499 research outputs found
Quasars Probing Quasars IV: Joint Constraints on the Circumgalactic Medium from Absorption and Emission
We have constructed a sample of 29 close projected quasar pairs where the
background quasar spectrum reveals absorption from optically thick HI gas
associated with the foreground quasar. These unique sightlines allow us to
study the quasar circumgalactic medium (CGM) in absorption and emission
simultaneously, because the background quasar pinpoints large concentrations of
gas where Ly-a emission, resulting from quasar-powered fluorescence, resonant
Ly-a scattering, and/or cooling radiation, is expected. A sensitive
slit-spectroscopic search (1-sigma limits of SB_Lya ~= 3e-18
erg/s/cm^2/arcsec^2) for diffuse Ly-a emission in the environments of the
foreground quasars is conducted. We fail to detect large-scale ~ 100 kpc Ly-a
emission, either at the location of the optically thick absorbers or in the
foreground quasar halos, in all cases except a single system. We interpret
these non-detections as evidence that the gas detected in absorption is
shadowed from the quasar UV radiation due to obscuration effects, which are
frequently invoked in unified models of AGN. Small-scale R_perp <~ 50 kpc
extended Ly-a nebulosities are detected in 34% of our sample, which are likely
the high-redshift analogs of the extended emission-line regions commonly
observed around low-redshift (z < 0.5) quasars. We also detect a compact high
rest-frame equivalent width (W_Lya > 50 A) Ly-alpha-emitter with luminosity
L_Lya =2.1+-0.32e41 erg/s at small impact parameter R_perp=134 kpc from one
foreground quasar, and argue that it is more likely to result from
quasar-powered fluorescence, than simply be a star-forming galaxy clustered
around the quasar. Our observations imply that much deeper integrations with
upcoming integral-field spectrometers such as MUSE and KCWI will be able to
routinely detect a diffuse Ly-a glow around bright quasars on scales R ~ 100
kpc and thus directly image the CGM. [abridged]Comment: 39 pages, 5 figures, 1 table, accepted by ApJ:
http://adsabs.harvard.edu/abs/2013ApJ...766...58
Quasars Probing Quasars IX. The Kinematics of the Circumgalactic Medium Surrounding z ~ 2 Quasars
We examine the kinematics of the gas in the environments of galaxies hosting
quasars at . We employ 148 projected quasar pairs to study the
circumgalactic gas of the foreground quasars in absorption. The sample selects
foreground quasars with precise redshift measurements, using emission-lines
with precision and average offsets from the
systemic redshift . We stack the background
quasar spectra at the foreground quasar's systemic redshift to study the mean
absorption in \ion{C}{2}, \ion{C}{4}, and \ion{Mg}{2}. We find that the mean
absorptions exhibit large velocity widths . Further, the mean absorptions appear to be asymmetric about the
systemic redshifts. The mean absorption centroids exhibit small redshift
relative to the systemic , with large
intrinsic scatter in the centroid velocities of the individual absorption
systems. We find the observed widths are consistent with gas in gravitational
motion and Hubble flow. However, while the observation of large widths alone
does not require galactic-scale outflows, the observed offsets suggest that the
gas is on average outflowing from the galaxy. The observed offsets also suggest
that the ionizing radiation from the foreground quasars is anisotropic and/or
intermittent.Comment: 13 pages, 9 figures, accepted to Ap
Quasars Probing Quasars VII. The Pinnacle of the Cool Circumgalactic Medium Surrounds Massive z~2 Galaxies
We survey the incidence and absorption strength of the metal-line transitions
CII 1334 and CIV from the circumgalactic medium (CGM) surrounding z~2 quasars,
which act as signposts for massive dark matter halos M_halo~10^12.5 Msun. On
scales of the virial radius (Mvir~160kpc), we measure a high covering fraction
fC=0.73+/-0.10 to strong CII absorption (rest equivalent width W1334>0.2A),
implying a massive reservoir of cool (T~10^4K) metal enriched gas. We
conservatively estimate a metal mass exceeding 10^8 Msun. We propose these
metals trace enrichment of the incipient intragroup/intracluster medium that
these halos eventually inhabit. This cool CGM around quasars is the pinnacle
amongst galaxies observed at all epochs, as regards covering fraction and
average equivalent width of HI Lya and low-ion metal absorption. We argue that
the properties of this cool CGM primarily reflect the halo mass, and that other
factors such as feedback, star-formation rate, and accretion from the
intergalactic medium are secondary. We further estimate, that the CGM of
massive, z~2 galaxies accounts for the majority of strong MgII absorption along
random quasar sightlines. Lastly, we detect an excess of strong CIV absorption
(W1548>0.3A) over random incidence to 1Mpc physical impact parameter and
measure the quasar-CIV cross-correlation function: xi(r)=(r/r0)^-g with r0 =
7.5Mpc and g=1.7. Consistent with previous work on larger scales, we infer that
this highly ionized CIV gas traces massive (10^12 Msun) halos.Comment: 27 pages, 16 figures; Re-submitted to ApJ following referee comments;
Additional comments encouraged prior to publicatio
He II Proximity Effect and the Lifetime of Quasars
The lifetime of quasars is fundamental for understanding the growth of
supermassive black holes, and is an important ingredient in models of the
reionization of the intergalactic medium. However, despite various attempts to
determine quasar lifetimes, current estimates from a variety of methods are
