Nuclear Star Clusters and Bulges

Nuclear star clusters are among the densest stellar systems known and are common in both early- and late-type galaxies. They exhibit scaling relations with their host galaxy which may be related to those of supermassive black holes. These may therefore help us to unravel the complex physical processes occurring at the centres of galaxies. The properties of nuclear stellar systems suggest that their formation requires both dissipational and dissipationless processes. They have stellar populations of different ages, from stars as old as their host galaxy to young stars formed in the last 100 Myr. Therefore star formation must be happening either directly in the nuclear star cluster or in its vicinity. The secular processes that fuel the formation of pseudobulges very likely also contributes to nuclear star cluster growth.Comment: To appear in "Galactic Bulges", E. Laurikainen, R.F. Peletier, and D.A. Gadotti (eds.), Springer Publishin

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

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

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

Double barred galaxies at intermediate redshifts: A feasibility study

Despite the increasing number of studies of barred galaxies at intermediate and high redshifts, double-barred (S2B) systems have only been identified in the nearby (z<0.04) universe thus far. In this feasibility study we demonstrate that the detection and analysis of S2Bs is possible at intermediate redshifts (0.1 < z < 0.5) with the exquisite resolution of the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). We identify barred galaxies in the HST/ACS data of the Great Observatories Origins Deep Survey (GOODS) using a novel method. The radial profile of the Gini coefficient -- a model-independent structure parameter -- is able to detect bars in early-type galaxies that are large enough that they might host an inner bar of sufficient angular size. Using this method and subsequent examination with unsharp masks and ellipse fits we identified the two most distant S2Bs currently known (at redshifts z=0.103 and z=0.148). We investigate the underlying stellar populations of these two galaxies through a detailed colour analysis, in order to demonstrate the analysis that could be performed on a future sample of intermediate-redshift S2Bs. We also identify two S2Bs and five S2B candidates in the HST/ACS data of the Cosmic Evolution Survey (COSMOS). Our detections of distant S2Bs show that deep surveys like GOODS and COSMOS have the potential to push the limit for S2B detection and analysis out by a factor of ten in redshift and lookback time (z=0.5, t=5Gyr) compared to the previously known S2Bs. This in turn would provide new insight into the formation of these objects.Comment: 9 pages + 10 figures. Accepted for publication in MNRAS. Main change from version 1 is an extension of the introduction/motivation and discussion section. A full resolution version including colour figures is available at http://www.astro.unibas.ch/~tlisker/papers/lisker2006_s2b.pd

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

Bulges or Bars from Secular Evolution?

We use high resolution collisionless $N$-body simulations to study the secular evolution of disk galaxies and in particular the final properties of disks that suffer a bar and perhaps a bar-buckling instability. Although we find that bars are not destroyed by the buckling instability, when we decompose the radial density profiles of the secularly-evolved disks into inner S\'ersic and outer exponential components, for favorable viewing angles, the resulting structural parameters, scaling relations and global kinematics of the bar components are in good agreement with those obtained for bulges of late-type galaxies. Round bulges may require a different formation channel or dissipational processes.Comment: Accepted to ApJL. 4 figures, 2 in color Corrected minor typos and reference lis