158 research outputs found
Feedback from quasars in star-forming galaxies and the triggering of massive galactic winds
The shining of quasars is a likely trigger of massive galatic winds, able to
remove most ISM from a star-forming spheroid. However, the mechanism
responsible for the deposition of energy into the ISM is still unclear.
Starting from a model for feedback in galaxy formation with a two-phase medium
(Monaco 2004a), we propose that the perturbation induced by radiative heating
from a quasar on the ISM triggers a critical change of feedback regime. In the
feedback model, SNRs expanding in the hot and pressurized phase of a
star-forming spheroid tipically become pressure-confined before the hot
interior gas is able to cool. Whenever the evaporation flow due to radiative
heating of the quasar is significant with respect to the star-formation rate,
the SNRs reach the point where their interior gas cools before being confined,
forming a thick cold shell. We show that in this conditions the shells
percolate into a super-shell of cold gas that sweeps the whole galaxy.
Radiation pressure then pushes the shell out of the galaxy. This self-limiting
mechanism leads to a correlation between black hole and bulge masses. The
insertion of a motivated wind trigger criterion in a hierarchical galaxy
formation model shows however that winds are not necessary to obtain a good
black hole--bulge correlation. In absence of winds, good results are obtained
if the mechanism responsible for the creation of a reservoir of low-angular
momentum gas (able to accrete onto the black hole) deposits mass at a rate
proportional to the star-formation rate. Using a novel galaxy formation model,
we show under which conditions black hole masses are self-limited by the wind
mechanism described above, and outline the possible observational consequences
of this self-limitation. [Abridged]Comment: 14 pages, 7 ps figures included, uses mn2e.cls. MNRAS, in pres
The Luminosity Function of high-redshift QSOs - A combined analysis of GOODS and SDSS
Aims: In this work the luminosity function of QSOs is measured in the
redshift range 3.5<z<5.2 for the absolute magnitude interval -21<M_{145}<-28.
The determination of the faint end of the luminosity function at these
redshifts provides important constraints on models of joint evolution of
galaxies and AGNs. Methods: We have defined suitable criteria to select faint
QSOs in the GOODS fields, checking in detail their effectiveness and
completeness. Spectroscopic follow-up of the resulting QSO candidates has been
carried out. The confirmed sample of faint QSOs is compared with a brighter one
derived from the SDSS. We have used a Monte-Carlo technique to estimate the
properties of the luminosity function, checking various parameterizations for
its shape and evolution. Results: Models based on Pure Density Evolution show
better agreement with observation than models based on Pure Luminosity
Evolution. However a different break magnitude with respect to z~2.1 is
required at 3.5<z<5.2. Models with a steeper faint end score a higher
probability. We do not find any evidence for a flattening of the bright end at
redshift z>3.5. Conclusions: The estimated space density evolution of QSOs
indicates a suppression of the formation and/or feeding of Supermassive Black
Holes at these redshifts. The QSO contribution to the UV background is
insufficient to ionize the IGM at 3.5<z<5.2.Comment: 17 pages, 13 ps figures, A&A accepted. Updated to journal versio
Wide and deep near-UV (360nm) galaxy counts and the extragalactic background light with the Large Binocular Camera
Deep multicolour surveys are the main tool to explore the formation and
evolution of the faint galaxies which are beyond the spectroscopic limit with
the present technology. The photometric properties of these faint galaxies are
usually compared with current renditions of semianalytical models to provide
constraints on the fundamental physical processes involved in galaxy formation
and evolution, namely the mass assembly and the star formation. Galaxy counts
over large sky areas in the near-UV band are important because they are
difficult to obtain given the low efficiency of near-UV instrumentation, even
at 8m class telescopes. A large instrumental field of view helps in minimizing
the biases due to the cosmic variance. We have obtained deep images in the
360nm U band provided by the blue channel of the Large Binocular Camera at the
prime focus of the Large Binocular Telescope. We have derived over an area of
~0.4 sq. deg. the galaxy number counts down to U=27 in the Vega system
(corresponding to U=27.86 in the AB system) at a completeness level of 30%
reaching the faintest current limit for this wavelength and sky area. The shape
of the galaxy counts in the U band can be described by a double power-law, the
bright side being consistent with the shape of shallower surveys of comparable
or greater areas. The slope bends over significantly at U>23.5 ensuring the
convergence of the contribution by star forming galaxies to the EBL in the
near-UV band to a value which is more than 70% of the most recent upper limits
derived for this band. We have jointly compared our near-UV and K band counts
collected from the literature with few selected hierarchical CDM models
emphasizing critical issues in the physical description of the galaxy formation
and evolution.Comment: Accepted for publication in A&A. Uses aa.cls, 9 pages, 4 figures.
