281 research outputs found

    The wide-field infrared survey explorer properties of complete samples of radio-loud active galactic nucleus

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    We present an analysis of four complete samples of radio-loud active galactic nucleus (AGN; 3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by theWide-field Infrared Survey Explorer (WISE). The combined sample consists of 79 quasars and 273 radio galaxies, and covers a redshift range 0.003<z<3.395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs and HERGs) is analysed for the first time using large complete samples. Our results demonstrate that a division in the accretion modes of LERGs and HERGs clearly stands out in the mid-IR-radio plane (L22μm = 5 × 1043 erg s-1). This means that WISE data can be effectively used to diagnose accretion modes in radio-loud AGN. The mid-IR properties of all objects were analysed to test the unification between quasars and radio galaxies, consistent with earlier work, and we argue that smooth torus models best reproduce the observation. Quasars are found to have higher mid-IR luminosities than radio galaxies. We also studied all the sources in the near-IR to gain insights into evolution of AGN host galaxies. A relation found between the near-IR luminosity and redshift, well known in the near-IR, is apparent in the two near-IR WISE bands, supporting the idea that radio sources are hosted by massive elliptical galaxies that formed their stars at high redshifts and evolved passively thereafter. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN.Peer reviewe

    On the evolution of clustering of 24um-selected galaxies

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    This paper investigates the clustering properties of a complete sample of 1041 24um-selected sources brighter than F[24um]=400 uJy in the overlapping region between the SWIRE and UKIDSS UDS surveys. We have concentrated on the two (photometric) interval ranges z=[0.6-1.2] (low-z sample) and z>1.6 (high-z sample) as it is in these regions were we expect the mid-IR population to be dominated by intense dust-enshrouded activity such as star formation and black hole accretion. Investigations of the angular correlation function produce a correlation length are r0~15.9 Mpc for the high-z sample and r0~8.5 Mpc for the low-z one. Comparisons with physical models reveal that the high-z sources are exclusively associated with very massive (M>~10^{13} M_sun)haloes, comparable to those which locally host groups-to-clusters of galaxies, and are very common within such (rare) structures. Conversely, lower-z galaxies are found to reside in smaller halos (M_min~10^{12} M_sun) and to be very rare in such systems. While recent studies have determined a strong evolution of the 24um luminosity function between z~2 and z~0, they cannot provide information on the physical nature of such an evolution. Our clustering results instead indicate that this is due to the presence of different populations of objects inhabiting different structures, as active systems at z<~1.5 are found to be exclusively associated with low-mass galaxies, while very massive sources appear to have concluded their active phase before this epoch. Finally, we note that the small-scale clustering data seem to require steep profiles for the distribution of galaxies within their halos. This is suggestive of close encounters and/or mergers which could strongly favour both AGN and star-formation activity.Comment: 13 pages, 8 figures, to appear in MNRA

    Observing Supermassive Black Holes across cosmic time: from phenomenology to physics

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    In the last decade, a combination of high sensitivity, high spatial resolution observations and of coordinated multi-wavelength surveys has revolutionized our view of extra-galactic black hole (BH) astrophysics. We now know that supermassive black holes reside in the nuclei of almost every galaxy, grow over cosmological times by accreting matter, interact and merge with each other, and in the process liberate enormous amounts of energy that influence dramatically the evolution of the surrounding gas and stars, providing a powerful self-regulatory mechanism for galaxy formation. The different energetic phenomena associated to growing black holes and Active Galactic Nuclei (AGN), their cosmological evolution and the observational techniques used to unveil them, are the subject of this chapter. In particular, I will focus my attention on the connection between the theory of high-energy astrophysical processes giving rise to the observed emission in AGN, the observable imprints they leave at different wavelengths, and the methods used to uncover them in a statistically robust way. I will show how such a combined effort of theorists and observers have led us to unveil most of the SMBH growth over a large fraction of the age of the Universe, but that nagging uncertainties remain, preventing us from fully understating the exact role of black holes in the complex process of galaxy and large-scale structure formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and Treves A. (Eds), 2015, Springer International Publishing AG, Cha

    Herschel-ATLAS/GAMA: What determines the far-infrared properties of radio galaxies?

