arXiv subjects

H. Andernach

Publications and source records attributed to H. Andernach.

At least 37 records · Page 2Linked to original sources

arXiv2020

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

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 $\sim$50$'$ or $\sim$ 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 $\langle J \rangle_{1\,{\rm GHz}}< 1.2 \times 10^{-44} {\rm W}\, {\rm Hz}^{-1} {\rm m}^{-3}$. 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 $\approx 1.7$ 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.

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arXiv2020

PKS 2250$-$351: A Giant Radio Galaxy in Abell 3936

We present a detailed analysis of the radio galaxy PKS 2250-351, a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and infra-red data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS 2250-351 lies at z=0.2115 in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, `red and dead' elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-infrared emission. Assuming it lies on the local M-sigma relation it has an Eddington accretion rate of lambda_EDD~0.014. We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission (~300 Myr) the accretion has switched from an inefficient advection dominated mode to a thin-disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS 2250-351 lying to the west of the densest concentration of galaxies in Abell 3936.

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arXiv2019

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science case and survey design

The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ($\approx$2.5"), sensitivity (a 1$σ$ goal of 70 $μ$Jy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination $>-40^{\circ}$), a total of 33,885 deg$^2$. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.

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arXiv2019

The ASKAP-EMU Early Science Project:Radio Continuum Survey of the Small Magellanic Cloud

We present two new radio continuum images from the Australian Square Kilometre Array Pathfinder (ASKAP) survey in the direction of the Small Magellanic Cloud (SMC). These images are part of the Evolutionary Map of the Universe (EMU) Early Science Project (ESP) survey of the Small and Large Magellanic Clouds. The two new source lists produced from these images contain radio continuum sources observed at 960 MHz (4489 sources) and 1320 MHz (5954 sources) with a bandwidth of 192 MHz and beam sizes of 30.0"x30.0" and 16.3"x15.1", respectively. The median Root Mean Squared (RMS) noise values are 186$μ$Jy beam$^{-1}$ (960 MHz) and 165$μ$Jy beam$^{-1}$ (1320 MHz). To create point source catalogues, we use these two source lists, together with the previously published Molonglo Observatory Synthesis Telescope (MOST) and the Australia Telescope Compact Array (ATCA) point source catalogues to estimate spectral indices for the whole population of radio point sources found in the survey region. Combining our ASKAP catalogues with these radio continuum surveys, we found 7736 point-like sources in common over an area of 30 deg$^2$. In addition, we report the detection of two new, low surface brightness supernova remnant candidates in the SMC. The high sensitivity of the new ASKAP ESP survey also enabled us to detect the bright end of the SMC planetary nebula sample, with 22 out of 102 optically known planetary nebulae showing point-like radio continuum emission. Lastly, we present several morphologically interesting background radio galaxies.

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arXiv2019

Differences in Faraday Rotation Between Adjacent Extragalactic Radio Sources as a Probe of Cosmic Magnetic Fields

Faraday rotation measures (RMs) of extragalactic radio sources provide information on line-of-sight magnetic fields, including contributions from our Galaxy, source environments, and the intergalactic medium (IGM). Looking at differences in RMs, $Δ$RM, between adjacent sources on the sky can help isolate these different components. In this work, we classify adjacent polarized sources in the NVSS as random or physical pairs. We recompute and correct the uncertainties in the NVSS RM catalog, since these were significantly overestimated. Our sample contains 317 physical and 5111 random pairs, all with Galactic latitudes $|b|\ge20^{\circ}$, polarization fractions $\ge2\%$, and angular separations between $1.^{'},$ and $20^{'}$. We find an rms $Δ$RM of $14.9\pm0.4\,$rad m$^{-2}$ and $4.6\pm1.1\,$rad m$^{-2}$ for random and physical pairs, respectively. This means polarized extragalactic sources that are close on the sky, but at different redshifts, have larger differences in RM than two components of one source. This difference of $\sim10\,$rad m$^{-2}$ is significant at $5σ$, and persists in different data subsamples. While there have been other statistical studies of $Δ$RM between adjacent polarized sources, this is the first unambiguous demonstration that some of this RM difference must be extragalactic, thereby providing a firm upper limit on the RM contribution of the IGM. If the $Δ$RMs originate local to the sources, then the local magnetic field difference between random sources is a factor of two larger than between components of one source. Alternatively, attributing the difference in $Δ$RMs to the intervening IGM yields an upper limit on the IGM magnetic field strength of $40\,$nG.

