arXiv subjects

H. R. de Ruiter

Publications and source records attributed to H. R. de Ruiter.

At least 19 recordsLinked to original sources

arXiv2012

Chandra observations of dying radio sources in galaxy clusters

The dying radio sources represent a very interesting and largely unexplored stage of the active galactic nucleus (AGN) evolution. They are considered to be very rare, and almost all of the few known ones were found in galaxy clusters. However, considering the small number detected so far, it has not been possible to draw any firm conclusions about their X-ray environment. We present X-ray observations performed with the Chandra satellite of the three galaxy clusters Abell 2276, ZwCl 1829.3+6912, and RX J1852.1+5711, which harbor at their center a dying radio source with an ultra-steep spectrum that we recently discovered. We analyzed the physical properties of the X-ray emitting gas surrounding these elusive radio sources. We determined the global X-ray properties of the clusters, derived the azimuthally averaged profiles of metal abundance, gas temperature, density, and pressure. Furthermore, we estimated the total mass profiles. The large-scale X-ray emission is regular and spherical, suggesting a relaxed state for these systems. Indeed, we found that the three clusters are also characterized by significant enhancements in the metal abundance and declining temperature profiles toward the central region. For all these reasons, we classified RX J1852.1+5711, Abell 2276, and ZwCl 1829.3+6912 as cool-core galaxy clusters.

astro-ph.CO↗

arXiv2010

Molecular Disks in Radio Galaxies: The pathway to ALMA

Context. It has recently been proposed that the jets of low-luminosity radio galaxies are powered by direct accretion of the hot phase of the IGM onto the central black hole. Cold gas remains a plausible alternative fuel supply, however. The most compelling evidence that cold gas plays a role in fueling radio galaxies is that dust is detected more commonly and/or in larger quantities in (elliptical) radio galaxies compared with radio-quiet elliptical galaxies. On the other hand, only small numbers of radio galaxies have yet been detected in CO (and even fewer imaged), and whether or not all radio galaxies have enough cold gas to fuel their jets remains an open question. If so, then the dynamics of the cold gas in the nuclei of radio galaxies may provide important clues to the fuelling mechanism. Aims. The only instrument capable of imaging the molecular component on scales relevant to the accretion process is ALMA, but very little is yet known about CO in southern radio galaxies. Our aim is to measure the CO content in a complete volume-limited sample of southern radio galaxies, in order to create a well-defined list of nearby targets to be imaged in the near future with ALMA. Methods. APEX has recently been equipped with a receiver (APEX-1) able to observe the 230 GHz waveband. This allows us to search for CO(2-1) line emission in our target galaxies. Results. Here we present the results for our first three southern targets, proposed for APEX-1 spectroscopy during science verification: NGC3557, IC4296 and NGC1399. The experiment was successful with two targets detected, and possible indications for a double-horned CO line profile, consistent with ordered rotation. These early results are encouraging, demonstrating that APEX can efficiently detect CO in nearby radio galaxies. We therefore plan to use APEX to obtain CO spectroscopy for all our southern targets.

astro-ph.CO↗

arXiv2010

Dying Radio Sources in Clusters

We present the study of five `dying' nearby radio galaxies belonging to the WENSS minisurvey and to the B2 bright catalogs: WNB1734+6407, WNB1829+6911, WNB1851+5707, B2 0120+33, and B2 1610+29. These sources have been selected on the basis of their extremely steep broad-band radio spectra. The modeling of the integrated spectra and the deep spectral index images obtained with the VLA confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime although their extended lobes have not yet completely faded away. We found that WNB1851+5707 is in reality composed by two distinct dying galaxies, which appear blend together as a single source in the WENSS. In the cases of WNB1829+6911 and B2 0120+33, the fossil radio lobes are seen in conjunction with a currently active core. A very faint core is detected also in a MERLIN image of WNB1851+5707a, one of the two dying sources composing WNB1851+5707. We found that all sources of our sample are located (at least in projection) at the center of an X-ray emitting cluster. Our results suggest that the duration of the dying phase for a radio source in cluster can be significantly higher with respect to that of a radio galaxy in the field. The simplest interpretation is that the low-frequency emission from the fading radio lobes last longer if their expansion is somewhat reduced or even stopped. Another possibility is that the occurrence of dying sources is higher in galaxy clusters. Radio sources in dense environment, like e.g. the center of cooling core clusters, may have a peculiar accretion mode which results in a bursting duty cycle sequence of active and quiescent periods. This result could have important implications for theories of the life cycles of radio sources and AGN feedback in clusters of galaxies but awaits confirmation from future observations of larger samples of objects.

