm{30~GHz}} = 2.4\pm0.4 \times 10^{-2}$ MJy sr$^{-1}$ (M$_\odot$ pc$^{-2}$)$^{-1}$ in units of dust surface density (equivalently, $1.4\pm0.2 \times 10^{-18}$ Jy sr$^{-1}$ (H cm$^{-2}$)$^{-1}$ in units of H column density). We finally suggest to search for AME in quiescent spirals with relatively low radio luminosity, such as M~31.

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arXiv2021

The mm-to-cm SED of spiral galaxies. Synergies between NIKA2 and SRT instruments

The mm-to-cm range of the Spectral Energy Distribution of spiral galaxies remains largely unexplored. Its coverage is required to disentangle the contribution of dust emission, free-free and synchrotron radiation and can provide constraints on dust models, star-formation rates and ISM properties. We present the case for a synergy between NIKA2 observations of nearby spirals and those from planned and current instrumentation at the Sardinia Radio Telescope, and report on a pilot K-band program to search for Anomalous Microwave Emission, an elusive emission component which is presumably related to dust.

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arXiv2021

Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz

The Andromeda galaxy is the best-known large galaxy besides our own Milky Way. Several images and studies exist at all wavelengths from radio to hard X-ray. Nevertheless, only a few observations are available in the microwave range where its average radio emission reaches the minimum. In this paper, we want to study the radio morphology of the galaxy, decouple thermal from nonthermal emission, and extract the star formation rate. We also aim to derive a complete catalog of radio sources for the mapped patch of sky. We observed the Andromeda galaxy with the Sardinia Radio Telescope at 6.6 GHz with very high sensitivity and angular resolution, and an unprecedented sky coverage. Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, we confirm that, globally, the spectral index is $\sim 0.7-0.8$, while in the star forming regions it decreases to $\sim 0.5$. By disentangling (gas) thermal and nonthermal emission, we find that at 6.6 GHz, thermal emission follows the distribution of HII regions around the ring. Nonthermal emission within the ring appears smoother and more uniform than thermal emission because of diffusion of the cosmic ray electrons away from their birthplaces. This causes the magnetic fields to appear almost constant in intensity. Furthermore, we calculated a map of the star formation rate based on the map of thermal emission. Integrating within a radius of $R_{max}=15$ kpc, we obtained a total star formation rate of $0.19 \pm 0.01$ $M_{\odot}$/yr in agreement with previous results in the literature. Finally, we correlated our radio data with infrared images of the Andromeda galaxy. We find an unexpectedly high correlation between nonthermal and mid-infrared data in the central region, with a correlation parameter $r=0.93$.

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arXiv2020

A perfect power-law spectrum even at highest frequencies: The Toothbrush relic

Radio relics trace shock fronts generated in the intracluster medium (ICM) during cluster mergers. The particle acceleration mechanism at the shock fronts is not yet completely understood. We observed the Toothbrush relic with the Effelsberg and Sardinia Radio Telescope at 14.25 GHz and 18.6 GHz, respectively. Unlike previously claimed, the integrated spectrum of the relic closely follows a power law over almost three orders of magnitude in frequency, with a spectral index of $α_{\rm 58\,MHz}^{\rm 18.6\,GHz}=-1.16\pm0.03$. Our finding is consistent with a power-law injection spectrum, as predicted by diffusive shock acceleration theory. The result suggests that there is only little magnetic field strength evolution downstream to the shock. From the lack of spectral steepening, we find that either the Sunyaev-Zeldovich decrement produced by the pressure jump is less extended than $\sim$ 600\,kpc along the line of sight or, conversely, that the relic is located far behind in the cluster. For the first time, we detect linearly polarized emission from the "brush" at 18.6 GHz. Compared to 8.3 GHz, the degree of polarization across the brush increases at 18.6 GHz, suggesting a strong Faraday depolarization towards lower frequencies. The observed depolarization is consistent with an intervening magnetized screen that arise from the dense ICM containing turbulent magnetic fields. The depolarization, corresponding to a standard deviation of the Rotation Measures as high as $σ_{\rm RM}= 212\pm23\rm \,rad\,m^{-2}$, suggests that the brush is located in or behind the ICM. Our findings indicate that the Toothbrush can be consistently explained by the standard scenario for relic formation