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arXiv2018

The intergalactic magnetic field probed by a giant radio galaxy

Cosmological simulations predict that an intergalactic magnetic field (IGMF) pervades the large scale structure (LSS) of the Universe. Measuring the IGMF is important to determine its origin (i.e. primordial or otherwise). Using data from the LOFAR Two Metre Sky Survey (LoTSS), we present the Faraday rotation measure (RM) and depolarisation properties of the giant radio galaxy J1235+5317, at a redshift of $z = 0.34$ and 3.38 Mpc in size. We find a mean RM difference between the lobes of $2.5\pm0.1$ rad/m$^2$ , in addition to small scale RM variations of ~0.1 rad/m$^2$ . From a catalogue of LSS filaments based on optical spectroscopic observations in the local universe, we find an excess of filaments intersecting the line of sight to only one of the lobes. Associating the entire RM difference to these LSS filaments leads to a gas density-weighted IGMF strength of ~0.3 μG. However, direct comparison with cosmological simulations of the RM contribution from LSS filaments gives a low probability (~5%) for an RM contribution as large as 2.5 rad/m$^2$ , for the case of IGMF strengths of 10 to 50 nG. It is likely that variations in the RM from the Milky Way (on 11' scales) contribute significantly to the mean RM difference, and a denser RM grid is required to better constrain this contribution. In general, this work demonstrates the potential of the LOFAR telescope to probe the weak signature of the IGMF. Future studies, with thousands of sources with high accuracy RMs from LoTSS, will enable more stringent constraints on the nature of the IGMF.

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arXiv2018

Radio Galaxy Zoo: Observational evidence for environment as the cause of radio source asymmetry

We investigate the role of environment on radio galaxy properties by constructing a sample of large ($\gtrsim100$~kpc), nearby ($z<0.3$) radio sources identified as part of the Radio Galaxy Zoo citizen science project. Our sample consists of 16 Fanaroff-Riley Type II (FR-II) sources, 6 FR-I sources, and one source with a hybrid morphology. FR-I sources appear to be hosted by more massive galaxies, consistent with previous studies. In the FR-II sample, we compare the degree of asymmetry in radio lobe properties to asymmetry in the radio source environment, quantified through optical galaxy clustering. We find that the length of radio lobes in FR-II sources is anti-correlated with both galaxy clustering and lobe luminosity. These results are in quantitative agreement with predictions from radio source dynamical models, and suggest that galaxy clustering provides a useful proxy for the ambient gas density distribution encountered by the radio lobes.

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arXiv2018

Radio Galaxy Zoo: Machine learning for radio source host galaxy cross-identification

We consider the problem of determining the host galaxies of radio sources by cross-identification. This has traditionally been done manually, which will be intractable for wide-area radio surveys like the Evolutionary Map of the Universe (EMU). Automated cross-identification will be critical for these future surveys, and machine learning may provide the tools to develop such methods. We apply a standard approach from computer vision to cross-identification, introducing one possible way of automating this problem, and explore the pros and cons of this approach. We apply our method to the 1.4 GHz Australian Telescope Large Area Survey (ATLAS) observations of the Chandra Deep Field South (CDFS) and the ESO Large Area ISO Survey South 1 (ELAIS-S1) fields by cross-identifying them with the Spitzer Wide-area Infrared Extragalactic (SWIRE) survey. We train our method with two sets of data: expert cross-identifications of CDFS from the initial ATLAS data release and crowdsourced cross-identifications of CDFS from Radio Galaxy Zoo. We found that a simple strategy of cross-identifying a radio component with the nearest galaxy performs comparably to our more complex methods, though our estimated best-case performance is near 100 per cent. ATLAS contains 87 complex radio sources that have been cross-identified by experts, so there are not enough complex examples to learn how to cross-identify them accurately. Much larger datasets are therefore required for training methods like ours. We also show that training our method on Radio Galaxy Zoo cross-identifications gives comparable results to training on expert cross-identifications, demonstrating the value of crowdsourced training data.

