1,354,545 research outputs found
The BL Lac heart of Centaurus A
No full textEmission from the nucleus of the closest radio galaxy, Centaurus A, is observed from the radio to the gamma ray band. We build, for the first time, its overall Spectral Energy Distribution (SED) that appears to be intriguingly similar to those of blazars, showing two broad peaks located in the far-infrared band and at ~0.1 MeV respectively. The whole nuclear emission of Centaurus A is successfully reproduced with a synchrotron self-Compton model. The estimated physical parameters of the emitting source are similar to those of BL Lacs, except for a much smaller beaming factor, as qualitatively expected when a relativistic jet is orientated at a large angle to the line of sight. These results represent strong evidence that Centaurus A is indeed a misoriented BL Lac and provide strong support in favour of the unification scheme for low luminosity radio-loud AGNs. Modeling of the SED of Centaurus A also provides further and independent indications of the presence of velocity structures in sub-pc scale jets
Understanding the nature of FR II optical nuclei: a new diagnostic plane of radio galaxies
No full textWe extend our study of the nuclei of 3CR FR II radio galaxies through HST optical images up to z=0.3. In the majority of them an unresolved nucleus (central compact core, CCC) is found. We analyze their position in the plane formed by the radio and optical nuclear luminosities in relation to their optical spectral properties. The broad-lined objects (BLO) have the brightest nuclei: they are present only at optical luminosities nu L_nu > 4 X 10^42 erg s^-1 which we suggest might represent a threshold in the radiative efficiency combined to a small range of black hole masses. About 40 % of the high and low excitation galaxies (HEG and LEG) show CCC which resemble those previously detected in FR I galaxies, in apparent contrast to the unification model. The equivalent width of the [OIII] emission line (with respect to the nuclear luminosity) reveals the nature of these nuclei, indicating that the nuclei of HEG are obscured to our line of sight and only scattered radiation is observed. This implies that the population of FR II is composed by objects with different nuclear properties, and only a fraction of them can be unified with quasars
Testing the FR I/BL Lac unifying model with HST observations
No full textHubble Space Telescope (HST) observations provide a novel way of testing unified models for FR I radio sources and BL Lac objects. The detection of extended dust discs in some radio galaxies provides information on their jet orientation. Given this, the strength of the compact nuclear sources of FR I and BL Lacs can be compared with model predictions.
As a pilot project towards using HST information in testing unified models, we selected five radio galaxies that show extended nuclear discs in the HST images. The relative orientation of the projected radio jets and of the extended nuclear discs indicates that they are not perpendicular, as the simplest geometrical model would suggest, but that they form an angle of similar to 20-40 degrees with the symmetry axis of the disc: a significant change of orientation occurs between the innermost AGN structure and the kiloparsec scale. Nevertheless, the discs appear to be useful indicators of the orientation of the radio source, since the angles formed by the disc axis and the jet with the line of sight differ by only similar to 10-20 degrees
Probing the nuclear obscuration in radio-galaxies with near infrared imaging
No full textWe present the first near-infrared (K'-band) homogeneous observations of a complete sub-sample of the 3CR radio catalogue comprising all High Excitation Galaxies (HEGs) at z<0.3. After showing that the surface brightness decomposition technique to measure central point-like sources is affected by significant uncertainties for the objects in the studied sample, we present a new, more accurate method based on the R-K' color profile. Via this method we find a substantial nuclear K'-band excess in all but two HEGs -- most likely directly associated to their nuclear emission -- and we measure the corresponding 2.12 μm nuclear luminosities. Within the frame of the unification scheme for radio-loud active galactic nuclei, it appears that obscuration alone is not able to account for the different nuclear properties of the majority of the HEGs and Broad Line Radio Galaxies (BLRGs), and also scattering of the (optically) hidden nuclear light from a compact region must be invoked. More precisely, for ~70% of the HEGs the observed point-like optical emission is dominated by the scattered component, while in the K'-band both scattered and direct light passing through the torus contribute to the observed nuclear luminosity. The estimated fraction of scattered light ranges from a few tenths to a few percent, while the torus extinction is between 15<A_{V,torus}<50 mag with only a few exceptions with lower obscuration
