Tuesday, July 25, 2017

Understanding Active Galactic Nuclei

About 0.5% of galaxies have Active Galactic Nuclei (AGNs), the most famous of which are quasars, for which there are dozens of different categories under often overlapping classification schemes. A new review paper considers the "AGN Zoo" and tries to make sense of the relatively modest number of parameters that sum up the variation in AGN types. The body text of the paper begins by introducing the concept of an AGN (most citations omitted).  
The discovery of quasars (Schmidt, 1963) opened up a whole new branch of astronomy . Twenty years earlier Seyfert (1943) had reported the presence of broad and strong emission lines in the nuclei of six spiral nebulae (including some by now “classical” AGN, like NGC 1068 and NGC 4151). However, his work remained largely ignored until Baade & Minkowski (1954) pointed out the similarities between the spectra of the galaxies studied by Seyfert and that of the galaxy they had associated with the Cygnus A radio source.

As implicit in the name, AGN are stronger emitters than the nuclei of “normal” galaxies. This “extra” component is unrelated to the nuclear fusion powering stars and is now universally accepted to be connected instead to the presence of an actively accreting central supermassive (greater than equal to 10^6 stellar masses) black hole (SMBH).

AGN have many interesting properties. These include: (1) very high luminosities (up to Lbol ≈ 1048 erg s−1 ), which make them the most powerful non-explosive sources in the Universe and therefore visible up to very high redshifts (currently z = 7.1); (2) small emitting regions in most bands, of the order of a milliparsec, as inferred from their rapid variability, implying high energy densities; (3) strong evolution of their luminosity functions (LFs); (4) detectable emission covering the whole electromagnetic spectrum.
The review paper and its abstract are as follows:
Active Galactic Nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different "flavours" in the literature that now comprise a complex and confusing AGN "zoo". It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN, and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their "big picture" through observations in each electromagnetic band from radio to gamma-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress. 
P. Padovani, et al. "Active Galactic Nuclei: what's in a name?" (July 22, 2017).

Improved Methods Of Simulating Supernovas In Simulations Makes Galaxies Form Faster

One of the surprises discovered when improved telescopes made it possible to see very old galaxies is that galaxies formed much more rapidly after the Big Bang than simulations had expected. One reason the simulations were inaccurate is that their model of mechanical feedback from supernovaes was flawed. An improved method of simulating this has resolved the paradox. 
We study the implementation of mechanical feedback from supernovae (SNe) and stellar mass loss in galaxy simulations, within the Feedback In Realistic Environments (FIRE) project. We present the FIRE-2 algorithm for coupling mechanical feedback, which can be applied to any hydrodynamics method (e.g. fixed-grid, moving-mesh, and mesh-less methods), and black hole as well as stellar feedback. This algorithm ensures manifest conservation of mass, energy, and momentum, and avoids imprinting 'preferred directions' on the ejecta. We show that it is critical to incorporate both momentum and thermal energy of mechanical ejecta in a self-consistent manner, accounting for SNe cooling radii when they are not resolved. Using idealized simulations of single SNe explosions, we show that the FIRE-2 algorithm, independent of resolution, reproduces converged solutions in both energy and momentum. In contrast, common 'fully-thermal' (energy-dump) or 'fully-kinetic' (particle-kicking) schemes in the literature depend strongly on resolution: when applied at mass resolution ≳100M⊙, they diverge by orders-of-magnitude from the converged solution. In galaxy-formation simulations, this divergence leads to orders-of-magnitude differences in galaxy properties, unless those models are adjusted in a resolution-dependent way. We show that all models that individually time-resolve SNe converge to the FIRE-2 solution at sufficiently high resolution (<100M⊙). However, in both idealized single-SNe simulations and cosmological galaxy-formation simulations, the FIRE-2 algorithm converges much faster than other sub-grid models without re-tuning parameters.
Philip F. Hopkins, et al. "How To Model Supernovae in Simulations of Star and Galaxy Formation" (July 21, 2017)

Monday, July 24, 2017

Key Cosmological Measurement Related To Inflation Revised

A certain kind of gravitational waves evident in the cosmic background radiation are predicted by all but a handful of the hundreds of versions of cosmological inflation theory. A reanalysis of observations designed to see evidence of these waves now concludes that they are either absent or only negligible in magnitude.
A joint analysis of data collected by the Planck and BICEP2+Keck teams has previously given r=0.09+0.060.04 for BICEP2 and r=0.02+0.040.02 for Keck. Analyzing BICEP2 using its published noise estimate, we had earlier (Colley & Gott 2015) found r=0.09±0.04, agreeing with the final joint results for BICEP2. With the Keck data now available, we have done something the joint analysis did not: a correlation study of the BICEP2 vs. Keck B-mode maps. Knowing the correlation coefficient between the two and their amplitudes allows us to determine the noise in each map (which we check using the E-modes). We find the noise power in the BICEP2 map to be twice the original BICEP2 published estimate, explaining the anomalously high r value obtained by BICEP2. We now find r=0.004±0.04for BICEP2 and r=0.01±0.04 for Keck. Since r0 by definition, this implies a maximum likelihood value of r=0, or no evidence for gravitational waves. Starobinsky Inflation (r=0.0036) is not ruled out, however. Krauss & Wilzcek (2014) have already argued that "measurement of polarization of the CMB due to a long-wavelength stochastic background of gravitational waves from Inflation in the early Universe would firmly establish the quantization of gravity," and, therefore, the existence of gravitons. We argue it would also constitute a detection of gravitational Hawking radiation (explicitly from the causal horizons due to Inflation).
J. Richard Gott III and Wesley N. Colley, "Reanalysis of the BICEP2, Keck and Planck Data: No Evidence for Gravitational Radiation" (July 21, 2017).

Starobinsky Inflation (which implies f(R) gravity) is also supported by another recent study:

We obtain a differential equation which allows to reconstruct a f(R) theory from the α-Attractors class of inflationary models and solve it in the limit of high energies, showing an analogy between a f(R)=R+aRn1+bR2 theory, with ab and n free parameters, and the α-Attractors. We then calculate the predictions of the model f(R)=R+aRn1+bR2 on the scalar spectral index ns and the tensor-to-scalar ratio r and show that the power law correction Rn1allows for a production of gravitational waves enhanced with respect to the one in the Starobinsky model, while maintaining a viable prediction on ns. We also investigate the case of a single power law f(R)=γR2δ theory, with γ and δ free parameters. We calculate analytically the predictions of this model on the scalar spectral index ns and the tensor-to-scalar ratio r and the values of δ which are allowed from the current observational results. We find that 0.015<δ<0.016, confirming once again the excellent agreement between the Starobinsky model and observation.

T. Miranda, J. C. Fabris, O. F. Piattella, "Reconstructing a f(R) theory from the α-Attractors" (July 19, 2017)