astro-ph.CO↗

arXiv2009

The ATESP 5 GHz radio survey. III. 4.8, 8.6 and 19 GHz follow-up observations of radio galaxies

[Abridged] Physical and evolutionary properties of the sub-mJy radio population are not entirely known. The radio/optical analysis of the ATESP 5 GHz sample has revealed a significant class of compact flat/inverted radio-spectrum sources associated to early-type galaxies up to redshift 2. Such sources are most plausibly triggered by an AGN, but their observational properties are not entirely consistent with those of standard radio galaxy populations. In the present work we aim at a better understanding of the radio spectra of such sources and ultimately of the nature of AGNs at sub-mJy flux levels. We used the ATCA to get multi-frequency (4.8, 8.6 and 19 GHz) quasi-simultaneous observations for a representative sub-sample of ATESP radio sources associated with early-type galaxies (26 objects with S>0.6 mJy). This can give us insight into the accretion/radiative mechanism that is at work, since different regimes display different spectral signatures in the radio domain. From the analysis of the radio spectra, we find that our sources are most probably jet-dominated systems. ADAF models are ruled out by the high frequency data, while ADAF+jet scenarios are still consistent with flat/moderately inverted-spectrum sources, but are not required to explain the data. We compared our sample with high (>20 GHz) frequency selected surveys, finding spectral properties very similar to the ones of much brighter (S>500 mJy) radio galaxies extracted from the Massardi et al. (2008) sample. Linear sizes of ATESP 5 GHz sources associated with early type galaxies are also often consistent with the ones of brighter B2 and 3C radio galaxies, with possibly a very compact component that could be associated at least in part to (obscured) radio-quiet quasar-like objects and/or low power BL Lacs.

astro-ph.CO↗

arXiv2009

Is cold gas fuelling the radio galaxy NGC 315?

We present WSRT, VLA and VLBI observations of the HI absorption in the radio galaxy NGC 315. The main result is that two HI absorbing systems are detected against the central region. In addition to the known highly redshifted, very narrow component, we detect relatively broad (FWZI 150 km/s) absorption. This broad component is redshifted by ~80 km/s compared to the systemic velocity, while the narrow absorption is redshifted ~ 490 km/s. Both HI absorption components are spatially resolved at the pc-scale of the VLBI observations. The broad component shows strong gradients in density (or excitation) and velocity along the jet. We conclude that this gas is physically close to the AGN, although the nature of the gas resulting in the broad absorption is not completely clear. The possibility that it is entrained by the radio jet appears unlikely. Gas located in a thick circum-nuclear toroidal structure cannot be completely ruled out although it appears difficult to reconcile with the observed morphology and kinematics of the HI. A perhaps more likely scenario is that the gas producing the broad absorption could be (directly or indirectly) connected with the fuelling of the AGN, i.e. gas that is falling into the nucleus. If this is the case, the accretion rate derived is similar to that found for other X-ray luminous elliptical galaxies, although lower than that derived from the radio core luminosity for NGC 315. The density distribution of the narrow component is, featureless. Moreover, we detect a small amount of HI in emission a few kpc SW of the AGN, coincident with faint optical absorption features and at velocities very similar to the narrow absorption. This suggests that the gas causing the narrow absorption is not close to the AGN and is more likely caused by clouds falling into NGC 315.

astro-ph.CO↗

arXiv2008

A view of the sub-mJy populations, modelling and perspectives for future deep surveys

We use deep multi-colour (UBVRIJK) images mostly taken in the framework of the ESO Deep Public Survey (DPS) to optically identify and derive photometric redshifts for a complete sample of 131 radio sources with S>0.4 mJy, observed at both 1.4 and 5 GHz as part of the ATESP radio survey. The availability of multi-wavelength radio and optical information is exploited to infer the physical properties of the faint radio population. In particular we find that, considering both early-type galaxies and quasars as sources with an active nucleus, AGNs largely dominate our sample sub-mJy sample (78%). Further radio/optical analysis of such AGN component has revealed a somewhat unexpected class of flat/inverted-spectrum sources with low radio-to-optical ratios (R<100), which are preferentially identified with early-type galaxies. Such sources are quite compact (d<10-30 kpc), suggesting core-dominated radio emission triggered by low luminosity AGNs. This intriguing class of objects deserves further analysis, and new higher resolution radio observations are currently under way. In parallel we are developing radio source models, for both the AGN and the star-forming components of the sub-mJy radio pupulation. Here we discuss the first results.