FR0 CAT: a FIRST catalog of FR 0 radio galaxies
No full textWith the aim of exploring the properties of the class of FR 0 radio galaxies, we selected a sample of 108 compact radio sources, called FR0CAT, by combining observations from the NVSS, FIRST, and SDSS surveys. We included in the catalog sources with redshift ≤0.05, with a radio size ≲5 kpc, and with an optical spectrum characteristic of low-excitation galaxies. Their radio luminosities at 1.4 GHz are in the range 1038 ≲ νL1.4 ≲ 1040 erg s-1. The FR0CAT hosts are mostly (86%) luminous (−21 ≳ Mr ≳ −23) red early-type galaxies with black hole masses 108 ≲ MBH ≲ 109M⊙. These properties are similar to those seen for the hosts of FR I radio galaxies, but they are on average a factor ~1.6 less massive. The number density of FR0CAT sources is ~5 times higher than that of FR Is, and thus they represent the dominant population of radio sources in the local Universe. Different scenarios are considered to account for the smaller sizes and larger abundance of FR 0s with respect to FR Is. An age-size scenario that considers FR 0s as young radio galaxies that will all eventually evolve into extended radio sources cannot be reconciled with the large space density of FR 0s. However, the radio activity recurrence, with the duration of the active phase covering a wide range of values and with short active periods strongly favored with respect to longer ones, might account for their large density number. Alternatively, the jet properties of FR 0s might be intrinsically different from those of the FR Is, the former class having lower bulk Lorentz factors, possibly due to lower black hole spins. Our study indicates that FR 0s and FR I/IIs can be interpreted as two extremes of a continuous population of radio sources that is characterized by a broad distribution of sizes and luminosities of their extended radio emission, but shares a single class of host galaxies
Simplified dataset for 'Preliminary one-year data from the AntiCROWN Study'
No full textSource data for the paper titled: "One year durability of anti-spike antibodies to SARS-CoV-2: preliminary data from the AntiCROWN prospective observational study" submitted to Lance
The HST view of FR I radio galaxies: evidence for non--thermal sources
No full textUnresolved nuclear sources are detected by the Hubble Space Telescope in the great majority of a complete sample of 33 FR I radio galaxies belonging to the 3CR catalogue.
The optical flux of these Central Compact Cores (CCC) shows a striking linear correlation with the radio core one over four decades, arguing for a non-thermal synchrotron origin of the CCC radiation. We also find evidence that this emission is anisotropic, which leads us to identify CCCs with the misoriented relativistic jet component which dominates in BL Lac objects. This interpretation is also supported by the similarity in the radio-to-optical and optical spectral indices.
The high rate of CCC detection (85%) suggests that a "standard" pc scale, geometrically thick torus is not present in low luminosity radio-galaxies. Thus the lack of broad lines in FR I cannot be attributed to obscuration.
CCC fluxes also represent upper limits to any thermal/disc emission. For a 10(9)M(.) black hole, typical of FR I sources, these limits translate into a fraction as small as less than or similar to 10(-7) - 10(-5) of the Eddington luminosity
The HST view of the FR I FR II dichotomy
No full textIn order to explore how the FR I/FR II dichotomy is related to the nuclear properties of radio galaxies, we studied a complete sample of 26 nearby FR II radio galaxies using Hubble Space Telescope (HST) images and compared them with a sample of FR I previously analyzed. FR I nuclei lie in the radio-optical luminosity plane along a tight linear correlation, which argues for a common synchrotron origin. FR II show a more complex behavior, which is however clearly related to their optical spectral classification.
Broad line FR II radio galaxies (BLRG) are located overall well above the FR I correlation, suggesting that a contribution from thermal (disc) emission is present. Three narrow line (NLRG) and one weak line radio galaxy (WLRG), in which no nuclear source is seen, can be interpreted as the obscured counter-parts of BLRG, in agreement with the current unification schemes.
Conversely, in 5 sources of the sample, all of them NLRG or WLRG, optical cores are located on the same correlation defined by FR I and with similar radio and optical luminosities. This suggests that, in analogy to FR I, the emission is dominated by synchrotron radiation and represents the optical counter-part of the non-thermal radio cores. Interestingly, all these galaxies are located in clusters, an environment typical of FR I.
