Over the past four years I have carried out surveys for ultra-luminous quasars in the SDSS and Pan-STARRS 1 footprints. Our goal was to discover previously missed intermediate redshift (2.8 < z < 5.0) quasars with a new selection technique on optical and near-infrared photometry (SDSS/Pan-STARRS, 2MASS, WISE) using random forest classification and regression. The Extremely Luminous Quasar Survey (ELQS; in SDSS) and it's expansion to Pan-STARRS 1, PS-ELQS have discovered a total of 299 new ultra-luminous quasars at z>2.8. The ELQS and PS-ELQS quasar catalogs provide the most complete sample of luminous intermediate redshift quasars to date.
As part of this work we have re-examined the bright end of the quasar luminosity function at these redshifts and find the bright-end slope to be quite steep (power law exponents of beta~-4.1). This result contrasts with previous results in the same redshift range, which find a much flatter slope (beta~-2.5), but agrees with recent measurements of the bright-end slope at lower and higher redshifts. Our results encourage a more consistent picture of quasar evolution across all redshifts, but also indicate the bright-end slope as seen at z=3-5, will be virtually inacessible at the highest redshifts z>6.
Image Credit: ESO/M. Kornmesser
At redshifts beyond z∼1 measuring the black hole galaxy relations proves to be a difficult task. The bright light of the AGN aggravates deconvolution of black hole and galaxy properties. On the other hand high redshift data on these relations is vital to understand in what ways galaxies and black holes co-evolve and in what ways they don't.
In this work we use black hole (BHMDs) and stellar mass densities (SMDs) to constrain the possible co-evolution of black holes with their host galaxies since z∼5. The BHMDs are calculated from quasar luminosity functions (QLF) using the Soltan argument, while we use integrals over stellar mass functions (SMFs) or the star formation rate density to obtain values for the stellar mass density. We find that both the stellar mass density and the black hole mass density grow in lock-step below redshifts of z∼3 with a non-evolving BHMD to SMD ratio.
A fit to the data assuming a power law relation between the BHMD and the SMD yields exponents around unity (1.0−1.5). Up to z∼5 the BHMD to SMD ratio doesn't show a strong evolution given the larger uncertainty in the completeness of high-redshift datasets. Our results, always applying the same analysis technique, seem to be consistent across all adopted data sets.
Image Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)
Asteroseismology is a powerful technique to study the interior structure of distant stars. Recent detections of g-mode pulsations in evolved He-burning stars, subdwarf B stars (sdB) suggests convective cores of 0.22−0.28M⊙ , ≳45% of the total stellar mass. Since previous studies found found significantly smaller convective core masses (≲0.19M⊙), we evolved stellar models with MESA (Modules for Experiments in Stellar Astrophysics) to explore how well the interior structure inferred from asteroseismology can be reproduced by standard algorithms. As we were able to successfully reproduce observational properties and previous simulations results (qualitative evolutionary paths, position in the logg−Teff diagram, mass range), we then focused on the interior structure.
Our standard model (mixing length theory + atomic diffusion) produced too small convective core masses of ∼0.17−0.18M⊙, averaged over the entire sdB lifetime. With extreme values for additional convective overshoot, we were able to reproduce the core sizes from the asteroseismological analyses. However, this introduced physically unrealistic behavior at the convective boundary. High resolution three-dimensional (3D) simulations of turbulent convection in stars suggest that the Schwarzschild criterion for convective mixing sytematically underestimates the actual extent of mixing because a boundary layer forms. Accounting for this would decrease the errors in both sdB total and convective core masses.
Image Credit: ESO