astro-ph↗

arXiv2007

The intracluster magnetic field power spectrum in Abell 2382

The goal of this work is to put constraints on the strength and structure of the magnetic field in the cluster of galaxies A2382. We investigate the relationship between magnetic field and Faraday rotation effects in the cluster, using numerical simulations as a reference for the observed polarization properties. For this purpose we present Very Large Array observations at 20 cm and 6 cm of two polarized radio sources embedded in A2382, and we obtained detailed rotation measure images for both of them. We simulated random three-dimensional magnetic field models with different power spectra and thus produced synthetic rotation measure images. By comparing our simulations with the observed polarization properties of the radio sources, we can determine the strength and the power spectrum of intra-cluster magnetic field fluctuations that best reproduce the observations. The data are consistent with a power law magnetic field power spectrum with the Kolmogorov index $n=11/3$, while the outer scale of the magnetic field fluctuations is of the order of 35 kpc. The average magnetic field strength at the cluster center is about 3 $μ$G and decreases in the external region as the square root of the electron gas density. The average magnetic field strength in the central 1 Mpc$^{3}$ is about 1 $μ$G.

astro-ph↗

arXiv2007

The ATESP 5 GHz radio survey. II. Physical properties of the faint radio population

One of the most debated issues about sub-mJy radio sources, which are responsible for the steepening of the 1.4 GHz source counts, is the origin of their radio emission. Particularly interesting is the possibility of combining radio spectral index information with other observational properties to assess whether the sources are triggered by star formation or nuclear activity. The aim of this work is to study the optical and near infrared properties of a complete sample of 131 radio sources with S>0.4 mJy, observed at both 1.4 and 5 GHz as part of the ATESP radio survey. We use deep multi-colour (UBVRIJK) images, mostly taken in the framework of the ESO Deep Public Survey, to optically identify and derive photometric redshifts for the ATESP radio sources. Deep optical coverage and extensive colour information are available for 3/4 of the region covered by the radio sample. Typical depths of the images are U~25, B~26, V~25.4, R~25.5, I~24.3, 19.5 1000), typical of classical powerful radio galaxies and quasars. Flat-spectrum sources with low R values are preferentially identified with early type galaxies, where the radio emission is most probably triggered by low-luminosity active galactic nuclei. Considering both early type galaxies and quasars as sources with an active nucleus, such sources largely dominate our sample (78%). Flat-spectrum sources associated with early type galaxies are quite compact (d<10-30 kpc), suggesting core-dominated radio emission.

astro-ph↗

arXiv2007

In search of dying radio sources in the local universe

Up till now very few dying sources were known, presumably because the dying phase is short at centimeter wavelengths. We therefore have tried to improve the statistics on sources that have ceased to be active, or are intermittently active. The latter sources would partly consist of a fossil radio plasma left over from an earlier phase of activity, plus a recently restarted core and radio jets. Improving the statistics of dying sources will give us a better handle on the evolution of radio sources, in particular the frequency and time scales of radio activity. We have used the WENSS and NVSS surveys, in order to find sources with steep spectral indices, associated with nearby elliptical galaxies. In the cross correlation we presently used only unresolved sources, with flux densities at 1.4 GHz larger than 10 mJy. The eleven candidates thus obtained were observed with the VLA in various configurations, in order to confirm the steepness of the spectra, and to check whether active structures like flat-spectrum cores and jets are present, perhaps at low levels. We estimated the duration of the active and relic phases by modelling the integrated radio spectra using the standard models of spectral evolution. We have found six dying sources and three restarted sources, while the remaining two candidates remain unresolved also with the new VLA data and may be Compact Steep Spectrum sources, with an unusually steep spectrum. The typical age of the active phase, as derived by spectral fits, is in the range 10^7 - 10^8 years. For our sample of dying sources, the age of the relic phase is on average shorter by an order of magnitude than the active phase.

astro-ph↗

arXiv2006

A Molecular Gas Study of Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radio galaxies, selected from the B2 catalogue. We investigate whether the CO properties of our sample are correlated with the properties of the host galaxy, and in particular with the dust component. We find strong evidences for a physical link between the dust disks probed by HST in the galaxy cores and the molecular gas probed by the CO spectral lines, which in two cases display a double-horn shape, consistent with ordered rotation. On the other hand, from a preliminary comparison with other samples of radio sources we find no significant differences in molecular gas properties between FRI and FRII radio sources. In order to confirm the suggestion that the CO is dynamically associated with the core dust disks, the most suitable sources of our sample will be proposed for interferometric imaging at PdBI.

