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Garching bei München, Germany

Ceverino D.,Hebrew University of Jerusalem | Dekel A.,Hebrew University of Jerusalem | Mandelker N.,Hebrew University of Jerusalem | Bournaud F.,University Paris Diderot | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We address the internal support against total free-fall collapse of the giant clumps that form by violent gravitational instability in high-z disc galaxies. Guidance is provided by an analytic model, where the protoclumps are cut from a rotating disc and collapse to equilibrium while preserving angular momentum. This model predicts prograde clump rotation, which dominates the support if the clump has contracted to a surface density contrast ≳10. This is confirmed in hydro adaptive mesh refinement zoom-in simulations of galaxies in a cosmological context. In most high-z clumps, the centrifugal force dominates the support, , where V rot is the rotation velocity and the circular velocity V circ measures the potential well. The clump spin indeed tends to be in the sense of the global disc angular momentum, but substantial tilts are frequent, reflecting the highly warped nature of the high-z discs. Most clumps are in Jeans equilibrium, with the rest of the support provided by turbulence, partly driven by the gravitational instability itself. The general agreement between model and simulations indicates that angular momentum loss or gain in most clumps is limited to a factor of 2. Simulations of isolated gas-rich discs that resolve the clump substructure reveal that the cosmological simulations may overestimate by ~30 per cent, but the dominance of rotational support at high z is not a resolution artefact. In turn, isolated gas-poor disc simulations produce at z= 0 smaller gaseous non-rotating transient clouds, indicating that the difference in rotational support is associated with the fraction of cold baryons in the disc. In our current cosmological simulations, the clump rotation velocity is typically more than twice the disc dispersion, V rot~ 100kms -1, but when beam smearing of ≥0.1arcsec is imposed, the rotation signal is reduced to a small gradient of ≤30kms -1kpc -1 across the clump. The velocity dispersion in the simulated clumps is comparable to the disc dispersion so it is expected to leave only a marginal signal for any beam smearing. Retrograde minor-merging galaxies could lead to massive clumps that do not show rotation even when marginally resolved. A testable prediction of the scenario as simulated is that the mean stellar age and the stellar fraction of the clumps are declining linearly with distance from the disc centre. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Lusso E.,Max Planck Institute for Astronomy | Hennawi J.F.,Max Planck Institute for Astronomy | Comastri A.,National institute for astrophysics | Zamorani G.,National institute for astrophysics | And 9 more authors.
Astrophysical Journal | Year: 2013

The fraction of active galactic nucleus (AGN) luminosity obscured by dust and re-emitted in the mid-IR is critical for understanding AGN evolution, unification, and parsec-scale AGN physics. For unobscured (Type 1) AGNs, where we have a direct view of the accretion disk, the dust covering factor can be measured by computing the ratio of re-processed mid-IR emission to intrinsic nuclear bolometric luminosity. We use this technique to estimate the obscured AGN fraction as a function of luminosity and redshift for 513 Type 1 AGNs from the XMM-COSMOS survey. The re-processed and intrinsic luminosities are computed by fitting the 18 band COSMOS photometry with a custom spectral energy distribution fitting code, which jointly models emission from hot dust in the AGN torus, from the accretion disk, and from the host galaxy. We find a relatively shallow decrease of the luminosity ratio as a function of L bol, which we interpret as a corresponding decrease in the obscured fraction. In the context of the receding torus model, where dust sublimation reduces the covering factor of more luminous AGNs, our measurements require a torus height that increases with luminosity as . Our obscured-fraction- luminosity relation agrees with determinations from Sloan Digital Sky Survey censuses of Type 1 and Type 2 quasars and favors a torus optically thin to mid-IR radiation. We find a much weaker dependence of the obscured fraction on 2-10 keV luminosity than previous determinations from X-ray surveys and argue that X-ray surveys miss a significant population of highly obscured Compton-thick AGNs. Our analysis shows no clear evidence for evolution of the obscured fraction with redshift. © 2013. The American Astronomical Society. All rights reserved..

De Hoon A.,Leibniz Institute for Astrophysics Potsdam | Lamer G.,Leibniz Institute for Astrophysics Potsdam | Schwope A.,Leibniz Institute for Astrophysics Potsdam | Muhlegger M.,Max Planck Institute For Extraterrestische Physik | And 9 more authors.
Astronomische Nachrichten | Year: 2013

