The Milky Way's bright satellites as an apparent failure of LCDM

TL;DR

High-resolution Aquarius simulations show the brightest Milky Way dSphs live in relatively low-mass halos with $V_{ m max}\lesssim 25\,\mathrm{km\,s^{-1}}$, but LCDM predicts many denser subhalos with $V_{ m max}>25\,\mathrm{km\,s^{-1}}$ that cannot host these dwarfs. A Bayesian analysis links each dSph to halos with $V_{ m max}\sim 12$–$24\,\mathrm{km\,s^{-1}}$, $V_{\rm infall}\sim14$–$26\,\mathrm{km\,s^{-1}}$, and $M_{\rm infall}\sim10^{8}$–$10^{9}\,M_\odot$, leaving numerous massive subhalos unaccounted for as luminous satellites. The study argues that neither lowering the Milky Way’s mass nor standard baryonic feedback fully resolves the mismatch between observed densities and LCDM predictions, and it discusses stochastic galaxy formation and alternative DM physics as possible avenues, while noting that existing evidence remains inconclusive. The work highlights a persistent small-scale challenge to CDM, suggesting that a combination of modestly altered halo demographics, stochastic star formation, or new DM physics may be needed to reconcile theory with the observed MW satellite system.

Abstract

We use the Aquarius simulations to show that the most massive subhalos in galaxy-mass dark matter halos in LCDM are grossly inconsistent with the dynamics of the brightest Milky Way dwarf spheroidal galaxies. While the best-fitting hosts of the dwarf spheroidals all have 12 < Vmax < 25 km/s, LCDM simulations predict at least ten subhalos with Vmax > 25 km/s. These subhalos are also among the most massive at earlier times, and significantly exceed the UV suppression mass back to z ~ 10. No LCDM-based model of the satellite population of the Milky Way explains this result. The problem lies in the satellites' densities: it is straightforward to match the observed Milky Way luminosity function, but doing so requires the dwarf spheroidals to have dark matter halos that are a factor of ~5 more massive than is observed. Independent of the difficulty in explaining the absence of these dense, massive subhalos, there is a basic tension between the derived properties of the bright Milky Way dwarf spheroidals and LCDM expectations. The inferred infall masses of these galaxies are all approximately equal and are much lower than standard LCDM predictions for systems with their luminosities. Consequently, their implied star formation efficiencies span over two orders of magnitude, from 0.2% to 20% of baryons converted into stars, in stark contrast with expectations gleaned from more massive galaxies. We explore possible solutions to these problems within the context of LCDM and find them to be unconvincing. In particular, we use controlled simulations to demonstrate that the small stellar masses of the bright dwarf spheroidals make supernova feedback an unlikely explanation for their low inferred densities.

Figures (10)

• Figure 1: Observed $V_{\rm{circ}}$ values of the nine bright dSphs (symbols, with sizes proportional to $\log \, L_V$), along with rotation curves corresponding to NFW subhalos with $V_{\rm{max}}=(12, \,18, \, 24, \, 40)\, {\rm km \, s}^{-1}$. The shading indicates the $1\, \sigma$ scatter in $r_{\rm{max}}$ at fixed $V_{\rm{max}}$ taken from the Aquarius simulations. All of the bright dSphs are consistent with subhalos having $V_{\rm{max}} \leq 24 \,{\rm km \, s}^{-1}$, and most require $V_{\rm{max}} \lesssim 18\,{\rm km \, s}^{-1}$. Only Draco, the least luminous dSph in our sample, is consistent (within $2 \sigma$) with a massive CDM subhalo of $\approx 40\,{\rm km \, s}^{-1}$ at $z=0$.
• Figure 2: Left panel: circular velocity profiles at redshift zero for subhalos of the Aquarius B halo (top; $M_{\rm{vir}} = 9.5 \times 10^{11}\,M_{\odot}$) and E halo (bottom; $M_{\rm{vir}}=1.4 \times 10^{12}\,M_{\odot}$) that have $V_{\rm infall} > 30\,{\rm km \, s}^{-1}$ and $V_{\rm{max}}(z=0) > 10\,{\rm km \, s}^{-1}$ (excluding MC candidates). Measured $V_{\rm{circ}}(r_{1/2})$ values for the MW dSphs are plotted as data points with error bars. Each subsequent panel shows redshift zero rotation curves for subhalos from the left panel with the ten highest values of $V_{\rm{max}}(z=0)$ (second panel), $V_{\rm infall}$ (third panel), or $V_{\rm{max}}(z=10)$ (fourth panel). In none of the three scenarios are the most massive subhalos dynamically consistent with the bright MW dSphs: there are always several subhalos more massive than all of the MW dSphs. (Analogous results are found for the other four halos.)
• Figure 3: Rotation curves for all subhalos with $V_{\rm infall} > 30\,{\rm km \, s}^{-1}$ and $V_{\rm{max}} > 10 \,{\rm km \, s}^{-1}$, after excluding Magellanic Cloud analogs, in each of the six Aquarius simulations (top row, from left to right: A, B, C; bottom row: D, E, F). Subhalos that are at least $2\,\sigma$ denser than every bright MW dwarf spheroidal are plotted with solid curves, while the remaining subhalos are plotted as dotted curves. Data points with errors show measured $V_{\rm{circ}}$ values for the bright MW dSphs. Not only does each halo have several subhalos that are too dense to host any of the dSphs, each halo also has several massive subhalos (nominally capable of forming stars) with $V_{\rm{circ}}$ comparable to the MW dSphs that have no bright counterpart in the MW. In total, between 7 and 22 of these massive subhalos are unaccounted for in each halo.
• Figure 4: The median mass of $z=0$ subhalos having $V_{\rm infall} > 30 \,{\rm km \, s}^{-1}$ (excluding MC analogs) as function of redshift (solid curve), along with the 68% confidence range, symmetric about the median (shaded region). The hatched region marked "UV-suppressed" shows where halos are expected to have lost at least 50% of their baryonic mass owing to the UV background okamoto2008. Subhalos at $z=0$ having $V_{\rm infall} > 30 \,{\rm km \, s}^{-1}$ are more massive than the photo-suppression scale at all redshifts.
• Figure 5: The distribution of masses at $z=6$ for $z=0$ subhalos with $V_{\rm infall} > 30\,{\rm km \, s}^{-1}$ (excluding MC candidates). The open black histogram shows the "massive failures" (subhalos that are $2\,\sigma$ more dense than all of the MW's bright dSphs), while the filled gray histogram shows the remaining subhalos, which we deem to be potentially luminous satellites. Even at $z=6$, the massive failures are typically more massive than subhalos consistent with the bright dSphs. They are also all more massive than characteristic scale below which the UV background significantly affects the baryon content of halos (hatched region). At $z=6$, this characteristic mass is comparable to $T_{\rm{vir}}=10^{4}\,{\rm K}$, the threshold for atomic cooling.