arXiv subjects

H. T. Intema

Publications and source records attributed to H. T. Intema.

At least 73 records · Page 4Linked to original sources

arXiv2015

Radio halos in SZ-selected clusters of galaxies: the making of a halo?

Radio halos are synchrotron radio sources detected in some massive galaxy clusters. Their Mpc-size indicates that (re)acceleration processes are taking place in the host cluster. X-ray catalogues of galaxy clusters have been used in the past to search for radio halos and to understand their connection with cluster-cluster mergers and with the thermal component of the intra-cluster medium. More recently, the Sunyaev-Zel'dovich effect has been proven to be a better route to search for massive clusters in a wider redshift range. With the aim of discovering new radio halos and understanding their connection with cluster-cluster mergers, we have selected from the Planck Early source catalog the most massive clusters, and we have observed with the Giant Metrewave Radio Telescope at 323 MHz those objects for which deep observations were not available. We have discovered new peculiar radio emission in three of the observed clusters finding: (i) a radio halo in the cluster RXCJ0949.8+1708; (ii) extended emission in Abell 1443 that we classify as a radio halo plus a radio relic, with a bright filament embedded in the radio halo; (iii) low-power radio emission is found in CIZA J1938.3+5409 which is ten times below the radio - X-ray correlation, and represents the first direct detection of the radio emission in the "upper-limit" region of the radio - X-ray diagram. We discuss the properties of these new radio halos in the framework of theoretical models for the radio emission.

astro-ph.CO↗

arXiv2015

A powerful double radio relic system discovered in PSZ1 G108.18-11.53: evidence for a shock with non-uniform Mach number?

Diffuse radio emission in the form of radio halos and relics has been found in a number of merging galaxy clusters. These structures indicate that shock and turbulence associated with the merger accelerate electrons to relativistic energies. We report the discovery of a radio relic + radio halo system in PSZ1 G108.18-11.53 (z=0.335). This cluster hosts the second most powerful double radio relic system ever discovered. We observed PSZ1 G108.18-11.53 with the Giant Meterwave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). We obtained radio maps at 147, 323, 607 and 1380 MHz. We also observed the cluster with the Keck telescope, obtaining the spectroscopic redshift for 42 cluster members. From the injection index we obtained the Mach number of the shocks generating the two radio relics. For the southern shock we found M = 2.33^{+0.19}_{-0.26}, while the northern shock Mach number goes from M = 2.20^{+0.07}_{-0.14} in the north part down to M = 2.00^{+0.03}_{-0.08} in the southern region. If the relation between the injection index and the Mach number predicted by diffusive shock acceleration (DSA) theory holds, this is the first observational evidence for a gradient in the Mach number along a galaxy cluster merger shock.

astro-ph.HE↗

arXiv2015

Resolving the Merging Planck Cluster PLCK G147.3-16.6 with GISMO

The $Planck$ satellite has recently completed an all-sky galaxy cluster survey exploiting the thermal Sunyaev-Zel'dovich (SZ) effect to locate some of the most massive systems observable. With a median redshift of $\left =0.22$, the clusters found by $Planck$ at $z>0.3$ are proving to be exceptionally massive and/or disturbed systems. One notable $Planck$ discovery at $z=0.645$, PLCK G147.3-16.6, has a dual core and hosts a radio halo, indicating it is likely in the process of merging. We present a 16."5 resolution SZ observation of this high-$z$ merger using the Goddard-IRAM Superconducting 2 Millimeter Observer (GISMO), and compare it to X-ray follow-up observations with XMM-$Newton$. We find the SZ pressure substructure is offset from the core components seen in X-ray. We interpret this offset as possible line of sight temperature or density substructure due to the on-going merger.

