Manodeep Sinha

A FIRST LOOK AT GALAXY FLYBY INTERACTIONS. I. CHARACTERIZING THE FREQUENCY OF FLYBYS IN A COSMOLOGICAL CONTEXT

Hierarchical structure formation theory is based on the notion that mergers drive galaxy evolution, so a considerable framework of semi-analytic models and N-body simulations has been constructed to calculate how mergers transform a growing galaxy. However, galaxy mergers are only one type of major dynamical interaction between halos—another class of encounter, a close flyby, has been largely ignored. We use cosmological N-body simulations to reconstruct the entire dynamical interaction history of dark matter halos. We present a careful method of identifying and tracking a dark matter halo which resolves the typical classes of anomalies that occur in N-body data. This technique allows us to robustly follow halos and several hierarchical levels of subhalos as they grow, dissolve, merge, and flyby one another—thereby constructing both a census of the dynamical interactions in a volume and an archive of the dynamical evolution of an individual halo. In addition to a census of mergers, our tool characterizes the frequency of close flyby interactions in the universe. We find that the number of close flyby interactions is comparable to, or even surpasses, the number of mergers for halo masses 1011 M ☉ h –1 at z 2. Halo flybys occur so frequently to high-mass halos that they are continually perturbed, unable to reach a dynamical equilibrium. In particular, we find that Milky Way type halos undergo a similar number of flybys as mergers irrespective of mass ratio for z 2. We also find tentative evidence that at high redshift, z 14, flybys are as frequent as mergers. Our results suggest that close halo flybys can play an important role in the evolution of the earliest dark matter halos and their galaxies, and can still influence galaxy evolution at the present epoch. Our companion paper quantifies the effect of close flyby interactions on galaxies and their dark matter hosts.

Numerical simulations of hot halo gas in galaxy mergers

Galaxy merger simulations have explored the behaviour of gas within the galactic disc, yet the dynamics of hot gas within the galaxy halo have been neglected. We report on the results of high-resolution hydrodynamic simulations of colliding galaxies with metal-free hot halo gas. To isolate the effect of the halo gas, we simulate only the dark matter halo and the hot halo gas over a range of mass ratios, gas fractions and orbital configurations to constrain the shocks and gas dynamics within the progenitor haloes. We find that (i) a strong shock is produced in the galaxy haloes before the first passage, increasing the temperature of the gas by almost an order of magnitude to T ∼ 10 6.3 K. (ii) The X-ray luminosity of the shock is strongly dependent on the gas fraction; it is ≥ 10 39 erg s ―1 for halo gas fractions larger than 10 per cent. (iii) The hot diffuse gas in the simulation produces X-ray luminosities as large as 10 42 erg s ―1 . This contributes to the total X-ray background in the Universe. (iv) We find an analytic fit to the maximum X-ray luminosity of the shock as a function of merger parameters. This fit can be used in semi-analytic recipes of galaxy formation to estimate the total X-ray emission from shocks in merging galaxies. (v) ∼10-20 per cent of the initial gas mass is unbound from the galaxies for equal-mass mergers, while 3-5 per cent of the gas mass is released for the 3:1 and 10: mergers. This unbound gas ends up far from the galaxy and can be a feasible mechanism to enrich the intergalactic medium with metals.

BAR FORMATION FROM GALAXY FLYBYS

Recently, both simulations and observations have revealed that flybys—fast, one-time interactions between two galaxy halos—are surprisingly common, nearing/comparable to galaxy mergers. Since these are rapid, transient events with the closest approach well outside the galaxy disk, it is unclear if flybys can transform the galaxy in a lasting way. We conduct collisionless N-body simulations of three coplanar flyby interactions between pure-disk galaxies to take a first look at the effects flybys have on disk structure, with particular focus on stellar bar formation. We find that some flybys are capable of inciting a bar with bars forming in both galaxies during our 1:1 interaction and in the secondary during our 10:1 interaction. The bars formed have ellipticities ≳ 0.5, sizes on the order of the host disks scale length, and persist to the end of our simulations, ∼5 Gyr after pericenter. The ability of flybys to incite bar formation implies that many processes associated with secular bar evolution may be more closely tied with interactions than previously thought.