Citations update
The probabilistic random forest applied to the selection of quasar candidates in the QUBRICS survey
The number of known, bright (i 2.5) QSOs in the Southern hemisphere is considerably lower than the corresponding number in the Northern hemisphere due to the lack of multiwavelength surveys at δ 2.5 QSOs. The performances of the PRF, currently comparable to those of the CCA, are expected to improve as the number of high-z QSOs available for the training sample grows: results are however already promising, despite this being one of the first applications of this method to an astrophysical context
The contribution of faint AGNs to the ionizing background at z~4
Finding the sources responsible for the hydrogen reionization is one of the
most pressing issues in cosmology. Bright QSOs are known to ionize their
surrounding neighborhood, but they are too few to ensure the required HI
ionizing background. A significant contribution by faint AGNs, however, could
solve the problem, as recently advocated on the basis of a relatively large
space density of faint active nuclei at z>4. We have carried out an exploratory
spectroscopic program to measure the HI ionizing emission of 16 faint AGNs
spanning a broad U-I color interval, with I~21-23 and 3.6<z<4.2. These AGNs are
three magnitudes fainter than the typical SDSS QSOs (M1450<~-26) which are
known to ionize their surrounding IGM at z>~4. The LyC escape fraction has been
detected with S/N ratio of ~10-120 and is between 44 and 100% for all the
observed faint AGNs, with a mean value of 74% at 3.6<z<4.2 and
-25.1<M1450<-23.3, in agreement with the value found in the literature for much
brighter QSOs (M1450<~-26) at the same redshifts. The LyC escape fraction of
our faint AGNs does not show any dependence on the absolute luminosities or on
the observed U-I colors. Assuming that the LyC escape fraction remains close to
~75% down to M1450~-18, we find that the AGN population can provide between 16
and 73% (depending on the adopted luminosity function) of the whole ionizing UV
background at z~4, measured through the Lyman forest. This contribution
increases to 25-100% if other determinations of the ionizing UV background are
adopted. Extrapolating these results to z~5-7, there are possible indications
that bright QSOs and faint AGNs can provide a significant contribution to the
reionization of the Universe, if their space density is high at M1450~-23.Comment: Accepted for publication on A&A, 16 pages, 22 figure
BRDF characterization of Al-coated thermoplastic polymer surfaces
In this paper, we present a combined morphological and optical characterization of aluminum-coated thermoplastic polymer surfaces. Flat plastic substrates, obtained by means of an injection molding process starting from plastic granules, were coated with ultra-thin aluminum films evaporated in vacuo, on top of which a silicon-based protective layer was plasma deposited in order to prevent oxidation of the metal reflective surface. Different sample treatments were studied to unravel the influence of substrate chemistry, substrate thickness, aluminum and protective layer thickness, and surface roughness on the actual optical reflectance properties. Bidirectional reflectance distribution function measurements, corroborated by surface morphological information obtained by means of atomic force microscopy, correlate reflectance characteristics with the root-mean-square surface roughness, providing evidence for\ua0the role of the substrate and the thin films\u2019 morphology. The results unravel information of interest within many applicative fields involving metal coating processes of plastic substrates as an example in the case of automotive lighting
High-redshift QSOs in the GOODS
The Great Observatories Origins Deep Survey provides significant constraints
on the space density of less luminous QSOs at high redshift, which is
particularly important to understand the interplay between the formation of