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arXiv2015

Physical properties of z>4 submillimeter galaxies in the COSMOS field

We study the physical properties of a sample of 6 SMGs in the COSMOS field, spectroscopically confirmed to lie at z>4. We use new GMRT 325 MHz and 3 GHz JVLA data to probe the rest-frame 1.4 GHz emission at z=4, and to estimate the sizes of the star-forming (SF) regions of these sources, resp. Combining our size estimates with those available in the literature for AzTEC1 and AzTEC3 we infer a median radio-emitting size for our z>4 SMGs of (0.63"+/-0.12")x(0.35"+/-0.05") or 4.1x2.3 kpc^2 (major times minor axis; assuming z=4.5) or lower if we take the two marginally resolved SMGs as unresolved. This is consistent with the sizes of SF regions in lower-redshift SMGs, and local normal galaxies, yet higher than the sizes of SF regions of local ULIRGs. Our SMG sample consists of a fair mix of compact and more clumpy systems with multiple, perhaps merging, components. With an average formation time of ~280 Myr, derived through modeling of the UV-IR SEDs, the studied SMGs are young systems. The average stellar mass, dust temperature, and IR luminosity we derive are M*~1.4x10^11 M_sun, T_dust~43 K, and L_IR~1.3x10^13L_sun, resp. The average L_IR is up to an order of magnitude higher than for SMGs at lower redshifts. Our SMGs follow the correlation between dust temperature and IR luminosity as derived for Herschel-selected 0.1 =1.95+/-0.26 for our sample, compared to q~2.6 for IR luminous galaxies at z<2). In summary, we find that the physical properties derived for our z>4 SMGs put them at the high end of the L_IR-T_dust distribution of SMGs, and that our SMGs form a morphologically heterogeneous sample. Thus, further in-depth analyses of large, statistical samples of high-redshift SMGs are needed to fully understand their role in galaxy formation and evolution.

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arXiv2014

Tracing the merger history of MACS clusters using the GMRT

Galaxy-cluster merger shocks efficiently accelerate the ambient electrons through diffusive shock acceleration and amplify magnetic field by compressing the Inter Cluster Medium. As a result, such objects produce a significant amount of synchrotron radio emission. Radio halo and Mpc scale peripheral radio relics thus help us to trace back the cluster formation history. To study the dynamical state of the galaxy clusters and their relation to the merging scenario, as a pilot project, we thus observed four suitable candidates from the MACS (Massive Cluster Survey) cluster sample. Observations were carried out simultaneously at 610 and 235 MHz with the GMRT. We observed a rare phenomena in the cluster MACSJ0014.3-3022, which hosts both a peripheral spectacular radio relic and an unusually flat spectrum central giant radio halo of more than 1.5 Mpc dimension. We also report that another cluster MACSJ0152.5-2852 is possibly showing the sign of one of the earliest and young merging system where, we saw a faint 0.5 Mpc radio relic at the cluster periphery.

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arXiv2014

LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources

We present LOFAR Low Band observations of the Bootes and 3C295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam$^{-1}$, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17 to 52 deg$^{2}$. From the observations we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and VLA 74 MHz differential source counts, scaling with a spectral index of $-0.7$. We find that a spectral index scaling of $-0.5$ is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens towards lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum ($α< -1.1$) radio sources, that could be associated with massive high redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz and 1.4 GHz for sources in the Boötes field. We cross-correlate these radio sources with optical and infrared catalogues and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the $ 0.7 \lesssim z \lesssim 2.5$ range.

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arXiv2014

A new double radio relic in PSZ1 G096.89+24.17 and a radio relic mass-luminosity relation

Radio relics are diffuse synchrotron sources in galaxy clusters that are believed to trace large-scale shock waves. We have discovered a new double radio relic system in PSZ1 G096.89+24.17 (z=0.3) and have carried out a full-polarization radio observation using the Westerbork Synthesis Radio Telescope (WSRT) at 1.4 GHz. The observation revealed the presence of two relics located on the two diametrically opposite sides of the cluster and hints of a central radio halo. The linear sizes of the relics are $\sim 0.9$ and $\sim 1.4$ Mpc. We carried out an analysis of all known double radio relics by using radio, X-ray and Sunyaev-Zeldovich (SZ) data. We find that the radio luminosity of double relics is a steep function of the cluster mass, with $L_R \propto M^{2.83\pm0.39}$. If we include single radio relics, this relation is maintained. This dependence has implications for the origin of magnetic fields at the relic's locations.

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arXiv2014

A giant radio halo in the cool core cluster CL1821+643

Giant radio halos are Mpc-size sources found in some merging galaxy clusters. The synchrotron emitting electrons are thought to be (re)accelerated by plasma turbulence induced by the merging of two massive clusters. Cool core galaxy clusters have a low temperature core, likely an indication that a major merger has not recently occurred. CL1821+643 is one of the strongest cool core clusters known so far. Surprisingly, we detect a giant radio halo with a largest linear size of $\sim$ 1.1 Mpc. We discuss the radio and X-ray properties of the cluster in the framework of the proposed models for giant radio halos. If a merger is causing the radio emission, despite the presence of a cool-core, we suggest that it should be off-axis, or in an early phase, or a minor one.

