Dealing with gravity on galactic scales

Sascha TRIPPE 한국천문학회 2013 天文學會報 Vol.38 No.1

Probing the millimeter/radio polarization of active galactic nuclei

Sascha Trippe 한국천문학회 2012 天文學會報 Vol.37 No.1

The long-term mm/radio activity of active galactic nuclei

Sascha Trippe 한국천문학회 2011 天文學會報 Vol.36 No.2

AGN BROAD LINE REGIONS SCALE WITH BOLOMETRIC LUMINOSITY

Sascha Trippe 한국천문학회 2015 Journal of The Korean Astronomical Society Vol.48 No.3

The masses of supermassive black holes in active galactic nuclei (AGN) can be derived spec- troscopically via virial mass estimators based on selected broad optical/ultraviolet emission lines. These estimates commonly use the line width as a proxy for the gas speed and the monochromatic continuum luminosity, λLλ, as a proxy for the radius of the broad line region. However, if the size of the broad line region scales with the bolometric AGN luminosity rather than λLλ, mass estimates based on different emission lines will show a systematic discrepancy which is a function of the color of the AGN continuum. This has actually been observed in mass estimates based on Hα/Hβ and Civ lines, indicating that AGN broad line regions indeed scale with bolometric luminosity. Given that this effect seems to have been overlooked as yet, currently used single-epoch mass estimates are likely to be biased.

CAN MASSIVE GRAVITY EXPLAIN THE MASS DISCREPANCY-ACCELERATION RELATION OF DISK GALAXIES?

Trippe, Sascha The Korean Astronomical Society 2013 Journal of The Korean Astronomical Society Vol.46 No.3

The empirical mass discrepancy-acceleration (MDA) relation of disk galaxies provides a key test for models of galactic dynamics. In terms of modified laws of gravity and/or inertia, the MDA relation quantifies the transition from Newtonian to modified dynamics at low centripetal accelerations $a_c{\lesssim}10^{-10}ms^{-2}$. As yet, neither dynamical models based on dark matter nor proposed modifications of the laws of gravity/inertia have predicted the functional form of the MDA relation. In this work, I revisit the MDA data and compare them to four different theoretical scaling laws. Three of these scaling laws are entirely empirical; the fourth one - the "simple ${\mu}$" function of Modified Newtonian Dynamics - derives from a toy model of gravity based on massive gravitons (the "graviton picture"). All theoretical MDA relations comprise one free parameter of the dimension of an acceleration, Milgrom's constant aM. I find that the "simple ${\mu}$" function provides a good fit to the data free of notable systematic residuals and provides the best fit among the four scaling laws tested. The best-fit value of Milgrom's constant is $a_M=(1.06{\pm}0.05){\times}10^{-10}ms^{-2}$. Given the successful prediction of the functional form of the MDA relation, plus an overall agreement with the observed kinematics of stellar systems spanning eight orders of magnitude in size and 14 orders of magnitude in mass, I conclude that the "graviton picture" is sufficient (albeit probably not a necessary nor unique approach) to describe galactic dynamics on all scales well beyond the scale of the solar system. This suggests that, at least on galactic scales, gravity behaves as if it was mediated by massive particles.

POLARIZATION AND POLARIMETRY: A REVIEW

Sascha Trippe 한국천문학회 2014 Journal of The Korean Astronomical Society Vol.47 No.1

Polarization is a basic property of light and is fundamentally linked to the internal geometry of a source of radiation. Polarimetry complements photometric, spectroscopic, and imaging analyses of sources of radiation and has made possible multiple astrophysical discoveries. In this article I review (i) the physical basics of polarization: electromagnetic waves, photons, and parameterizations; (ii) astrophysical sources of polarization: scattering, synchrotron radiation, active media, and the Zeeman, Goldreich-Kylafis, and Hanle effects, as well as interactions between polarization and matter (like birefringence, Faraday rotation, or the Chandrasekhar-Fermi effect); (iii) observational methodology: on-sky geometry, influence of atmosphere and instrumental polarization, polarization statistics, and observational techniques for radio, optical, and X/$\gamma$ wavelengths; and (iv) science cases for astronomical polarimetry: solar and stellar physics, planetary system bodies, interstellar matter, astrobiology, astronomical masers, pulsars, galactic magnetic fields, gamma-ray bursts, active galactic nuclei, and cosmic microwave background radiation.

