Rare Halos Research Articles

Contrast-Enhanced Ultrasound Features of Primary Hepatic Lymphoepithelioma-Like Carcinoma: Comparison with Hepatocellular Carcinoma

Abstract Background: Primary hepatic lymphoepithelioma-like carcinoma (LELC) is a malignant tumor with a low incidence, but the number of case reports has increased in recent years. The prognosis of hepatic LELC is better than hepatocellular carcinoma (HCC). The differentiation between hepatic LELC and HCC has clinical value during follow-up treatment. The purpose of our study was to compare contrast-enhanced ultrasound (CEUS) imaging features in patients with hepatic LELC and HCC. Methods: Twelve patients with an average age of 60.1±9.5 years and histopathologically confirmed hepatic LELC were included in the study. Forty-three patients with an average age of 57.4±9.0 years and a histopathological diagnosis of HCC were designated as the control group by means of propensity score matching (1:4). The clinical data, B-mode ultrasound (BMUS), and CEUS features were retrospectively analyzed between patients with hepatic LELC and HCC. Results: The serum a-fetoprotein (58.1% [25/43] vs.16.7% [2/12]; p=0.017) and des-gamma-carboxy prothrombin levels (74.4% [32/43] vs.16.7% [2/12]; p=0.001) were not significantly elevated in patients with hepatic LELCs compared to HCCs. LELCs were mainly hypoechoic based on BMUS, while the echogenicity of HCCs varied (p=0.016). A halo sign was less common in patients with hepatic LELCs than HCCs (16.7% [2/12] vs. 58.1% [25/43]; p=0.011). Of hepatic LELCs, 75% (9/12) had homogeneous hyperenhancement based on CEUS, whereas 58.1% (25/43) of HCCs had heterogeneous hyperenhancement (p=0.004). Early washout was noted in 91.7% (11/12) of hepatic LELCs compared to 46.5% (20/43) of HCCs (p=0.005). Furthermore, hepatic LELCs were more likely to exhibit peripheral rim-like hyperenhancement (83.3% [10/12] vs. 11.6% [5/43]; p < 0.001). Conclusion: BMUS and CEUS are helpful in discriminating between hepatic LELC and HCC. A hypoechoic mass, the rare halo sign, homogeneous hyperenhancement in the arterial phase, higher frequencies of early washout, and peripheral rim-like hyperenhancement are useful ultrasound features that can help differentiate hepatic LELCs from HCCs.

The ultrasonographic characteristics of fibrocystic breast change

Objective To investigate the ultrasonographic features of fibrocystic breast change (FBC) and improve the ultrasonographic diagnosis and differential diagnosis of FBC. Methods Fifty-five patients of FBC with 60 lesions and 39 patients of invasive ductal carcinoma (IDC) with 42 lesions, which were confirmed by pathology after operation in the Affiliated Union Hospital of Fujian Medical University from January 2014 to February 2019 were enrolled. The preoperative sonographic findings of FBC and IDC were retrospectively analyzed and compared. Results There was no significant difference between the two groups in the rates of showing irregular shape, which were 86.7% in FBC group and 88.1% in IDC group, respectively (P>0.05), but the rate of showing crab feet or burrs on the edge of lesions in FBC group was lower than that in IDC group(P<0.05). The occurrence rates of posterior echo enhancement and cystic degeneration in FBC group were 81.7% and 71.7% respectively, which were significantly higher than those in IDC group (38.1% and 16.7%)(P<0.001). In addition, the FBC group showed fewer features such as hyperechoic halo and more features such as hypovascular supply than that of IDC group (all P<0.001). Conclusions In terms of ultrasonic features, including irregular shape or even crab feet and burrs, FBC can be easily misdiagnosed as malignant tumors. However, posterior echo enhancement, interior scattered small cysts, lack of blood supply and rare hyperechoic halo may be the characteristics of FBC, which can be differentiated from malignant tumors. Key words: Ultrasonography; Fibrocystic breast change; Invasive ductal carcinoma

