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Register a new userDynamical friction-driven orbital circularisation in rotating discs: a semi-analytical description
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We present and validate a novel semi-analytical approach to study the effect of dynamical friction on the orbits of massive perturbers in rotating stellar discs. We find that dynamical friction efficiently circularises the orbit of co-rotating perturbers, while it constantly increases the eccentricity of counter-rotating ones until their angular momenta reverse, then once again promoting circularisation. Such drag toward circular corotation could shape the distribution of orientations of kinematically decoupled cores in disc galaxies, naturally leading to the observed larger fraction of co-rotating cores.
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We present semi-analytical models and simplified $N$-body simulations with $10^4$ and $10^5$ particles aimed at probing the role of dynamical friction (DF) in determining the radial distribution of Blue Straggler Stars (BSSs) in globular clusters. The semi-analytical models show that DF (which is the only evolutionary mechanism at work) is responsible for the formation of a bimodal distribution with a dip progressively moving toward the external regions of the cluster. However, these models fail to reproduce the formation of the long-lived central peak observed in all dynamically evolved clusters. The results of $N$-body simulations confirm the formation of a sharp central peak, which remains as a stable feature over the time regardless of the initial concentration of the system. In spite of a noisy behavior, a bimodal distribution forms in many cases, with the size of the dip increasing as a function of time. In the most advanced stages the distribution becomes monotonic. These results are in agreement with the observations. Also the shape of the peak and the location of the minimum (which in most of the cases is within 10 core radii) turn out to be consistent with observational results. For a more detailed and close comparison with observations, including a proper calibration of the timescales of the dynamical processes driving the evolution of the BSS spatial distribution, more realistic simulations will be necessary.
In this paper, we explore the impact of a galactic bar on the inspiral time-scale of a massive perturber (MP) within a Milky Way-like galaxy. We integrate the orbit of MPs in a multi-component galaxy model via a semi-analytical approach including an accurate treatment for dynamical friction generalized to rotationally supported backgrounds. We compare the MP evolution in a galaxy featuring a Milky Way-like rotating bar to the evolution within an analogous axisymmetric galaxy without the bar. We find that the bar presence may significantly affect the inspiral, sometimes making it shorter by a factor of a few, sometimes hindering it for a Hubble time, implying that dynamical friction alone is greatly insufficient to fully characterize the orbital decay. The effect of the bar is more prominent for initially in-plane, prograde MPs, especially those crossing the bar co-rotation radius or outer Lindblad resonance. In the barred galaxy, we find the sinking of the most massive MPs (>~10^7.5 Msun) approaching the galaxy from large separations (>~8 kpc) to be most efficiently hampered. Neglecting the effect of global torques associated to the non-symmetric mass distribution is thus not advisable even within our idealized, smooth Milky Way model, and it should be avoided when dealing with more complex and realistic galaxy systems. This has important implications for the orbital decay of massive black holes in late-type spirals, the natural candidate sources to be detected with the Laser Interferometer Space Antenna (LISA).
The High Energy Stereoscopic System (H.E.S.S.) is an array of five Imaging Atmospheric Cherenkov Telescopes (IACTs) designed to detect cosmogenic gamma-rays with very high energies. Originally consisting of just four identical IACTs (CT1-4) with an effective mirror diameter of 12$,$m each, it was expanded with a fifth IACT (CT5) with a mirror diameter of 28$,$m in 2012. Being the largest IACT worldwide, CT5 allows to lower the energy threshold of H.E.S.S., making the array sensitive at energies where space-based detectors run out of statistics. Events can be analysed either monoscopically (i.e. using only information of CT5) or stereoscopically (requiring at least two triggered telescopes per event). To achieve a good performance, a sophisticated event reconstruction and analysis framework is indispensable. This is particularly important for H.E.S.S. since it is now the first IACT array that consists of different telescope types. An advanced reconstruction method is based on a semi-analytical model of electromagnetic particle showers in the atmosphere (model analysis). The properties of the primary particle are reconstructed by comparing the image recorded by each triggered telescope with the Cherenkov emission from the shower model using a log-likelihood maximisation. Due to its performance, this method has become one of the standard analysis techniques applied to CT1-4 data. Now it has been modified for use with the five-telescope array. We present the adapted model analysis and its performance in both monoscopic and stereoscopic analysis mode.
The most massive and luminous galaxies in the Universe serve as powerful probes to study the formation of structure, the assembly of mass, and cosmology. However, their detailed formation and evolution is still barely understood. Here we extract a sample of massive mock galaxies from the semi-analytical model of galaxy formation (SAM) GALACTICUS from the MultiDark-Galaxies, by replicating the CMASS photometric selection from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). The comparison of the GALACTICUS CMASS-mock with BOSS-CMASS data allows us to explore different aspects of the massive galaxy population at $0.5<z<0.6$, including the galaxy-halo connection and the galaxy clustering. We find good agreement between our modelled galaxies and observations regarding the galaxy-halo connection, but our CMASS-mock over-estimates the clustering amplitude of the 2-point correlation function, due to a smaller number density compared to BOSS, a lack of blue objects, and a small intrinsic scatter in stellar mass at fixed halo mass of $<0.1$ dex. To alleviate this problem, we construct an alternative mock catalogue mimicking the CMASS colour-magnitude distribution by randomly down-sampling the SAM catalogue. This CMASS-mock reproduces the clustering of CMASS galaxies within 1$sigma$ and shows some environmental dependency of star formation properties that could be connected to the quenching of star formation and the assembly bias.
The motion of a point like object of mass $M$ passing through the background potential of massive collisionless particles ($m << M$) suffers a steady deceleration named dynamical friction. In his classical work, Chandrasekhar assumed a Maxwellian velocity distribution in the halo and neglected the self gravity of the wake induced by the gravitational focusing of the mass $M$. In this paper, by relaxing the validity of the Maxwellian distribution due to the presence of long range forces, we derive an analytical formula for the dynamical friction in the context of the $q$-nonextensive kinetic theory. In the extensive limiting case ($q = 1$), the classical Gaussian Chandrasekhar result is recovered. As an application, the dynamical friction timescale for Globular Clusters spiraling to the galactic center is explicitly obtained. Our results suggest that the problem concerning the large timescale as derived by numerical $N$-body simulations or semi-analytical models can be understood as a departure from the standard extensive Maxwellian regime as measured by the Tsallis nonextensive $q$-parameter.
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