Ultracold quantum gases in triangular optical lattices

Over the last years the exciting developments in the field of ultracold atoms confined in optical lattices have led to numerous theoretical proposals devoted to the quantum simulation of problems e.g. known from condensed matter physics. Many of those ideas demand for experimental environments with non-cubic lattice geometries. In this paper we report on the implementation of a versatile three-beam lattice allowing for the generation of triangular as well as hexagonal optical lattices. As an important step the superfluid-Mott insulator (SF-MI) quantum phase transition has been observed and investigated in detail in this lattice geometry for the first time. In addition to this we study the physics of spinor Bose-Einstein condensates (BEC) in the presence of the triangular optical lattice potential, especially spin changing dynamics across the SF-MI transition. Our results suggest that below the SF-MI phase transition, a well-established mean-field model describes the observed data when renormalizing the spin-dependent interaction. Interestingly this opens new perspectives for a lattice driven tuning of a spin dynamics resonance occurring through the interplay of quadratic Zeeman effect and spin-dependent interaction. We finally discuss further lattice configurations which can be realized with our setup.Comment: 19 pages, 7 figure

The Large-Scale Structure of the X-ray Background and its Cosmological Implications

A careful analysis of the HEAO1 A2 2-10 keV full-sky map of the X-ray background (XRB) reveals clustering on the scale of several degrees. After removing the contribution due to beam smearing, the intrinsic clustering of the background is found to be consistent with an auto-correlation function of the form (3.6 +- 0.9) x 10^{-4} theta^{-1} where theta is measured in degrees. If current AGN models of the hard XRB are reasonable and the cosmological constant-cold dark matter cosmology is correct, this clustering implies an X-ray bias factor of b_X ~ 2. Combined with the absence of a correlation between the XRB and the cosmic microwave background, this clustering can be used to limit the presence of an integrated Sachs-Wolfe (ISW) effect and thereby to constrain the value of the cosmological constant, Omega_Lambda < 0.60 (95 % C.L.). This constraint is inconsistent with much of the parameter space currently favored by other observations. Finally, we marginally detect the dipole moment of the diffuse XRB and find it to be consistent with the dipole due to our motion with respect to the mean rest frame of the XRB. The limit on the amplitude of any intrinsic dipole is delta I / I < 5 x 10^{-3} at the 95 % C.L. When compared to the local bulk velocity, this limit implies a constraint on the matter density of the universe of Omega_m^{0.6}/b_X(0) > 0.24.Comment: 15 pages, 8 postscript figures, to appear in the Astrophysical Journal. The postscript version appears not to print, so use the PDF versio

Where are all the gravastars? Limits upon the gravastar model from accreting black holes

The gravastar model, which postulates a strongly correlated thin shell of anisotropic matter surrounding a region of anti-de Sitter space, has been proposed as an alternative to black holes. We discuss constraints that present-day observations of well-known black hole candidates place on this model. We focus upon two black hole candidates known to have extraordinarily low luminosities: the supermassive black hole in the Galactic Center, Sagittarius A*, and the stellar-mass black hole, XTE J1118+480. We find that the length scale for modifications of the type discussed in Chapline et al. (2003) must be sub-Planckian.Comment: 11 pages, 4 figure

The black hole mass distribution in early-type galaxies: cusps in HST photometry interpreted through adiabatic black hole growth

The surface brightness profiles of early-type galaxies have central cusps. Two characteristic profile types are observed with HST: `core' profiles have a break at a resolved radius and logarithmic cusp slope gamma < 0.3 inside that radius; `power-law' profiles have no clear break and gamma > 0.3. With few exceptions, galaxies with M_V -20.5 have power-law profiles. Both profile types occur in galaxies with -22 < M_V < -20.5. We show that these results are consistent with the hypothesis that: (i) all early-type galaxies have black holes (BHs) that grew adiabatically in homogeneous isothermal cores; and (ii) these `progenitor' cores followed scaling relations similar to those of the fundamental plane. The models studied here are the ones first proposed by Young. Models with BH masses and progenitor cores that obey established scaling relations predict (at Virgo) that galaxies with M_V < -21.2 have core profiles and galaxies with M_V > -21.2 have power-law profiles. This reproduces both the sense and the absolute magnitude of the observed transition. Intrinsic scatter in BH and galaxy properties can explain why both types of galaxies are observed around the transition magnitude. The observed bimodality in cusp slopes may be due to a bimodality in M_bh/L, with rapidly rotating disky galaxies having larger M_bh/L than slowly rotating boxy galaxies. Application to individual galaxies with HST photometry yields a roughly linear correlation between BH mass and V-band galaxy luminosity, log M_bh = -1.83 + log L (solar units). This agrees with the average relation for nearby galaxies with kinematically determined BH masses, and also with predictions from quasar statistics (shortened abstract).Comment: 41 pages, LaTeX, with 11 PostScript figures. Submitted to the Astronomical Journal. Postscript version also available from http://sol.stsci.edu/~marel/abstracts/abs_R23.htm

