arXiv subjects

K. T. Chyzy

Publications and source records attributed to K. T. Chyzy.

At least 19 recordsLinked to original sources

arXiv2023

The LOFAR LBA Sky Survey II. First data release

The Low Frequency Array (LOFAR) is the only existing radio interferometer able to observe at ultra-low frequencies (<100 MHz) with high resolution (<15") and high sensitivity (<1 mJy/beam). To exploit these capabilities, the LOFAR Surveys Key Science Project is using the LOFAR Low Band Antenna (LBA) to carry out a sensitive wide-area survey at 41-66 MHz named the LOFAR LBA Sky Survey (LoLSS). LoLSS is covering the whole northern sky above declination 24 deg with a resolution of 15" and a sensitivity of 1-2 mJy/beam (1 sigma) depending on declination, field properties, and observing conditions. Here we present the first data release. An automated pipeline was used to reduce the 95 fields included in this data release. The data reduction procedures developed for this project have general application and are currently being used to process LOFAR LBA interferometric observations. Compared to the preliminary release, direction-dependent errors have been corrected for during the calibration process. This results in a typical sensitivity of 1.55 mJy/beam at the target resolution of 15". The first data release of the LOFAR LBA Sky Survey covers 650 sqdeg in the HETDEX spring field. The resultant data products released to the community include mosaic images (I and V Stokes) of the region, and a catalogue of 42463 detected sources and related Gaussian components used to describe sources' morphologies. Separate catalogues for 6 in-band frequencies are also released. The first data release of LoLSS shows that, despite the influences of the ionosphere, LOFAR can conduct large-scale surveys in the frequency window 42-66 MHz with unprecedentedly high sensitivity and resolution. The data can be used to derive unique information on the low-frequency spectral properties of many thousands of sources with a wide range of applications in extragalactic and galactic astronomy.

astro-ph.IM↗

arXiv2022

CHANG-ES XXVI: Insights into cosmic-ray transport from radio halos in edge-on galaxies

Galactic winds play a key role in regulating the evolution of galaxies over cosmic time. In recent years, the role of cosmic rays (CR) in the formation of the galactic wind has increasingly gained attention. Therefore, we use radio continuum data to analyse the cosmic ray transport in edge-on galaxies. Data from the LOFAR Two-metre Sky Survey (LoTSS) data release 2 at 144 MHz (HBA) and reprocessed VLA data at 1.6 GHz (L-band) from the Continuum Halos in Nearby Galaxies - an EVLA Survey (CHANG-ES) enable us to increase the extent of the analysed radio continuum profile significantly (up to a factor of 2) compared to previous studies. We compute thermal emission maps using a mixture approach of H-alpha and near infrared data, which is then subtracted to yield radio synchrotron emission maps. Then we compile non-thermal spectral index maps and compute intensity profiles using a box integration approach. Lastly, we perform 1D cosmic ray transport modelling. The non-thermal spectral index maps show evidence that the LoTSS maps are affected by thermal absorption, in star forming regions. The scale height analysis reveals that most of the galaxies are equally well fitted with an one-component instead of a twocomponent exponential profile. We find a bi-modality within our sample. While NGC 3432 and NGC 4013 have similar scale heights in the L-band and HBA, the low-frequency scale heights of NGC 891, NGC 4157, and NGC 4631 exceed their high-frequency counterpart significantly. The 1D CR transport modelling shows agreement of the predicted magnetic field strength and the magnetic field strength estimates of equipartition measurements. Additionally we find an increasing difference of wind velocities (with increasing height over the galactic disk) between central and outer regions of the analysed galaxies.

astro-ph.GA↗

arXiv2021

The LOFAR LBA Sky Survey I. survey description and preliminary data release

LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (<1 mJy/b and <15") observations at ultra-low frequencies (<100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66 MHz, at a resolution of 15 arcsec and at a sensitivity of 1 mJy/b. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740 sqdeg around the HETDEX spring field region at a resolution of 47" with a median noise level of 5 mJy/b. The images and the catalogue with 25,247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1 mJy/b and the resolution of 15" once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity.