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arXiv2017

Radio Galaxy Zoo: A Search for Hybrid Morphology Radio Galaxies

Hybrid morphology radio sources are a rare type of radio galaxy that display different Fanaroff-Riley classes on opposite sides of their nuclei. To enhance the statistical analysis of hybrid morphology radio sources, we embarked on a large-scale search of these sources within the international citizen science project, Radio Galaxy Zoo (RGZ). Here, we present 25 new candidate hybrid morphology radio galaxies. Our selected candidates are moderate power radio galaxies (L_median = 4.7x10^{24} W/(Hz sr) at redshifts 0.14 1Mpc) radio galaxies, one resides at a centre of a galaxy cluster, and one is hosted by a rare green bean galaxy. Although the origin of the hybrid morphology radio galaxies is still unclear, this type of radio source starts depicting itself as a rather diverse class. We discuss hybrid radio morphology formation in terms of the radio source environment (nurture) and intrinsically occurring phenomena (nature; activity cessation and amplification), showing that these peculiar radio galaxies can be formed by both mechanisms. While high angular resolution follow-up observations are still necessary to confirm our candidates, we demonstrate the efficacy of the Radio Galaxy Zoo in the pre-selection of these sources from all-sky radio surveys, and report the reliability of citizen scientists in identifying and classifying complex radio sources.

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arXiv2017

Radio Galaxy Zoo: Cosmological Alignment of Radio Sources

We study the mutual alignment of radio sources within two surveys, FIRST and TGSS. This is done by producing two position angle catalogues containing the preferential directions of respectively $30\,059$ and $11\,674$ extended sources distributed over more than $7\,000$ and $17\,000$ square degrees. The identification of the sources in the FIRST sample was performed in advance by volunteers of the Radio Galaxy Zoo project, while for the TGSS sample it is the result of an automated process presented here. After taking into account systematic effects, marginal evidence of a local alignment on scales smaller than $2.5°$ is found in the FIRST sample. The probability of this happening by chance is found to be less than $2$ per cent. Further study suggests that on scales up to $1.5°$ the alignment is maximal. For one third of the sources, the Radio Galaxy Zoo volunteers identified an optical counterpart. Assuming a flat $Λ$CDM cosmology with $Ω_m = 0.31, Ω_Λ= 0.69$, we convert the maximum angular scale on which alignment is seen into a physical scale in the range $[19, 38]$ Mpc $h_{70}^{-1}$. This result supports recent evidence reported by Taylor and Jagannathan of radio jet alignment in the $1.4$ deg$^2$ ELAIS N1 field observed with the Giant Metrewave Radio Telescope. The TGSS sample is found to be too sparsely populated to manifest a similar signal.

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arXiv2016

Radio Galaxy Zoo: discovery of a poor cluster through a giant wide-angle tail radio galaxy

We have discovered a previously unreported poor cluster of galaxies (RGZ-CL J0823.2+0333) through an unusual giant wide-angle tail radio galaxy found in the Radio Galaxy Zoo project. We obtained a spectroscopic redshift of $z=0.0897$ for the E0-type host galaxy, 2MASX J08231289+0333016, leading to M$_r = -22.6$ and a $1.4\,$GHz radio luminosity density of $L_{\rm 1.4} = 5.5\times10^{24}$ W Hz$^{-1}$. These radio and optical luminosities are typical for wide-angle tailed radio galaxies near the borderline between Fanaroff-Riley (FR) classes I and II. The projected largest angular size of $\approx8\,$arcmin corresponds to $800\,$kpc and the full length of the source along the curved jets/trails is $1.1\,$Mpc in projection. X-ray data from the XMM-Newton archive yield an upper limit on the X-ray luminosity of the thermal emission surrounding RGZ J082312.9+033301,at $1.2-2.6\times10^{43}$ erg s$^{-1}$ for assumed intra-cluster medium temperatures of $1.0-5.0\,$keV. Our analysis of the environment surrounding RGZ J082312.9+033301 indicates that RGZ J082312.9+033301 lies within a poor cluster. The observed radio morphology suggests that (a) the host galaxy is moving at a significant velocity with respect to an ambient medium like that of at least a poor cluster, and that (b) the source may have had two ignition events of the active galactic nucleus with $10^7\,$yrs in between. This reinforces the idea that an association between RGZ J082312.9+033301, and the newly discovered poor cluster exists.