A tiny host galaxy for the first giant black hole:z = 7.5 quasar in BlueTides

The most distant known quasar recently discovered by Ba\~nados et al. (2018) is at $z=7.5$ (690 Myr after the Big Bang), at the dawn of galaxy formation. We explore the host galaxy of the brightest quasar in the large volume cosmological hydrodynamic simulation BlueTides, which in Phase II has reached these redshifts. The brightest quasar in BlueTides has a luminosity of a $\sim$ few $10^{13} L_{\odot}$ and a black hole mass of $6.4 \times 10^{8} M_{\odot}$ at $z \sim 7.5$, comparable to the observed quasar (the only one in this large volume). The quasar resides in a rare halo of mass $M_{H} \sim 10^{12} M_{\odot}$ and has a host galaxy of stellar mass of $4 \times 10^{10}M_{\odot}$ with an ongoing (intrinsic) star formation rate of $\sim 80 M_{\odot} yr^{-1}$. The corresponding intrinsic UV magnitude of the galaxy is $-23.1$, which is roughly $2.7$ magnitudes fainter than the quasar's magnitude of $-25.9$. We find that the galaxy is highly metal enriched with a mean metallicity equal to the solar value. We derive quasar and galaxy spectral energy distribution (SED) in the mid and near infrared JWST bands. We predict a significant amount of dust attenuation in the rest-frame UV corresponding to $A_{1500} \sim 1.7$ giving an UV based SFR of $\sim 14 M_{\odot} yr^{-1}$. We present mock JWST images of the galaxy with and without central point source, in different MIRI and NIRCam filters. The host galaxy is detectable in NIRCam filters, but it is extremely compact ($R_{E}=0.35$ kpc). It will require JWST's exquisite sensitivity and resolution to separate the galaxy from the central point source. Finally within the FOV of the quasar in BlueTides there are two more sources that would be detectable by JWST.

Halo occupation distribution (HOD) modelling of high redshift galaxies using the BlueTides simulation

We construct halo occupation distribution (HOD) models of high redshift ($z \gtrsim 7.5$) galaxies with $M_{*}>10^8~M_{\odot}/h$ using the BlueTides hydrodynamic simulation suite, with a particular emphasis on modelling the small scale / 1-halo clustering ($0.01\lesssim r \lesssim 1~ h^{-1}\rm{Mpc}$). Similar to low redshift studies, we find that the central and satellite mean HODs ($\left<N_{\mathrm{cen}}\right>$ and $\left<N_{\mathrm{sat}}\right>$) can be modeled by a smoothed step function and a power law respectively. The number density of satellite galaxies is however significantly suppressed compared to low redshift (satellite fractions drop from $\sim 50 \%$ at $z=0$ to $\lesssim 10 \%$ at $z=7.5$). The mean number of satellites, $\left<N_{\mathrm{sat}}\right> < 1$ for halo masses below $3 \times 10^{11} M_{\odot}/h$ (a rare halo at these redshifts). For the radial number density profiles, satellites with $10^8 \lesssim M^* \lesssim 10^{9} M_{\odot}/h$ in halos with $M_H \gtrsim 3 \times10^{11} M_{\odot}/h$ are consistent with NFW (with concentrations $c_{\mathrm{sat}} \sim 10-40$). Within halos of mass $M_H\lesssim 3 \times 10^{11} M_{\odot}/h$ satellites exhibit a power law profile with slope -3. Because these halos dominate the small scale clustering, the resulting 1-halo term is steeper than predicted using standard NFW profiles. Using this power-law profile for satellites, we can successfully reproduce the small-scale clustering exhibited by BlueTides galaxies using HOD modelling. We predict the highest probability of detecting satellites at $z>7.5$ is around centrals of $M^*\sim 3 \times 10^{10} M_{\odot}/h$ (with $M^{*}\gtrsim$ a few $10^{7} M_{\odot}/h$ ). This should be achievable with the James Webb Space Telescope (JWST).