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arXiv2020

Diffuse Radio Sources in a Statistically Complete Sample of High Redshift Galaxy Clusters

Non-thermal properties of galaxy clusters have been studied with detailed and deep radio images in comparison with X-ray data. While much progress has been made, most of the studied clusters are at a relatively low redshift (z < 0.3). We here investigate the evolutionary properties of the non-thermal cluster emission using two statistically complete samples at z > 0.3. We obtained short JVLA observations at L-band of the statistically complete sample of very X-ray luminous clusters from the Massive Cluster Survey (MACS) presented by Ebeling et al. (2010), and redshift range 0.3 - 0.5. We add to this list the complete sample of the 12 most distant MACS clusters (z > 0.5) presented in Ebeling et al. (2007). Most clusters show evidence of emission in the radio regime. We present the radio properties of all clusters in our sample and show images of newly detected diffuse sources. A radio halo is detected in 19 clusters, and five clusters contain a relic source. Most of the brightest cluster galaxies (BCG) in relaxed clusters show radio emission with powers typical of FRII radio galaxies, and some are surrounded by a radio mini-halo. The high frequency of radio emission from the BCG in relaxed clusters suggests that BCG feedback mechanisms are in place already at z about 0.6. The properties of radio halos and the small number of detected relics suggest redshift evolution in the properties of diffuse sources. The radio power (and size) of radio halos could be related to the number of past merger events in the history of the system. In this scenario, the presence of a giant and high-power radio halo is indicative of an evolved system with a large number of past major mergers.

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arXiv2020

The recurrent nuclear activity of Fornax A and its interaction with the cold gas

Sensitive (noise $\sim 16\,μ$\Jyb), high-resolution ($\sim 10''$) MeerKAT observations of Fornax A show that its giant lobes have a double-shell morphology, where dense filaments are embedded in a diffuse and extended cocoon, while the central radio jets are confined within the host galaxy. The spectral radio properties of the lobes and jets of Fornax A reveal that its nuclear activity is rapidly flickering. Multiple episodes of nuclear activity must have formed the radio lobes, for which the last stopped $12$ Myr ago. More recently ($\sim 3$ Myr ago), a less powerful and short ($\lesssim 1$ Myr) phase of nuclear activity generated the central jets. The distribution and kinematics of the neutral and molecular gas in the centre give insights on the interaction between the recurrent nuclear activity and the surrounding interstellar medium.

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arXiv2020

Collimated synchrotron threads linking the radio lobes of ESO137-006

We present MeerKAT 1000 MHz and 1400 MHz observations of a bright radio galaxy in the southern hemisphere, ESO~137-006. The galaxy lies at the centre of the massive and merging Norma galaxy cluster. The MeerKAT continuum images (rms ~0.02 mJy/beam at ~10" resolution) reveal new features that have never been seen in a radio galaxy before: collimated synchrotron threads of yet unknown origin, which link the extended and bent radio lobes of ESO~137-006. The most prominent of these threads stretches in projection for about 80 kpc and is about 1 kpc in width. The radio spectrum of the threads is steep, with a spectral index of up to $α\simeq 2$ between 1000 MHz and 1400 MHz.

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arXiv2019

Sardinia Radio Telescope observations of Local Group dwarf galaxies -- I. The cases of NGC6822, IC1613, and WLM