uncertain by orders of magnitude. This work combines cosmological
hydrodynamical simulations and 1D radiative transfer to investigate the
structure and evolution of the He II Ly proximity zones around quasars
at . We show that the time evolution in the proximity zone can be
described by a simple analytical model for the approach of the He II fraction
to ionization equilibrium, and use this picture
to illustrate how the transmission profile depends on the quasar lifetime,
quasar UV luminosity, and the ionization state of helium in the ambient IGM
(i.e. the average He II fraction, or equivalently the metagalactic He II
ionizing background). A significant degeneracy exists between the lifetime and
the average He II fraction, however the latter can be determined from
measurements of the He II Ly optical depth far from quasars, allowing
the lifetime to be measured. We advocate stacking existing He II quasar spectra
at , and show that the shape of this average proximity zone profile is
sensitive to lifetimes as long as Myr. At higher redshift
where the He II fraction is poorly constrained, degeneracies will make it
challenging to determine these parameters independently. Our analytical model
for He II proximity zones should also provide a useful description of the
properties of H I proximity zones around quasars at .Comment: 26 pages, 18 figures, accepted to Ap
Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time
We measure the effective optical depth of HeII Ly\alpha\ absorption
\tau at 2.3<z<3.5 in 17 UV-transmitting quasars observed
with UV spectrographs on the Hubble Space Telescope (HST). The median
\tau values increase gradually from 1.95 at z=2.7 to 5.17
at z=3.4, but with a strong sightline-to-sightline variance. Many 35
comoving Mpc regions of the z>3 intergalactic medium (IGM) remain transmissive
(\tau<4), and the gradual trend with redshift appears
consistent with density evolution of a fully reionized IGM. These modest
optical depths imply average HeII fractions of x<0.01 and HeII
ionizing photon mean free paths of 50 comoving Mpc at z3.4,
thus requiring that a substantial volume of the helium in the Universe was
already doubly ionized at early times; this stands in conflict with current
models of HeII reionization driven by luminous quasars. Along 10 sightlines we
measure the coeval HI Ly\alpha\ effective optical depths, allowing us to study
the density dependence of \tau at z3. We establish
that the dependence of \tau on increasing
\tau is significantly shallower than expected from simple
models of an IGM reionized in HeII. This requires higher HeII photoionization
rates in overdense regions or underdense regions being not in photoionization
equilibrium. Moreover, there are very large fluctuations in
\tau at all , which greatly exceed
the expectations from these simple models. These data present a distinct
challenge to scenarios of HeII reionization - an IGM where HeII appears to be
predominantly ionized at z3.4, and with a radiation field strength that
may be correlated with the density field, but exhibits large fluctuations at
all densities.Comment: 31 pages, 15 figures; accepted to ApJ; systematic errors in the data
corrected, conclusions unchange
Quasar Quartet Embedded in Giant Nebula Reveals Rare Massive Structure in Distant Universe
All galaxies once passed through a hyperluminous quasar phase powered by
accretion onto a supermassive black hole. But because these episodes are brief,
quasars are rare objects typically separated by cosmological distances. In a
survey for Lyman-alpha emission at redshift z ~ 2, we discovered a physical
association of four quasars embedded in a giant nebula. Located within a
substantial overdensity of galaxies, this system is probably the progenitor of
a massive galaxy cluster. The chance probability of finding a quadruple quasar
is estimated to be ~10^-7, implying a physical connection between Lyman-alpha
nebulae and the locations of rare protoclusters. Our findings imply that the
most massive structures in the distant universe have a tremendous supply (~
10^11 solar masses) of cool dense (volume density ~1 cm^-3) gas, which is in
conflict with current cosmological simulations.Comment: accepted for publication in Science. Main text: 11 pages, 4 figures;
Supplementary Online Material: 52 pages, 10 figures, 6 table
LSST: a Complementary Probe of Dark Energy
The number of mass clusters and their distribution in redshift are very
sensitive to the density of matter Omega_m and the equation of state of dark
energy w. Using weak lens gravitational tomography one can detect clusters of
dark matter, weigh them, image their projected mass distribution, and determine
their 3-D location. The degeneracy curve in the Omega_m - w plane is nearly
orthogonal to that from CMB or SN measurements. Thus, a combination of CMB data
with weak lens tomography of clusters can yield precision measurements of
Omega_m and w, independently of the SN observations. The Large Synoptic Survey
Telescope (LSST) will repeatedly survey 30,000 square degrees of the sky in
multiple wavelengths. LSST will create a 3-D tomographic assay of mass
overdensities back to half the age of the universe by measuring the shear and
color-redshift of billions of high redshift galaxies. By simultaneously
measuring several functions of cosmic shear and mass cluster abundance, LSST
will provide a number of independent constraints on the dark energy density and
the equation of state.Comment: 9 pages, 6 figures, for Proc. 5th International UCLA Symposium on
Sources and Detection of Dark Matter, Marina del Rey, February 2002, ed. D.
