On the Efficiency of the Tidal Stirring Mechanism for the Origin of Dwarf Spheroidals: Dependence on the Orbital and Structural Parameters of the Progenitor Disky Dwarfs

Abstract

The tidal stirring model posits the formation of dwarf spheroidal galaxies (dSphs) via the tidal interactions between late-type, rotationally-supported dwarfs and Milky Way-sized host galaxies. Using a comprehensive set of collisionless N-body simulations, we investigate the efficiency of the tidal stirring mechanism for the origin of dSphs. In particular, we examine the degree to which the tidal field of the primary galaxy affects the sizes, masses, shapes, and kinematics of the disky dwarfs for a range of dwarf orbital and structural parameters. Our study is the first to employ self-consistent, equilibrium models for the progenitor dwarf galaxies constructed from a composite distribution function and consisting of exponential stellar disks embedded in massive, cosmologically-motivated dark matter halos. Exploring a wide variety of dwarf orbital configurations and initial structures, we demonstrate that in the majority of cases the disky dwarfs experience significant mass loss and their stellar distributions undergo a dramatic morphological, as well as dynamical, transformation. Specifically, the stellar components evolve from disks to bars and finally to pressure-supported, spheroidal systems with kinematic and structural properties akin to those of the classic dSphs in the Local Group (LG) and similar environments. The self-consistency of the adopted dwarf models is crucial for confirming this complex transformation process via tidally-induced dynamical instabilities and impulsive tidal heating of the stellar distribution. Our results suggest that such tidal transformations should be common occurrences within the currently favored cosmological paradigm and highlight the key factor responsible for an effective metamorphosis to be the strength of the tidal shocks at the pericenters of the orbit. We also demonstrate that the combination of short orbital times and small pericentric distances, characteristic of dwarfs being accreted by their hosts at high redshift, induces the strongest and most complete transformations. Our models also indicate that the efficiency of the transformation via tidal stirring is affected significantly by the structure of the progenitor disky dwarfs. While the mass-to-light ratios, M/L, of the dwarf galaxies typically decrease monotonically with time as the extended dark matter halos are efficiently tidally stripped, we identify a few cases where this trend is reversed later in the evolution when stellar mass loss becomes more effective. We also find that the dwarf remnants satisfy the relation V max = √ 3 σ * , where σ * is the one-dimensional, central stellar velocity dispersion and V max is the maximum halo circular velocity, which has intriguing implications for the missing satellites problem. Assuming that the distant dSphs in the LG, such as Leo I, Tucana, and Cetus are the products of tidal stirring, our findings suggest that these galaxies should have only been partially stirred by the tidal field of their hosts. We thus predict that these remote dwarfs should exhibit higher val...

An optical and H i study of the dwarf Local Group galaxy VV124 = UGC4879

“…For example, the relatively high recession velocity (V g = +100 km s −=1 ) of the gas-poor isolated dwarf Tucana (Fraternali et al 2009) may indicate that it has been ejected to large distances by a three-body encounter, as envisaged by Sales et al (2007). Such a scenario cannot be invoked in the case of VV124, which has a much smaller galactocentric velocity (V g = +17 km s −=1 ; see also D'Onghia et al 2009;Kazantzidis et al 2011, for other scenarios for the transformation of dwarf galaxies and discussion).…”

“…All the values of surface brightness are corrected for extinction. Mayer et al (2006Mayer et al ( , 2007 explicitly postulates dwarf disk galaxies as the progenitors of dSphs (see Kazantzidis et al 2011, for a recent and thorough discussion). Indeed, observational evidence of the existence of the expected intermediate stages of such transformation process is beginning to emerge, from study of various environments (see, for example, the disky dEs identified in the Virgo cluster by Lisker et al 2007).…”

On the Tidal Radius of Satellites on Prograde and Retrograde Orbits

“…Interactions of galaxies with the potential of larger objects have proved capable of transforming their morphology and kinematics on various scales: those of dwarf galaxies orbiting the Milky-Way-like galaxy (Mayer et al 2001;Kazantzidis et al 2011), galaxies in groups (Villalobos et al 2012), and clusters (Mastropietro et al 2005;Bialas et al 2015). One of the factors influencing the final outcome of such a process is the initial inclination of the galaxy disk with respect to its orbit (Villalobos et al 2012;Łokas et al 2015).…”

A Virtual Observatory Census to Address Dwarfs Origins (AVOCADO)

“…Internal mechanisms -such as gas consumption via star formation or feedback effects -appear to be insufficient to completely cease the star formation activity in dwarf galaxies. External processes need therefore to be invoked, and they most likely involve a combination of tidal and gas-stripping mechanisms (e.g., Mayer et al 2001bMayer et al ,a, 2006Grebel et al 2003;Peñarrubia et al 2008;Kazantzidis et al 2011). One might argue that the fact that quiescent lowmass galaxies are found out to projected distances larger than 1 Mpc -corresponding to several virial radii for haloes hosting L L * galaxies -is not consistent with the importance of environmental effects.…”