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arXiv2014

A distant radio mini-halo in the Phoenix galaxy cluster

We report the discovery of extended radio emission in the Phoenix cluster (SPT-CL J2344-4243, z=0.596) with the GMRT at 610 MHz. The diffuse emission extends over a region of at least 400-500 kpc and surrounds the central radio source of the Brightest Cluster Galaxy, but does not appear to be directly associated with it. We classify the diffuse emission as a radio mini-halo, making it the currently most distant mini-halo known. Radio mini-halos have been explained by synchrotron emitting particles re-accelerated via turbulence, possibly induced by gas sloshing generated from a minor merger event. Chandra observations show a non-concentric X-ray surface brightness distribution, which is consistent with this sloshing interpretation. The mini-halo has a flux density of $17\pm5$ mJy, resulting in a 1.4 GHz radio power of ($10.4\pm3.5) \times 10^{24}$ W Hz$^{-1}$. The combined cluster emission, which includes the central compact radio source, is also detected in a shallow GMRT 156 MHz observation and together with the 610 MHz data we compute a spectral index of $-0.84\pm0.12$ for the overall cluster radio emission. Given that mini-halos typically have steeper radio spectra than cluster radio galaxies, this spectral index should be taken as an upper limit for the mini-halo.

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arXiv2014

The diffuse radio emission around NGC 5580 and NGC 5588

The galaxy pair NGC 5580 and NGC 5588 are part of a loose group of galaxies. They are surrounded by steep-spectrum, extended radio emission which was previously suggested to be a down-scaled example of Mpc-size radio haloes present in galaxies clusters. We present a multi-frequency study of the radio-emission aimed to clarify its nature. The source has been observed with the Giant Meterwave Radio Telescope at 235, 325 and 610 MHz and the images obtained were combined with archival data to cover the frequency range 150-1400 MHz. The new observations revealed the presence of a second, fainter lobe on the South-East of NGC 5580. The spectral index study of the source shows a flattening of the spectrum (which implies a younger particle population) close to the two galaxies. We argue that the extended radio emission is the remnant of a past activity cycle of the active galactic nucleus present in NGC 5580 and therefore a notable example of a dying radio galaxy located outside a dense environments.

astro-ph.GA↗

arXiv2014

SPAM: A data reduction recipe for high-resolution, low-frequency radio-interferometric observations

High-resolution astronomical imaging at sub-GHz radio frequencies has been available for more than 15 years, with the VLA at 74 and 330 MHz, and the GMRT at 150, 240, 330 and 610 MHz. Recent developments include wide-bandwidth upgrades for VLA and GMRT, and commissioning of the aperture-array-based, multi-beam telescope LOFAR. A common feature of these telescopes is the necessity to deconvolve the very many detectable sources within their wide fields-of-view and beyond. This is complicated by gain variations in the radio signal path that depend on viewing direction. One such example is phase errors due to the ionosphere. Here I discuss the inner workings of SPAM, a set of AIPS-based data reduction scripts in Python that includes direction-dependent calibration and imaging. Since its first version in 2008, SPAM has been applied to many GMRT data sets at various frequencies. Many valuable lessons were learned, and translated into various SPAM software modifications. Nowadays, semi-automated SPAM data reduction recipes can be applied to almost any GMRT data set, yielding good quality continuum images comparable with (or often better than) hand-reduced results. SPAM is currently being migrated from AIPS to CASA with an extension to handle wide bandwidths. This is aimed at providing users of the VLA low-band system and the upcoming wide-bandwidth GMRT with the necessary data reduction tools.

astro-ph.IM↗

arXiv2014

Discovery of large-scale diffuse radio emission and of a new galaxy cluster in the surroundings of MACSJ0520.7-1328