Ice halos in weather lore and meteorology, and a rare halo display in San Francisco

AbstractFrom Aristotle to modern science, this article presents several accounts of ice halo displays in weather lore and answers questions such as Could a sighting of a halo be an omen of a shipwreck? Does a halo herald a hurricane? How are ice halos observed in Oklahoma connected to plankton from the Pacific Ocean? and others. The article also offers images and descriptions of a very rare odd‐radius solar halo display, observed in California on 23 February 2016, in which the Sun appeared to be surrounded by five concentric circles. Finally, the article analyses the meteorological conditions associated with this phenomenon, alongside those of other displays, and answers a question that has been asked for many thousands of years: can halo displays be used as indicators of the weather?

Feedback Limits to Maximum Seed Masses of Black Holes

Abstract The most massive black holes observed in the universe weigh up to ∼1010 M ⊙, nearly independent of redshift. Reaching these final masses likely required copious accretion and several major mergers. Employing a dynamical approach that rests on the role played by a new, relevant physical scale—the transition radius—we provide a theoretical calculation of the maximum mass achievable by a black hole seed that forms in an isolated halo, one that scarcely merged. Incorporating effects at the transition radius and their impact on the evolution of accretion in isolated halos, we are able to obtain new limits for permitted growth. We find that large black hole seeds (M • ≳ 104 M ⊙) hosted in small isolated halos (M h ≲ 109 M ⊙) accreting with relatively small radiative efficiencies (ϵ ≲ 0.1) grow optimally in these circumstances. Moreover, we show that the standard M •–σ relation observed at z ∼ 0 cannot be established in isolated halos at high-z, but requires the occurrence of mergers. Since the average limiting mass of black holes formed at z ≳ 10 is in the range 104–6 M ⊙, we expect to observe them in local galaxies as intermediate-mass black holes, when hosted in the rare halos that experienced only minor or no merging events. Such ancient black holes, formed in isolation with subsequent scant growth, could survive, almost unchanged, until present.

Light or heavy supermassive black hole seeds: the role of internal rotation in the fate of supermassive stars

Supermassive black holes are a key ingredient of galaxy evolution. However, their origin is still highly debated. In one of the leading formation scenarios, a black hole of ∼100 M⊙ results from the collapse of the inner core of a supermassive star (≳104–5 M⊙), created by the rapid accumulation (≳0.1 M⊙ yr−1) of pristine gas at the centre of newly formed galaxies at z ∼ 15. The subsequent evolution is still speculative: the remaining gas in the supermassive star can either directly plunge into the nascent black hole or part of it can form a central accretion disc, whose luminosity sustains a surrounding, massive, and nearly hydrostatic envelope (a system called a ‘quasi-star’). To address this point, we consider the effect of rotation on a quasi-star, as angular momentum is inevitably transported towards the galactic nucleus by the accumulating gas. Using a model for the internal redistribution of angular momentum that qualitatively matches results from simulations of rotating convective stellar envelopes, we show that quasi-stars with an envelope mass greater than a few 105 M| $_{\odot } \times (\rm black\ hole\ mass/100\,\mathrm{M}_{\odot })^{0.82}$ | have highly sub-Keplerian gas motion in their core, preventing gas circularization outside the black hole's horizon. Less massive quasi-stars could form but last for only ≲104 yr before the accretion luminosity unbinds the envelope, suppressing the black hole growth. We speculate that this might eventually lead to a dual black hole seed population: (i) massive (>104 M⊙) seeds formed in the most massive (>108 M⊙) and rare haloes; (ii) lighter (∼102 M⊙) seeds to be found in less massive and therefore more common haloes.

Separate universe consistency relation and calibration of halo bias

Linear halo bias is the response of dark matter halo number density to a long wavelength fluctuation in the dark matter density. Using abundance matching between separate universe simulations which absorb the latter into a change in the background, we test the consistency relation between the change in a one point function, the halo mass function, and a two point function, the halo-matter cross correlation in the long wavelength limit. We find excellent agreement between the two at the $1-2\%$ level for average halo biases between $1 \lesssim \bar b_1 \lesssim 4$ and no statistically significant deviations at the $4-5\%$ level out to $\bar b_1 \approx 8$. Halo bias inferred assuming instead a universal mass function is significantly different and inaccurate at the 10\% level or more. The separate universe technique provides a way of calibrating linear halo bias efficiently for even highly biased rare halos in the $\Lambda$CDM model. Observational violation of the consistency relation would indicate new physics, e.g.~in the dark matter, dark energy or primordial non-Gaussianity sectors.