astro-ph.IM↗

arXiv2019

A LOFAR-IRAS cross-match study: the far-infrared radio correlation and the 150-MHz luminosity as a star-formation rate

Aims. We aim to study the far-infrared radio correlation (FIRC) at 150 MHz in the local Universe (at a median redshift z~0:05) and improve the use of the rest-frame 150-MHz luminosity, L150, as a star-formation rate (SFR) tracer, which is unaffected by dust extinction. Methods. We cross-match the 60-um selected Revised IRAS Faint Source Survey Redshift (RIFSCz) catalogue and the 150-MHz selected LOFAR value-added source catalogue in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) Spring Field. We estimate L150 for the cross-matched sources and compare it with the total infrared (IR) luminosity, LIR, and various SFR tracers. Results. We find a tight linear correlation between log L150 and log LIR for star-forming galaxies, with a slope of 1.37. The median qIR value (defined as the logarithm of the LIR to L150 ratio) and its rms scatter of our main sample are 2.14 and 0.34, respectively. We also find that log L150 correlates tightly with the logarithm of SFR derived from three different tracers, i.e., SFR_Halpha based on the Halpha line luminosity, SFR_60 based on the rest-frame 60-um luminosity and SFR_IR based on LIR, with a scatter of 0.3 dex. Our best-fit relations between L150 and these SFR tracers are, log L150 (Lsun) = 1.35(0.06) x log SFR_Halpha (Msun/yr) + 3.20(0.06), log L150 (Lsun) = 1.31(0.05) x log SFR_60 (Msun/yr) + 3.14(0.06), and log L150 (Lsun) = 1.37(0.05) x log SFR_IR (Msun/yr) + 3.09(0.05), which show excellent agreement with each other.

astro-ph.GA↗

arXiv2018

The Far-Infrared Radio Correlation at low radio frequency with LOFAR/H-ATLAS

The radio and far-infrared luminosities of star-forming galaxies are tightly correlated over several orders of magnitude; this is known as the far-infrared radio correlation (FIRC). Previous studies have shown that a host of factors conspire to maintain a tight and linear FIRC, despite many models predicting deviation. This discrepancy between expectations and observations is concerning since a linear FIRC underpins the use of radio luminosity as a star-formation rate indicator. Using LOFAR 150MHz, FIRST 1.4 GHz, and Herschel infrared luminosities derived from the new LOFAR/H-ATLAS catalogue, we investigate possible variation in the monochromatic (250$\mathrm{μm}$) FIRC at low and high radio frequencies. We use statistical techniques to probe the FIRC for an optically-selected sample of 4,082 emission-line classified star-forming galaxies as a function of redshift, effective dust temperature, stellar mass, specific star formation rate, and mid-infrared colour (an empirical proxy for specific star formation rate). Although the average FIRC at high radio frequency is consistent with expectations based on a standard power-law radio spectrum, the average correlation at 150MHz is not. We see evidence for redshift evolution of the FIRC at 150MHz, and find that the FIRC varies with stellar mass, dust temperature and specific star formation rate, whether the latter is probed using MAGPHYS fitting, or using mid-infrared colour as a proxy. We can explain the variation, to within 1$σ$, seen in the FIRC over mid-infrared colour by a combination of dust temperature, redshift, and stellar mass using a Bayesian partial correlation technique.

astro-ph.GA↗

arXiv2018

Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

Low-frequency radio continuum observations of edge-on galaxies are ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129-163 MHz with the LOw Frequency ARray (LOFAR) and at 13-18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1-2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free-free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 +- 2 $μ$G. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 +- 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation.

astro-ph.GA↗

arXiv2017

The LOFAR window on star-forming galaxies and AGN - curved radio SEDs and IR-radio correlation at $0 < z < 2.5$