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    We perform a stacking analysis of Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS) data in order to obtain isothermal dust temperatures and rest-frame luminosities at 250 μm (L_250), for a well-defined sample of 1599 radio sources over the H-ATLAS Phase 1/Galaxy and Mass Assembly (GAMA) area. The radio sample is generated using a combination of NRAO VLA Sky Survey data and K-band United Kingdom Infrared Telescope Deep Sky Survey–Large Area Survey data, over the redshift range 0.01 < z < 0.8. The far-infrared (FIR) properties of the sample are investigated as a function of 1.4-GHz luminosity, redshift, projected radio-source size and radio spectral index. In order to search for stellar-mass-dependent relations, we split the parent sample into those sources which are below and above 1.5 L∗_(K). After correcting for stellar mass and redshift, we find no relation between the 250-μm luminosity and the 1.4-GHz radio luminosity of radio active galactic nuclei. This implies that a galaxy's nominal radio luminosity has little or no bearing on the star formation rate (SFR) and/or dust mass content of the host system, although this does not mean that other variables (e.g. radio source size) related to the jets do not have an effect. The L_250 of both the radio detected and non-radio-detected galaxies (defined as those sources not detected at 1.4 GHz but detected in the Sloan Digital Sky Survey with r′ 30 kpc) counterparts. The higher dust temperature suggests that this may be attributed to enhanced SFRs in compact radio galaxies, but whether this is directly or indirectly due to radio activity (e.g. jet-induced or merger-driven star formation) is as yet unknown. For matched samples in L_K and g′–r′, sub-1.5 L∗_K and super-1.5 L∗_K radio-detected galaxies have 0.89±0.18 and 0.49±0.12 times the 250 μm luminosity of their non-radio-detected counterparts. Thus, while no difference in L_250 is observed in sub-1.5 L∗_K radio-detected galaxies, a strong deficit is observed in super-1.5 L∗_K radio-detected galaxies. We explain these results in terms of the hotter, denser and richer halo environments massive radio galaxies maintain and are embedded in. These environments are expected to quench the cold gas and dust supply needed for further star formation and therefore dust production. Our results indicate that all massive radio galaxies (>1.5 L∗_K) may have systematically lower FIR luminosities (∼25 per cent) than their colour-matched non-radio-detected counterparts. Finally, no relation between radio spectral index and L_250 is found for the subset of 1.4-GHz radio sources with detections at 330 MHz

    Evolution of faint radio sources in the VIDEO-XMM3 field

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    © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical SocietyIt has been speculated that low-luminosity radio-loud active galactic nuclei (AGN) have the potential to serve as an important source of AGN feedback, and may be responsible for suppressing star formation activity in massive elliptical galaxies at late times. As such the cosmic evolution of these sources is vitally important to understand the significance of such AGN feedback processes and their influence on the global star formation history of the Universe. In this paper, we present a new investigation of the evolution of faint radio sources out to z ~ 2.5. We combine a 1 square degree Very Large Array radio survey, complete to a depth of 100 μJy, with accurate 10 band photometric redshifts from the following surveys: Visible and Infrared Survey Telescope for Astronomy Deep Extragalactic Observations and Canada-France-Hawaii Telescope Legacy Survey. The results indicate that the radio population experiences mild positive evolution out to z ~ 1.2 increasing their space density by a factor of ~3, consistent with results of several previous studies. Beyond z = 1.2, there is evidence of a slowing down of this evolution. Star-forming galaxies drive the more rapid evolution at low redshifts, z 1.2. The evolution is best fitted by pure luminosity evolution with star-forming galaxies evolving as (1 + z)2.47 ± 0.12 and AGN as (1 + z)1.18 ± 0.21M.Peer reviewe

    Multiwavelength characterization of faint ultra steep spectrum radio sources: a search for high-redshift radio galaxies

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    Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts. Aims. Using our 325 MHz GMRT observations (5σ ∼ 800 μJy) and 1.4 GHz VLA observations (5σ ∼ 80−100 μJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature. Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Results. Redshift estimates are available for 86/116 (∼74%) USS sources in the VLA-VVDS field and for 39/44 (∼87%) USS sources in the SXDF fields with median values (zmedian) ∼1.18 and ∼1.57, respectively, which are higher than estimates for non-USS radio sources (zmedian non−USS ∼ 0.99 and ∼0.96), in the two subfields. The MIR color–color diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z > 0.5) being AGN. The flux ratio of radio to mid-IR (S 1.4 GHz/S 3.6 μm) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ∼ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (∼26% in the VLA-VVDS and ∼13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates. Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments

    Reporting through the lens of the past: from Challenger to Columbia

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    Collective memory, the publicly shared meaning of a common past, can structure both news stories and reporters' search for information within the broader context of journalistic practices. It can also provide reporters with an independent perspective, balancing elite-dominated news frames. Following the space shuttle Columbia's crash, journalists turned repeatedly to the ‘lessons' of the accident that claimed the Challenger shuttle 17 years earlier both in formulating questions at NASA briefings and in reporting Columbia's destruction and the subsequent investigation in print. In many instances, journalists' reliance on these memories is entirely implicit in the finished news stories, making Challenger a ghostly presence that led reporters to focus on NASA's inadequacies rather than on the mechanical causes of Columbia's demise.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline

    Multi vegetation model evaluation of the Green Sahara climate regime

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    During the Quaternary, the Sahara desert was periodically colonized by vegetation, likely because of orbitally induced rainfall increases. However, the estimated hydrological change is not reproduced in climate model simulations, undermining confidence in projections of future rainfall. We evaluated the relationship between the qualitative information on past vegetation coverage and climate for the mid-Holocene using three different dynamic vegetation models. Compared with two available vegetation reconstructions, the models require 500–800 mm of rainfall over 20°–25°N, which is significantly larger than inferred from pollen but largely in agreement with more recent leaf wax biomarker reconstructions. The magnitude of the response also suggests that required rainfall regime of the early to middle Holocene is far from being correctly represented in general circulation models. However, intermodel differences related to moisture stress parameterizations, biases in simulated present-day vegetation, and uncertainties about paleosoil distributions introduce uncertainties, and these are also relevant to Earth system model simulations of African humid periods
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