1711.05272
The stellar populations of two ultra-diffuse galaxies from optical and near-infrared photometry
Pandya, et al
Present observational constraints on the stellar populations of 2 UDGs using optical through NIR data, from Spitzer IRAC 3.6um and 4.5um imaging, archival optical imaging, and with a Bayesian SED fitting framework. 3 samples : 1 field UDG, 1 cluster UDG, and one cluster dwarf elliptical. All 3 galaxies have NIR colors that are significantly different form each other. The Virgo UDG is old (~7.7 Gyr) and significantly metal poor (Z/Zsun <~ -1.0). The field UDG is probably younger than the Virgo UDG, with an extended SFH and an age posterior extending down to ~3 Gyr. The stellar metallicity of the field UDG is sub-solar, but higher than that of the Virgo UDG, with Z/Zsun=-0.63+0.35-0.62; in the case of exactly zero diffuse interstellar dust, the field UDG may even have solar metallicity. The spectroscopically confirmed globular clusters of the Virgo UDG have similar optical-NIR colors as the UDG itself, with empirical color relations suggesting sub-solar metallicities and supporting the metal-poor nature of it. With it and several Coma UDGs, a general picture is emerging where cluster UDGs may be "failed" galaxies, but the field UDG seems more consistent with a stellar feedback-induced expansion scenario.
1711.05276
Disruption of dark matter substructure: fact or fiction?
van den Bosch, Ogiya, Hahn, Burkert
Use both analytical estimates and idealized numerical simulations to investigate whether the DM substructure disruption in N-body sims is mainly physical (tidal heating and stripping) or numerical (i.e., artificial). Show that, contrary to naive expectation, sub haloes that experience a tidal shock Delta E that exceeds the smbhalo's binding energy, E_b, do not undergo disruption, even when Delta E/E_b is as large as 100. Along the same line, and contrary to existing claims in the literature, instantaneously stripping matter from the outskirts of a DM subhalo also does not result in its complete disruption, even when the instantaneous remnant has positive binding energy. In addition, show that today heating due to high-speed (impulsive) encounters with other sub haloes ('harassment'), is negligible compared to the tidal effects due to the host halo. Hence conclude that in the absence of baryonic processes, the complete, physical disruption of CDM substructure is extremely rare, and that most disruption in numerical simulations therefore must be artificial. Discuss various processes that have been associated with numerical over merging, and conclude that inadequate force-softening is the most likely culprit.
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