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Notes on the Missing Satellites Problem  [PDF]
James S. Bullock
Physics , 2010,
Abstract: The Missing Satellites Problem (MSP) broadly refers to the overabundance of predicted Cold Dark Matter (CDM) subhalos compared to satellite galaxies known to exist in the Local Group. The most popular interpretation of the MSP is that the smallest dark matter halos in the universe are extremely inefficient at forming stars. The question from that standpoint is to identify the feedback source that makes small halos dark and to identify any obvious mass scale where the truncation in the efficiency of galaxy formation occurs. Among the most exciting developments in near-field cosmology in recent years is the discovery of a new population satellite galaxies orbiting the Milky Way and M31. Wide field, resolved star surveys have more than doubled the dwarf satellite count in less than a decade, revealing a population of ultrafaint galaxies that are less luminous that some star clusters. For the first time, there are empirical reasons to believe that there really are missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to have been surveyed. Both kinematic studies and completeness-correction studies seem to point to a characteristic potential well depth for satellite subhalos that is quite close to the mass scale where photoionization and atomic cooling should limit galaxy formation. Among the more pressing problems associated with this interpretation is to understand the selection biases that limit our ability to detect the lowest mass galaxies. The least massive satellite halos are likely to host stealth galaxies with very-low surface brightness and this may be an important limitation in the hunt for low-mass fossils from the epoch of reionization.
Galaxy Formation: Warm Dark Matter, Missing Satellites, and the Angular Momentum Problem  [PDF]
Martin Goetz,Jesper Sommer-Larsen
Physics , 2002, DOI: 10.1023/A:1024073909753
Abstract: We present warm dark matter (WDM) as a possible solution to the missing satellites and angular momentum problem in galaxy formation and introduce improved initial conditions for numerical simulations of WDM models, which avoid the formation of unphysical haloes found in earlier simulations. There is a hint, that the mass function of satellite haloes has been overestimated so far, pointing to higher values for the WDM particle mass.
The Missing Mass Problem  [PDF]
Angel Fierros Palacios
Journal of High Energy Physics, Gravitation and Cosmology (JHEPGC) , 2016, DOI: 10.4236/jhepgc.2016.23026
Abstract: In this paper a possible solution to the missing mass problem is proposed.
Mechanisms of Baryon Loss for Dark Satellites in Cosmological SPH Simulations  [PDF]
S. Nickerson,G. Stinson,H. M. P. Couchman,J. Bailin,J. Wadsley
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.18700.x
Abstract: We present a study of satellites in orbit around a high-resolution, smoothed particle hydrodynamics (SPH) galaxy simulated in a cosmological context. The simulated galaxy is approximately the same mass as the Milky Way. The cumulative number of luminous satellites at z = 0 is similar to the observed system of satellites orbiting the Milky Way although an analysis of the satellite mass function reveals an order of magnitude more dark satellites than luminous. Some of the dark subhalos are more massive than some of the luminous subhalos at z = 0. What separates luminous and dark subhalos is not their mass at z = 0, but the maximum mass the subhalos ever achieve. We study the effect of four mass loss mechanisms on the subhalos: ultraviolet (UV) ionising radiation, ram pressure stripping, tidal stripping, and stellar feedback, and compare the impact of each of these four mechanisms on the satellites. In the lowest mass subhalos, UV is responsible for the majority of the baryonic mass loss. Ram pressure stripping removes whatever mass remains from the low mass satellites. More massive subhalos have deeper potential wells and retain more mass during reionisation. However, as satellites pass near the centre of the main halo, tidal forces cause significant mass loss from satellites of all masses. Satellites that are tidally stripped from the outside can account for the luminous satellites that are lower mass than some of the dark satellites. Stellar feedback has the greatest impact on medium mass satellites that had formed stars, but lost all their gas by z = 0. Our results demonstrate that the missing satellite problem is not an intractable issue with the cold dark matter cosmology, but is rather a manifestation of baryonic processes.
