Galactic Tides and the Shape and Orientation of Dwarf Galaxy Satellites (original) (raw)
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Dwarf Spheroidal Satellite Galaxies and the Galactic Tidal Field
The Milky Way is surrounded by nine or more dwarf-spheroidal (dSph) satellite galaxies that appear to consist primarily of dark matter. Here I summarise research that shows that initially spherical bound low-mass satellites without dark matter, that are on orbits within a massive Galactic dark corona, can evolve into remnants that are non-spherical, have a non-isotropic velocity dispersion tensor and are not in virial equilibrium, but are bright enough for sufficiently long times to be mistaken for dark-matter dominated dSph galaxies.
Monthly Notices of the Royal Astronomical Society, 2009
We conduct high-resolution collisionless N -body simulations to investigate the tidal evolution of dwarf galaxies on an eccentric orbit in the Milky Way (MW) potential. The dwarfs originally consist of a low surface brightness stellar disk embedded in a cosmologically motivated dark matter halo. During 10 Gyr of dynamical evolution and after 5 pericentre passages the dwarfs suffer substantial mass loss and their stellar component undergoes a major morphological transformation from a disk to a bar and finally to a spheroid. The bar is preserved for most of the time as the angular momentum is transferred outside the galaxy. A dwarf spheroidal (dSph) galaxy is formed via gradual shortening of the bar. This work thus provides a comprehensive quantitative explanation of a potentially crucial morphological transformation mechanism for dwarf galaxies that operates in groups as well as in clusters. We compare three cases with different initial inclinations of the disk and find that the evolution is fastest when the disk is coplanar with the orbit. Despite the strong tidal perturbations and mass loss the dwarfs remain dark matter dominated. For most of the time the 1D stellar velocity dispersion, σ, follows the maximum circular velocity, V max , and they are both good tracers of the bound mass. Specifically, we find that M bound ∝ V 3.5 max and V max ∼ √ 3σ in agreement with earlier studies based on pure dark matter simulations. The latter relation is based on directly measuring the stellar kinematics of the simulated dwarf and may thus be reliably used to map the observed stellar velocity dispersions of dSphs to halo circular velocities when addressing the missing satellites problem.
The Astrophysical Journal, 2007
We present a wide-field (4.5 deg 2 ) photometric and spectroscopic survey of the Leo I dwarf spheroidal (dSph) galaxy to explore its extended morphology and dynamics. As in previous papers in this series, we take advantage of photometry in the M, T 2 , and DDO51 filter system to select Leo I red giant branch star candidates, and, so far, this selection technique has proven 100% reliable in selecting actual Leo I members among more than 100 M < 21.5 Leo I giant candidates having previous or new Keck DEIMOS spectroscopy to a radius > 1.3 times the limiting radius of the fitted, central King profile. The two-dimensional distribution of all similarlyselected Leo I giant candidates is well fitted by a central single-component King profile of limiting radius 13.3 arcmin, but many giant stars are found outside this newly derived King limiting radius. The density profile thus shows a break at a major axis radial distance of ∼ 10 arcmin produced by an excess of stars at and beyond the King limiting radius (spectroscopically confirmed to be made of true Leo I members), and primarily along the major axis of the main body of the rather elongated satellite. This spatial configuration, a rather flat velocity dispersion profile and an asymmetric radial velocity (RV) distribution among the Leo I members at large radii together support a picture where Leo I has been tidally disrupted on at least one, but at most two, perigalactic passages of a massive Local Group member. We demonstrate this hypothesis using mass-follows-light, N-body simulations of satellites in a Milky Way-like potential that reproduce the observed structural and dynamical properties of Leo I remarkably well. These models include ∼ 3 × 10 7 solar mass, tidally disrupting dSph systems on bound orbits with rather high eccentricity (0.93-0.96) and small perigalactica (10-15 kpc). The simulations allow the first observationally constrained orbit for Leo I without the measurement of its proper motion and show that the observed RV distribution is more consistent with a two Milky Way orbit history for the satellite while ruling out a Leo I orbit that includes a previous association with M31 within the last 10 Gyr. Given the overall success of tidally disrupting mass-follows-light satellite models to account for the observed properties of Leo I, we conclude that there is no need to invoke an extended dark matter halo around the satellite (e.g., as one explanation of the velocity dispersion and radial profiles at large radii), and that an overall modest M/L for the satellite is consistent with the available data. That a satellite on such a large (apogalacticon of ∼ 450 kpc), long period (P ∼ 6 Gyr) orbit as Leo I can experience tidal disruption suggests that similarly structured satellites with even smaller (eccentric) orbits will experience even greater tidally-induced mass loss rates.
