AST4320 - Cosmology and extragalactic astronomy Lecture 13 Structure of Dark Matter Halos & The Missing Satellites Problem. 1 AST4320 - Cosmology and extragalactic astronomy Structure of Dark Matter Halos & The Missing Satellites Problem. Outline The `Too-Big-to-Fail Problem’ the contemporary version of the missing satellite problem. Dark matter: why cold, why pressureless? Dark matter candidates / prospects for detection / possible detections? 2 Missing Satellite Problem (MSP) (see review by Weinberg et al. 2013, arXiv:1306.0913) 12 Left: simulated dark matter distribution in dark matter halo with M=10 Msun. Circles denote 9 most massive substructures or `satellites’. Right: Spatial distribution of observed Milky Way `satellites’. 3 Missing Satellite Problem (MSP) A quantitative comparison of # satellites at r < 400 kpc. Klypin et al. 1999 simulated Observed Number Discrepancy apparent at vcirc < 40 km/s. 10 At vcirc<40 km/s (or M < 10 Msun) feedback mechanisms become efficient. 4 Feedback Mechanism I: Photoionization Feedback. Observations indicate that gas in most of the volume outside of galaxies is photoionized. More appropriate would be the `filter mass’ (takes into account time evolution of T and density during collapse of a cloud). Gnedin 2000 Jeans Mass Filter Mass Not possible (or more difficult) for baryons to collapse into Dark 9 matter halos with M < 10 Msun. This corresponds to 5 Feedback Mechanism II: Stellar Feedback. Photoionization feedback provides natural explanation for Missing Satellite Problem, but... though plausible, no direct evidence exists for photoionization feedback in action in contrast, there exists ample evidence for so-called `stellar’ feedback, e.g. M82, nearby dwarf galaxy with a recent star burst. Red indicates H-alpha emission from outflowing gas. Outflow driven by supernova explosions, and/or radiation pressure. 6 Feedback Mechanism II: Stellar Feedback. Stellar feedback is increasingly efficient in removing gas from galaxies towards lower mass dark matter halos (derive on board). Assume that we convert some fraction f* of baryonic matter into stars. The higher f*, the more stars explode as supernovae. If supernovae drive out gas from galaxies, then this limits how much gas is available to star formation. In other words, supernova-feedback can put a limit on how large f* can be. We derive (on the board + lecture notes 12) that this maximum f* scales with Vcirc as In other words, supernova feedback suppresses star formation preferentially in low mass halos. 7 Missing Satellite Problem in Models that Include Feedback Munoz et al. 2009 Predicted number of satellites with no feedback Predicted number of satellites with feedback* * Note: feedback tuned to reproduce observations. 8 Missing Satellite Problem: Observational Advancements 2005: Discovery of new Milky Way companion Willman 1 (dwarf galaxy). 9 Missing Satellite Problem: Observational Advancements 2005: Discovery of new Milky Way companion Willman 1. This object is classified as an `ultra-faint dwarf’ galaxy (MV=-3.0; 400.000 times fainter than faintest, most distant galaxies we talked about earlier!) Difficult to find these structures. Circles highlight an overdensity of faint, blue stars. 10 Missing Satellite Problem: Observational Advancements 2005: Discovery of new Milky Way companion Willman 1. This object is classified as an `ultra-faint dwarf’ galaxy (MV=-3.0; 400.000 times fainter than faintest, most distant galaxies we talked about earlier!) Difficult to find these structures. Circles highlight an overdensity of faint, blue stars. To date, another 15 ultra-faint dwarfs have been found! 11 Missing Satellite Problem in Summary The Missing Satellite Problem refers to the apparent discrepancy between the predicted number of dark matter satellites and observed number of dwarf galaxies around the Milky Way There is no shortage of baryonic processes which suppress the efficiency at which stars can form in low mass satellites of Milky Way. New ultra-faint dwarf galaxies have only recently been discovered. Stellar kinematics suggests that there exists a large spread in relation between luminosity + dark matter halo mass. Reasonable to (for now) regard the relation between low mass dark matter halos & `ultra-faint dwarfs’ as puzzle of galaxy formation physics (feedback) instead of a contradiction of the standard cold-dark matter paradigm. A bigger - more pressing - problem is related to the most luminous satellites. 12 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012 Boylan-Kolchin took 6 hydrodynamical simulations designed to simulate `Milky-Way dark matter halos’ with a variety of mass & force resolution. Compared the properties of the most massive simulated satellites with the most luminous satellites. 