arXiv subjects

Steven S. Vogt

Publications and source records attributed to Steven S. Vogt.

76 records · Page 5Linked to original sources

arXiv1998

The Internal Kinematics of the Leo I Dwarf Spheroidal Galaxy: Dark Matter at the Fringe of the Milky Way

We present radial velocities of 33 red giants in the Leo I dSph galaxy obtained from spectra taken with the HIRES echelle spectrograph on the Keck I telescope. These data have a mean precision of 2.2 km/s and lead to estimates of the central velocity dispersion and systemic velocity of Leo I of 8.8 +- 1.3 km/s, and 287.0 +- 1.9 km/s, respectively. The systemic velocity confirms past results that Leo I has an unusually large galactocentric velocity, implying the presence of a massive dark halo in the Milky Way or an extended dark component pervading the Local Group. The V-band (M/L) ratio of Leo I is in the range 3.5-5.6. We have produced a set of models that account for the effects of stellar evolution on the global mass-to-light ratio of a composite population. Because Leo I contains a dominant intermediate-age population, we find that the V-band mass-to-light ratio of Leo I would be in the range 6-13 if it were composed exclusively of old stars such as found in globular clusters. This suggests that Leo I probably does contain a significant dark halo. The mass of this halo is approximately 2 x10^7 solar masses, similar to the dark halo masses inferred for all other Galactic dSph galaxies. Because Leo I is isolated and has passed the Milky Way at most once in the past, external tides could not plausibly have inflated its centraldispersion to the observed value. We also considered whether MOdified Newtonian Dynamics (MOND) could account for the internal kinematics of Leo I and conclude that this alternative gravitational model can account for the Leo I kinematics adequately without requiring a dark halo.

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arXiv1997

Doppler Imagery of the Spotted RS CVn Star HR 1099 (= V711 Tau) from 1981 - 1992

We present 23 Doppler images of the spotted RS CVn star HR 1099 from 1981 to 1992. HR 1099 shows a large cool polar spot which has persisted for the 11 years of this study and other low-latitude spots which come and go on relatively short timescales. The polar spot has variable protuberances which look very similar to the time-variable vertical extensions of the Sun's polar coronal hole. The area of the polar spot and its extensions shows marginal evidence of being periodically variable in time with a period of ~3 years. On several occasions, isolated prominent spots emerged and then disappeared near the equator. The tracks of two long-lived spots suggest that some spots which emerge at low or intermediate latitudes migrate up to the pole in a clockwise spiral (slower than the orbit), and then apparently merge with the polar spot. Differential rotation as measured from spot movements showed that intermediate and low latitudes rotate slightly slower than the orbital angular velocity, while the pole and highest latitudes appear to be synchronized to the orbit. The differential rotation is of opposite sign and a factor of 50 less than for the Sun. We propose that the starspots on HR 1099 and other rapidly rotating RS CVn stars are, by analogy with solar coronal holes, large unipolar magnetic regions tightly frozen into multi-kilogauss axisymmetric dipole fields in these stars. Since the large cool polar spots, the signature of these dipoles, are not present on more slowly rotating RS CVn stars, we believe that they must be dynamo-induced rather than remnant fossil fields.

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arXiv1996

On the Spatial and Kinematic Distributions of Mg II Absorbing Gas in =0.7 Galaxies

(Abridged) We present HIRES/Keck spectra having resolution 6 km/s of Mg II 2796 absorption profiles which arise in the gas associated with 15 identified galaxies over the redshift range 0.5 < z < 0.9. Using non-parametric rank correlation tests, we searched for correlations of the absorption strengths, saturation, and line-of-sight kinematics with the galaxy redshifts, rest frame B and K luminosities, rest colors, and impact parameters D. We found no correlations at the 2.5-sigma level between these properties. Of primary significance is the fact that the QSO-galaxy impact parameter apparently does not provide the primary distinguishing factor by which absorption properties can be characterized. The galaxy absorption properties exhibit a large scatter, which, we argue, is suggestive of a picture in which the gas arises from a variety of on-going dynamical events. Inferences from our study include: (1) The spatial distribution of absorbing gas in galaxies does not appear to follow a simple galactocentric functional dependence. (2) A single systematic kinematic model apparently cannot describe the observed velocity spreads in the absorbing gas. It is more that a heterogeneous population of sub-galaxy scale structures are giving rise to the observed cloud velocities. (3) The absorbing gas spatial distribution and kinematics may depend upon gas producing events and mechanisms that are recent to the epoch at which the absorption is observed. These distributions likely change over a few Gyr timescale. Based upon these inferences, we note that any evolution in the absorption gas properties over a larger redshift range should be directly quantifiable from a larger dataset of high-resolution absorption profiles.

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arXiv1996

The Halo from the Outside In: QSO Absorption Lines

Studies of high resolution quasar absorption lines (QALs), arising from gas in local and z < 1.5 galaxies, provide direct probes of the kinematic, chemical, and ionization conditions and evolution of Milky Way-like galaxies. Inferences drawn from these studies provide a powerful analog for those based upon direct studies of the Halo. The observed gas kinematics and velocity evolution reveal that galactic halos are complex dynamical components, consistent with Searle--Zinn formation scenarios.

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