Halo expansion in cosmological hydro simulations: towards a baryonic solution of the cusp/core problem in massive spirals  [PDF]
Andrea V. Maccio',Greg Stinson,Chris B. Brook,James Wadsley,H. M. P. Couchman,Sijing Shen,Brad K. Gibson,Tom Quinn
Physics , 2011, DOI: 10.1088/2041-8205/744/1/L9
Abstract: A clear prediction of the Cold Dark Matter model is the existence of cuspy dark matter halo density profiles on all mass scales. This is not in agreement with the observed rotation curves of spiral galaxies, challenging on small scales the otherwise successful CDM paradigm. In this work we employ high resolution cosmological hydro-dynamical simulations to study the effects of dissipative processes on the inner distribution of dark matter in Milky-Way like objects (M~1e12 Msun). Our simulations include supernova feedback, and the effects of the radiation pressure of massive stars before they explode as supernovae. The increased stellar feedback results in the expansion of the dark matter halo instead of contraction with respect to N-body simulations. Baryons are able to erase the dark matter cuspy distribution creating a flat, cored, dark matter density profile in the central several kpc of a massive Milky-Way like halo. The profile is well fit by a Burkert profile, with fitting parameters consistent with the observations. In addition, we obtain flat rotation curves as well as extended, exponential stellar disk profiles. While the stellar disk we obtain is still partially too thick to resemble the MW thin disk, this pilot study shows that there is enough energy available in the baryonic component to alter the dark matter distribution even in massive disc galaxies, providing a possible solution to the long standing problem of cusps vs. cores.
An assessment of the "too big to fail" problem for field dwarf galaxies in view of baryonic feedback effects  [PDF]
Emmanouil Papastergis,Francesco Shankar
Physics , 2015,
Abstract: Recent studies have established that extreme dwarf galaxies --whether satellites or field objects-- suffer from the so called "too big to fail" (TBTF) problem. Put simply, the TBTF problem consists of the fact that it is difficult to explain both the measured kinematics of dwarfs and their observed number density within the LCDM framework. The most popular proposed solutions to the problem involve baryonic feedback processes. For example, reionization and baryon depletion can decrease the abundance of halos that are expected to host dwarf galaxies. Moreover, feedback related to star formation can alter the dark matter density profile in the central regions of low-mass halos. In this article we assess the TBTF problem for field dwarfs, taking explicitly into account the baryonic effects mentioned above. We find that 1) reionization feedback cannot resolve the TBTF problem on its own, because the halos in question are too massive to be affected by it, and that 2) the degree to which profile modification can be invoked as a solution to the TBTF problem depends on the radius at which galactic kinematics are measured. Based on a literature sample of about 90 dwarfs with interferometric observations in the 21cm line of atomic hydrogen (HI), we conclude that the TBTF problem persists despite baryonic effects. However, the preceding statement assumes that the sample under consideration is representative of the general population of field dwarfs. In addition, the unexplained excess of dwarf galaxies in LCDM could be as small as a factor of ~ 1.8, given the current uncertainties in the measurement of the galactic velocity function. Both of these caveats highlight the importance of upcoming uniform surveys with HI interferometers for advancing our understanding of the issue.
Dwarf Cosmology with the Stromlo Missing Satellites Survey  [PDF]
Helmut Jerjen
Advances in Astronomy , 2010, DOI: 10.1155/2010/434390
Abstract: The standard Lambda Cold Dark Matter model is considered to be a triumph of theoretical astrophysics but observations of the Milky Way and its system of satellite galaxies irresistibly signal that theory is incomplete on galactic and subgalactic scales. The Stromlo Missing Satellites (SMS) Survey is a critical endeavor to investigate at what level predictions of CDM cosmology are consistent with the observed matter distribution in the Milky Way halo. It will be the deepest, most extended search for optically elusive satellite galaxies to date, covering 20?000 square degrees of sky. The international SMS Survey collaboration will exploit 150 TB of CCD images in six filters acquired by the new SkyMapper telescope of the Australian National University over the next five years, expecting on completion photometric limits 0.5–1.0 mag fainter than the Sloan Digital Sky Survey. The primary objective of the program is to characterise the baryonic and dark matter components of a complete sample of MW satellites in the Southern hemisphere to provide stringent observational constraints for improving our understanding of how the Milky Way formed and what physical processes governed galaxy formation and evolution in general. 