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Register a new userExtended radio emission in MOJAVE Blazars: Challenges to Unification
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We present the results of a study on the 1.4 GHz kpc-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud AGNs. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. Many BL Lacs exhibit extended radio power and kpc-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs. This poses challenges to the simple radio-loud unified scheme, which links BL Lacs to FRIs and quasars to FRIIs. We find a significant correlation between extended radio emission and pc-scale jet speeds: the more radio powerful sources possess faster jets. This indicates that the 1.4 GHz (or low frequency) radio emission is indeed related to jet kinetic power. Various properties such as extended radio power and apparent pc-scale jet speeds vary smoothly between different blazar subclasses, suggesting that, at least in terms of radio jet properties, the distinction between quasars and BL Lac objects, at an emission-line equivalent width of 5 Angstrom is essentially an arbitrary one. Based on the assumption that the extended radio luminosity is affected by the kpc-scale environment, we define the ratio of extended radio power to absolute optical magnitude as a proxy for environmental effects. Trends with this parameter suggest that the pc-scale jet speeds and the pc-to-kpc jet misalignments are not affected by the large-scale environment, but are more likely to depend upon factors intrinsic to the AGN, or its local pc-scale environment. We suggest that some of the extremely misaligned MOJAVE blazar jets could be hybrid morphology sources, with an FRI jet on one side and an FRII jet on the other. (Abridged)
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Unified schemes of radio sources, which account for different types of radio AGN in terms of anisotropic radio and optical emission, together with different orientations of the ejection axis to the line of sight, have been invoked for many years. Recently, large samples of optical quasars, mainly from the Sloan Digital Sky Survey, together with large radio samples, such as FIRST, have become available. These hold the promise of providing more stringent tests of unified schemes but, compared to previous samples, lack high resolution radio maps. Nevertheless they have been used to investigate unified schemes, in some cases yielding results which appear inconsistent with such theories. Here we investigate using simulations how the selection effects to which such investigations are subject can influence the conclusions drawn. In particular, we find that the effects of limited resolution do not allow core-dominated radio sources to be fully represented in the samples, that the effects of limited sensitivity systematically exclude some classes of sources and the lack of deep radio data make it difficult to decide to what extent closely separated radio sources are associated. Nevertheless, we conclude that relativistic unified schemes are entirely compatible with the current observational data. For a sample selected from SDSS and FIRST which includes weak-cored triples we find that the equivalent width of the [OIII] emission line decreases as core-dominance increases, as expected, and also that core-dominated quasars are optically brighter than weak-cored quasars.
The F-GAMMA program is among the most comprehensive programs that aim at understanding the physics in active galactic nuclei through the multi-frequency monitoring of Fermi blazars. Here we discuss monthly sampled broad-band radio spectra (2.6 - 142 GHz). Two different studies are presented. (a) We discuss that the variability patterns traced can be classified into two classes: (1) to those showing intense spectral-evolution and (2) those showing a self-similar quasi-achromatic behaviour. We show that a simple two-component model can very well reproduce the observed phenomenologies. (b) We present the cm-to-mm behaviour of three gamma-ray bright Narrow Line Seyfert 1 galaxies over time spans varying between ~1.5 and 3 years and compare their variability characteristics with typical blazars.
We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3sigma level. Although most features within a jet typically have speeds within ~40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc de-projected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 +- 0.2. The parent jets of the brightest radio quasars have a space density of 261 +- 19 Gpc$^{-3}$ and unbeamed 15 GHz luminosities above ~$10^{24.5}$ W/Hz, consistent with FR II class radio galaxies.
We present a new 400ks Chandra X-ray observation and a GMRT radio observation at 325MHz of the merging galaxy cluster Abell 2146. The Chandra observation reveals detailed structure associated with the major merger event including the Mach M=2.1+/-0.2 bow shock located ahead of the dense subcluster core and the first known example of an upstream shock (M=1.6+/-0.1). Surprisingly, the deep GMRT observation at 325MHz does not detect any extended radio emission associated with either shock front. All other merging galaxy clusters with X-ray detected shock fronts, including the Bullet cluster, Abell 520, Abell 754 and Abell 2744, and clusters with candidate shock fronts have detected radio relics or radio halo edges coincident with the shocks. We consider several possible factors which could affect the formation of radio relics, including the shock strength and the presence of a pre-existing electron population, but do not find a favourable explanation for this result. We calculate a 3sigma upper limit of 13mJy on extended radio emission, which is significantly below the radio power expected by the observed P_{radio}-L_{X} correlation for merging systems. The lack of an extended radio halo in Abell 2146 maybe due to the low cluster mass relative to the majority of merging galaxy clusters with detected radio halos.
X-ray properties of galaxy groups can unlock some of the most challenging research topics in modern extragalactic astronomy: the growth of structure and its influence on galaxy formation. Only with the advent of the Chandra and XMM facilities have X-ray observations reached the depths required to address these questions in a satisfactory manner. Here we present an X-ray imaging study of two patches from the CNOC2 spectroscopic galaxy survey using combined Chandra and XMM data. A state of the art extended source finding algorithm has been applied, and the resultant source catalog, including redshifts from a spectroscopic follow-up program, is presented. The total number of spectroscopically identified groups is 25 spanning a redshift range 0.04-0.79. Approximately 50% of CNOC2 spectroscopically selected groups in the deeper X-ray (RA14h) field are likely X-ray detections, compared to 20% in the shallower (RA21h) field. Statistical modeling shows that this is consistent with expectations, assuming an expected evolution of the Lx-M relation. A significant detection of a stacked shear signal for both spectroscopic and X-ray groups indicates that both samples contain real groups of about the expected mass. We conclude that the current area and depth of X-ray and spectroscopic facilities provide a unique window of opportunity at z~0.4 to test the X-ray appearance of galaxy groups selected in various ways. There is at present no evidence that the correlation between X-ray luminosity and velocity dispersion evolves significantly with redshift, which implies that catalogs based on either method can be fairly compared and modeled.
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