H2 suppression with shocking inflows: testing a pathway for supermassive black hole formation

The presence of quasars at redshifts z > 6 indicates the existence of supermassive black holes (SMBHs) as massive as a few times 10^9 Msun, challenging models for SMBH formation. One pathway is through the direct collapse of gas in T_{vir} > 10^4 K halos; however, this requires the suppression of H_2 cooling to prevent fragmentation. In this paper, we examine a proposed new mechanism for this suppression which relies on cold-mode accretion flows leading to shocks at high densities (n > 10^4 cm^{-3}) and temperatures (T > 10^4 K). In such gas, H_2 is efficiently collisionally dissociated. We use high-resolution numerical simulations to test this idea, demonstrating that such halos typically have lower temperature progenitors, in which cooling is efficient. Those halos do show filamentary flows; however, the gas shocks at or near the virial radius (at low densities), thus preventing the proposed collisional mechanism from operating. We do find that, if we artificially suppress H_2 formation with a high UV background, so as to allow gas in the halo center to enter the high-temperature, high-density "zone of no return", it will remain there even if the UV flux is turned off, collapsing to high density at high temperature. Due to computational limitations, we simulated only three halos. However, we demonstrate, using Monte Carlo calculations of 10^6 halo merger histories, that a few rare halos could assemble rapidly enough to avoid efficient H_2 cooling in all of their progenitor halos, provided that the UV background exceeds J_{21} ~ few at redshifts as high as z ~ 20.

Formation of galactic nuclei with multiple supermassive black holes at high redshifts

We examine the formation of groups of multiple supermassive black holes (SMBHs) in gas-poor galactic nuclei due to high merger rate of galaxies at high redshifts. We calculate the relative likelihood of binary, triple and quadruple SMBH systems, by considering the time-scales for relevant processes and by combining merger trees with N-body simulations for the dynamics of stars and SMBHs in galactic nuclei. Typical haloes today with mass M0≈ 1014 M⊙ have an average mass of Mz= 6= 5 × 1011 M⊙ at z∼ 6, while rare haloes with current mass M0≳ 1015 M⊙ have an average mass of Mz= 6= 5 × 1012 M⊙ at that redshift. These cluster-size haloes are expected to host single galaxies at z∼ 6. We expect about 30 per cent galaxies within haloes with present-day mass M0≈ 1014 M⊙ to contain more than two SMBHs at redshifts 2 ≲z≲ 6. For larger present-day haloes, with M0≳ 1015 M⊙, this fraction is almost 60 per cent. The existence of multiple SMBHs at high redshifts can potentially explain the mass deficiencies observed in cores of massive elliptical galaxies, which are up to five times the mass of their central BHs. Multiple SMBHs would also lead to an enhanced rate of tidal disruption of stars, modified gravitational wave signals compared to isolated BH binaries and slingshot ejection of SMBHs from galaxies at high speeds in excess of 2000 km s−1.

THE BINARY FREQUENCY OF r -PROCESS-ELEMENT-ENHANCED METAL-POOR STARS AND ITS IMPLICATIONS: CHEMICAL TAGGING IN THE PRIMITIVE HALO OF THE MILKY WAY

A few rare halo giants in the range [Fe/H] $\simeq -2.9\pm0.3$ exhibit {\it r}-process element abundances that vary as a group by factors up to [$r$/Fe] $\sim80$, relative to those of the iron peak and below. Yet, the astrophysical production site of these {\it r}-process elements remains unclear. We report initial results from four years of monitoring the radial velocities of 17 {\it r}-process-enhanced metal-poor giants to detect and characterise binaries in this sample. We find three (possibly four) spectroscopic binaries with orbital periods and eccentricities that are indistinguishable from those of Population I binaries with giant primaries, and which exhibit no signs that the secondary components have passed through the AGB stage of evolution or exploded as supernovae. The other 14 stars in our sample appear to be single -- including the prototypical $r$-process-element enhanced star CS 22892-052, which is also enhanced in carbon, but not in {\it s}-process elements. We conclude that the {\it r}-process (and potentially carbon) enhancement of these stars was not a local event due to mass transfer or winds from a binary companion, but was imprinted on the natal molecular clouds of these (single and binary) stars by an external source. These stars are thus spectacular chemical tracers of the inhomogeneous nature of the early Galactic halo system.

