1508.04138
The 21-cm BAO signature of enriched low-mass galaxies during cosmic reionization
Cohen, Fialkov, Barkana
Studies of the formation of the first stars have established that they formed in small haloes of ~1e5-6 Msun via molecular H cooling. Since a low level of UV radiation from stars suffices to dissociate H2, under the usually-assumed scenario this primordial mode of SF ended by z~15 and much more massive haloes came to dominate star formation. However, metal enrichment from the first stars may have allowed the smaller haloes to continue to form stars efficiently, a possibility that has been boosted by recent numerical simulations. In this Letter, explore the possible effect of SF in metal-rich low-mass haloes on the redshifted 21-cm signal of H2 from z=6-40. These haloes are significantly affected by the supersonic streaming velocity, with its characteristic BAO signature. Show that enrichment of low-mass galaxies can produce a strong signature in the 21-cm PS over a wide range of redshifts, and can allow the effect of the streaming velocity to survive until the midpoint of reoinization. The predictions, therefore, are relevant for current and upcoming radio telescopes.
1508.04143
Dark matter cores all the way down
Read, Agertz, Collins
Use high resolution sims of isolated dwarf galaxies to study the physics of DM cusp-core transformation at the edge of galaxy formation (Mvir=1e7-9 Msun). Work at a resolution (4pc) at which the impact from individual supernovae explosions can be resolved, becoming insensitive to even large changes in the numerical 'sub-grid' parameters. Find that the dwarf galaxies give a remarkable match to the stellar light profile; SFH; metallicity distribution function; and star/gas kinematics of isolated dwarf irregular galaxies. The key result is that DM cores of size comparable to the half light radius r_1/2 always form if SF proceeds for long enough. Cores fully form in less than 4 Gyrs for the Mvir=1e8Msun and 14 Gyrs for the 1e9 Msun dwarf. Provide a convenient two parameter 'coreNFW' fitting function that captures this DM core growth as a function of SF time and the projected HLR. The results have several important implications: (i) a strong prediction is made that if LCDM is correct, then 'pristine' DM cusps will be found either in systems that have truncated SF and/or at radii r>r_1/2; (ii) complete core formation lowers the projected velocity dispersion at r_1/2 by a factor ~2, which is sufficient to fully explain the 'too big to fail problem' (though it is stressed that a full solution likely also involves unmodelled environmental effects); and (iii) cored dwarfs will be much more susceptible to tides, leading to a dramatic scouring of the sub halo mass function inside galaxies and groups. Explore such environmental effects in a forthcoming paper.
1508.04149
Investigating galaxy-filament alignments in hydrodynamic simulations using density ridges
Chen, Ho, et al
Study the filamentary structures and the galaxy alignment along filaments at z=0.06 in the MassiveBlack-II simulation, a state-of-the-art, high-res hydro cosmo sims which includes stellar and AGN feedback in a volume of (100 Mpc/h)^3. The filaments are constructed using the subspace constrained mean shift (SCMS). First, show that reconstructed filaments using galaxies and reconstructed filaments using DM particles are similar to each other; over 50% of the points on the galaxy filaments have corresponding point on the DM filaments within distance 0.13 Mpc/h (and vice versa) and this distance is even smaller at high-density regions. Second, observe the alignment of the major principal axis of a galaxy with respect to the orientation of its nearest filament and detect a 2.5 Mpc/h critical radius for filament's influence on the alignment when the sub halo mass of this galaxy is between 1e9Msun/h and 1e12 Msun/h. Moreover, find the alignment signal to increase significantly with the sub halo mass. Third, when a galaxy is close to filaments (less than 0.25 Mpc/h), the galaxy alignment toward the nearest galaxy group depends on the galaxy sub halo mass. Finally, find that galaxies close to filaments or groups tend to be rounder than those away from filaments or groups.
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