Creating, moving and merging Dirac points with a Fermi gas in a tunable honeycomb lattice

Dirac points lie at the heart of many fascinating phenomena in condensed matter physics, from massless electrons in graphene to the emergence of conducting edge states in topological insulators [1, 2]. At a Dirac point, two energy bands intersect linearly and the particles behave as relativistic Dirac fermions. In solids, the rigid structure of the material sets the mass and velocity of the particles, as well as their interactions. A different, highly flexible approach is to create model systems using fermionic atoms trapped in the periodic potential of interfering laser beams, a method which so far has only been applied to explore simple lattice structures [3, 4]. Here we report on the creation of Dirac points with adjustable properties in a tunable honeycomb optical lattice. Using momentum-resolved interband transitions, we observe a minimum band gap inside the Brillouin zone at the position of the Dirac points. We exploit the unique tunability of our lattice potential to adjust the effective mass of the Dirac fermions by breaking inversion symmetry. Moreover, changing the lattice anisotropy allows us to move the position of the Dirac points inside the Brillouin zone. When increasing the anisotropy beyond a critical limit, the two Dirac points merge and annihilate each other - a situation which has recently attracted considerable theoretical interest [5-9], but seems extremely challenging to observe in solids [10]. We map out this topological transition in lattice parameter space and find excellent agreement with ab initio calculations. Our results not only pave the way to model materials where the topology of the band structure plays a crucial role, but also provide an avenue to explore many-body phases resulting from the interplay of complex lattice geometries with interactions [11, 12]

Quantum phase transition to unconventional multi-orbital superfluidity in optical lattices

Orbital physics plays a significant role for a vast number of important phenomena in complex condensed matter systems such as high-T$_c$ superconductivity and unconventional magnetism. In contrast, phenomena in superfluids -- especially in ultracold quantum gases -- are commonly well described by the lowest orbital and a real order parameter. Here, we report on the observation of a novel multi-orbital superfluid phase with a {\it complex} order parameter in binary spin mixtures. In this unconventional superfluid, the local phase angle of the complex order parameter is continuously twisted between neighboring lattice sites. The nature of this twisted superfluid quantum phase is an interaction-induced admixture of the p-orbital favored by the graphene-like band structure of the hexagonal optical lattice used in the experiment. We observe a second-order quantum phase transition between the normal superfluid (NSF) and the twisted superfluid phase (TSF) which is accompanied by a symmetry breaking in momentum space. The experimental results are consistent with calculated phase diagrams and reveal fundamentally new aspects of orbital superfluidity in quantum gas mixtures. Our studies might bridge the gap between conventional superfluidity and complex phenomena of orbital physics.Comment: 5 pages, 4 figure

Unveiling obscured accretion in the Chandra Deep Field South

A large population of heavily obscured, Compton Thick AGNs is predicted by models of galaxy formation, models of Cosmic X-ray Background and by the ``relic'' super-massive black-hole mass function measured from local bulges. However, so far only a handful of Compton thick AGNs have been possibly detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can be recovered thanks to the reprocessing of the AGN UV emission in the infrared by selecting sources with AGN luminosity's in the mid-infrared and faint near-infrared and optical emission. To this purpose, we make use of deep HST, VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the number of Compton thick AGN in this field. We show that sources with high 24$\mu$m to optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80$\pm15$) are indeed likely to be highly obscured, Compton thick AGNs. The number of infrared selected, Compton thick AGNs with 5.8$\mu$m luminosity higher than $10^{44.2}$ erg s$^{-1}$ turns out to be similar to that of X-ray selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity higher than $10^{43}$ erg s$^{-1}$ in the redshift bin 1.2-2.6. This ``factor of 2'' source population is exactly what it is needed to solve the discrepancies between model predictions and X-ray AGN selection.Comment: Revised version, to be published by The Astrophysical Journa

Quasar clustering: evidence for an increase with redshift and implications for the nature of AGNs

