1308.2670
Spitzer, Gaia, and the Potential of the Milky Way
Price-Whelan, Johnston
Near-future data from ESA's Gaia mission will provide precise, full phase-space information for hundreds of millions of stars out to heliocentric distances of ~10 kpc [a good chunk of the MW]. This "horizon" for full phase-space measurements is imposed by the Gaia parallax errors degrading to worse than 10%, and could be significantly extended by an accurate distance indicator. Recent work has demonstrated how Spitzer observations of RR Lyrae stars can be used to make distance estimates accurate to 2%, effectively extending the Gaia, precise-data horizon by a factor of ten in distance and a factor of 1000 in volume. This Letter presents one approach to exploit data of such accuracy to measure the Galactic potential using small samples of stars associated with debris from satellite [satellite galaxies] destruction. The method is tested with synthetic observations of 100 stars from the end point of a simulation of satellite destruction: the shape, orientation, and depth of the potential used in the simulation are recovered to within a few percent. The success of this simple test with such a small sample in a single debris stream [I guess this stream probes the MW gravitational potential?] suggests that constraints from multiple streams could be combined to examine the Galaxy's DM halo in even more detail -- a truly unique opportunity that is enabled by the combination of Spitzer and Gaia with an intimate perspective on the Galaxy.
1308.2682
The stellar halos of massive elliptical galaxies II: detailed abundance ratios at large radius
Greene, Murphy, Graves, Gunn, Raskutti, Comerford, Gebhardt
Observations consistent with a picture in which the stellar outskirts are built up through minor mergers with disky galaxies whose SF is truncated early (z~1.5-2). Study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions sigma*>150 km/s. Measure stellar population properties in composite spectra, and use ratios of these composites to highlight the largest spectral changes as a function of radius. Based on stellar population modeling, The typical star at 2 R_e is old (~10 Gyr), relatively metal poor ([Fe/H]-0.5), and alpha-enhanced ([Mg/Fe]~0.3). The stars were made rapidly at z~1.5-2 in shallow potential wells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise from rapid SF timescales due to declining carbon yields from low-metallicity massive stars. In contrast, [N/Fe] remains high at large radius. Stars at large radius have different abundance ratio patterns from stars in the center of any present-day galaxy, but are similar to MW thick disk stars.
1308.2869
Do intermediate-mass black holes exist in globular clusters?
Sun, et al
Current radio observations (discrepancy between radio observation and dynamical modeling) cannot rule out the possibility that IMBHs may exist in GCs.
1308.2964
The formation of eccentric compact binary inspirals and the role of gravitational wave emission in binary-single stellar encounters
Samsing, MacLeod, Ramirez-Ruiz
The insprial and merger of eccentric binaries leads to gravitational waveforms distinct from those generated by circularly merging binaries. Study binary-single stellar scatterings occurring in dense stellar systems as a source of eccentrically-inspiraling binaries. Many interactions between compact binaries and single objects are characterized by chaotic resonances in which the binary-single system undergoes many exchanges before reaching a final state. During these chaotic resonances, a pair of objects has a non-negligible probability of experiencing a very close passage. Significant orbital energy and angular momentum are carried away from the system by GW radiation in these close passages and in some cases this implies an inspiral time shorter than the orbital period of the bound third body. Derive the cross section for such dynamical inspiral outcomes through analytical arguments and through numerical scattering experiments including GW losses. Show that the cross section for dynamical inspirals grows with increasing target binary semi-major axis, a, and that for equal-mass binaries it scales as a^2/7. Thus, expect wide target binaries to predominantly contribute to the production of these relativistic outcomes. Estimate that eccentric inspirals account for approximately one percent of dynamically assembled non-eccentric merging binaries. While these events are rare, show that binary-single scatterings are a more effective formation channel than single-single captures for the production of eccentrically-inspiraling binaries, even given modest binary fractions.
1308.2974
Evolution of the stellar-to-dark matter relation: separating star-forming and passive galaxies from z=1 to 0
Tinker, Leauthaud, Bundy, George, Behroozi, Massey, Rhodes, Wechsler
Use measurements of stellar MF, galaxy clustering, and gg lensing within COSMOS to constrain the stellar-to-halo mass relation (SHMR) of SF and quiescent galaxies over the redshift range z=0.2-1.0. For massive galaxies, M*>1e10.6 Msun, results indicate that SF galaxies grow proportionately as fast as their DM haloes while quiescent galaxies are outpaced by DM growth. At lower masses, there is minimal difference in the SHMRs, implying that the majority low-mass quiescent galaxies have only recently been quenched of their SF. Analysis also affords a breakdown of all COSMOS galaxies into the relative numbers of central and satellite galaxies for both populations. At z=1, satellite galaxies dominate the red sequence below the knee in the stellar mass function. But the number of quiescent satellites exhibits minimal redshift evolution; all evolution in the red sequence is due to low-mass central galaxies being quenched of their star formation. At M*~1e10 Msun, the fraction of central galaxies on the red sequence increases by a factor of ten over redshift baseline, while the fraction of quenched satellite galaxies at that mass is constant with redshift. Define a "migration rate" to the red sequence as the time derivative of the passive galaxy abundances. Find that the migration rate of central galaxies to the red sequence increases by nearly an order of magnitude from z=1 to 0. These results imply that the efficiency of quenching SF for centrals is increasing with cosmic time, while the mechanisms that quench the SF of satellite galaxies in groups and clusters is losing efficiency.
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