800 research outputs found
The Gains from Privatization in Transition Economies: Is "Change of Ownership" Enough?
This paper seeks to clarify what factors contributed to the macroeconomic gains and losses from privatization in transition economies over the past decade. In contrast to the original "Washington Consensus," which had a tendency to equate change-of-title with privatization, we find that economic performance gains come only from "deep" privatization, that is, when change-of-title reforms occur once key institutional and "agency"-related reforms have exceeded certain threshold levels. We also find that as a result of different initial conditions the economic performance responses of countries to the same policies are different. Copyright 2002, International Monetary Fund
Performative Recommendation: Diversifying Content via Strategic Incentives
The primary goal in recommendation is to suggest relevant content to users,
but optimizing for accuracy often results in recommendations that lack
diversity. To remedy this, conventional approaches such as re-ranking improve
diversity by presenting more diverse items. Here we argue that to promote
inherent and prolonged diversity, the system must encourage its creation.
Towards this, we harness the performative nature of recommendation, and show
how learning can incentivize strategic content creators to create diverse
content. Our approach relies on a novel form of regularization that anticipates
strategic changes to content, and penalizes for content homogeneity. We provide
analytic and empirical results that demonstrate when and how diversity can be
incentivized, and experimentally demonstrate the utility of our approach on
synthetic and semi-synthetic data
Moderate-Luminosity Growing Black Holes From 1.25 < z < 2.7: Varied Accretion In Disk-Dominated Hosts
We compute black hole masses and bolometric luminosities for 57 active
galactic nuclei (AGN) in the redshift range 1.25 < z < 2.67, selected from the
GOODS-South deep multi-wavelength survey field via their X-ray emission. We
determine host galaxy morphological parameters by separating the galaxies from
their central point sources in deep HST images, and host stellar masses and
colors by multi-wavelength SED fitting. 90% of GOODS AGN at these redshifts
have detected rest-frame optical nuclear point sources; bolometric luminosities
range from 2e43 - 2e46 erg/s. The black holes are growing at a range of
accretion rates, with at least 50% of the sample having L/L_Edd < 0.1. 70% of
host galaxies have stellar masses M* > 1e10 M_sun, with a range of colors
suggesting a complex star formation history. We find no evolution of AGN
bolometric luminosity within the sample, and no correlation between AGN
bolometric luminosity and host stellar mass, color or morphology. Fully half
the sample of host galaxies is disk-dominated, with another 25% having strong
disk components. Fewer than 15% of the systems appear to be at some stage of a
major merger. These moderate-luminosity AGN hosts are therefore inconsistent
with a dynamical history dominated by mergers strong enough to destroy disks,
indicating minor mergers or secular processes dominate the co-evolution of
galaxies and their central black holes at z ~ 2.Comment: 11 pages, 6 figures, accepted to ApJ. Sersic indices, AGN/galaxy
luminosity ratios, stellar masses etc. provided in Table
Major Mergers Host the Most Luminous Red Quasars at z ~ 2: A Hubble Space Telescope WFC3/IR Study
We used the Hubble Space Telescope WFC3 near-infrared camera to image the
host galaxies of a sample of eleven luminous, dust-reddened quasars at z ~ 2 --
the peak epoch of black hole growth and star formation in the Universe -- to
test the merger-driven picture for the co-evolution of galaxies and their
nuclear black holes. The red quasars come from the FIRST+2MASS red quasar
survey and a newer, deeper, UKIDSS+FIRST sample. These dust-reddened quasars
are the most intrinsically luminous quasars in the Universe at all redshifts,
and may represent the dust-clearing transitional phase in the merger-driven
black hole growth scenario. Probing the host galaxies in rest-frame visible
light, the HST images reveal that 8/10 of these quasars have actively merging
hosts, while one source is reddened by an intervening lower redshift galaxy
along the line-of-sight. We study the morphological properties of the quasar
hosts using parametric Sersic fits as well as the non-parametric estimators
(Gini coefficient, M_{20} and asymmetry). Their properties are heterogeneous
but broadly consistent with the most extreme morphologies of local merging
systems such as Ultraluminous Infrared galaxies. The red quasars have a
luminosity range of log(L_bol) = 47.8 - 48.3 (erg/s) and the merger fraction of
their AGN hosts is consistent with merger-driven models of luminous AGN
activity at z=2, which supports the picture in which luminous quasars and
galaxies co-evolve through major mergers that trigger both star formation and
black hole growth.Comment: Submitted to ApJ. This version includes the response to the referee
repor
Discovery of Two Spectroscopically Peculiar, Low-Luminosity Quasars at z~4
We report the discovery of two low-luminosity quasars at z~4, both of which
show prominent N IV] 1486A emission. This line is extremely rare in quasar
spectra at any redshift; detecting it in two out of a sample of 23 objects
(i.e., ~ 9% of the sample) is intriguing and is likely due to the
low-luminosity, high-redshift quasar sample we are studying. This is still a
poorly explored regime, where contributions from associated, early starbursts
may be significant. One interpretation of this line posits photoionization by
very massive young stars. Seeing N IV] 1486A emission in a high-redshift quasar
may thus be understood in the context of co-formation and early co-evolution of
galaxies and their supermassive black holes. Alternatively, we may be seeing a
phenomenon related to the early evolution of quasar broad emission line
regions. The non-detection (and possibly even broad absorption) of N V 1240A
line in the spectrum of one of these quasars may support that interpretation.
These two objects may signal a new faint quasar population or an early AGN
evolutionary stage at high redshifts.Comment: 15 pages, 5 figures, Accepted for publicated in ApJ Letter
The Faint End of the Quasar Luminosity Function at z ~ 4: Implications for Ionization of the Intergalactic Medium and Cosmic Downsizing
We present an updated determination of the z ~ 4 QSO luminosity function (QLF), improving the quality of the determination of the faint end of the QLF presented by Glikman et al. (2010). We have observed an additional 43 candidates from our survey sample, yielding one additional QSO at z = 4.23 and increasing the completeness of our spectroscopic follow-up to 48% for candidates brighter than R = 24 over our survey area of 3.76 deg^2. We study the effect of using K-corrections to compute the rest-frame absolute magnitude at 1450 Å compared with measuring M_(1450) directly from the object spectra. We find a luminosity-dependent bias: template-based K-corrections overestimate the luminosity of low-luminosity QSOs, likely due to their reliance on templates derived from higher luminosity QSOs. Combining our sample with bright quasars from the Sloan Digital Sky Survey and using spectrum-based M 1450 for all the quasars, we fit a double power law to the binned QLF. Our best fit has a bright-end slope, α = 3.3 ± 0.2, and faint-end slope, β = 1.6^(+0.8)_(–0.6). Our new data revise the faint-end slope of the QLF down to flatter values similar to those measured at z ~ 3. The break luminosity, though poorly constrained, is at M* = –24.1^(+0.7)_(–1.9), approximately 1-1.5 mag fainter than at z ~ 3. This QLF implies that QSOs account for about half the radiation needed to ionize the intergalactic medium at these redshifts
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
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