The evolution of quasar clustering is investigated with a new sample of 388 quasars with 0.3<z<=2.2, B<=20.5 and Mb<-23, selected over an area of 24.6 sq. deg. in the South Galactic Pole. Assuming a two-point correlation function of the form xi(r) = (r/r_o)^-1.8, we detect clustering with r_0=6.2 +/- 1.6 h^-1 comoving Mpc at an average redshift of z=1.3. We find a 2 sigma significant increase of the quasar clustering between z=0.95 and z=1.8, independent of the quasar absolute magnitude and inconsistent with recent evidence on the evolution of galaxy clustering. If other quasar samples are added (resulting in a total data-set of 737 quasars) the increase of the quasar clustering is still favoured although it becomes less significant. We find epsilon=-2.5. Evolutionary parameters epsilon>0.0 are excluded at a 0.3% probability level, to be compared with epsilon=0.8 found for galaxies. The observed clustering properties appear qualitatively consistent with a scenario of Omega=1 CDM in which a) the difference between the quasar and the galaxy clustering can be explained as a difference in the effective bias and redshift distributions, and b) the quasars, with a lifetime of t~10^8 yr, sparsely sample halos of mass greater than M_min~10^12-10^13 h^-1 M_sun. We discuss also the possibility that the observed change in the quasar clustering is due to an increase in the fraction of early-type galaxies as quasar hosts at high z.Comment: 8 pages including 2 eps figures, LaTeX (AAS v4.0), ApJ in pres

Biases in Virial Black Hole Masses: An SDSS Perspective

We compile black hole (BH) masses for $\sim 60,000$ quasars in the redshift range $0.1 \lesssim z \lesssim 4.5$ included in the Fifth Data Release of the Sloan Digital Sky Survey (SDSS), using virial BH mass estimators based on the \hbeta, \MgII, and \CIV emission lines. We find that: (1) within our sample, the widths of the three lines follow log-normal distributions, with means and dispersions that do not depend strongly on luminosity or redshift;(2) the \MgII- and \hbeta-estimated BH masses are consistent with one another; and (3) the \CIV BH mass estimator may be more severely affected by a disk wind component than the \MgII and \hbeta estimators, giving a positive bias in mass correlated with the \CIV-\MgII blueshift. Most SDSS quasars have virial BH masses in the range $10^8-10^9 M_\odot$. There is a clear upper mass limit of $\sim 10^{10} M_\odot$ for active BHs at $z \gtrsim 2$, decreasing at lower redshifts. Making the reasonable assumptions that the underlying BH mass distribution decreases with mass and that the Eddington ratio distribution at fixed BH mass has non-zero width, we show that the measured virial BH mass distribution and Eddington ratio distribution are subject to Malmquist bias. A radio quasar subsample (with $1.5\lesssim z\lesssim 2.3$) has mean virial BH mass larger by $\sim 0.12$ dex than the whole sample. A broad absorption line (BAL) quasar subsample (with $1.7\lesssim z\lesssim 2.2$) has identical virial mass distribution as the nonBAL sample, with no mean offset. (Abridged)Comment: Updated virial mass measurements; improved presentation of the MC simulation; added new discussion sections; conclusions unchanged. The full table1 is available at http://www.astro.princeton.edu/~yshen/BH_mass/datafile1.txt.tar.g

Black Holes in Galaxy Mergers: Evolution of Quasars

Based on numerical simulations of gas-rich galaxy mergers, we discuss a model in which quasar activity is tied to the self-regulated growth of supermassive black holes in galaxies. Nuclear inflow of gas attending a galaxy collision triggers a starburst and feeds black hole growth, but for most of the duration of the starburst, the black hole is heavily obscured by surrounding gas and dust which limits the visibility of the quasar, especially at optical and UV wavelengths. Eventually, feedback energy from accretion heats the gas and expels it in a powerful wind, leaving a 'dead quasar'. Between buried and dead phases there is a window during which the galaxy would be seen as a luminous quasar. Because the black hole mass, radiative output, and distribution of obscuring gas and dust all evolve strongly with time, the duration of this phase of observable quasar activity depends on both the waveband and imposed luminosity threshold. We determine the observed and intrinsic lifetimes as a function of luminosity and frequency, and calculate observable lifetimes ~10 Myr for bright quasars in the optical B-band, in good agreement with empirical estimates and much smaller than the black hole growth timescales ~100 Myr, naturally producing a substantial population of 'buried' quasars. However, observed and intrinsic energy outputs converge in the IR and hard X-ray bands as attenuation becomes weaker and chances of observation greatly increase. We obtain the distribution of column densities along sightlines in which the quasar is seen above a given luminosity, and find that our result agrees remarkably well with observed estimates of the column density distribution from the SDSS for appropriate luminosity thresholds. (Abridged)Comment: 12 pages, 7 figures. Accepted for publication in ApJ (September 2005). Replacement with minor revisions from referee repor