We report the discovery of large-scale diffuse radio emission South-East of the galaxy cluster MACS J0520.7-1328, detected through high sensitivity Giant Metrewave Radio Telescope 323 MHz observations. This emission is dominated by an elongated diffuse radio source and surrounded by other features of lower surface brightness. Patches of these faint sources are marginally detected in a 1.4 GHz image obtained through a re-analysis of archival NVSS data. Interestingly, the elongated radio source coincides with a previously unclassified extended X-ray source. We perform a multi-wavelength analysis based on archival infrared, optical and X-ray Chandra data. We find that this source is a low-temperature (~3.6 keV) cluster of galaxies, with indications of a disturbed dynamical state, located at a redshift that is consistent with the one of the main galaxy cluster MACS J0520.7-132 (z=0.336). We suggest that the diffuse radio emission is associated with the non-thermal components in the intracluster and intergalactic medium in and around the newly detected cluster. We are planning deeper multi-wavelength and multi-frequency radio observations to accurately investigate the dynamical scenario of the two clusters and to address more precisely the nature of the complex radio emission.

astro-ph.CO↗

arXiv2014

Evidence for particle re-acceleration in the radio relic in the galaxy cluster PLCKG287.0 +32.9

Radio relics are diffuse radio sources observed in galaxy clusters, probably produced by shock acceleration during cluster-cluster mergers. Their large size, of the order of 1 Mpc, indicates that the emitting electrons need to be (re)accelerated locally. The usually invoked Diffusive Shock Acceleration models have been challenged by recent observations and theory. We report the discovery of complex radio emission in the Galaxy cluster PLCKG287.0 +32.9, which hosts two relics, a radio halo, and several radio filamentary emission. Optical observations suggest that the cluster is elongated, likely along an intergalactic filament, and displays a significant amount of substructure. The peculiar features of this radio relic are that (i) it appears to be connected to the lobes of a radio galaxy and (ii) the radio spectrum steepens on either side of the radio relic. We discuss the origins of these features in the context of particle re-acceleration.

astro-ph.CO↗

arXiv2014

The discovery of a radio halo in PLCK G147.3-16.6 at z=0.65

Recent X-ray and Sunyaev-Zel'dovich (SZ) observations have dramatically increased the number of known distant galaxy clusters. In some merging, low-redshift (z<0.4) clusters, centrally-located, diffuse, extended radio emission (called radio halos) has been found. Using the Giant Metrewave Radio Telescope (GMRT), we report the detection of diffuse radio emission in the binary-merging cluster PLCK G147.3-16.6 located at z = 0.65. We classify the emission as a giant radio halo due to the large physical extent of about 0.9 Mpc and low-surface brightness. We measure an integrated flux density of $7.3\pm1.1$ mJy at 610 MHz for the radio halo, resulting in a 1.4 GHz radio power of $5.1 \times 10^{24}$ W/Hz. The radio halo power is consistent with that expected from the known correlation between X-ray luminosity or the cluster integrated SZ signal and radio power. Our observations also suggest that more of these distant radio halos could be discovered with the GMRT.

astro-ph.CO↗

arXiv2013

Discovery of spectral curvature in the shock downstream region: CIZA J2242.8+5301

Giant cluster radio relics are thought to form at shock fronts in the course of collisions between galaxy clusters. Via processes that are still poorly understood, these shocks accelerate or re-accelerate cosmic-ray electrons and might amplify magnetic fields. The best object to study this phenomenon is the galaxy cluster CIZA J2242.8+5301 as it shows the most undisturbed relic. By means of Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT) data at seven frequencies spanning from 153 MHz to 2272 MHz, we study the synchrotron emission in this cluster. We aim at distinguishing between theoretical injection and acceleration models proposed for the formation of radio relics. We also study the head-tail radio sources to reveal the interplay between the merger and the cluster galaxies. We produced spectral index, curvature maps and radio colour-colour plots and compared our data with predictions from models. We present one of the deepest 153 MHz maps of a cluster ever produced, reaching a noise level of 1.5 mJy/beam. We derive integrated spectra for four relics in the cluster, discovering extremely steep spectrum diffuse emission concentrated in multiple patches. We find a possible radio phoenix embedded in the relic to the south of the cluster. The spectral index of the northern relic retains signs of steepening from the front towards the back of the shock also at the radio frequencies below 600 MHz. The spectral curvature in the same relic also increases in the downstream area. The data is consistent with the Komissarov-Gubanov injection models, meaning that the emission we observe is produced by a single burst of spectrally-aged accelerated radio electrons.