Approach to photorealistic halo simulations

A multiple-scattering Monte Carlo model that can produce near-photographic quality images is developed and used to simulate several dramatic halo displays. The model atmosphere contains an absorbing ozone layer plus two clear, molecular air layers with Rayleigh scattering surrounding a cloud layer and an atmospheric boundary layer with aerosol particles subject to Lorentz-Mie scattering. Halos are produced by right hexagonal or pyramidal crystals that reflect and refract according to geometric optics without diffraction, although "junk" crystals with a pronounced forward-scattering peak but no halo peaks may be included to simulate typical, faint halos. Model parameters include ozone height and content, surface and cloud pressure, cloud optical thickness, crystal shapes, orientations and abundances, atmospheric turbidity, aerosol radius, and albedo. Beams for each wavelength are sorted into small bins as halo beams if they have been scattered once only by a single crystal and otherwise as sky beams, which are smoothed and combined with the halo beams to produce images. Multiple scattering generally vitiates halos, but extremely rare halos, such as Kern's arc, can be produced if a significant fraction of crystals in optically thick clouds have identical shapes and are highly oriented. Albedo is a model by-product with potential value in climate studies.

Suppression of HD cooling in protogalactic gas clouds by Lyman-Werner radiation

It has been shown that HD molecules can form efficiently in metal-free gas collapsing into massive protogalactic haloes at high redshift. The resulting radiative cooling by HD can lower the gas temperature to that of the cosmic microwave background (CMB), TCMB= 2.7(1 +z) K, significantly below the temperature of a few ×100 K achievable via H2 cooling alone, and thus reduce the masses of the first generation of stars. Here we consider the suppression of HD cooling by ultraviolet (UV) irradiation in the Lyman–Werner (LW) bands. We include photodissociation of both H2 and HD, and explicitly compute the self-shielding and shielding of both molecules by neutral hydrogen, H i, as well as the shielding of HD by H2. We use a simplified dynamical collapse model, and follow the chemical and thermal evolution of the gas, in the presence of a UV background. We find that a LW flux of Jcrit,HD≈ 10−22 erg cm−2 sr−1 s−1 Hz−1 is able to suppress HD cooling and thus prevent collapsing primordial gas from reaching temperatures below ∼100 K. The main reason for the lack of HD cooling for J > Jcrit,HD is the partial photodissociation of H2, which prevents the gas from reaching sufficiently low temperatures (T < 150 K) for HD to become the dominant coolant; direct HD photodissociation is unimportant except for a narrow range of fluxes and column densities. Since the prevention of HD cooling requires only partial H2 photodissociation, the critical flux Jcrit,HD is modest, and is below the UV background required to re-ionize the Universe at z∼ 10–20. We conclude that HD cooling can reduce the masses of typical stars only in rare haloes forming well before the epoch of re-ionization.

THE LARGE-SCALE BIAS OF DARK MATTER HALOS: NUMERICAL CALIBRATION AND MODEL TESTS

We measure the clustering of dark matter halos in a large set of collisionless cosmological simulations of the flat LCDM cosmology. Halos are identified using the spherical overdensity algorithm, which finds the mass around isolated peaks in the density field such that the mean density is Delta times the background. We calibrate fitting functions for the large scale bias that are adaptable to any value of Delta we examine. We find a ~6% scatter about our best fit bias relation. Our fitting functions couple to the halo mass functions of Tinker et. al. (2008) such that bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare halos is higher than that predicted in the modified ellipsoidal collapse model of Sheth, Mo, & Tormen (2001), and approaches the predictions of the spherical collapse model for the rarest halos. Halo bias results based on friends-of-friends halos identified with linking length 0.2 are systematically lower than for halos with the canonical Delta=200 overdensity by ~10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak-background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but ~20% residuals remain, both at high and low masses.

