David R. Cole

Weakening dark matter cusps by clumpy baryonic infall

We consider the infall of a massive clump into a dark matter halo as a simple and extreme model for the effect of baryonic physics (neglected in gravity-only simulations of large-scale structure formation) on the dark matter. We find that such an infalling clump is extremely efficient in altering the structure of the halo and reducing its central density: a clump of 1 per cent the mass of the halo can remove about twice its own mass from the inner halo and transform a cusp into a core or weaker cusp. If the clump is subsequently removed, mimicking a galactic wind, the central halo density is further reduced and the mass removed from the inner halo doubled. Lighter clumps are even more efficient: the ratio of removed mass to clump mass increases slightly towards smaller clump masses. This process becomes more efficient the more radially anisotropic the initial dark matter velocities are. While such a clumpy infall may be somewhat unrealistic, it demonstrates that the baryons need to transfer only a small fraction of their initial energy to the dark matter via dynamical friction to explain the discrepancy between predicted dark matter density profiles and those inferred from observations of dark-matter-dominated galaxies.

The mass distribution of the Fornax dSph: constraints from its globular cluster distribution

Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical friction has not yet dragged any of Fornax’s five globular clusters to the centre, and also why there is no evidence that any similar star cluster has been in the past (for Fornax or any other tidally undisrupted dSph). We set up a suite of 2800 N-body simulations that sample the full range of globular cluster orbits and mass models consistent with all existing observational constraints for Fornax. In agreement with previous work, we find that if Fornax has a large dark matter core, then its globular clusters remain close to their currently observed locations for long times. Furthermore, we find previously unreported behaviour for clusters that start inside the core region. These are pushed out of the core and gain orbital energy, a process we call ‘dynamical buoyancy’. Thus, a cored mass distribution in Fornax will naturally lead to a shell-like globular cluster distribution near the core radius, independent of the initial conditions. By contrast, cold dark matter-type

YOUNG STARS IN AN OLD BULGE: A NATURAL OUTCOME OF INTERNAL EVOLUTION IN THE MILKY WAY

The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body+ smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look.

The formation of stellar nuclear discs in bar-induced gas inflows

ABSTRACT The role of gas in the mass assembly at the nuclei of galaxies is still subject to some un-certainty. Stellar nuclear discs bridge the gap between the large-scale galaxy and the centralmassive objects that reside there. Using a high resolution simulation of a galaxy forming outof gas cooling and settling into a disc, we study the formation and properties of nuclear discs.Gas, driven to the centre by a bar, settles into a rotating star-forming nuclear disc (ND). ThisND is thinner, younger, kinematically cooler, and more metal-rich than the surrounding bar.The ND is elliptical and orthogonal to the bar. The complex kinematics in the region of theND are a result of the superposition of older stars streaming along the bar and younger starscirculating within the ND. The signature of the ND is therefore subtle in the kinematics. In-stead the ND stands out clearly in metallicity and age maps. We compare the model to thedensity and kinematics of real galaxies with NDs finding qual itative similarities. Our resultssuggest that gas dissipation is very important for forming nuclear structures.Key words: galaxies: bulges — galaxies: evolution — galaxies:kinemat ics and dynamics —galaxies: nuclei —galaxies: structure

Thermal instabilities in cooling galactic coronae: fuelling star formation in galactic discs

We investigate the means by which cold gas can accrete onto Milky Way mass galaxies from a hot corona of gas, using a new smoothed particle hydrodynamics code, ‘SPHS’. We find that the ‘cold clumps’ seen in many classic SPH simulations in the literature are not present in our SPHS simulations. Instead, cold gas condenses from the halo along filaments that form at the intersection of supernovae-driven bubbles from previous phases of star formation. This positive feedback feeds cold gas to the galactic disc directly, fuelling further star formation. The resulting galaxies in the SPH and SPHS simulations differ greatly in their morphology, gas phase diagrams, and stellar content. We show that the classic SPH cold clumps owe to a numerical thermal instability caused by an inability for cold gas to mix in the hot halo. The improved treatment of mixing in SPHS suppresses this instability leading to a dramatically different physical outcome. In our highest resolution SPHS simulation, we find that the cold filaments break up into bound clumps that form stars. The filaments are overdense by a factor of 10-100 compared to the surrounding gas, suggesting that the fragmentation results from a physical non-linear instability driven by the overdensity. This ‘fragmenting filament’ mode of disc growth has important implications for galaxy formation, in particular the role of star formation in bringing cold gas into disc galaxies.