astro-ph↗

arXiv2006

A Search for Molecular Gas in Low Luminosity Radio Galaxies

We discuss CO spectral line data of a volume-limited sample of 23 nearby (z<0.03) low luminosity radi galaxies, selected from the B2 catalogue. Most of such objects (16/23) have HST imaging. Our aim is to establish the distribution of molecular gas masses in low luminosity radio galaxies, in comparison with other radio source samples, confirm the suggestion that the CO is in ordered rotation, determine its relation to the dust disks observed in these objects with HST and establish targets for future interferometric imaging.

astro-ph↗

arXiv2006

The ATESP 5 GHz radio survey I. Source counts and spectral index properties of the faint radio population

[Abridged] The nature and evolutionary properties of the faint radio population, responsible for the steepening observed in the 1.4 GHz source counts below 1 milliJy, are not yet entirely clear. Radio spectral indices may help to constrain the origin of the radio emission in such faint radio sources and may be fundamental in understanding eventual links to the optical light. We study the spectral index behaviour of sources that were found in the 1.4 GHz ATESP survey (Prandoni et al. 2000a,b). Using the Australia Telescope Compact Array we observed at 5 GHz part of the region covered by the sub-mJy ATESP survey. In particular we imaged a one square degree area for which deep optical imaging in UBVRIJK is available. In this paper we present the 5 GHz survey and source catalogue, we derive the 5 GHz source counts and we discuss the 1.4-5 GHz spectral index properties of the ATESP sources. The analysis of the optical properties of the sample will be the subject of a following paper. The 5 GHz survey has produced a catalogue of 111 radio sources, complete down to a (6 sigma) limit S_lim(5 GHz)~0.4 mJy. We take advantage of the better spatial resolution at 5 GHz (~2 arcsec compared to ~8 arcsec at 1.4 GHz) to infer radio source structures and sizes. The 5 GHz source counts derived by the present sample are consistent with those reported in the literature, but improve significantly the statistics in the flux range 0.4< S(5 GHz)<1 mJy. The ATESP sources show a flattening of the 1.4-5 GHz spectral index with decreasing flux density, which is particularly significant for the 5 GHz selected sample. Such a flattening confirm previous results coming from smaller samples and is consistent with a flattening of the 5 GHz source counts occurring at fluxes <=0.5 mJy.

astro-ph↗

arXiv2005

The VVDS-VLA Deep Field II. Optical and near infrared identifications of VLA S(1.4GHz)>80 microJy sources in the VIMOS VLT Deep Survey VVDS-02h field

In this paper we present the optical and near-infrared identifications of the 1054 radio sources detected in the 20cm deep radio survey down to a 5sigma flux limit of about 80 microJy obtained with the VLA in the VIMOS VLT Deep Survey VVDS-02h deep field. Using U,B,V,R,I and K data, we identified 718 radio sources (~74% of the whole sample). The photometric redshift analysis shows that, in each magnitude bin, the radio sample has a higher median photometric redshift than the whole optical sample, while the median (V-I) color of the radio sources is redder than the median color of the whole optical sample. These results suggest that radio detection is preferentially selecting galaxies with higher intrinsic optical luminosity. From the analysis of the optical properties of the radio sources as function of the radio flux, we found that while about 35% of the radio sources are optically unidentified in the higher radio flux bin (S> 1.0 mJy), the percentage of unidentified sources decreases to about 25% in the faintest bins (S< 0.5 mJy). The median I magnitude for the total sample of radio sources,i.e. including also the unidentified ones, is brighter in the faintest radio bins than in the bin with higher radio flux. This suggests that most of the faintest radio sources are likely to be associated to relatively lower radio luminosity objects at relatively modest redshift, rather than radio-powerful, AGN type objects at high redshift.

astro-ph↗

arXiv2005

Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles

We present a study of the optical brightness profiles of early type galaxies, using a number of samples of radio galaxies and optically selected elliptical galaxies. For the radio galaxy samples--B2 of Fanaroff-Riley type I and 3C of Fanaroff-Riley type II-- we determined a number of parameters that describe a "Nuker-law" profile, which were compared with those already known for the optically selected objects. We find that radio active galaxies are always of the "core" type (i.e. an inner Nuker law slope gamma < 0.3). However, there are core-type galaxies which harbor no significant radio source and which are indistinguishable from the radio active galaxies. We do not find any radio detected galaxy with a power law profile (gamma > 0.5). This difference is not due to any effect with absolute magnitude, since in a region of overlap in magnitude the dichotomy between radio active and radio quiescent galaxies remains. We speculate that core-type objects represent the galaxies that have been, are, or may become, radio active at some stage in their lives; active and non-active core-type galaxies are therefore identical in all respects except their eventual radio-activity: on HST scales we do not find any relationship between boxiness and radio-activity. There is a fundamental plane, defined by the parameters of the core (break radius r_b and break brightness mu_b), which is seen in the strong correlation between r_b and mu_b. The break radius is also linearly proportional to the optical Luminosity in the $I$ band. Moreover, for the few galaxies with an independently measured black hole mass, the break radius turns out to be tightly correlated with M_{BH}. The black hole mass correlates even better with the combination of fundamental plane parameters r_b and mu_b, which represents the central velocity dispersion.