HOW WELL DO COSMOLOGICAL SIMULATIONS REPRODUCE INDIVIDUAL HALO PROPERTIES?

Cosmological simulations of galaxy formation often rely on prescriptions for star formation and feedback that depend on halo properties such as halo mass, central over-density, and virial temperature. In this paper we address the convergence of individual halo properties, based on their number of particles N, focusing in particular on the mass of halos near the resolution limit of a simulation. While it has been established that the halo mass function is sampled on average down to N~30 particles, we show that individual halo properties exhibit significant scatter, and some systematic biases, as one approaches the resolution limit. We carry out a series of cosmological simulations using the Gadget2 and Enzo codes with N_p=64^3 to N_p=1024^3 total particles, keeping the same large-scale structure in the simulation box. We consider boxes from l_{box} = 8 Mpc/h to l_{box} = 512 Mpc/h to probe different halo masses and formation redshifts. We cross-identify dark matter halos in boxes at different resolutions and measure the scatter in their properties. The uncertainty in the mass of single halos depends on the number of particles (scaling approximately as N^{-1/3}), but the rarer the density peak, the more robust its identification. The virial radius of halos is very stable and can be measured without bias for halos with N>30. In contrast, the average density within a sphere containing 25% of the total halo mass is severely underestimated (by more than a factor 2) and the halo spin is moderately overestimated for N<100. If sub-grid physics is implemented upon a cosmological simulation, we recommend that rare halos (~3sigma peaks) be resolved with N>100 particles and common halos (~1sigma peaks) with N>400 particles to avoid excessive numerical noise and possible systematic biases in the results.

Supermassive black hole formation by direct collapse: keeping protogalactic gas H2free in dark matter haloes with virial temperaturesTvir&gt;rsim 104K

In the absence of H_2 molecules, the primordial gas in early dark matter halos with virial temperatures just above T_vir >~ 10^4 K cools by collisional excitation of atomic H. Although it cools efficiently, this gas remains relatively hot, at a temperature near T ~ 8000 K, and consequently might be able to avoid fragmentation and collapse directly into a supermassive black hole (SMBH). In order for H_2--formation and cooling to be strongly suppressed, the gas must be irradiated by a sufficiently intense ultraviolet (UV) flux. We performed a suite of three--dimensional hydrodynamical adaptive mesh refinement (AMR) simulations of gas collapse in three different protogalactic halos with T_vir >~ 10^4 K, irradiated by a UV flux with various intensities and spectra. We determined the critical specific intensity, Jcrit, required to suppress H_2 cooling in each of the three halos. For a hard spectrum representative of metal--free stars, we find (in units of 10^{-21} erg s^{-1} Hz^{-1} sr^{-1} cm^{-2}) 10^4<Jcrit<10^5, while for a softer spectrum, which is characteristic of a normal stellar population, and for which H^{-} --dissociation is important, we find 30<Jcrit<300. These values are a factor of 3--10 lower than previous estimates. We attribute the difference to the higher, more accurate H_2 collisional dissociation rate we adopted. The reduction in Jcrit exponentially increases the number of rare halos exposed to super--critical radiation. When H_2 cooling is suppressed, gas collapse starts with a delay, but it ultimately proceeds more rapidly. The infall velocity is near the increased sound speed, and an object as massive as M ~ 10^5 solar mass may form at the center of these halos, compared to the M ~ 10^2 solar mass stars forming when H_2--cooling is efficient.

