arXiv subjects

P. Parma

Publications and source records attributed to P. Parma.

At least 37 records · Page 2Linked to original sources

arXiv2008

The VVDS-VLA Deep Field - IV: Radio-optical properties

(abridged) We use the 1.4 GHz VIMOS-VLA Deep Survey and the optical VVDS and the CFHT-LS to compare the properties of radio loud galaxies with respect to the whole population of optical galaxies. The availability of multiband photometry and high quality photometric redshifts allows to derive rest frame colors and radio luminosity functions down to a limit of a B rest-frame magnitude of M=-20. Galaxy properties and luminosity functions (LFs) are estimated up to z~1 for radio loud and radio quiet early and late type galaxies. Radio loud late type galaxies are redder than radio quiet objects of the same class and this is an effect related to the presence of more dust in stronger star forming galaxies. Moreover, we estimate optical LFs, stellar masses and star formation rate distributions for radio sources and compare them with those derived for a well defined control sample, finding that the probability for a galaxy to be a radio emitter significantly increases at high values of these parameters. Radio loud early type galaxies show luminosity evolution of their bivariate radio-optical LF, due to an evolution in the radio-optical ratio. The lack of evolution of the mass function of radio loud early type galaxies means that no new AGN are formed at z<1. On the contrary, radio loud late type objects show a strong evolution, both in luminosity and in density, of the radio LF for z>0.7. This evolution is the direct effect of the strong optical evolution of this class and no significant change with redshift of the radio-optical ratio is required. With the knowledge of the radio-optical ratio and the optical and radio LFs for late type galaxies, we estimated the star formation history of the Universe up to z~1.5, using optical galaxies as tracers of the global radio emission.

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arXiv2008

Structures of the magnetoionic media around the FR I radio galaxies 3C 31 and Hydra A

We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365 to 8440MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the North and South sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron-emitting plasma. We establish that the residual degree of polarization in the short-wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We show that the observed RM variations over selected areas of 3C 31 are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power-law slope changes significantly on the scales sampled by our data. The power spectrum can only have the form expected for Kolmogorov turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also compare the global variations of RM across 3C 31 with the results of three-dimensional simulations of the magnetic-field fluctuations in the surrounding magnetoionic medium. We show that our data are consistent with a field distribution that favours the plane perpendicular to the jet axis - probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X-ray data. (Abridged)

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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.

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arXiv2008

Multifrequency VLA observations of the FR I radio galaxy 3C 31: morphology, spectrum and magnetic field

We present high-quality VLA images of the FR I radio galaxy 3C 31 in the frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40 arcsec. Our new images reveal complex, well resolved filamentary substructure in the radio jets and tails. We also use these images to explore the spectral structure of 3C 31 on large and small scales. We infer the apparent magnetic field structure by correcting for Faraday rotation. Some of the intensity substructure in the jets is clearly related to structure in their apparent magnetic field: there are arcs of emission where the degree of linear polarization increases, with the apparent magnetic field parallel to the ridges of the arcs. The spectral indices are significantly steeper (0.62) within 7 arcsec of the nucleus than between 7 and 50 arcsec (0.52 - 0.57). The spectra of the jet edges are also slightly flatter than the average for their surroundings. At larger distances, the jets are clearly delimited from surrounding larger-scale emission both by their flatter radio spectra and by sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of 3C 31's jets is very close to that found in other FR I galaxies where their jets first brighten in the radio and where X-ray synchrotron emission is most prominent. Farther from the nucleus, where the spectra flatten, X-ray emission is fainter relative to the radio. The brightest X-ray emission from FR I jets is therefore not associated with the flattest radio spectra, but with a particle-acceleration process whose characteristic energy index is 2.24. The spectral flattening with distance from the nucleus occurs where our relativistic jet models require deceleration, and the flatter-spectra at the jet edges may be associated with transverse velocity shear. (Slightly abridged)

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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.

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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.

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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.

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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.

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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.

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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.

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arXiv2006

Faraday rotation variations along radio jets: the magnetic field in galaxy and group halos

Our modelling of FR I radio jets as decelerating, relativistic flows allows us to derive their orientations accurately. We present images of Faraday rotation for two of these these objects (3C 31 and NGC 315) and show that the fluctuations of rotation measure (RM) are larger in the fainter (receding) jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas density is much lower in NGC 315 and the RM fluctuations are only just detectable.

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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.

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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.

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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.

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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.

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arXiv2003

The VLA-VIRMOS Deep Field I. Radio observations probing the microJy source population

We have conducted a deep survey (r.m.s noise 17 microJy) with the Very Large Array (VLA) at 1.4 GHz, with a resolution of 6 arcsec, of a 1 square degree region included in the VIRMOS VLT Deep Survey. In the same field we already have multiband photometry down to I(AB)=25, and spectroscopic observations will be obtained during the VIRMOS VLT survey. The homogeneous sensitivity over the whole field has allowed to derive a complete sample of 1054 radio sources (5 sigma limit). We give a detailed description of the data reduction and of the analysis of the radio observations, with particular care to the effects of clean bias and bandwidth smearing, and of the methods used to obtain the catalogue of radio sources. To estimate the effect of the resolution bias on our observations we have modelled the effective angular-size distribution of the sources in our sample and we have used this distribution to simulate a sample of radio sources. Finally we present the radio count distribution down to 0.08 mJy derived from the catalogue. Our counts are in good agreement with the best fit derived from earlier surveys, and are about 50 % higher than the counts in the HDF. The radio count distribution clearly shows, with extremely good statistics, the change in the slope for the sub-mJy radio sources.

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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.

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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.

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