The XMM-Newton Distant Cluster Project (XDCP) aims at the identification of a well defined sample of X-ray selected clusters of galaxies at redshifts z ≥ 0.8. As part of this project, we analyse the deep archival XMM-Newton exposure of LBQS 2212-1759 to quantify the cluster content. We validate the optical follow-up strategy as well as the X-ray selection function. We base the cluster identification of the extended X-ray sources on deep imaging with the ESO-VLT and on the CFHT-LS. The confirmation of cluster candidates is done by VLT/FORS2 spectroscopy. Photometric redshifts from CFTH-LS D4 are utilised to confirm the effectiveness of the X-ray cluster selection method. The survey sensitivity is computed to have a flux limit of S0.5-2.0 keV ~ 2.5×10-15 erg s-1 for 50% completeness in an area ~0.13 deg2. We detect six clusters of galaxies above this level both in X-rays and the optical. Two newly discovered X-ray luminous clusters of galaxies in this work are at z ≥ 1.0 and one is at z = 0.41. The constructed log N -log S tends to favour a scenario where no evolution in the cluster X-ray luminosity function takes place. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Buat V.,Aix - Marseille University | Oi N.,Japan Aerospace Exploration Agency | Heinis S.,University of Maryland University College | Ciesla L.,University of Crete | And 17 more authors.
Astronomy and Astrophysics | Year: 2015

Aims. We aim to study the evolution of dust attenuation in galaxies selected in the infrared (IR) in the redshift range in which they are known to dominate the star formation activity in the universe. The comparison with other measurements of dust attenuation in samples selected using different criteria will give us a global picture of the attenuation at work in star-forming galaxies and its evolution with redshift. Methods. We selected galaxies in the mid-IR from the deep survey of the North Ecliptic Field performed by the AKARI satellite. Using multiple filters of IRC instrument, we selected more than 4000 galaxies from their rest-frame emission at 8 μm, from z ≃ 0.2 to ∼2. We built spectral energy distributions from the rest-frame ultraviolet (UV) to the far-IR by adding ancillary data in the optical-near IR and from GALEX and Herschel surveys. We fit spectral energy distributions with the physically-motivated code CIGALE. We test different templates for active galactic nuclei (AGNs) and recipes for dust attenuation and estimate stellar masses, star formation rates, amount of dust attenuation, and AGN contribution to the total IR luminosity. We discuss the uncertainties affecting these estimates on a subsample of galaxies with spectroscopic redshifts. We also define a subsample of galaxies with an IR luminosity close to the characteristic IR luminosity at each redshift and study the evolution of dust attenuation of this selection representative of the bulk of the IR emission. Results. The AGN contribution to the total IR luminosity is found to be on average approximately 10%, with a slight increase with redshift. The determination of AGN contribution does not depend significantly on the assumed AGN templates except for galaxies detected in X-ray. The choice of attenuation law has a marginal impact on the determination of stellar masses and star formation rates. Dust attenuation in galaxies dominating the IR luminosity function is found to increase from z = 0 to z = 1 and to remain almost constant from z = 1 to z = 1.5. Conversely, when galaxies are selected at a fixed IR luminosity, their dust attenuation slightly decreases as redshift increases but with a large dispersion, confirming previous results obtained at lower redshift.The attenuation in our mid-IR selected sample is found ∼2 mag higher than that found globally in the universe or in UV and Hα line selections in the same redshift range. This difference is well explained by an increase of dust attenuation with the stellar mass, in global agreement with other recent studies. Starbursting galaxies do not systematically exhibit a high attenuation. We conclude that the galaxies selected in IR and dominating the star formation exhibit a higher attenuation than those measured on average in the universe because they are massive systems. Conversely UV selected galaxies exhibit a large range of stellar masses leading in a lower average attenuation than that found in an IR selection. © 2015 ESO.

Lusso E.,University of Bologna | Lusso E.,National institute for astrophysics | Comastri A.,National institute for astrophysics | Vignali C.,University of Bologna | And 18 more authors.
Astronomy and Astrophysics | Year: 2011

We present a study of the multi-wavelength properties, from the mid-infrared to the hard X-rays, of a sample of 255 spectroscopically identified X-ray selected type-2 AGN from the XMM-COSMOS survey. Most of them are obscured and the X-ray absorbing column density is determined by either X-ray spectral analyses (for 45% of the sample), or from hardness ratios. Spectral energy distributions (SEDs) are computed for all sources in the sample. The average SEDs in the optical band are dominated by the host-galaxy light, especially at low X-ray luminosities and redshifts. There is also a trend between X-ray and mid-infrared luminosity: the AGN contribution in the infrared is higher at higher X-ray luminosities. We calculate bolometric luminosities, bolometric corrections, stellar masses and star formation rates (SFRs) for these sources using a multi-component modeling to properly disentangle the emission associated to stellar light from that due to black hole accretion. For 90% of the sample we also have the morphological classifications obtained with an upgraded version of the Zurich estimator of structural types (ZEST+). We find that on average type-2 AGN have lower bolometric corrections than type-1 AGN. Moreover, we confirm that the morphologies of AGN host-galaxies indicate that there is a preference for these type-2 AGN to be hosted in bulge-dominated galaxies with stellar masses greater than 1010 solar masses. © 2011 ESO.

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