OPTICAL MULTI-CHANNEL INTENSITY INTERFEROMETRY – OR: HOW TO RESOLVE O-STARS IN THE MAGELLANIC CLOUDS

Sascha Trippe,김재영,이방원,최창수,오정환,이태석,윤성철,임명신,박용선 한국천문학회 2014 Journal of The Korean Astronomical Society Vol.47 No.6

Intensity interferometry, based on the Hanbury Brown–Twiss effect, is a simple and inexpensivemethod for optical interferometry at microarcsecond angular resolutions; its use in astronomywas abandoned in the 1970s because of low sensitivity. Motivated by recent technical developments, weargue that the sensitivity of large modern intensity interferometers can be improved by factors up toapproximately 25 000, corresponding to 11 photometric magnitudes, compared to the pioneering NarrabriStellar Interferometer. This is made possible by (i) using avalanche photodiodes (APD) as light detectors,(ii) distributing the light received from the source over multiple independent spectral channels, and (iii)use of arrays composed of multiple large light collectors. Our approach permits the construction of large(with baselines ranging from few kilometers to intercontinental distances) optical interferometers at thecost of (very) long-baseline radio interferometers. Realistic intensity interferometer designs are able toachieve limiting R-band magnitudes as good as mR ≈ 14, sufficient for spatially resolved observations ofmain-sequence O-type stars in the Magellanic Clouds. Multi-channel intensity interferometers can addressa wide variety of science cases: (i) linear radii, effective temperatures, and luminosities of stars, via directmeasurements of stellar angular sizes; (ii) mass–radius relationships of compact stellar remnants, via directmeasurements of the angular sizes of white dwarfs; (iii) stellar rotation, via observations of rotationflattening and surface gravity darkening; (iv) stellar convection and the interaction of stellar photospheresand magnetic fields, via observations of dark and bright starspots; (v) the structure and evolution of multiplestars, via mapping of the companion stars and of accretion flows in interacting binaries; (vi) directmeasurements of interstellar distances, derived from angular diameters of stars or via the interferometricBaade–Wesselink method; (vii) the physics of gas accretion onto supermassive black holes, via resolvedobservations of the central engines of luminous active galactic nuclei; and (viii) calibration of amplitudeinterferometers by providing a sample of calibrator stars.

A DERIVATION OF MODIFIED NEWTONIAN DYNAMICS

Trippe, Sascha The Korean Astronomical Society 2013 Journal of The Korean Astronomical Society Vol.46 No.2

Modified Newtonian Dynamics (MOND) is a possible solution for the missing mass problem in galactic dynamics; its predictions are in good agreement with observations in the limit of weak accelerations. However, MOND does not derive from a physical mechanism and does not make predictions on the transitional regime from Newtonian to modified dynamics; rather, empirical transition functions have to be constructed from the boundary conditions and comparisons with observations. I compare the formalism of classical MOND to the scaling law derived from a toy model of gravity based on virtual massive gravitons (the "graviton picture") which I proposed recently. I conclude that MOND naturally derives from the "graviton picture" at least for the case of non-relativistic, highly symmetric dynamical systems. This suggests that-to first order-the "graviton picture" indeed provides a valid candidate for the physical mechanism behind MOND and gravity on galactic scales in general.

Does the Jet Production Efficiency of Radio Galaxies Control Their Optical AGN Types?

Sascha Trippe 한국천문학회 2014 Journal of The Korean Astronomical Society Vol.47 No.4

The jet production efficiency of radio galaxies can be quantified by comparison of their kinetic jet powers Pjet and Bondi accretion powers PB. These two parameters are known to be related linearly, with the jet power resulting from the Bondi power by multiplication with an efficiency factor of order 1%. Using a recently published (Nemmen & Tchekhovskoy 2014 high-quality sample of 27 radio galaxies, I construct a PB-Pjet diagram that includes information on optical AGN types as far as available. This diagram indicates that the jet production efficiency is a function of AGN type: Seyfert 2 galaxies seem to be systematically (with a false alarm probability of 4.3×10-4 less efficient, by about one order of magnitude, in powering jets than Seyfert 1 galaxies, LINERs, or the remaining radio galaxies. This suggests an evolutionary sequence from Sy2s to Sy,1s and LINERs, controlled by an interplay of jets on the one hand and dust and gas in galactic nuclei on the other hand. When taking this effect into account, the PB-Pjet relation is probably much tighter intrinsically than currently assumed.

Capella: A Space-only High-frequency Radio VLBI Network Formed by a Constellation of Small Satellites

Sascha Trippe 한국천문학회 2023 天文學會報 Vol.48 No.1