We present a new analytic estimate for the energy required to create a constant density core within a dark matter halo. Our new estimate, based on more realistic assumptions, leads to a required energy that is orders of magnitude lower than is claime
d in earlier work. We define a core size based on the logarithmic slope of the dark matter density profile so that it is insensitive to the functional form used to fit observed data. The energy required to form a core depends sensitively on the radial scale over which dark matter within the cusp is redistributed within the halo. Simulations indicate that within a region of comparable size to the active star forming regions of the central galaxy that inhabits the halo, dark matter particles have their orbits radially increased by a factor of 2--3 during core formation. Thus the inner properties of the dark matter halo, such as halo concentration, and final core size, set the energy requirements. As a result, the energy cost increases slowly with halo mass as M$_{rm{h}}^{0.3-0.7}$ for core sizes $lesssim1$ kpc. We use the expected star formation history for a given dark matter halo mass to predict dwarf galaxy core sizes. We find that supernovae alone would create well over 4 kpc cores in $10^{10}$ M$_{odot}$ dwarf galaxies emph{if} 100% of the energy were transferred to dark matter particle orbits. We can directly constrain the efficiency factor by studying galaxies with known stellar content and core size, such as Fornax. We find that the efficiency of coupling between stellar feedback and dark matter orbital energy need only be at the 1% level or less to explain Fornaxs 1 kpc core.
We present a new model for the formation of stellar halos in dwarf galaxies. We demonstrate that the stars and star clusters that form naturally in the inner regions of dwarfs are expected to migrate from the gas rich, star forming centre to join the
stellar spheroid. For dwarf galaxies, this process could be the dominant source of halo stars. The effect is caused by stellar feedback-driven bulk motions of dense gas which, by causing potential fluctuations in the inner regions of the halo, couple to all collisionless components. This effect has been demonstrated to generate cores in otherwise cuspy cold dark matter profiles and is particularly effective in dwarf galaxy haloes. It can build a stellar spheroid with larger ages and lower metallicities at greater radii without requiring an outside-in formation model. Globular cluster-type star clusters can be created in the galactic ISM and then migrate to the spheroid on 100thinspace Myr timescales. Once outside the inner regions they are less susceptible to tidal disruption and are thus long lived; clusters on wider orbits may be easily unbound from the dwarf to join the halo of a larger galaxy during a merger. A simulated dwarf galaxy ($text{M}_{vir}simeq10^{9}text{M}_{odot}$ at $z=5$) is used to examine this gravitational coupling to dark matter and stars.
White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff<6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms s
hortward of the bandheads of the C_2 Swan bands. These peculiar DQ bands have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere. The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behavior of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak>45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.
