15 research outputs found
Modeling quasar accretion disc temperature profiles
Microlensing observations indicate that quasar accretion discs have
half-light radii larger than expected from standard theoretical predictions
based on quasar fluxes or black hole masses. Blackburne and colleagues have
also found a very weak wavelength dependence of these half-light radii. We
consider disc temperature profile models that might match these observations.
Nixon and colleagues have suggested that misaligned accretion discs around
spinning black holes will be disrupted at radii small enough for the
Lense-Thirring torque to overcome the disc's viscous torque. Gas in precessing
annuli torn off a disc will spread radially and intersect with the remaining
disc, heating the disc at potentially large radii. However, if the intersection
occurs at an angle of more than a degree or so, highly supersonic collisions
will shock-heat the gas to a Compton temperature of T~10^7 K, and the spectral
energy distributions (SEDs) of discs with such shock-heated regions are poor
fits to observations of quasar SEDs. Torn discs where heating occurs in
intermittent weak shocks that occur whenever the intersection angle reaches a
tenth of a degree pose less of a conflict with observations, but do not have
significantly larger half-light radii than standard discs. We also study two
phenomenological disc temperature profile models. We find that discs with a
temperature spike at relatively large radii and lowered temperatures at radii
inside the spike yield improved and acceptable fits to microlensing sizes in
most cases. Such temperature profiles could in principle occur in sub-Keplerian
discs partially supported by magnetic pressure. However, such discs overpredict
the fluxes from quasars studied with microlensing except in the limit of
negligible continuum emission from radii inside the temperature spike.Comment: Submitted to MNRAS. Comments welcome. 20 pages, 5 figure
Sub-LET Threshold SEE cross section dependency with ion energy
This study focuses on the ion species and energy dependence of the heavy ion SEE cross section in the sub-LET threshold region through a set of experimental data. In addition, a Monte Carlo based model is introduced and applied, showing a good agreement with the data in the several hundred MeV/n range while evidencing large discrepancies with the measurements in the 10-30 MeV/n interval, notably for the Ne ion. Such discrepancies are carefully analyzed and discussed
The Future Landscape of High-Redshift Galaxy Cluster Science
Large scale structure and cosmolog
Quiescent galaxies in a virialized cluster at redshift 2:evidence for accelerated size growth
The XXL survey. XLIX. Linking the members star formation histories to the cluster mass assembly in the z=1.98 galaxy cluster XLSSC 122
International audienceThe most massive protoclusters virialize to become clusters at , which is also a critical epoch for the evolution of their member galaxies. XLSSC 122 is a galaxy cluster with 37 spectroscopically confirmed members. We aim to characterize their star formation histories and to put them in the context of the cluster accretion history. We measure their photometry in 12 bands and create a PSF-matched catalogue of the cluster members. We employ BAGPIPES to fit star formation histories characterized by exponentially decreasing star-forming rates. Stellar masses, metal and dust contents are also treated as free parameters. The oldest stars in the red-sequence galaxies display a range of ages, from 0.5 Gyr to over 3 Gyrs. Characteristic times are between 0.1 and 0.3 Gyr, and the oldest members present the longest times. Using MultiDark Planck 2 dark matter simulations, we calculate the assembly of XLSSC 122-like haloes, weighted by the age posteriors of the oldest members. We found that 74% of these haloes were less than 10% assembled at the onset of star formation, declining to 67% of haloes when such galaxies had formed 50% of their z=1.98 stellar masses. When 90% of their stellar masses were formed, 75% of the haloes were less than 30% assembled. The star formation histories of the red-sequence galaxies seem consistent with episodes of star formation with short characteristic times. Onset and cessation of star formation in the oldest galaxies are both likely to precede XLSSC 122 virialization
Spectroscopic confirmation of a mature galaxy cluster at a redshift of 2
Galaxy clusters are the most massive virialized structures in the Universe
and are formed through the gravitational accretion of matter over cosmic time.
The discovery of an evolved galaxy cluster at redshift z=2, corresponding to a
look-back time of 10.4 billion years, provides an opportunity to study its
properties. The galaxy cluster XLSSC 122 was originally detected as a faint,
extended X-ray source in the XMM Large Scale Structure survey and was revealed
to be coincident with a compact over-density of galaxies with photometric
redshifts of 1.9 +/- 0.2. Subsequent observations at millimetre wavelengths
detected a Sunyaev-Zel'dovich decrement along the line of sight to XLSSC 122,
thus confirming the existence of hot intracluster gas, while deep imaging
spectroscopy from the European Space Agency's X-ray Multi-Mirror Mission
(XMM-Newton) revealed an extended, X-ray bright gaseous atmosphere with a
virial temperature of 60 million Kelvin, enriched with metals to the same
extent as are local clusters. Here we report rest frame optical spectroscopic
observations of XLSSC 122 and identify 37 member galaxies at a mean redshift of
1.98, corresponding to a look-back time of 10.4 billion years. We use
photometry to determine a mean, dust-free stellar age of 2.98 billion years,
indicating that star formation commenced in these galaxies at a mean redshift
of 12, when the Universe was only 370 million years old. The full range of
inferred formation redshifts, including the effects of dust, covers the
interval from 7 to 13. These observations confirm that XLSSC 122 is a
remarkably mature galaxy cluster with both evolved stellar populations in the
member galaxies and a hot, metal-rich gas composing the intracluster medium.Comment: To appear in Nature, January 1st 2020. Authors formatted version, 20
pages, 7 figure
Galaxy populations in the most distant SPT-SZ clusters: I. Environmental quenching in massive clusters at 1.4 ≲ z ≲ 1.7
© 2019 ESO. We present the first results from a galaxy population study in the highest redshift galaxy clusters identified in the 2500 deg 2 South Pole Telescope Sunyaev Zel'dovich effect (SPT-SZ) survey, which is sensitive to M 500 ≳ 3 × 10 14 M · clusters from z ∼ 0.2 out to the highest redshifts where such massive structures exist. The cluster selection is to first order independent of galaxy properties, making the SPT-SZ sample particularly well suited for cluster galaxy population studies. We carried out a four-band imaging campaign with the Hubble and Spitzer Space Telescopes of the five z≳ 1.4, S/N SZE > 5 clusters, that are among the rarest most massive clusters known at this redshift. All five clusters show clear overdensities of red galaxies whose colors agree with the initial cluster redshift estimates, although one (SPT-CLJ0607-4448) shows a galaxy concentration much less prominent than the others. The highest redshift cluster in this sample, SPT-CLJ0459-4947 at z ∼ 1.72, is the most distant M 500 > 10 14 M · cluster discovered thus far through its intracluster medium, and is one of only three known clusters in this mass range at z≳ 1.7, regardless of selection. Based on UVJ-like photometric classification of quiescent and star-forming galaxies, we find that the quiescent fraction in the cluster central regions (r/r 500 10.85. We have explored the impact of emission from star formation on the selection of this sample, concluding that all five clusters studied here would still have been detected with S/N SZE > 5, even if they had the same quiescent fraction as measured in the field. Our results thus point towards an efficient suppression of star formation in the central regions of the most massive clusters, occurring already earlier than z ∼ 1.5