We present a study of the low-frequency radio properties of star forming (SF) galaxies and active galactic nuclei (AGN) up to redshift $z=2.5$. The new spectral window probed by the Low Frequency Array (LOFAR) allows us to reconstruct the radio continuum emission from 150 MHz to 1.4 GHz to an unprecedented depth for a radio-selected sample of $1542$ galaxies in $\sim 7~ \rm{deg}^2$ of the LOFAR Boötes field. Using the extensive multi-wavelength dataset available in Boötes and detailed modelling of the FIR to UV spectral energy distribution (SED), we are able to separate the star-formation (N=758) and the AGN (N=784) dominated populations. We study the shape of the radio SEDs and their evolution across cosmic time and find significant differences in the spectral curvature between the SF galaxy and AGN populations. While the radio spectra of SF galaxies exhibit a weak but statistically significant flattening, AGN SEDs show a clear trend to become steeper towards lower frequencies. No evolution of the spectral curvature as a function of redshift is found for SF galaxies or AGN. We investigate the redshift evolution of the infrared-radio correlation (IRC) for SF galaxies and find that the ratio of total infrared to 1.4 GHz radio luminosities decreases with increasing redshift: $ q_{\rm 1.4GHz} = (2.45 \pm 0.04) \times (1+z)^{-0.15 \pm 0.03} $. Similarly, $q_{\rm 150MHz}$ shows a redshift evolution following $ q_{\rm 150GHz} = (1.72 \pm 0.04) \times (1+z)^{-0.22 \pm 0.05}$. Calibration of the 150 MHz radio luminosity as a star formation rate tracer suggests that a single power-law extrapolation from $q_{\rm 1.4GHz}$ is not an accurate approximation at all redshifts.

astro-ph.GA↗

arXiv2015

AGN duty cycle estimates for the ultra-steep spectrum radio relic VLSS J1431.8+1331

Steep spectrum radio sources associated with active galactic nuclei (AGN) may contain remnants of past AGN activity episodes. Novel instruments like the LOw Frequency ARray (LOFAR) are enabling studies of these fascinating structures to be made at tens to hundreds of MHz with sufficient resolution to analyse their complex morphology. Our goal is to characterize the integrated and resolved spectral properties of VLSS J1431+1331 and estimate source ages based on synchrotron radio emission models, thus putting constraints on the AGN duty cycle. Using a broad spectral coverage, we have derived spectral and curvature maps, and used synchrotron ageing models to determine the time elapsed from the last time the source plasma was energized. We used LOFAR, Giant Metrewave Radio Telescope (GMRT) and Jansky Very Large Array (VLA) data. Based on our ageing analysis, we infer that the AGN that created this source currently has very low levels of activity or that it is switched off. The derived ages for the larger source component range from around 60 to 130 Myr, hinting that the AGN activity decreased or stopped around 60 Myr ago. Our analysis suggests that VLSS J1431.8+1331 is an intriguing, two-component source. The larger component seems to host a faint radio core, suggesting that the source may be an AGN radio relic. The spectral index we observe from the smaller component is distinctly flatter at lower frequencies than the spectral index of the larger component, suggesting the possibility that the smaller component may be a shocked plasma bubble. From the integrated source spectrum, we deduce that its shape and slope can be used as tracers of the activity history of this type of steep spectrum radio source.

astro-ph.GA↗

arXiv2015

The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (~4"), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (~30"), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect HI in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 km/s), similar to what is found in other clusters. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2013

Large-scale radio continuum properties of 19 Virgo cluster galaxies The influence of tidal interactions, ram pressure stripping, and accreting gas envelopes

