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Observable Properties Of Double-Barred Galaxies In N-Body Simulations
Although at least one quarter of early-type barred galaxies host secondary stellar bars embedded in their large-scale primary counterparts, the dynamics of such double-barred galaxies are still not well understood. Recently we reported success at simulating such systems in a repeatable way in collisionless systems. In order to further our understanding of double-barred galaxies, here we characterize the density and kinematics of the N-body simulations of these galaxies. This will facilitate comparison with observations and lead to a better understanding of the observed double-barred galaxies. We find the shape and size of our simulated secondary bars are quite reasonable compared to the observed ones. We demonstrate that an authentic decoupled secondary bar may produce only a weak twist of the kinematic minor axis in the stellar velocity field, due to the relatively large random motion of stars in the central region. We also find that the edge-on nuclear bars are probably not related to boxy peanut-shaped bulges which are most likely to be edge-on primary large-scale bars. Another kinematic feature often present in our double-barred models is a ring-like feature in the fourth-order Gauss-Hermite moment h(4) maps. Finally, we demonstrate that the non-rigid rotation of the secondary bar causes its pattern speed to not be derived with great accuracy using the Tremaine-Weinberg method. We also compare with observations of NGC 2950, a prototypical double-barred early-type galaxy, which suggest that the nuclear bar may be rotating in the opposite sense as the primary.H.J.S. fellowshipUniversity of WashingtonNSF ITR PHY-0205413McDonald Observator
Spatially Resolved Spectroscopic Star Formation Histories of Nearby Disks: Hints of Stellar Migration
We use the Mitchell Spectrograph (formerly VIRUS-P) to observe 12 nearby disk
galaxies. We successfully measure ages in the outer disk in six systems. In
three cases (NGC 2684, NGC 6155, and NGC 7437), we find that a downward break
in the disk surface brightness profile corresponds with a change in the
dominant stellar population with the interior being dominated by active star
formation and the exterior having older stellar populations that are best-fit
with star formation histories that decline with time. The observed increase in
average stellar ages beyond a profile break is similar to theoretical models
that predict surface brightness breaks are caused by stellar migration, with
the outer disk being populated from scattered old interior stars. In three more
cases (IC 1132, NGC 4904, and NGC 6691), we find no significant change in the
stellar population as one crosses the break radius. In these galaxies, both the
inner and outer disks are dominated by active star formation and younger
stellar populations. While radial migration can contribute to the stellar
populations beyond the break, it appears more than one mechanism is required to
explain all of our observed stellar profile breaks.Comment: 18 pages, 21 figures. Accepted for publication in Ap
Large-Scale Model of the Milky Way: Stellar Kinematics and Microlensing Event Timescale Distribution in the Galactic Bulge
We build a stellar-dynamical model of the Milky Way barred bulge and disk,
using a newly implemented adaptive particle method. The underlying mass model
has been previously shown to match the Galactic near-infrared surface
brightness as well as gas-kinematic observations. Here we show that the new
stellar-dynamical model also matches the observed stellar kinematics in several
bulge fields, and that its distribution of microlensing event timescales
reproduces the observed timescale distribution of the {\it MACHO} experiment
with a reasonable stellar mass function. The model is therefore an excellent
basis for further studies of the Milky Way. We also predict the observational
consequences of this mass function for parallax shifted events.Comment: 13 pages, 3 figures. Accepted to ApJ
On position angle errors in the Tremaine-Weinberg method
I show that Tremaine-Weinberg (TW) measurements of bar pattern speeds are sensitive to errors in the position angle of the disc, PAdisc. I use an N-body experiment to measure these errors; for typical random PAdisc errors, the resulting scatter in the measured values of the dimensionless bar speed parameter (defined as the ratio of the corotation radius to the bar semi-major axis) is of the order of the scatter in the observed values. I also consider how the systematic PAdisc errors produced by disc ellipticities affect TW measurements. The scatter produced by these errors may be significant, depending on the ellipticity distribution. Conversely, by using the sample of TW observations, I find that an upper limit of the typical disc (density) ellipticity is 0.07 at the 90 per cent confidence level, which is in good agreement with previous measurements. Taken together, the random and systematic scatter suggest that the intrinsic distribution of of gas-poor early-type barred galaxies may be as narrow as that of the gas-rich later type
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