We study the dark and luminous mass distributions, circular velocity curves (CVC), line-of-sight kinematics, and angular momenta for a sample of 42 cosmological zoom simulations of galaxies with stellar masses from 2.0 × 10 10 M ⊙ h −1 to 3.4 × 10 11 M ⊙ h −1 . Using a temporal smoothing technique, we are able to reach large radii. We find that: (i) The dark matter halo density profiles outside a few kpc follow simple power-law models, with flat dark matter CVCs for lower-mass systems, and rising CVCs for high-mass haloes. The projected stellar density distributions at large radii can be fitted by Sérsic functions with n ∼ > 10, larger than for typical early-type galaxies (ETGs). (ii) The massive systems have nearly flat total (luminous plus dark matter) CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the circular velocity at large radii correlates with circular velocity itself. (iii) The dark matter fractions within the projected stellar half mass radius R e are in the range 15-30% and increase to 40-65% at 5 R e . Larger and more massive galaxies have higher dark matter fractions. The fractions and trends with mass and size are in agreement with observational estimates, even though the stellar-to-total mass ratio is ∼2-3 times higher than estimated for ETGs. (iv) The short axes of simulated galaxies and their host dark matter haloes are well aligned and their short-to-long axis ratios are correlated. (v) The stellar root mean square velocity v rms (R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi) The line-of-sight velocity fieldsv show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter λ(R) are nearly flat or slightly rising, with values in [0.06, 0.75] from 2 R e to 5 R e . A few cases show local maxima in |v|/σ(R). These properties agree with observations of ETGs at large radii. (vii) Stellar mass, ellipticity at large radii ǫ(5 R e ), and λ(5 R e ) are correlated: the more massive systems have less angular momentum and are rounder, as for observed ETGs. (viii) More massive galaxies with a large fraction of accreted stars have radially anisotropic velocity distributions outside R e . Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars.
We study a merger of the NGC 4839 group with the Coma cluster using X-ray observations from the XMM-Newton and Chandra telescopes. X-ray data show two prominent features: (i) a long (∼600 kpc in projection) and bent tail of cool gas trailing (towards south-west) the optical center of NGC 4839, and ii) a 'sheath' region of enhanced X-ray surface brightness enveloping the group, which is due to hotter gas. While at first glance the X-ray images suggest that we are witnessing the first infall of NGC 4839 into the Coma cluster core, we argue that a post-merger scenario provides a better explanation of the observed features and illustrate this with a series of numerical simulations. In this scenario, the tail is formed when the group, initially moving to the south-west, reverses its radial velocity after crossing the apocenter, the ram pressure ceases and the ram-pressure-displaced gas falls back toward the center of the group and overshoots it. Shortly after the apocenter passage, the optical galaxy, dark matter and gaseous core move in a north-east direction, while the displaced gas continues moving to the south-west. The 'sheath' is explained as being due to interaction of the re-infalling group with its own tail of stripped gas mixed with the Coma gas. In this scenario, the shock, driven by the group before reaching the apocenter, has already detached from the group and would be located close to the famous relic to the south-west of the Coma cluster.
This is the first paper in a series of studies of the Coma cluster using the SRG/eROSITA X-ray data obtained in the course of the calibration and performance verification observations. The data cover a ~3° × 3° area around the cluster with a typical exposure time of more than 20 ks. The stability of the instrumental background and operation of the SRG observatory in the scanning mode provided us with an excellent data set for studies of the diffuse emission up to a distance of ~1.5R200 from the Coma center. In this study, we discuss the rich morphology revealed by the X-ray observations (also in combination with the SZ data) and argue that the most salient features can be naturally explained by a recent (ongoing) merger with the NGC 4839 group. In particular, we identify a faint X-ray bridge connecting the group with the cluster, which is convincing proof that NGC 4839 has already crossed the main cluster. The gas in the Coma core went through two shocks, first through the shock driven by NGC 4839 during its first passage through the cluster some gigayear ago and, more recently, through the “mini-accretion shock” associated with the gas settling back to quasi-hydrostatic equilibrium in the core. After passing through the primary shock, the gas should spend much of the time in a rarefaction region, where radiative losses of electrons are small, until the gas is compressed again by the mini-accretion shock. Unlike “runway” merger shocks, the mini-accretion shock does not feature a rarefaction region downstream and, therefore, the radio emission can survive longer. Such a two-stage process might explain the formation of the radio halo in the Coma cluster.