Deep scaled array VLA 20 and 6cm observations including polarization of 19 Virgo spirals are presented. This sample contains 6 galaxies with a global minimum of 20cm polarized emission at the receding side of the galactic disk and quadrupolar type large-scale magnetic fields. In the new sample no additional case of a ram-pressure stripped spiral galaxy with an asymmetric ridge of polarized radio continuum emission was found. In the absence of a close companion, a truncated HI disk, together with a ridge of polarized radio continuum emission at the outer edge of the HI disk, is a signpost of ram pressure stripping. 6 out of the 19 observed galaxies display asymmetric 6cm polarized emission distributions. Three galaxies belong to tidally interacting pairs, two galaxies host huge accreting HI envelopes, and one galaxy had a recent minor merger. Tidal interactions and accreting gas envelopes can lead to compression and shear motions which enhance the polarized radio continuum emission. In addition, galaxies with low average star formation rate per unit area have a low average degree of polarization. Shear or compression motions can enhance the degree of polarization. The average degree of polarization of tidally interacting galaxies is generally lower than expected for a given rotation velocity and star formation activity. This low average degree of polarization is at least partly due to the absence of polarized emission from the thin disk. Ram pressure stripping can decrease whereas tidal interactions most frequently decreases the average degree of polarization of Virgo spiral galaxies. We found that moderate active ram pressure stripping has no influence on the spectral index, but enhances the global radio continuum emission with respect to the FIR emission, while an accreting gas envelope can but not necessarily enhances the radio continuum emission with respect to the FIR emission.

astro-ph.CO↗

arXiv2012

The magnetic fields of large Virgo cluster spirals: Paper II

The Virgo cluster of galaxies provides excellent conditions for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated in detail. We extend our systematic search for possible anomalies in the magnetic field structures of Virgo cluster spirals in order to characterize a variety of effects and attribute them to different disturbing agents. Six angularly large Virgo cluster spiral galaxies (NGC4192, NGC4302, NGC4303, NGC4321, NGC4388, and NGC4535) were targets of a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85GHz and 8.35GHz (except for NGC4388 observed only at 4.85GHz, and NGC4535 observed only at 8.35GHz). Magnetic field structures distorted to various extent are found in all galaxies. Three galaxies (NGC4302, NGC4303, and NGC4321) show some signs of possible tidal interactions, while NGC4388 and NGC4535 have very likely experienced strong ram-pressure and shearing effects, respectively, visible as distortions and asymmetries of polarized intensity distributions. As in our previous study, even strongly perturbed galaxies closely follow the radio-far-infrared correlation. In NGC4303 and NGC4321, we observe symmetric spiral patterns of the magnetic field and in NGC4535 an asymmetric pattern. Magnetic fields allow us to trace even weak interactions that are difficult to detect with other observations. Our results show that the degree of distortions of a galaxy is not a simple function of the distance to the cluster center but reflects also the history of its interactions. The angle between the velocity vector and the rotation vector of a galaxy may be a general parameter that describes the level of distortions of galactic magnetic fields.

astro-ph.CO↗

arXiv2012

Tidal interaction vs. ram pressure stripping effects as seen in X-rays. Hot gas in group and cluster galaxies

The hot intracluster/intragroup medium (ICM/IGM) and a high galaxy density can lead to perturbations of the galactic interstellar medium (ISM) due to ram pressure and/or tidal interaction effects. In radio polarimetry observations, both phenomena may manifest similar features. X-ray data can help to determine the real origin of the perturbation. We analyse the distribution and physical properties of the hot gas in the Virgo cluster spiral galaxies NGC 4254 and NGC 4569, which indicate that the cluster environment has had a significant influence on their properties. By performing both spatial and spectral analyses of X-ray data, we try to distinguish between two major phenomena: tidal and ram pressure interactions. We compare our findings with the case of NGC 2276, in which a shock was reported, by analysing XMM-Newton X-ray data for this galaxy. We use archival XMM-Newton observations of NGC 4254, NGC 4569, and NGC 2276. Maps of the soft diffuse emission in the energy band 0.2 - 1 keV are obtained. For the three galaxies, especially at the position of magnetic field enhancements we perform a spectral analysis to derive gas temperatures and thus to look for shock signatures. A shock is a signature of ram pressure resulting from supersonic velocities; weak tidal interactions are not expected to influence the temperature of the ionized gas. In NGC 4254, we do not observe any temperature increase. This suggests tidal interactions rather than ram pressure stripping. In NGC 4569 the radio polarized ridge shows a higher temperature, which may indicate ram-pressure effects. For NGC 2276, we do not find clear indications of a shock. The main driver of the observed distortions is most likely tidal interaction. Determining gas temperatures via sensitive X-ray observations seems to be a good method for distinguishing between ram pressure and tidal interaction effects acting upon a galaxy.