The Effect of Tidal Stripping on Composite Stellar Populations in Dwarf Spheroidal Galaxies
Advances in Astronomy, 2010
We use N-body simulations to study the effects of tides on the kinematical structure of satellite galaxies orbiting a Milky Way-like potential. Our work is motivated by observations of dwarf spheroidal galaxies in the Local Group, for which often a distinction is possible between a cold centrally concentrated metal rich and a hot, extended metal poor population. Here we focus on the evolution of a spherical satellite with two stellar components set ab-initio to be spatially and kinematically segregated, and which are embedded in an extended dark matter halo. We find that an important attenuation of the initial differences in the distribution of the two stellar components occurs for orbits with small pericentric radii (rper ≤ 20 kpc). This is mainly due to: i) the loss of the gravitational support provided by the dark matter component after tidal stripping takes place, which forces a re-configuration of the luminous components, and ii) tides preferentially affect the more extended stellar component, leading to a net decrease in its velocity dispersion as a response for the mass loss, which thus shrinks the kinematical gap. We apply these ideas to the Sculptor and Carina dwarf spheroidals. Differences in their orbits might help to explain, under the assumption of similar initial configurations, why in the former a clear kinematical separation between metal poor and metal rich stars is apparent, while in Carina this segregation is significantly more subtle.
Tidal debris of dwarf spheroidals as a probe of structure formation models
Monthly Notices of the Royal Astronomical Society, 2002
Recent observations suggest that Carina and other nearby dwarf spheroidal galaxies (dSphs) are surrounded by unbound stars tidally stripped by the Milky Way. We run high-resolution N-Body simulations of dwarf galaxies orbiting within the Milky Way halo to determine if such observations can be explained with dark matter potentials as those implied by current structure formation models. We show that tidal forces acting on dwarfs with constant density cores or with cuspy profiles having a low concentration parameter (c < 5) lead to flat outer stellar density profiles like that of Carina for a variety of orbital configurations. On the contrary, it is more difficult to remove stars from cuspy dark matter halos with concentrations as high as predicted by CDM models at the mass scale of dwarf galaxies (c > ∼ 10) and the data can only be reproduced assuming nearly radial orbits. Our simulations show that Carina is losing mass at a fractional rate < 0.1 Gyr −1 and its mass-to-light ratio could be inflated by at most a factor of 2 due to unbound stars projected along the line of sight. We follow the evolution of the tidal debris within a triaxial clumpy cold dark matter Milky Way halo which causes differential precession and small scale heating of the stellar streams. This renders their use as a dynamical tracer of the Galactic potential practically useless, but does provide a novel test of the nature of the dark matter. Models with warm dark matter (WDM) or collisional fluid dark matter (FDM) produce dwarf halos with lower central densities than CDM and would be consistent with the observed tidal tails even for orbits with eccentricities as low as indicated by current data on nearby dwarf spheroidals. Galactic halos in FDM are smooth and spherical and would be favored by the detection of cold coherent streams such as that associated with the Sagittarius dwarf spheroidal.
The Metamorphosis of Tidally Stirred Dwarf Galaxies
The Astrophysical Journal, 2001
We present results from high-resolution N-Body/SPH simulations of rotationally supported dwarf irregular galaxies moving on bound orbits in the massive dark matter halo of the Milky Way. The dwarf models span a range in disk surface density and the masses and sizes of their dark halos are consistent with the predictions of cold dark matter cosmogonies. We show that the strong tidal field of the Milky Way determines severe mass loss in their halos and disks and induces bar and bending instabilities that transform low surface brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The final central velocity dispersions of the remnants are in the range 8 − 30 km/s and their final v/σ falls to values < 0.5, matching well the kinematics of early-type dwarfs. The transformation requires the orbital time of the dwarf to be < ∼ 3 − 4 Gyr, which implies a halo as massive and extended as predicted by hierarchical models of galaxy formation to explain the origin of even the farthest dSph satellites of the Milky Way, Leo I and Leo II. We show that only dwarfs with central dark matter densities as high as those of Draco and Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way: this is naturally achieved within hierarchical models, where the densest objects should have small orbital times due to their early formation epochs. Part of the gas is stripped and part is funneled to the center due to the bar, generating one strong burst of star formation in HSBs and smaller, multiple bursts in LSBs. Therefore, the large variety of star formation histories observed in LG dSphs naturally arises because different types of dIrr progenitors respond differently to the external perturbation of the Milky Way. Our evolutionary model automatically explains the morphology-density relation observed in the LG and in other nearby loose groups. Extended low-surface brightness stellar and gaseous streams originate from LSBs and follow the orbit of the dwarfs for several Gyr. Due to their high velocities, unbound stars projected along the line of sight can lead to overestimate the mass-to-light ratio of the bound remnant by a factor < ∼ 2, but this does not eliminate the need of extremely high dark matter contents in some of the dSphs.