Covariance of cross-correlations: towards efficient measures for large-scale structure

We study the covariance of the cross-power spectrum of different tracers for the large-scale structure. We develop the counts-in-cells framework for the multi-tracer approach, and use this to derive expressions for the full non-Gaussian covariance matrix. We show, that for the usual auto-power statistic, besides the off-diagonal covariance generated through gravitational mode-coupling, the discreteness of the tracers and their associated sampling distribution can generate strong off-diagonal covariance, and that this becomes the dominant source of covariance as k>>k_f=2 pi/L. On comparison with the derived expressions for the cross-power covariance, we show that the off-diagonal terms can be suppressed, if one cross-correlates a high tracer-density sample with a low one. Taking the effective estimator efficiency to be proportional to the signal-to-noise ratio (SN), we show that, to probe clustering as a function of physical properties of the sample, i.e. cluster mass or galaxy luminosity, then the cross-power approach can out perform the auto-power one by factors of a few. We confront the theory with measurements of the mass-mass, halo-mass, and halo-halo power spectra from a large ensemble of N-body simulations. We show that there is a significant SN advantage to be gained from using the cross-power approach when studying the bias of rare haloes. The analysis is repeated in configuration space and again SN improvement is found. We estimate the covariance matrix for these samples, and find strong off-diagonal contributions. The covariance depends on halo mass, with higher mass samples having stronger covariance. In agreement with theory, we show that the covariance is suppressed for the cross-power. This work points the way towards improved estimators for clustering studies.

The legendary Rome halo displays

The two Rome halo displays of 1629 and 1630 are prominent in the early halo literature, and the 1629 display is still cited today for having contained a 28 degrees circular halo. We have examined seventeenth century correspondence and publications in order to learn as much as possible about the existing documentation of the two displays. We find the documentation to be too weak to support a definitive interpretation of either display, and we see little evidence for a 28 degrees halo or for other rare halos. The two displays remain important for their role in initiating modern halo science.

Halo Assembly Bias in Hierarchical Structure Formation

We investigate the origin of halo assembly bias, the dependence of halo clustering on assembly history. We relate halo assembly to peak properties measured in the Lagrangian space of the initial linear Gaussian random density field, and show how these same Lagrangian properties determine large-scale bias. We focus on the two regimes where assembly bias has been observed to be significant: at masses very large and very small compared to the nonlinear mass scale. At high masses, we show that assembly bias is expected from the statistics of the peaks of Gaussian random fluctuations, and we show that the extent of assembly bias found in N-body simulations of rare halos is in excellent agreement with our theoretical prediction. At low masses, we argue that assembly bias largely arises from a sub-population of low mass halos whose mass accretion has ceased. Due to their arrested development, these halos naturally become unbiased, in contrast to their anti-biased peers. We show that a simple toy model incorporating these effects can roughly reproduce the bias trends found in N-body simulations.

Host Galaxy Bulge Predictors of Supermassive Black Hole Mass

We investigate the physical conditions for the growth of intermediate mass seed black holes assumed to have formed from remnants of the first generation of massive stars. We follow the collapse of high-sigma halos with Tvir > 1e4 K using cosmological, smooth-particle hydrodynamic (SPH) simulations in the standard LCDM model. During collapse of the parent halo the seed holes are incorporated through mergers into larger systems and accrete mass from the surrounding gas. We include a self-consistent treatment of star formation, black hole accretion and associated feedback processes. Even under optimistic assumptions for the seed black hole mass and for efficient merger rates, we find that seed holes in halos M<1e10 Msun never reach the conditions for critical Eddington growth. Most of the black hole growth in this regime is determined by the initial mass and the merger rates. Critical accretion rates are reached, albeit only after a significant delay, at the time of collapse z~7) for 3-4 sigma halos of M~1e11 Msun. Our results imply M_BH = 5e6 Msun (M_halo/1e11 Msun)^0.78 at the time of collapse. The required conditions of Eddington growth to explain the build-up of supermassive black holes (~1e9 Msun), as implied by Sloan quasars at z>6, are therefore hard to meet in such a scenario. Without a 'jump-start' these conditions may be only achieved in extremely rare halos with M_halo > 1e13 Msun that collapsed before z~6. The sub-Eddington regime in which black holes holes accrete at early time implies a small contribution to the reionization by miniquasar but still sufficient to cause appreciable heating of the IGM at z<15-18.