astro-ph.CO↗

arXiv2013

153 MHz GMRT follow-up of steep-spectrum diffuse emission in galaxy clusters

In this paper we present new high sensitivity 153 MHz Giant Meterwave Radio Telescope follow-up observations of the diffuse steep spectrum cluster radio sources in the galaxy clusters Abell 521, Abell 697, Abell 1682. Abell 521 hosts a relic, and together with Abell 697 it also hosts a giant very steep spectrum radio halo. Abell 1682 is a more complex system with candidate steep spectrum diffuse emission. We imaged the diffuse radio emission in these clusters at 153 MHz, and provided flux density measurements of all the sources at this frequency. Our new flux density measurements, coupled with the existing data at higher frequencies, allow us to study the total spectrum of the halos and relic over at least one order of magnitude in frequency. Our images confirm the presence of a very steep "diffuse component" in Abell 1682. We found that the spectrum of the relic in Abell 521 can be fitted by a single power-law with $α=1.45\pm0.02$ from 153 MHz to 5 GHz. Moreover, we confirm that the halos in Abell 521 and Abell 697 have a very steep spectrum, with $α=1.8-1.9$ and $α=1.52\pm0.05$ respectively. Even with the inclusion of the 153 MHz flux density information it is impossible to discriminate between power-law and curved spectra, as derived from homogeneous turbulent re-acceleration. The latter are favored on the basis of simple energetic arguments, and we expect that LOFAR will finally unveil the shape of the spectra of radio halos below 100 MHz, thus providing clues on their origin.

astro-ph.CO↗

arXiv2012

T-RaMiSu: The Two-meter Radio Mini Survey I. The Bootes Field

We present wide area, deep, high-resolution 153 MHz GMRT observations of the NOAO Bootes field, adding to the extensive, multi-wavelength data of this region. The observations, data reduction, and catalogue construction and description are described here. The seven pointings produced a final mosaic covering 30 square degrees with a resolution of 25". The rms noise is 2 mJy/beam in the centre of the image, rising to 4-5 mJy/beam on the edges, with an average of 3 mJy/beam. Seventy-five per cent of the area has an rms < 4 mJy/beam. The extracted source catalogue contains 1289 sources detected at 5σ, of which 453 are resolved. We estimate the catalogue to be 92 per cent reliable and 95 per cent complete at an integrated flux density limit of 14 mJy. The flux densities and astrometry have been corrected for systematic errors. We calculate the differential source counts {which are in good agreement with those in the literature and provide an important step forward in quantifying the source counts at these low frequencies and low flux densities}. The GMRT 153 MHz sources have been matched to the 1.4 GHz NVSS and 327 MHz WENSS catalogues and spectral indices were derived.

astro-ph.CO↗

arXiv2012

The "toothbrush-relic": evidence for a coherent linear 2-Mpc scale shock wave in a massive merging galaxy cluster?

Some merging galaxy clusters host diffuse extended radio emission, so-called radio halos and relics. Here we present observations between 147 MHz and 4.9 GHz of a new radio-selected galaxy cluster 1RXS J0603.3+4214 (z=0.225). The cluster is also detected as an extended X-ray source in the RASS. It hosts a large bright 1.9 Mpc radio relic, an elongated ~2 Mpc radio halo, and two smaller radio relics. The large radio relic has a peculiar linear morphology. For this relic we observe a clear spectral index gradient, in the direction towards the cluster center. We performed Rotation Measure (RM) Synthesis between 1.2 and 1.7 GHz. The results suggest that for the west part of the large relic some of the Faraday rotation is caused by ICM and is not only due to galactic foregrounds. We also carried out a detailed spectral analysis of this radio relic and created radio color-color diagrams. We find (i) an injection spectral index of -0.6 to -0.7, (ii) steepening spectral index and increasing spectral curvature in the post-shock region, and (iii) an overall power-law spectrum between 74 MHz and 4.9 GHz with α=-1.10 \pm 0.02. Mixing of emission in the beam from regions with different spectral ages is probably the dominant factor that determines the shape of the radio spectra. Changes in the magnetic field, total electron content, or adiabatic gains/losses do not play a major role. A model in which particles are (re)accelerated in a first order Fermi process at the front of the relic provides the best match to the observed spectra. We speculate that in the post-shock region particles are re-accelerated by merger induced turbulence to form the radio halo as the relic and halo are connected. The 1RXS J0603.3+4214 merger is probably more complex than the "simple'" binary merger events that are thought to give rise to symmetric double radio relics.

astro-ph.CO↗