Tidal Stirring and the Origin of Dwarf Spheroidals in the Local Group
The Astrophysical Journal, 2001
N-body + SPH simulations are used to study the evolution of dwarf irregular galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on plunging orbits. We propose a new dynamical mechanism driving the evolution of gas rich, rotationally supported dIrrs, mostly found at the outskirts of the Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs) or dwarf ellipticals (dEs), observed to cluster around the two giant spirals. The initial model galaxies are exponential disks embedded in massive dark matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the pericenter of their orbit partially strip their halo and disk and trigger dynamical instabilities that dramatically reshape their stellar component. After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This evolutionary mechanism naturally leads to the morphology-density relation observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest and most dark matter dominated among LG dSphs. If disks include a gaseous component, this is both tidally stripped and consumed in periodic bursts of star formation. The resulting star formation histories are in good qualitative agreement with those derived using HST color-magnitude diagrams for local dSphs.
Dwarf spheroidal satellites: are they of tidal origin?
Monthly Notices of the Royal Astronomical Society, 2007
The Milky Way and Andromeda must have formed through an initial epoch of substructure merging. As a result of fundamental physical conservation laws tidal-dwarf galaxies (TDGs) have likely been produced. Here we show that such TDGs appear, after a Hubble-time of dynamical evolution in the host dark-matter halo, as objects that resemble known dSph satellite galaxies. We discuss the possibility that some of the Milky Way's satellites may be of tidal origin.
The early evolution of tidal dwarf galaxies
Astronomy and Astrophysics, 2007
Context. Dwarf galaxies can arise from self-gravitating structures emerging from tidal tails. What fraction of the known dwarf galaxies in the Local Universe can have this origin is still a matter of debate. Aims. In our effort to understand the origin and evolution of tidal dwarf galaxies and their correspondence with local objects, the first step is to understand how these galaxies (which are supposed to have a limited amount of dark matter) react to the feedback of the ongoing star formation. Methods. We make use of 2-D chemodynamical calculations in order to study the early evolution of isolated, dark matter-free dwarf galaxies. We present models in which feedback parameters are varied. We also compare the results with dark matter-dominated dwarf galaxy models. Results. All the considered models show that the star formation proceeds for more than 300 Myr, therefore dwarf galaxies without large dark matter halos are not necessarily quickly destroyed. The chemical evolution of these objects is consistent with the main chemical properties of the dSphs of the Local Group. Models with large dark matter halos show results consistent with models free of dark matter, indicating that the distribution of gas is more important than the depth of the potential well in determining the global behaviour of dSph-sized dwarf galaxies.
Monthly Notices of the Royal Astronomical Society, 2012
The long term time evolution of tidal dwarf satellite galaxies with two different initial densities orbiting a host galaxy that resembles the Milky Way has been studied using a large set of Newtonian N-Body simulations. From the simulations two maps of the orbital conditions that lead to quasi-equilibrium objects were constructed. It has been found that several orbits of the satellites allow for the existence, for about 1 Gyr or more, of out-of-equilibrium bodies with high apparent mass-to-light ratios. Within this framework the satellites in the quasi-stable phase reproduce the observed satellite properties for about 16% of the orbit for high density progenitors, and for about 66% for progenitors with lower densities An additional simulation for a single satellite with initial mass of 10 7 M ⊙ and Plummer radius of 0.15 kpc leads to remnants in the quasiequilibrium phase that simultaneously reproduce remarkably well the observational quantities of the UFDGs of the Milky Way. This satellite in the quasi-stable phase reproduces the observed satellite properties for about 42% of the orbit. The results suggest that a fraction of the observed satellites could plausibly be galaxies without dark matter that have true M/L ratios much lower than those measured. The inflated M/L ratios arise because they are observed at the right time, along the right orbit and during the quasi-equilibrium phase of their evolution. This is a viable explanation for the high M/L ratios observed in all satellites as long as the satellites are preferentially on certain orbits and are observed at certain times. This could arise within the TDG scenario if all satellites are created at the same time along a few specific orbits that are particularly susceptible to the quasi-equilibrium phase.