1. Satellite Galaxies According to cosmological theory, overdensities of cold dark matter gravitationally collapsed and formed the first structures in the Universe one billion years after the Big Bang. The gravitational pull of those dark matter clumps drew in primordial baryons in the form of hydrogen gas, providing the seeds for galaxy formation. The observational Universe today is populated with galaxies, the prime repositories of stars, the shining baryonic matter. For obvious reasons, most of the detected and catalogued galaxies are intrinsically the largest and the brightest, those that can be seen from the greatest distance and are most easily studied against the night sky. Ironically, a major limitation on our ability to develop a physically consistent model that describes how galaxies evolved out of the dark matter and baryonic ingredients comes from our incomplete picture of the vicinity of the Milky Way, in particular from the lack of a good understanding of the phenomenon “satellite galaxies." This term is broadly used for dwarf galaxy companions of the Milky Way (Figure 1), some of which contain only a few thousand stars. They cover a stellar mass range of and consist of up to 99.9% of dark matter (e.g., [1–7]). At the low end of the mass scale, satellites exhibit extreme low star densities so they are completely resolved, that is,
Where are the missing galactic satellites?  [PDF]
Anatoly A. Klypin,Andrey V. Kravtsov,Octavio Valenzuela,Francisco Prada
Physics , 1999, DOI: 10.1086/307643
Abstract: Using published data, we have compiled the circular velocity (Vc) distribution function (VDF) of galaxy satellites in the Local Group. We find that within the volumes of radius of 570 kpc (400/h kpc for h=0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as n(>Vc)~ 45(Vc/10 km/s)^{-1} h^3 Mpc^{-3} for Vc in the range ~10-70 km/s. The observed VDF is compared with results of high-resolution cosmological simulations. We find that the VDF in models is very different from the observed one: n(>Vc)~1200(Vc/10 km/s)^{-2.75}h^3 Mpc^{-3}. Cosmological models thus predict that a halo of the size of our Galaxy should have about 50 dark matter satellites with circular velocity >20 km/s and mass >3x10^8/h Msun within a 570 kpc radius. This number is significantly higher than the approximate dozen satellites actually observed around our Galaxy. The observed and predicted VDFs cross at ~50 km/s, indicating that the predicted abundance of satellites with Vc> 50 km/s is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identification of some satellites with the High Velocity Clouds observed in the Local Group, and the existence of dark satellites that failed to accrete gas and form stars due either to the expulsion of gas in the supernovae-driven winds or to gas heating by the intergalactic ionizing background. (Abridged)
The Missing Massive Satellites of the Milky Way  [PDF]
Jie Wang,Carlos S. Frenk,Julio F. Navarro,Liang Gao,Till Sawala
Physics , 2012, DOI: 10.1111/j.1365-2966.2012.21357.x
Abstract: Recent studies suggest that only three of the twelve brightest satellites of the Milky Way (MW) inhabit dark matter halos with maximum circular velocity, V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM simulations of the Aquarius Project, which suggest that MW-sized halos should have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites in such massive subhalos is thus puzzling and may present a challenge to the LCDM paradigm. We note, however, that the number of massive subhalos depends sensitively on the (poorly-known) virial mass of the Milky Way, and that their scarcity makes estimates of their abundance from a small simulation set like Aquarius uncertain. We use the Millennium Simulation series and the invariance of the scaled subhalo velocity function (i.e., the number of subhalos as a function of \nu, the ratio of subhalo V_max to host halo virial velocity, V_200) to secure improved estimates of the abundance of rare massive subsystems. In the range 0.1<\nu<0.5, N_sub(>\nu) is approximately Poisson-distributed about an average given by =10.2x(\nu/0.15)^(-3.11). This is slightly lower than in Aquarius halos, but consistent with recent results from the Phoenix Project. The probability that a LCDM halo has 3 or fewer subhalos with V_max above some threshold value, V_th, is then straightforward to compute. It decreases steeply both with decreasing V_th and with increasing halo mass. For V_th=30km/s, ~40% of M_halo=10^12 M_sun halos pass the test; fewer than 5% do so for M_halo>= 2x10^12 M_sun; and the probability effectively vanishes for M_halo>= 3x 10^12 M_sun. Rather than a failure of LCDM, the absence of massive subhalos might simply indicate that the Milky Way is less massive than is commonly thought.
Influence of baryonic physics in galaxy simulations: a semi-analytic treatment of the molecular component  [PDF]
A. Halle,F. Combes
Physics , 2012, DOI: 10.1051/0004-6361/201220952
Abstract: Recent work in galaxy formation has enlightened the important role of baryon physics, to solve the main problems encountered by the standard theory at the galactic scale, such as the galaxy stellar mass functions, or the missing satellites problem. The present work aims at investigating in particular the role of the cold and dense molecular phase, which could play a role of gas reservoir in the outer galaxy discs, with low star formation efficiency. Through TreeSPH simulations, implementing the cooling to low temperatures, and the inclusion of the molecular hydrogen component, several feedback efficiencies are studied, and results on the gas morphology and star formation are obtained. It is shown that molecular hydrogen allows some slow star formation (with gas depletion times of about 5 Gyr) to occur in the outer parts of the discs. This dense and quiescent phase might be a way to store a significant fraction of dark baryons, in a relatively long time-scale, in the complete baryonic cycle, connecting the galaxy discs to hot gaseous haloes and to the cosmic filaments.
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