GalNUC Research Group

Astrophysical Dynamics and

Statistical Mechanics of Galactic Nuclei

Past talks:

• September 17

Mohsen Shadmehri (Golestan University)

The accretion mechanisms in the protoplanetary discs

The protoplanetary discs (PPDs) are known as sites of planet formation. Mass of a PPD mostly consists of gas, however, dust particles also exist with an abundance around one percent. While it was believed that magnetorotational instability (MRI) is responsible for the mass accretion in the PPDs, recent observational evidence and theoretical arguments indicate that probably wind-driven accretion is dominant at least in the inner disc region. Although this feature has raised intense investigations, a big challenge is how to propose a robust diagnostic to discriminate the dominant mechanism of the accretion in the PPDs. In this talk, I first review the major mechanisms that generate PPDs turbulence. The most recent observational diagnostics supporting the existence of efficient magnetically wind-driven accretion are discussed. We then explore the evolution of the PPDs in the presence of the magnetically driven winds and the resulting isochrones tracks in the accretion rate and disc mass plane are confronted with the observed PPDs. Evolutions of the disc quantities such as its total mass and its radius are studied for different levels of the magnetic wind strength. While previous studies based on the viscous disc model were able to obtain isochrones tracks more or less consistent with the observed PPDs, our more sophisticated disc model which incorporates magnetic winds can also give us isochrones tracks consistent with the observations. Our results thereby reinforce that role of the magnetic winds cannot be neglected in PPDs.

Gerry Williger (Konkoly Observatory, University of Louisville)

LSST-Euclid Synergy

Euclid and LSST are two independent projects which complement each other very well scientifically, but which have had significant problems in working together due to non-scientific reasons. I will first give a head-to-head comparison of the two projects, then discuss several areas where collaboration would significantly improve the results compared to each project working independently. These areas include deblending objects, spectral information for/IR colors of variable sources, galaxy morphologies, improving galaxy ellipticity measurements for weak lensing studies, improved photometric redshifts, more precise cluster mass estimates etc. I will describe some of the possible areas of collaborations. Although Hungary is a member of LSST but not Euclid, I will discuss ways of accessing Euclid data to maximize scientific output. This talk is based largely on Rhodes et al. (2017, ApJS, 233, 21) but I may add more recent material.

• May 14

Jean-Baptiste Fouvry (IAS)

Resonant relaxation of stars around a supermassive black hole

In the vicinity of a supermassive black hole, stars move on nearly Keplerian orbits. Yet, because of the enclosed stellar mass and general relativity, the potential slightly deviates from the Keplerian one, which causes the stellar orbits to precess. Similarly, as a result of the finite number of stars, the mutual gravitational torques between pairs of stars also drive a rapid reorientation of the stars' orbital orientation, much faster than the standard two-body relaxation driven by local scatterings. Overall, the combination of these two effects leads to a stochastic evolution of stellar orbital angular momentum vectors, through a process named "resonant relaxation".  Owing to recent developments in the diffusion theory of long-range interacting systems, I will show how one can fully describe such dynamics, in particular scalar resonant relaxation (relaxation of the norm of the angular momentum) and vector resonant relaxation (relaxation of the direction of the angular momentum vector). I will also highlight some astrophysical applications of these new methods, for example to understand the inefficiency of resonant relaxation to induce stellar tidal disruptions.

• June 10

• April 2

Manuel Arca-Sedda (Heidelberg)

Stellar- and intermediate-mass black holes in star clusters and galactic nuclei:

dynamics and implications for GW astronomy

Globular and nuclear clusters are ideal nurseries for black holes (BHs) and other compact stellar remnants, being sufficiently crowded to favour the formation of tightly bound pairs that in occasionally can merge and release observable gravitational waves (GWs). Globular clusters are thought to be a perfect birthplace for intermediate mass BHs (IMBH, mass range 102-105 Msun), while some nuclear clusters offer a unique view on the interplay between stars and supermassive BHs (SMBH, mass range 105-109 Msun). Understanding the circumstances that regulate merging events involving BHs and IMBHs is crucial in the GW era. Unfortunately, defining a robust way to assess the presence of "dark" compact objects in dense clusters on an observational basis is a hard task to accomplish. In this talk, I will discuss the impact of stellar BHs and IMBHs evolution on globular and nuclear clusters observational properties. For globulars, it is possible to define a "fundamental plane" that connects their observational quantities to the main properties of their BH population. Regarding nuclear clusters, I will discuss how the processes that govern their formation can be connected to observations using our Galactic Centre as a reference case.

• March 26

Teruaki Suyama (Tokyo)

Probing ultra-light scalar field with gravitational-wave interferometers

An ultralight scalar field is one of the dark matter candidates. If it couples with Standard Model particles, it oscillates mirrors in gravitational-wave detectors and generates detectable signals. We study the spectra of the signals taking into account the motion of the detectors due to the Earth's rotation/the detectors' orbital motion around the Sun and formulate a suitable data-analysis method to detect it. We find that our method can improve the existing constraints given by fifth-force experiments on one of the scalar field's coupling constants. Our study demonstrates that experiments with gravitational-wave detectors play a complementary role to that Equivalence Principle tests do.

• February 26

Giacomo Fragione (Jerusalem)

Merging Black Holes of Any Size and Hierarchy

The recent discovery of gravitational waves has opened new horizons for physics. Current and upcoming missions, such as LIGO, VIRGO, KAGRA, and LISA, promise to shed light on black holes of every size, from stellar mass sizes (SBH) up to supermassive black holes which reside in galactic nuclei. The astrophysical origin of these mergers is among the most puzzling open questions of our time. Possibilities include isolated binary evolution through a common envelope phase, gas-assisted mergers, dynamically assembled binaries in dense stellar systems such as globular clusters or galactic nuclei. By using N-body simulations, I will discuss how the rich dynamics in galactic nuclei and star clusters catalyzes SBH mergers in binary and triple systems, and make prediction on their merger rate through cosmic time. Moreover, the intermediate mass black hole (IMBH) family has not been detected beyond any reasonable doubt neither directly nor indirectly. Recent analyses suggest observational evidence for the presence of IMBHs in the centers of two Galactic globular clusters. I investigate the possibility that globular clusters were born with a central IMBH, which undergo repeated merger events with stars and SBHs in the cluster core. By means of a semi-analytical method, I will show how the evolution of the primordial cluster population in the galactic potential can be investigates and make predictions on the Gravitational Wave mergers of the binary IMBH-SBH systems. Currently there are no LIGO/VIRGO upper limits for GW sources in this mass range, but at design sensitivity these instruments may detect these IMBH-SBH mergers in the coming years. LISA and the Einstein Telescope will be best suited to detect these GW events.

• February 19

Ákos Bogdán (Harvard)

Exploring the missing baryons using absorption studies

In the low-redshift (z<2) universe, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in filaments connecting galaxies in the form of warm-hot intergalactic medium (WHIM). Although UV absorption studies explored the warm phase of the WHIM, it is hypothesized that notable fraction of the missing baryons are in the hot (X-ray) phase. However, X-ray spectroscopy is limited by the low effective area of currently available instrumentation, thus the conclusive observational evidence is still lacking. In this work, we utilize Chandra LETG spectra of luminous AGN, along with previous redshift measurements of UV absorption line systems, and apply a stacking method to gain unparalleled sensitivity. Based on the stacked data, we probe the most abundant helium-like and hydrogen-like metal lines in the spectra of AGN. In addition, we constrain the contribution of the WHIM to the overall baryon budget.