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arXiv2015

Radio Galaxy Zoo: host galaxies and radio morphologies derived from visual inspection

We present results from the first twelve months of operation of Radio Galaxy Zoo, which upon completion will enable visual inspection of over 170,000 radio sources to determine the host galaxy of the radio emission and the radio morphology. Radio Galaxy Zoo uses $1.4\,$GHz radio images from both the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) and the Australia Telescope Large Area Survey (ATLAS) in combination with mid-infrared images at $3.4\,μ$m from the {\it Wide-field Infrared Survey Explorer} (WISE) and at $3.6\,μ$m from the {\it Spitzer Space Telescope}. We present the early analysis of the WISE mid-infrared colours of the host galaxies. For images in which there is $>\,75\%$ consensus among the Radio Galaxy Zoo cross-identifications, the project participants are as effective as the science experts at identifying the host galaxies. The majority of the identified host galaxies reside in the mid-infrared colour space dominated by elliptical galaxies, quasi-stellar objects (QSOs), and luminous infrared radio galaxies (LIRGs). We also find a distinct population of Radio Galaxy Zoo host galaxies residing in a redder mid-infrared colour space consisting of star-forming galaxies and/or dust-enhanced non star-forming galaxies consistent with a scenario of merger-driven active galactic nuclei (AGN) formation. The completion of the full Radio Galaxy Zoo project will measure the relative populations of these hosts as a function of radio morphology and power while providing an avenue for the identification of rare and extreme radio structures. Currently, we are investigating candidates for radio galaxies with extreme morphologies, such as giant radio galaxies, late-type host galaxies with extended radio emission, and hybrid morphology radio sources.

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arXiv2015

The Nature of Faint Blue Stars in the PHL and Ton Catalogues based on Digital Sky Surveys

We determined accurate positions for 3000 of the "faint blue stars" in the PHL (Palomar-Haro-Luyten) and Ton/TonS catalogues. These were published from 1957 to 1962, and, aimed at finding new white dwarfs, provide approximate positions for about 10750 blue stellar objects. Some of these "stars" had become known as quasars, a type of objects unheard-of before 1963. We derived subarcsec positions from a comparison of published finding charts with images from the first-epoch Digitized Sky Survey. Numerous objects are now well known, but unfortunately neither their PHL or Ton numbers, nor their discoverers, are recognized in current databases. A comparison with modern radio, IR, UV and X-ray surveys leads us to suggest that the fraction of extragalactic objects in the PHL and Ton catalogues is at least 15 per cent. However, because we failed to locate the original PHL plates or finding charts, it may be impossible to correctly identify the remaining 7726 PHL objects.

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arXiv2015

A Semi-automatic Search for Giant Radio Galaxy Candidates and their Radio-Optical Follow-up

We present results of a search for giant radio galaxies (GRGs) with a projected largest linear size in excess of 1 Mpc. We designed a computational algorithm to identify contiguous emission regions, large and elongated enough to serve as GRG candidates, and applied it to the entire 1.4-GHz NRAO VLA Sky survey (NVSS). In a subsequent visual inspection of 1000 such regions we discovered 15 new GRGs, as well as many other candidate GRGs, some of them previously reported, for which no redshift was known. Our follow-up spectroscopy of 25 of the brighter hosts using two 2.1-m telescopes in Mexico, and four fainter hosts with the 10.4-m Gran Telescopio Canarias (GTC), yielded another 24 GRGs. We also obtained higher-resolution radio images with the Karl G. Jansky Very Large Array for GRG candidates with inconclusive radio structures in NVSS.

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arXiv2015

Comparing narrow and broad-line AGNs, in a new diagnostic diagram for emission-line galaxies based on WISE data