astro-ph.CO↗

arXiv2011

Hot gas in Mach cones around Virgo Cluster spiral galaxies

The detailed comparison between observations and simulations of ram pressure stripped spiral galaxies in the Virgo cluster has led to a three dimensional view of the galaxy orbits within the hot intracluster medium. The 3D velocities and Mach numbers derived from simulations can be used to derive simple Mach cone geometries for Virgo spiral galaxies. We search for indications of hot gas within Mach cones in X-ray observations of selected Virgo Cluster spiral galaxies (NGC 4569, NGC 4388, and NGC 4501). We find extraplanar diffuse X-ray emission in all galaxies. Based on the 3D velocity vectors from dynamical modelling a simple Mach cone is fitted to the triangular shape of NGC 4569's diffuse X-ray emission. Assuming that all extraplanar diffuse X-ray emission has to be located inside the Mach cone, we also fit Mach cones to NGC 4388's and NGC 4501's extraplanar X-ray emission. For NGC 4569 it is hard to reconcile the derived Mach cone opening angle with a Mach number based on the sound speed alone. Instead, a Mach number involving the Alfvénic speed seems to be more appropriate, yielding a magnetic field strength of $\sim 3$-6 $μ$G for a intracluster medium density of $n \sim 10^{-4}$ cm$^{-3}$. Whereas the temperature of the hot component of NGC 4569's X-ray halo (0.5 keV) is at the high end but typical for a galactic outflow, the temperature of the hot gas tails of NGC 4388 and NGC 4501 are significantly hotter (0.7-0.9 keV). In NGC 4569 we find direct evidence for a Mach cone which is filled with hot gas from a galactic superwind. We suggest that the high gas temperatures in the X-ray tails of NGC 4388 and NGC 4501 are due to the mixing of the stripped ISM into the hot intracluster medium of the Virgo cluster.

astro-ph.CO↗

arXiv2010

The influence of the cluster environment on the large-scale radio continuum emission of 8 Virgo cluster spirals

The influence of the environment on the polarized and total power radio continuum emission of cluster spiral galaxies is investigated. We present deep scaled array VLA 20 and 6 cm observations including polarization of 8 Virgo spiral galaxies. These data are combined with existing optical, HI, and Halpha data. Ram pressure compression leads to sharp edges of the total power distribution at one side of the galactic disk. These edges coincide with HI edges. In edge-on galaxies the extraplanar radio emission can extend further than the HI emission. In the same galaxies asymmetric gradients in the degree of polarization give additional information on the ram pressure wind direction. The local total power emission is not sensitive to the effects of ram pressure. The radio continuum spectrum might flatten in the compressed region only for very strong ram pressure. This implies that neither the local star formation rate nor the turbulent small-scale magnetic field are significantly affected by ram pressure. Ram pressure compression occurs mainly on large scales (>=1 kpc) and is primarily detectable in polarized radio continuum emission.

astro-ph.CO↗

arXiv2007

Magnetic fields and gas in the cluster influenced spiral galaxy NGC4254 - I. Radio and X-rays observations