Almost all dwarf galaxies in the Local Group that are not satellites of the Milky Way or M31, belong to either one of two highly-symmetric planes. It is still a matter of debate, whether these planar structures are dynamically stable or whether they only represent a transient alignment. Proper motions, if they could be measured, could help to discriminate between these scenarios. Such motions could be determined with multi-epoch Very Long Baseline Interferometry (VLBI) of sources that show emission from water and methanol at frequencies of 22 and 6.7 GHz, respectively. We report searches for such masers. We have mapped three Local Group galaxies, NGC6822, IC1613 and WLM in the bands covering the water vapor and methanol lines. These systems are members of the two above mentioned planes of galaxies. We have produced deep radio continuum (RC) maps and spectral line cubes. The former have been used to identify star forming regions and to derive global galactic star formation rates (SFRs). These SFRs turn out to be lower than those determined at other wavelengths in two of our sources. This indicates that dwarf galaxies may follow predictions on the RC-SFR relation only in individual regions of enhanced radio continuum emission, but not when considering the entire optical body of the sources. No methanol or water maser emission has been confidently detected, down to line luminosity limits of ~0.004 and 0.01 solar luminosities, respectively. This finding is consistent with the small sizes, low SFRs and metallicities of these galaxies.

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arXiv2019

The flickering nuclear activity of Fornax A

We present new observations of Fornax A taken at 1 GHz with the MeerKAT telescope and at 6 GHz with the Sardinia Radio Telescope (SRT). The sensitive (noise ~16 micro-Jy beam$^{-1}$), high resolution ( < 10'') MeerKAT images show that the lobes of Fornax A have a double-shell morphology, where dense filaments are embedded in a diffuse and extended cocoon. We study the spectral properties of these components by combining the MeerKAT and SRT observations with archival data between 84 MHz and 217 GHz. For the first time, we show that multiple episodes of nuclear activity must have formed the extended radio lobes. The modelling of the radio spectrum suggests that the last episode of injection of relativistic particles into the lobes started ~ 24 Myr ago and stopped approximately 12 Myr ago. More recently (~ 3 Myr ago), a less powerful and short ( < 1 Myr) phase of nuclear activity generated the central jets. Currently, the core may be in a new active phase. It appears that Fornax A is rapidly flickering. The dense environment in which Fornax A lives has lead to a complex recent merger history for this galaxy, including mergers spanning a range of gas contents and mass ratios, as shown by the analysis of the galaxy's stellar- and cold-gas phases. This complex recent history may be the cause of the rapid, recurrent nuclear activity of Fornax A.

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arXiv2019

NGC 326: X-shaped no more

We present new 144-MHz LOFAR observations of the prototypical `X-shaped' radio galaxy NGC 326, which show that the formerly known wings of the radio lobes extend smoothly into a large-scale, complex radio structure. We argue that this structure is most likely the result of hydrodynamical effects in an ongoing group or cluster merger, for which pre-existing X-ray and optical data provide independent evidence. The large-scale radio structure is hard to explain purely in terms of jet reorientation due to the merger of binary black holes, a previously proposed explanation for the inner structure of NGC 326. For this reason, we suggest that the simplest model is one in which the merger-related hydrodynamical processes account for all the source structure, though we do not rule out the possibility that a black hole merger has occurred. Inference of the black hole-black hole merger rate from observations of X-shaped sources should be carried out with caution in the absence of deep, sensitive low-frequency observations. Some X-shaped sources may be signposts of cluster merger activity, and it would be useful to investigate the environments of these objects more generally.

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arXiv2019

Strong evidence of Anomalous Microwave Emission from the flux density spectrum of M31

We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7GHz with the Sardinia Radio Telescope. We mapped the radio emission in the C-band, re-analyzed WMAP and Planck maps, as well as other ancillary data, and we have derived an overall integrated flux density spectrum from the radio to the infrared. This allowed us to estimate the emission budget from M31. Integrating over the whole galaxy, we found strong and highly significant evidence for anomalous microwave emission (AME), at the level of (1.45+0.17-0.19)Jy at the peaking frequency of ~25GHz. Decomposing the spectrum into known emission mechanisms such as free-free, synchrotron, thermal dust, and AME arising from electric dipole emission from rapidly rotating dust grains, we found that the overall emission from M31 is dominated, at frequencies below 10GHz, by synchrotron emission with a spectral index of -1.10+0.10-0.08, with subdominant free-free emission. At frequencies >10GHz, AME has a similar intensity to that of synchrotron and free-free emission, overtaking them between 20GHz and 50GHz, whereas thermal dust emission dominates the emission budget at frequencies above 60GHz, as expected.

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