galaxies and super-massive black holes and to measure the QSO contribution to
the UV ionizing background. We present the results of a search for high-z QSOs,
identified in the two GOODS fields on the basis of deep imaging in the optical
(with HST) and X-ray (Chandra), and discuss the allowed space density of QSOs
in the early universe.Comment: Proceedings of 'Multiwavelength mapping of galaxy evolution'
conference held in Venice (Italy), October 2003, A. Renzini and R. Bender
(Eds.), 6 pages, 1 figur
Eddington accreting black holes in the epoch of reionization
The evolution of the luminosity function (LF) of active galactic nuclei (AGNs) at redshift z ≳ 5 represents a key constraint to understand their contribution to the ionizing photon budget necessary to trigger the last phase transition in the Universe, i.e. the epoch of reionization. Recent searches for bright high-z AGNs suggest that the space densities of this population at z > 4 have to be revised upwards, and spark new questions about their evolutionary paths. Gas accretion is the key physical mechanism to understand both the distribution of luminous sources and the growth of central supermassive black holes (SMBHs). In this work, we model the high-z AGN-LF assuming that high-z luminous AGNs shine at their Eddington limit: We derive the expected evolution as a function of the ‘duty cycle’ (fdc), i.e. the fraction of lifetime that a given SMBH spends accreting at the Eddington rate. Our results show that intermediate values (fdc ≃ 0.1) predict the best agreement with the ionizing background and photoionization rate, but do not provide enough ionizing photons to account for the observed evolution of the hydrogen neutral fraction. Smaller values (fdc≲ 0.05) are required for AGNs to be the dominant population responsible for hydrogen reionization in the early Universe. We then show that this low-fdc evolution can be reconciled with the current constraints on helium reionization, although it implies a relatively large number of inactive SMBHs at z ≳ 5, in tension with SMBH growth models based on heavy seeding
On the AGN nature of two UV bright sources at z_spec~5.5 in the CANDELS fields: an update of the AGN space density at M1450~-22.5
It is a widespread opinion that hydrogen reionization is mainly driven by
primeval star-forming galaxies, with a minor role of high-z active galactic
nuclei. Recent observations, however, challenge this notion, indicating a
number of issues related to a galaxy-driven reionization scenario. We provide
here an updated assessment of the space density of relatively faint
(M1450~-22.5) AGNs at zspec~5.5 in order to improve the estimate of the
photo-ionization rate contribution from accreting super massive black holes.
Exploiting deep UV rest-frame ground-based spectra collected at the Very Large
Telescope on the CANDELS/GOODS-South field and deep Chandra X-ray images in the
CANDELS/GOODS-North and EGS areas, we find two relatively bright (M1450~-22.5)
AGNs at zspec~5.5. We derive an AGN space density of Phi=1.29x10^-6 cMpc^-3 at
z~5.5 and M1450~-22.5 by simply dividing their observed number by the
cosmological volume in the range 5.0<z<6.1. Our estimate does not consider
corrections for incompleteness, therefore it represents a lower limit, although
uncertainties due to cosmic variance can still be significant. This value
supports a high space density of AGNs at z>5, in contrast with previous claims
mostly based on standard color selection, possibly affected by significant
incompleteness. Our estimate for the AGN photo-ionization rate at z~5.5 is in
agreement with the observed values at similar redshifts, which are needed to
keep the intergalactic medium highly ionized. Upcoming JWST and giant ground
based telescopes observations will improve the study of high-z AGNs and their
contribution to the reionization of the Universe.Comment: Accepted for publication in ApJ. 21 pages, 6 figures, 4 table
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