Radio observations can show how cluster galaxies are affected by various environmental factors. We made three frequency radio-polarimetric VLA observations and performed sensitive XMM-Newton observations in X-rays and UV light of NGC4254. The distribution of total radio intensity at 8.46 4.86 GHz reveals a global asymmetry with a more diffuse and almost two times larger extension to the north than to the south. The radio polarized intensity is even more asymmetric, showing a strange bright ridge in the southern disk edge, displaced to the downstream side of the local density wave. Magnetic arms can be also seen in other disk portions, mostly avoiding nearby optical spiral arms. Spatially resolved emission of hot X-ray gas from the whole galactic disk, with its soft component closely tracing star-forming regions, is detected. The slope of the local radio nonthermal-infrared relation is <1, thus smaller than for the radio thermal-infrared one (>1). Using the radio thermal emission-based star formation rate (SFR) we find higher extinction in more Halpha luminous star-forming regions with a power-law slope of 0.83. The galaxy's estimated mean SFR of 0.026 M_sun/yr/kpc^2 is three times larger than in other spirals of similar Hubble type. NGC4254 seems to belong to the class of `young' Virgo cluster members, which recently experienced a gravitational encounter at the cluster's periphery, which perturbed its spiral arms by tidal forces and triggered a burst of star-formation which still maintains strong radio and infrared emissions. Tidal forces could also sheared the magnetic field in the southern disk portion and led to the observed polarized ridge, though, magnetic field compression by weak ram pressure forces of the cluster gas cannot be excluded.

astro-ph↗

arXiv2007

The magnetic fields of large Virgo Cluster spirals

Because of its proximity the Virgo Cluster is an excellent target for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated. Optical and/or \ion{H}{i} observations do not always show effects of weak interactions between galaxies and their encounters with the cluster medium. For this reason we searched for possible anomalies in the magnetic field structure in Virgo Cluster spirals which could be attributed to perturbations in their gas distribution and kinematics. Five angularly large Virgo Cluster spiral galaxies (NGC 4501, NGC 4438, NGC 4535, NGC 4548 and NGC 4654) were the targets for a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85 GHz. For two objects polarization data at higher frequencies have been obtained allowing Faraday rotation analysis. Distorted magnetic field structures were identified in all galaxies. Interaction-induced magnetized outflows were found in NGC 4438 (due to nuclear activity) and NGC 4654 (a combination of tidal tails and ram pressure effects). Almost all objects (except the anaemic NGC 4548) exhibit distortions in polarized radio continuum attributable to influence of the ambient gas. For some galaxies they agree with observations of other species, but sometimes (NGC 4535) the magnetic field is the only tracer of the interaction with the cluster environment. The cluster environment clearly affects the evolution of the galaxies due to ram pressure and tidal effects. Magnetic fields provide a very long-lasting memory of past interactions. Therefore, they are a good tracer of weak interactions which are difficult to detect by other observations.

astro-ph↗

arXiv2006

Magnetic fields and ionized gas in nearby late type galaxies

In order to analyze the importance of the star formation rate in generating and amplifying magnetic fields in the interstellar medium we obtained deep total power and polarization maps at 4.85 GHz of three late type spiral galaxies NGC4236, NGC4656 and IC2574 using the 100-m Effelsberg radio telescope. This was accompanied by imaging in the Halpha line. We also observed these objects at 1.4 GHz to obtain their integrated fluxes at this frequency and to determine their radio spectra. All galaxies were found to possess weak but detectable total power emission at 4.85 GHz, coincident with regions of recent star formation as traced by bright Halpha regions. The surface brightness of the radio-strongest object of our sample (NGC4656) is comparable to the radio-weakest objects in a sample of more than 50 normally star-forming spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio telescope are available. The fractional polarization of the 2 galaxies of our sample is less than 2%, significantly lower than for spiral galaxies of intermediate types, suggesting that the magnetic fields are not only weaker, but also less ordered than in spiral galaxies. The radio spectra of galaxies in our small sample are indicative of a substantial fraction of thermal emission, with a higher thermal fraction than in spirals with high star formation rates (SFR), while the nonthermal emission in our sample is relatively weak compared to spiral galaxies. We propose an equipartition model where the nonthermal emission increases ~SFR^(~1.4) and the ratio of nonthermal to thermal emission increases ~SFR^(~0.4). The objects of our sample still follow the radio-FIR correlation of surface brightness of the total emission, extending it towards the lowest values measured so far.

astro-ph↗