arXiv subjects

Thomas S. Statler

Publications and source records attributed to Thomas S. Statler.

22 records · Page 2Linked to original sources

arXiv1998

Stellar Kinematics of the Double Nucleus of M31

We report observations of the double nucleus of M31 with the f/48 long-slit spectrograph of the HST Faint Object Camera. We obtain a total exposure of 19,000 sec. over 7 orbits, with the 0.063-arcsec-wide slit along the line between the two brightness peaks (PA 42). A spectrum of Jupiter is used as a spectral template. The rotation curve is resolved, and reaches a maximum amplitude of ~250 km/s roughly 0.3 arcsec either side of a rotation center lying between P1 and P2, 0.16 +/- 0.05 arcsec from the optically fainter P2. We find the velocity dispersion to be < 250 km/s everywhere except for a narrow ``dispersion spike'', centered 0.06 +/- 0.03 arcsec on the anti-P1 side of P2, in which sigma peaks at 440 +/- 70 km/s. At much lower confidence, we see local disturbances to the rotation curve at P1 and P2, and an elevation in sigma at P1. At very low significance we detect a weak asymmetry in the line-of-sight velocity distribution opposite to the sense usually encountered. Convolving our V and sigma profiles to CFHT resolution, we find good agreement with the results of Kormendy & Bender (1998, preprint), though there is a 20% discrepancy in the dispersion that cannot be attributed to the dispersion spike. Our results are not consistent with the location of the maximum dispersion as found by Bacon et al. We find that the sinking star cluster model of Emsellem & Combes (1997) does not reproduce either the rotation curve or the dispersion profile. The eccentric disk model of Tremaine (1995) fares better, and can be improved somewhat by adjusting the original parameters. However, detailed modeling will require dynamical models of significantly greater realism.

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arXiv1998

The Three-Dimensional Mass Distribution in NGC 1700

A variety of modeling techniques is used with surface photometry from the literature and recently acquired high-accuracy stellar kinematic data to constrain the three-dimensional mass distribution in the luminous cuspy elliptical galaxy NGC 1700. First, we model the radial velocity field and photometry, and, using a Bayesian technique, estimate the triaxiality T and short-to-long axis ratio c in five concentric annuli between approximately 1 and 3 effective radii. The results are completely consistent with T being constant inside about 2.5 r_e (36 arcsec; 6.7/h kpc). Adding an assumption of constant T as prior information gives an upper limit of T < 0.16 (95% confidence); this relaxes to T < 0.22 if it is also assumed that there is perfect alignment between the angular momentum and the galaxy's intrinsic short axis. Near axisymmetry permits us then to use axisymmetric models to constrain the radial mass profile. Using the Jeans (moment) equations, we demonstrate that 2-integral, constant-M/L models cannot fit the data; but a 2-integral model in which the cumulative enclosed M/L increases by a factor of roughly 2 from the center out to 12/h kpc can. Three-integral models constructed by quadratic programming show that, in fact, no constant-M/L model is consistent with the kinematics. Anisotropic 3-integral models with variable M/L, while not uniquely establishing a minimum acceptable halo mass, imply, as do the moment models, a cumulative M/L_B approximately 10 h at 12/h kpc. We conclude that NGC 1700 represents the best stellar dynamical evidence to date for dark matter in elliptical galaxies.

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arXiv1997

Streamlines of the Mean Stellar Motions in Elliptical Galaxies

The stellar velocity fields of elliptical galaxies hold clues to their dynamical structure and origin. The construction of velocity field models is greatly simplified by assuming an approximate geometrical form for the streamlines of the mean stellar motions. We test the conjecture that confocal streamlines are a valid approximation for realistic triaxial systems. We numerically integrate orbits in Schwarzschild's logarithmic potential, and fit confocal streamlines to their mean velocity fields by minimizing the RMS magnitude of the cross product between the velocity vectors and the streamlines. We find that most orbits at a given energy can be fitted by nearly identical confocal systems. There are statistically significant differences between the streamline parameters obtained for different orbit families, but the differences are small. The fitted parameters reproduce, to high accuracy, the boundary between short axis and outer long axis tubes, which is a direct measure of the triaxiality of the potential. These results strongly support efforts to obtain accurate statistical measurements of triaxiality from kinematic observations and reasonably simple velocity field models.

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arXiv1995

The Post-Merger Elliptical NGC 1700: Stellar Kinematic Fields to Four Effective Radii

We have measured the stellar motions in the elliptical galaxy NGC 1700 along four position angles, to very large radii, using absorption features in spectra obtained with the Multiple Mirror Telescope. Our data extend the coverage of the stellar velocity field by a factor of 5 (2.5 times further in radius and twice as many PAs) beyond previous work. We have attained 10 km/s accuracy in the mean velocity out to nearly 2 effective radii (r_e), and errors are < 15% of the maximum rotation speed out to nearly 3 r_e. The lack of detectable minor-axis rotation and the nearly identical kinematics on the +/- 45 degree PAs suggest that NGC 1700 is nearly oblate for r < 2.5 r_e. Beyond this radius, twisting of the morphological and kinematic axes indicate increasing triaxiality, an intrinsic twist, or both. The velocity distribution in the low-amplitude counterrotating core is weakly skewed in the direction of rotation, arguing against a central stellar disk. The small skewness and the depression of the central velocity dispersion are consistent with the accretion of a low-mass stellar companion in a retrograde orbit. Photometric fine structure at large radii (Schweizer \& Seitzer 1992) is also indicative of a merger; a velocity reversal 50 arcsec northeast suggests a major event. However, radially increasing prograde rotation in the main body of the galaxy implies that this was not the same event responsible for the counterrotating core. The strong rotation at large R and the nearly oblate shape are consistent with N-body simulations of group mergers (Weil 1995); that all disturbances inside 2.5 r_e have phase-mixed out suggests that NGC 1700 owes its present form to a merger of 3 or more stellar systems 2 to 4 h^-1 Gyr ago.

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