The lives of high-redshift mergers

We present a comparative study of recent works on merger time-scales with dynamical friction and find a strong contrast between idealized/isolated mergers and mergers from a cosmological volume. Our study measures the duration of mergers in a cosmological N-body simulation of dark matter, with emphasis on higher redshifts (z≤ 10) and a lower mass range. In our analysis we consider and compare two merger definitions: tidal disruption and coalescence. We find that the merger-time formula proposed by Jiang et al. describes our results well and conclude that cosmologically motivated merger-time formulae provide a more versatile and statistically robust approximation for practical applications such as semi-analytic/hybrid models.

Forward Modeling of Large-scale Structure: An Open-source Approach with Halotools

We present the first stable release of Halotools (v0.2), a community-driven Python package designed to build and test models of the galaxy-halo connection. Halotools provides a modular platform for creating mock universes of galaxies starting from a catalog of dark matter halos obtained from a cosmological simulation. The package supports many of the common forms used to describe galaxy-halo models: the halo occupation distribution (HOD), the conditional luminosity function (CLF), abundance matching, and alternatives to these models that include effects such as environmental quenching or variable galaxy assembly bias. Satellite galaxies can be modeled to live in subhalos, or to follow custom number density profiles within their halos, including spatial and/or velocity bias with respect to the dark matter profile. The package has an optimized toolkit to make mock observations on a synthetic galaxy population, including galaxy clustering, galaxy-galaxy lensing, galaxy group identification, RSD multipoles, void statistics, pairwise velocities and others, allowing direct comparison to observations. Halotools is object-oriented, enabling complex models to be built from a set of simple, interchangeable components, including those of your own creation. Halotools has an automated testing suite and is exhaustively documented on this http URL, which includes quickstart guides, source code notes and a large collection of tutorials. The documentation is effectively an online textbook on how to build and study empirical models of galaxy formation with Python.

Spatial clustering of dark matter haloes: secondary bias, neighbour bias, and the influence of massive neighbours on halo properties

Department of Energy Computational Science Graduate Fellowship Program of the Office of Science; National Nuclear Security Administration in the Department of Energy [DE-FG02-97ER25308]; NSF Career Award [AST-1151650]; Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) [CE170100013]; NSF [AST-1516997]; [NSF-REU 1263045]

Can a satellite galaxy merger explain the active past of the Galactic Centre

Observations of the Galactic centre (GC) have accumulated a multitude of “forensic” evidence indicating that several million years ago the centre of the Milky Way galaxy was teeming with star formation and accretion-powered activity ‐ this paint s a rather different picture from the GC as we understand it today. We examine a possibility that this epoch of activity could have been triggered by the infall of a satellite galaxy into the Mi lky-Way which began at the redshift of z = 8 and ended few million years ago with a merger of the Galactic supermassive black hole with an intermediate mass black hole brought in by the inspiralling satellite.

VORONOI TESSELLATION AND NON-PARAMETRIC HALO CONCENTRATION

We present and test TesseRACt, a non-parametric technique for recovering the concentration of simulated dark matter halos using Voronoi tessellation. TesseRACt is tested on idealized N-body halos that are axisymmetric, triaxial, and contain substructure and compared to traditional least-squares fitting as well as two non-parametric techniques that assume spherical symmetry. TesseRACt recovers halo concentrations within 0.3% of the true value regardless of whether the halo is spherical, axisymmetric, or triaxial. Traditional fitting and non-parametric techniques that assume spherical symmetry can return concentrations that are systematically off by as much as 10% from the true value for non-spherical halos. TesseRACt also performs significantly better when there is substructure present outside

Balancing the baryon budget: the fraction of the IGM due to galaxy mergers

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Small- and Large-Scale Galactic Conformity in SDSS DR7

. Given that cosmological halos are rarely spherical and often contain substructure, we discuss implications for studies of halo concentration in cosmological N-body simulations including how choice of technique for measuring concentration might bias scaling relations.