These results imply that, at least at low redshifts, the FR II population is not homogeneous. Furthermore, the traditional dichotomy between edge darkened and brightened radio morphology is not unequivocally connected with the innermost nuclear structure, as we find FR II with FR I-like nuclei and this has interesting bearings from the point of view of the AGN unified model
Examining the Radio-loud/Radio-quiet Dichotomy with New Chandra and VLA Observations of 13 UGC Galaxies
No full textWe present the results from new ~15 ks Chandra-ACIS and 4.9 GHz Very Large Array (VLA) observations of 13 galaxies hosting low-luminosity active galactic nuclei (AGNs). This completes the multiwavelength study of a sample of 51 nearby early-type galaxies described in Capetti & Balmaverde and Balmaverde & Capetti. The aim of the three previous papers was to explore the connection between the host galaxies and AGN activity in a radio-selected sample. We detect nuclear X-ray emission in eight sources and radio emission in all but one (viz., UGC 6985). The new VLA observations improve the spatial resolution by a factor of 10: the presence of nuclear radio sources in 12 of the 13 galaxies confirms their AGN nature. As previously indicated, the behavior of the X-ray and radio emission in these sources depends strongly on the form of their optical surface brightness profiles derived from Hubble Space Telescope imaging, i.e., on their classification as "core," "power-law," or "intermediate" galaxies. With more than twice the number of "power-law" and "intermediate" galaxies compared to previous work, we confirm with a much higher statistical significance that these galaxies lie well above the radio-X-ray correlation established in Fanaroff-Riley type I radio galaxies and the low-luminosity "core" galaxies. This result highlights the fact that the "radio-loud/radio-quiet" dichotomy is a function of the host galaxy's optical surface brightness profile. We present radio-optical-X-ray spectral indices for all 51 sample galaxies. Survival statistics point to significant differences in the radio-to-optical and radio-to-X-ray spectral indices between the "core" and "power-law" galaxies (Gehan's Generalized Wilcoxon test probability p for the two classes being statistically similar is <10-5), but not in the optical-to-X-ray spectral indices (p = 0.25). Therefore, the primary difference between the "core" and "power-law" galaxies is in their ability to launch powerful radio outflows. This result is consistent with the hypothesis of different formation processes and evolution histories in "core" and "power-law" galaxies: major mergers are likely to have created "core" galaxies, while minor mergers were instrumental in the creation of "power-law" galaxies
X-ray study of a sample of FR0 radio galaxies: unveiling the nature of the central engine
No full textFanaroff–Riley type 0 radio galaxies (FR0s) are compact radio sources that represent the bulk of the radio-loud active galactic nuclei (AGN) population, but they are still poorly understood. Pilot studies on these sources have been already performed at radio and optical wavelengths: here we present the first X-ray study of a sample of 19 FR0 radio galaxies selected from the Sloan Digital Sky Survey/NRAO VLA Sky Survey/Faint Images of the Radio Sky at Twenty-cm sample of Best & Heckman, with redshift ≤0.15, radio size ≤10 kpc, and optically classified as low-excitation galaxies. The X-ray spectra are modelled with a power-law component absorbed by Galactic column density with, in some cases, a contribution from thermal extended gas. The X-ray photons are likely produced by the jet as attested by the observed correlation between X-ray (2–10 keV) and radio (5 GHz) luminosities, similar to Fanaroff–Riley type I radio galaxies (FRIs). The estimated Eddington-scaled luminosities indicate a low accretion rate. Overall, we find that the X-ray properties of FR0s are indistinguishable from those of FRIs, thus adding another similarity between AGN associated with compact and extended radio sources. A comparison between FR0s and low-luminosity BL Lacs rules out important beaming effects in the X-ray emission of the compact radio galaxies. FR0s have different X-ray properties with respect to young radio sources (e.g. gigahertz-peaked spectrum/compact steep spectrum sources), generally characterized by higher X-ray luminosities and more complex spectra. In conclusion, the paucity of extended radio emission in FR0s is probably related to the intrinsic properties of their jets that prevent the formation of extended structures, and/or to intermittent activity of their engines
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