Mass determination of elliptical galaxies

The work presented here focuses on the investigation and further development of simple mass estimators for early-type galaxies which are suitable for large optical galaxy surveys with poor and/or noisy data. We consider simple and robust methods that provide an anisotropy-independent estimate of the galaxy mass relying on the stellar surface brightness and projected velocity dispersion profiles. Under reasonable assumptions a fundamental mass-anisotropy degeneracy can be circumvented without invoking any additional observational data, although at a special (characteristic) radius only, i.e these approaches do not recover the radial mass distribution. Reliable simple mass estimates at a single radius could be used (i) to cross-calibrate other mass determination methods; (ii) to estimate a non-thermal contribution to the total gas pressure when compared with the X-ray mass estimate at the same radius; (iii) to evaluate a dark matter fraction when compared with the luminous mass estimate; (iv) to derive the slope of the mass profile when combined with the mass estimate from strong lensing; (v) or as a virial mass proxy. Two simple mass estimators have been suggested recently - the local (Churazov et al. 2010) and the global (Wolf et al. 2010) methods - which evaluate mass at a particular radius and are claimed to be weakly dependent on the anisotropy of stellar orbits. One approach (Wolf et al. 2010) uses the total luminosity-weighted velocity dispersion and evaluates the mass at a deprojected half-light radius, i.e. relies on the global properties of a galaxy. In contrast, the Churazov et al. technique uses local properties: logarithmic slopes of the surface brightness and velocity dispersion profiles, and recovers the mass at a radius where the surface brightness declines as R^{-2} (see also Richstone and Tremaine 1984, Gerhard 1993). To test the robustness and accuracy of the methods I applied them to analytic models and to simulated galaxies from a sample of cosmological zoom-simulations which are similar in properties to nearby early-type galaxies. Both local and global simple mass estimates are found to be in good agreement with the true mass at the corresponding characteristic radius. Particularly, for slowly rotating simulated galaxies the local method gives an almost unbiased mass-estimate (when averaged over the sample) with a modest RMS-scatter of 12% (Chapter 2). When applied to massive simulated galaxies with a roughly flat velocity dispersion profile, the global approach on average also provides the almost unbiased mass-estimate, although the RMS-scatter is slightly larger (14-20 %) than for the local estimator (Chapter 4). A noticeable scatter in the determination of the characteristic radius is also expected since the half-light radius depends on the radial range used for the analysis and applied methodology. Next I tested the simple mass estimators on a sample of real early-type galaxies which had previously been analyzed in detail using state-of-the-art dynamical modeling. For this set of galaxies the simple mass estimates are in remarkable agreement with the results of the Schwarzschild modeling despite the fact that some of the considered galaxies are flattened and mildly rotating. When averaged over the sample the simple local method overestimates the best-fit mass from dynamical modeling by 10% with the RMS-scatter 13% between different galaxies. The bias is comparable to measurement uncertainties. Moreover, it is mainly driven by a single galaxy which has been found to be the most compact one in the sample. When this galaxy is excluded from the sample, the bias and the RMS-scatter are both reduced to 6%. The global estimator for the same sample gives the mean deviation 4% with the slightly larger RMS-scatter of 15% (Chapter 4). Given the encouraging results of the tests I apply the local mass estimation method to a sample of five X-ray bright early-type galaxies observed with the 6-m telescope BTA in Russia. Using publicly available Chandra data I derived the X-ray mass profile assuming spherical symmetry and hydrostatic equilibrium of hot gas. A comparison between the X-ray and optical mass estimates allowed me to put constraints on the non-thermal contribution (sample averaged value is 4%) to the total gas pressure arising from, for instance, microturbulent gas motions. Once the X-ray derived circular speed is corrected for the non-thermal contribution, the mismatch between the X-ray circular speed V_c^X and the optical circular velocity for isotropic stellar orbits V_c^{iso} provides a clue to the orbital structure of the galaxy. E.g., at small radii V_c^X > V_c^{iso} would suggest more circular orbits, while at larger radii this would correspond to more radial orbits. For two galaxies in our sample there is a clear indication that at radii larger than the half-light radius stellar orbits become predominantly radial. Finally, the difference between the optical mass-estimate at the characteristic radius and the stellar contribution to the total mass permitted the derivation of a dark-matter fraction. A typical dark matter fraction for our sample of early-type galaxies is 50% for Salpeter IMF and 70% for Kroupa IMF at the radius which is close to the half-light radius (Chapter 3)

The mass and angular momentum distribution of simulated massive early-type galaxies to large radii

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 massive galaxies. 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 Sersic functions with n>10, larger than for typical ETGs. (ii)The massive systems have nearly flat total CVCs at large radii, while the less massive systems have mildly decreasing CVCs. The slope of the CVC at large radii correlates with v_circ itself. (iii)The dark matter fractions within Re are in the range 15-30% and increase to 40-65% at 5Re. Larger and more massive galaxies have higher dark matter fractions. (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 vrms(R) profiles are slowly declining, in agreement with planetary nebulae observations in the outer haloes of most ETGs. (vi)The line-of-sight velocity fields v show that rotation properties at small and large radii are correlated. Most radial profiles for the cumulative specific angular momentum parameter lambda(R) are nearly flat or slightly rising, with values in [0.06,0.75] from 2Re to 5Re. (vii)Stellar mass, ellipticity at 5Re, and lambda(5Re) 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 Re. Tangential anisotropy is seen only for galaxies with high fraction of in-situ stars. (Full abstract in PDF)Comment: 17 pages, 15 figures, 2 tables, accepted by MNRA

A new class of x-ray tails of early-type galaxies and subclusters in galaxy clusters: Slingshot tails versus ram pressure stripped tails

© 2019. The American Astronomical Society. All rights reserved.. We show that there is a new class of gas tails - slingshot tails - that form as a subhalo (i.e., a subcluster or early-type cluster galaxy) moves away from the cluster center toward 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 regarding 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 and 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