The mass distribution of the Fornax dSph: Constraints from its globular cluster distribution

Uniquely among the dwarf spheroidal (dSph) satellite galaxies of the Milky Way, Fornax hosts globular clusters. It remains a puzzle as to why dynamical friction has not yet dragged any of Fornax’s five globular clusters to the centre, and also why there is no evidence that any similar star cluster has been in the past (for Fornax or any other tidally undisrupted dSph). We set up a suite of 2800 N-body simulations that sample the full range of globular cluster orbits and mass models consistent with all existing observational constraints for Fornax. In agreement with previous work, we find that if Fornax has a large dark matter core, then its globular clusters remain close to their currently observed locations for long times. Furthermore, we find previously unreported behaviour for clusters that start inside the core region. These are pushed out of the core and gain orbital energy, a process we call ‘dynamical buoyancy’. Thus, a cored mass distribution in Fornax will naturally lead to a shell-like globular cluster distribution near the core radius, independent of the initial conditions. By contrast, cold dark matter-type

The effect of bars on the M•-σe relation: offset, scatter and residuals correlations

We analyse a set of collisionless disc galaxy simulations to study the consequences of bar formation and evolution on the M•-σe relation of supermassive black holes (SMBHs). The redistribution of angular momentum driven by bars leads to a mass increase within the central region, raising the velocity dispersion of the bulge, σe, on average by ˜12 per cent and as much as ˜20 per cent. If a disc galaxy with an SMBH satisfying the M•-σe relation forms a bar, and the SMBH does not grow in the process, then the increase in σe moves the galaxy off the M•-σe relation. We explore various effects that can affect this result including contamination from the disc and anisotropy. The displacement from the M•-σe relation for individual model barred galaxies correlates with both the bulge-to-total stellar mass ratio, M(B)/M(B + D), and the 2D anisotropy, βφ(B + D), both measured within the effective radius of the bulge. Overall, this process leads to an M•-σe for barred galaxies offset from that of unbarred galaxies, as well as an increase in its scatter. We assemble samples of observed unbarred and barred galaxies with classical bulges and find tentative hints of an offset between the two consistent with the predicted. Including all barred galaxies, rather than just those with a classical bulge, leads to a significantly larger offset, which is mostly driven by the significantly larger offset of pseudo bulges

A centrally heated dark halo for our Galaxy

We construct a new family of models of our Galaxy in which dark matter and disc stars are both represented by distribution functions that are analytic functions of the action integrals of motion. The potential that is self-consistently generated by the dark matter, stars and gas is determined, and parameters in the distribution functions are adjusted until the model is compatible with observational constraints on the circularspeed curve, the vertical density profile of the stellar disc near the Sun, the kinematics of nearly 200 000 giant stars within 2 kpc of the Sun, and estimates of the optical depth to microlensing of bulge stars. We find that the data require a dark halo in which the phase-space density is approximately constant for actions |J| ≲ 140 kpc km s−1 . In real space these haloes have core radii ≃ 2 kpc.

A Kiloparsec-Scale Nuclear Stellar Disk in the Milky Way as a Possible Explanation of the High Velocity Peaks in the Galactic Bulge

Gas in the Milky Way is driven inwards by its bar, some of it settling into a disk extending to Galactocentric radius

Spatial and kinematic segregation in star cluster merger remnants

\sim 1.4 \kpc