Moderately strong shocks arise naturally when two subclusters merge. For instance, when a smaller subcluster falls into the gravitational potential of a more massive cluster, a bow shock is formed and moves together with the subcluster. After pericenter passage, however, the subcluster is decelerated by the gravity of the main cluster, while the shock continues moving away from the cluster center. These shocks are considered as promising candidates for powering radio relics found in many clusters. The aim of this paper is to explore the fate of such shocks when they travel to the cluster outskirts, far from the place where the shocks were initiated. In a uniform medium, such a "runaway" shock should weaken with distance. However, as shocks move to large radii in galaxy clusters, the shock is moving down a steep density gradient that helps the shock to maintain its strength over a large distance. Observations and numerical simulations show that, beyond R 500 , gas density profiles are as steep as, or steeper than, ∼ r −3 , suggesting that there exists a "Habitable zone" for moderately strong shocks in cluster outskirts where the shock strength can be maintained or even amplified. A characteristic feature of runaway shocks is that the strong compression, relative to the initial state, is confined to a narrow region just behind the shock. Therefore, if such a shock runs over a region with a pre-existing population of relativistic particles, then the boost in radio emissivity, due to pure adiabatic compression, will also be confined to a narrow radial shell.1 The boundary separating the gaseous atmosphere of the main cluster from that of the subcluster is a contact discontinuity, a.k.a. cold front.
We show that there is a new class of gas tails -slingshot tails -which form as a subhalo (i.e. a subcluster or early-type cluster galaxy) moves away from the cluster center towards the apocenter of its orbit. These tails can point perpendicular or even opposite to the subhalo direction of motion, not tracing the recent orbital path. Thus, the observed tail direction can be misleading, and we caution against naive conclusions regarding the subhalo's direction of motion based on the tail direction. A head-tail morphology of a galaxy's or subcluster's gaseous atmosphere is usually attributed to ram pressure stripping and the widely applied conclusion is that gas stripped tail traces the most recent orbit. However, during the slingshot tail stage, the subhalo is not being ram pressure stripped (RPS) and the tail is shaped by tidal forces more than just the ram pressure. Thus, applying a classic RPS scenario to a slingshot tail leads not only to an incorrect conclusion regarding the direction of motion, but also to incorrect conclusions in regard to the subhalo velocity, expected locations of shear flows, instabilities and mixing. We describe the genesis and morphology of slingshot tails using data from binary cluster merger simulations, discuss their observable features and how to distinguish them from classic RPS tails. We identify three examples from the literature that are not RPS tails but slingshot tails and discuss other potential candidates.
The accuracy and robustness of a simple method to estimate the total mass profile of a galaxy are tested using a sample of 65 cosmological zoom simulations of individual galaxies. The method only requires information on the optical surface brightness and the projected velocity dispersion profiles, and therefore can be applied even in the case of poor observational data. In the simulated sample, massive galaxies (σ≃ 200–400 km s−1) at redshift z= 0 have almost isothermal rotation curves for broad range of radii (rms ≃ 5 per cent for the circular speed deviations from a constant value over 0.5Reff < r < 3Reff). For such galaxies, the method recovers the unbiased value of the circular speed. The sample‐averaged deviation from the true circular speed is less than ∼1 per cent with the scatter of ≃5–8 per cent (rms) up to R≃ 5Reff. Circular speed estimates of massive non‐rotating simulated galaxies at higher redshifts (z= 1 and 2) are also almost unbiased and with the same scatter. For the least massive galaxies in the sample (σ < 150 km s−1) at z= 0, the rms deviation is ≃7–9 per cent and the mean deviation is biased low by about 1–2 per cent. We also derive the circular velocity profile from the hydrostatic equilibrium (HE) equation for hot gas in the simulated galaxies. The accuracy (rms) of this estimate is about 4–5 per cent for massive objects (M > 6.5 × 1012 M⊙) and the HE estimate is biased low by ≃ 3–4 per cent, which can be traced to the presence of gas motions. This implies that the simple mass estimate can be used to determine the mass of observed massive elliptical galaxies to an accuracy of 5–8 per cent and can be very useful for galaxy surveys.
We discuss constraints on the mass density distribution (parameterized as ρ ∝ r −γ ) in early-type galaxies provided by strong lensing and stellar kinematics data. The constraints come from mass measurements at two 'pinch' radii. One 'pinch' radius r 1 = 2.2R Einst is defined such that the Einstein (i.e. aperture) mass can be converted to the spherical mass almost independently of the mass-model. Another 'pinch' radius r 2 = R opt is chosen so that the dynamical mass, derived from the lineof-sight velocity dispersion, is least sensitive to the anisotropy of stellar orbits. We verified the performance of this approach on a sample of simulated elliptical galaxies and on a sample of 15 SLACS lens galaxies at 0.01 z 0.35, which have already been analysed in Barnabè et al. (2011) by the self-consistent joint lensing and kinematic code. For massive simulated galaxies the density slope γ is recovered with an accuracy of ∼ 13%, unless r 1 and r 2 happen to be close to each other. For SLACS galaxies, we found good overall agreement with the results of Barnabè et al. (2011) with a sample-averaged slope γ = 2.1 ± 0.05. While the two-pinch-radii approach has larger statistical uncertainties, it is much simpler and uses only few arithmetic operations with directly observable quantities.
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