astro-ph↗

arXiv2004

Modeling the faint radio population: the nanoJy radio sky

The apparent change in the composition of the parent optical objects of radio sources around 1 mJy (at 1.4 GHz) is now well established, although there is still some debate about the relative importance of classical radio galaxies and star-forming galaxies at sub-mJy levels (see e.g. Gruppioni et al. 1999; Prandoni et al. 2001b). It is clear, however, that at microJy levels star-forming galaxies are dominant (see Fomalont et al. 1997; Haarsma et al. 2000). Does this mean that SKA will basically tell us more about the history of star formation than about the space density (and its cosmological evolution) of active galactic nuclei? Using current best estimates of luminosity functions (and their evolution) of various classes of objects, we show that the increasing dominance of star-forming galaxies below 1 mJy is a natural consequence of the different luminosity functions, but that this does not at all mean that star-forming galaxies do necessarily dominate at all sub-mJy flux levels and all redshifts.

astro-ph↗

arXiv2004

Dying Radio Galaxies In Clusters

We report the recent discovery of three `dying' radio galaxies belonging to the WENSS minisurvey sample: WNB1734+6407, WNB1829+6911 and WNB1851+5707. These sources have been selected on the basis of their extremely steep broad-band radio spectra, which is a strong indication that these objects either belong to the rare class of dying radio galaxies or that we are observing `fossil' radio plasma remaining from a previous nuclear activity. Deep spectral index images obtained with the Very Large Array confirmed that in these sources the central engine has ceased to be active for a significant fraction of their lifetime although their extended lobes have not yet completely faded away. In one case, WNB1829+6911, fossil radio lobes are seen in conjunction with newly restarting jets. We found that each source is located (at least in projection) at the center of an X-ray emitting cluster. We argue that their intriguing association with clusters implies that the pressure of the dense intracluster medium, perhaps a cooling flow, prevents quick liquidation of a fossil radio lobe through adiabatic expansion. On statistical ground we deduce that the duration of the dying phase for a radio source in cluster is one order of magnitude higher with respect to that of a radio galaxy in the field.

astro-ph↗

arXiv2002

B2 0648+27: a radio galaxy in a major merger

We present WSRT observations of the neutral hydrogen in the nearby radio galaxy B2 0648+27. In emission, we detect a very large amount of HI (M_HI = 1.1 x 10^10 M_sun) that is distributed in a very extended disk, or ring-like structure, of about 160 kpc in size. We also detect HI absorption against the central radio continuum component. The detection of the HI, its structure and kinematics, give us key information for building a possible evolutionary scenario. The characteristics of the detected HI are explained as the result of a major merger event that is likely to have occurred <10^9 yr ago. Interestinly, we find that, when observed in radio continuum at higher resolution, this galaxy has a double lobed, steep spectrum structure of about 1 kpc in size. Thus, despite its low radio power, B2 0648+27 bears striking similarities with Compact Symmetric Objects, i.e. objects believed to represent the early phase of radio galaxies (< few thousand yrs old). B2~0648+27 is one of the few nearby radio galaxies where extended neutral hydrogen has been detected so far. This, and other recent results, appear however to indicate that nearby radio galaxies are more often gas rich than commonly assumed. The phenomena described are likely to be much more common at high redshift and galaxies like B2 0648+27 may provide valuable information on the evolution of high redshift radio sources.

astro-ph↗

arXiv2002

HST images of B2 radio galaxies: a link between circum-nuclear dust and radio properties?

Almost 60% of the B2 low luminosity radio galaxies have been observed with the Hubble Space Telescope. We present an analysis of the dust features, which are often present in the form of circum-nuclear disks or lanes, and show that there are correlations between radio source and dust properties. It is found that nearby radio sources in which a jet has been detected tend to have dust more often than sources without jets; the dust is often in the form of disks or lanes. Moreover the radio jets are close to perpendicular to the disk or lane in the weaker radio sources (with P < 10^{24} W/Hz). In stronger sources the orientation effect appears to be weak or even absent. Also the dust masses found in the weaker radio sources are smaller than in the stronger ones (log M/M_sun ~ 3 against 5 respectively). More generally it appears that there is a correlation between dust mass and total radio power (for those sources in which dust has been detected); we show that this correlation is not induced by redshift.

astro-ph↗