Using a new color-color diagnostic diagram in the mid infrared built from WISE data, the MIRDD, we compare narrow emission-line galaxies (NELGs) that exhibit different activity types (star-forming galaxies, SFGs, and AGNs, i.e.,LINERs, Sy2s and TOs), with broad-line AGNs (QSOs and Sy1s) and BL Lac objects at low redshift ($z \le 0.25$). We show that the BL Lac objects occupy in the MIRDD the same region as the LINERs, whereas the QSOs and Sy1s occupy an intermediate region, between the LINERs and the Sy2s.In the MIRDD these galaxies trace a sequence that can be reproduced by a power law, $F_ν= ν^α$, where the spectral index, $α$, varies from 0 to $-2$, which is similar to what is observed in the optical-ultraviolet part of the spectra of AGNs with different luminosities. For the NELGs, we perform a stellar population synthesis analysis, demonstrating that the ${\rm W}2-{\rm W}3$ color is tightly correlated with the level of star formation in their host galaxies. A comparison of their MIR colors with the colors yielded by energy distributions (SEDs) of galaxies with different activity types, shows that the SED of the LINERs is similar to the SEDs of the QSOs and Sy1s, consistent with AGN galaxies with mild star formation, whereas the SEDs of the Sy2s and TOs are consistent with AGN galaxies with strong star formation components. For the BL Lac objects, we can only fit a SED that has no star formation component, consistent with AGNs in elliptical-type galaxies. From their similarities in MIR colors and SEDs, we infer that, in the nearby universe, the level of star formation activity most probably increases in the host galaxies of emission-line galaxies with different activity types along the sequence BL Lac$\rightarrow$LINER$\rightarrow$QSO/Sy1$\rightarrow$Sy2$\rightarrow$TO$\rightarrow$SFG.

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arXiv2014

Two New Catalogues of Superclusters of Abell/ACO Galaxy Clusters out to redshift 0.15

We present two new catalogues of superclusters of galaxies out to a redshit of z = 0.15, based on the Abell/ACO cluster redshift compilation maintained by one of us (HA). The first of these catalogues, the all-sky Main SuperCluster Catalogue (MSCC), is based on only the rich (A-) Abell clusters, and the second one, the Southern SuperCluster Catalogue (SSCC), covers declinations delta < -17 deg and includes the supplementary Abell S-clusters. A tunable Friends-of-Friends (FoF) algorithm was used to account for the cluster density decreasing with redshift and for different selection functions in distinct areas of the sky. We present the full list of Abell clusters used, together with their redshifts and supercluster memberships and including the isolated clusters. The SSCC contains about twice the number of superclusters than MSCC for delta < -17 deg, which we found to be due to: (1) new superclusters formed by A-clusters in their cores and surrounded by S-clusters (50%), (2) new superclusters formed by S-clusters only (40%), (3) redistribution of member clusters by fragmentation of rich (multiplicity m > 15) superclusters (8%), and (4) new superclusters formed by the connection of A-clusters through bridges of S-clusters (2%). Power-law fits to the cumulative supercluster multiplicity function yield slopes of alpha = -2.0 and alpha = -1.9 for MSCC and SSCC respectively. This power-law behavior is in agreement with the findings for other observational samples of superclusters, but not with that of catalogues based on cosmological simulations.

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arXiv2014

A mid infrared study of low-luminosity AGNs with WISE

Using data from the Wide-field Infrared Survey Explorer (WISE) we show that the mid infrared (MIR) colors of low-luminosity AGNs (LLAGNs) are significanlty different from those of post-asymptotic giant branch stars (PAGBs). This is due to a difference in spectral energy distribution (SEDs), the LLAGNs showing a flat component due to an AGN. Consistent with this interpretation we show that in a MIR color-color diagram the LINERs and the Seyfert~2s follow a power law with specific colors that allow to distinguish them from each other, and from star forming galaxies, according to their present level of star formation. Based on this result we present a new diagnostic diagram in the MIR that confirms the classification obtained in the optical using standard diagnostic diagrams, clearly identifying LINERs and LLAGNs as genuine AGNs.

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arXiv2013

What do the Star Formation Histories of Galaxies tell us about the Starburst-AGN connection?

Using a sample of 229618 narrow emission-line galaxies, we have determined the normal star formation histories (SFHs) for galaxies with different activity types: star forming galaxies (SFGs), transition type objects (TOs), Seyfert 2s (Sy2s) and LINERs. We find that the variation of the SFH with the activity type is explained by the mass of the galaxies and the importance of their bulge: the LINERs reside in massive early-type galaxies, the Sy2s and TOs are hosted by intermediate mass galaxies with intermediate morphological types, and the SFGs are found in lower mass late-type spirals. Except for the Sy2s, the more massive galaxies formed the bulk of their stars more rapidly than the less massive ones. The Sy2s formed their stars more slowly and show presently an excess in star formation. We have also found that the maximum in star formation rate in the past increases with the virial mass within the aperture (VMA), the VMA increasing from the SFGs to the TOs, to the Sy2s, culminating in the LINERs. This correlation suggests that the bulges and the supermassive black holes at the center of galaxies grow in parallel, in good agreement with the M(BH)-sigma relation.

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