13 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012 Satellite Luminosity function observed simulated Models that best reproduce observed satellite luminosity function - and hence best `solves’ the missing satellite problems, predicts that all satellites have significantly larger rotational velocities! 14 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012 Satellite Luminosity function observed simulated Because rotational velocity provides a measure of enclosed mass, predicted satellites are too massive (`too big’). 15 `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012 Because rotational velocity provides a measure of enclosed mass, predicted satellites are too massive (`too big’). If the most luminous observed satellites around the Milky Way indeed reside in dark matter halos with vcirc < 25 km/s, then why do all 10 (!) more massive dark halos not have observed low-luminosity counter parts? Why is star formation(relatively) efficient SF in vcirc < 25 km/s, but much less so in the more massive objects (which presumably are more `immune’ to feedback effects, i.e. `too big to fail’). `One possible explanation is that the matter concentration is less concentrated than what current dissipationless simulations predict’ (see next slides.) 16 From previous lecture: Observational Constraints on Dark Matter Halo Profiles `Rotation curves’ of gas rich dwarf galaxies. I showed this to illustrate the `cusp- core’ problem. (from Moore 1994) Simulated profiles have cusps (density ~ r-1), while observed rotation curves favor `cored’ profiles (density is constant). vROT Radius (kpc) From previous lecture: Observational Constraints on Dark Matter Halo Profiles `Rotation curves’ of gas rich (from Moore 1994) dwarf galaxies. Cusp-core problem: simulated density profiles 1. are steeper. 2. have higher central densities 3. steeper rotation curves. vROT Appears related to the discrepancy in predicted and observed circular velocities? At fixed r, can get significant differences! Radius (kpc) `Too Big To Fail’ Problem. Boylan-Kolchin et al. 2011/2012 Models that best reproduce observed satellite luminosity function - and hence best `solves’ the missing satellite problems, predicts that all satellites have significantly larger rotational velocities (i.e. are more massive, or more centrally concentrated). There is a possible connection with the cusp-core problem, which also states that simulated dark matter profiles have higher central densities than what has been inferred from gas (and stellar) kinematics in dark-matter dominated galaxies (dwarf galaxies, and low-surface brightness galaxies) 19 The Small-Scales Crisis in Cold Dark Matter Cosmologies Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced: Problem 1 ‘cusp-core problem’: Simulated density profiles are `cuspy’, inferred profiles are `cored’. Some proposed solutions: 1. Cuspy profiles originally predicted in dark-matter only simulations. Baryonic physics can transform cusps into cores (e.g. via supernova feedback, but this requires a minimum stellar content). 2. Modify dark matter properties (next). 3. Modify gravity. 20 The Small-Scales Crisis in Cold Dark Matter Cosmologies Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced: Problem 2 ‘missing satellite problem’: The simulated and observed satellite distributions around the Milky Way are inconsistent, in the sense that simulations predict many more satellites. Solution: 1. Feedback. Can bring down the predicted number to the observed number - which has increased in recent years (about 15 ultra-faint dwarfs have been discovered in the past 10 years) Caveat: This problem has `transformed’ into the `too big to fail’ problem (discussed next). We are not missing satellites, rather simulations predict them to be too massive. 21 The Small-Scales Crisis in Cold Dark Matter Cosmologies Summary (Incl. Lecture 11.) When comparing the structure of dark matter halos as obtained from cosmological simulations to structure inferred from gas & stellar kinematics, some problems surfaced: Problem 2 (absorbed `missing satellite problem’) ‘too-big-to-fail- problem’: Predicted masses of Milky Way satellites are significantly higher than observationally inferred values. Solution: The problem appears connected to the cusp-core problems. So solutions range from baryonic physics, to modifying dark matter or gravity. Interestingly: The `severity’ of the problem depends on mass of Milky-Way halo, which is still somewhat uncertain (see video). 22 The Small-Scales Crisis in Cold