Clin
The Stacked Lyman-Alpha Emission Profile from the Circum-Galactic Medium of z~2 Quasars
In the context of the FLASHLIGHT survey, we obtained deep narrow band images
of 15 quasars with GMOS on Gemini-South in an effort to measure
Ly emission from circum- and inter-galactic gas on scales of hundreds
of kpc from the central quasar. We do not detect bright giant Ly
nebulae (SB~10 erg s cm arcsec at distances >50
kpc) around any of our sources, although we routinely (%) detect
smaller scale <50 kpc Ly emission at this SB level emerging from either
the extended narrow emission line regions powered by the quasars or by
star-formation in their host galaxies. We stack our 15 deep images to study the
average extended Ly surface brightness profile around quasars,
carefully PSF-subtracting the unresolved emission component and paying close
attention to sources of systematic error. Our analysis, which achieves an
unprecedented depth, reveals a surface brightness of SB erg s cm arcsec at kpc,
with a detection of Ly emission at SB erg s cm arcsec
within an annulus spanning 50 kpc <R< 500 kpc from the quasars. Assuming this
Ly emission is powered by fluorescence from highly ionized gas
illuminated by the bright central quasar, we deduce an average volume density
of cm on these large scales. Our results
are in broad agreement with the densities suggested by cosmological
hydrodynamical simulations of massive () quasar hosts,
however they indicate that the typical quasars at these redshifts are
surrounded by gas that is a factor of ~100 times less dense than the (~1
cm) gas responsible for the giant bright Ly nebulae around
quasars recently discovered by our group.Comment: 22 pages, 13 figures, 1 table, submitted to Ap
Deep HeII and CIV Spectroscopy of a Giant Lyman alpha Nebula: Dense Compact Gas Clumps in the Circumgalactic Medium of a z~2 Quasar
The recent discovery by Cantalupo et al. (2014) of the largest (~500 kpc) and
luminous Ly-alpha nebula associated with the quasar UM287 (z=2.279) poses a
great challenge to our current understanding of the astrophysics of the halos
hosting massive z~2 galaxies. Either an enormous reservoir of cool gas is
required , exceeding the expected baryonic mass
available, or one must invoke extreme gas clumping factors not present in
high-resolution cosmological simulations. However, observations of Ly-alpha
emission alone cannot distinguish between these two scenarios. We have obtained
the deepest ever spectroscopic integrations in the HeII and CIV lines with the
goal of detecting extended line emission, but detect neither line to a
3 limiting SB erg/s/cm/arcsec. We construct
models of the expected emission spectrum in the highly probable scenario that
the nebula is powered by photoionization from the central hyper-luminous
quasar. The non-detection of HeII implies that the nebular emission arises from
a mass of cool gas on ~200 kpc
scales, distributed in a population of remarkably dense (
cm) and compact ( pc) clouds, which would clearly be
unresolved by current cosmological simulations. Given the large gas motions
suggested by the Ly-alpha line ( 500 km/s), it is unclear how these
clouds survive without being disrupted by hydrodynamic instabilities. Our study
serves as a benchmark for future deep integrations with current and planned
wide-field IFU such as MUSE, KCWI, and KMOS. Our work suggest that a
10 hr exposure would likely detect ~10 rest-frame UV/optical emission lines,
opening up the possibility of conducting detailed photoionization modeling to
infer the physical state of gas in the CGM.Comment: 24 pages, 11 figures, submitted to Ap
Observational Constraints on the Self Interacting Dark Matter Scenario and the Growth of Supermassive Black Holes
We consider the consequences of SIDM for a velocity dependent cross section per unit mass. Accretion of SIDM onto seed black holes can produce supermassive black holes that are too large for certain combinations of parameters,which is used to obtain a new constraint on the dark matter interaction. Constraints due to other considerations are presented and previous ones are generalized. The black hole constraint is extremely sensitive to the slope \alpha, of the inner density profile of dark halos. For the most probable value of \alpha=1.3, there exists a narrow range in parameter space, such that all constraints are satisfied. However, the adiabatic compression of the dark halo by baryons as they cool and contract in normal galaxies yields a steeper cusp, \alpha=1.7. This gives a tighter constraint, which would exclude SIDM as a possible solution to the purported problems with CDM in the absence of other dynamical processes. Nevertheless, SIDM with parameters consistent with this stronger constraint, can explain the ubiquity of supermassive black holes in the centers of galaxies. A ``best fit'' model is presented which reproduces the supermassive black hole masses and their observed correlations with the velocity dispersion of the host bulges. Specifically, the fourth power dependence of black hole mass on velocity dispersion is a direct consequence of the power spectrum having an index of n=-2. Although the dark matter collision rates for this model are too small to directly remedy problems with CDM, mergers between dark halos harboring supermassive black holes at high redshift could ameliorate the cuspy halo problem. This scenario also explains the lack of comparable supermassive black holes in bulgeless galaxies like M33
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