### Question --- Describe the following galaxy-formation problems: (i) the missing satellite problem; (ii) the core- cusp problem; (iii) the cooling flow problem. Which of these are solved? What are their solutions? ### Answer --- ##### (i) Missing Satellite Problem [[Dark Matter#CDM|CDM]] simulations (like [[Galaxy Profiles#Navarro-Frenk-White Profile|NFW]]) overpredict the number of small satellite galaxies around larger galaxies in comparison to observations. In other words, simulations produce more satellite halos than there are observed satellite galaxies. This could mean that the smallest DM halos are inefficient at forming stars, creating “dark subhalos". In this case, they exist, but we can't observe them directly and we must use lensing or stellar streams to infer their existence. Recently though, there have been many more satellites found as our observational techniques improve. ![[missing_satellites.png|align:center|450]] **Resolution?** - Recently deep surveys ([[Catalogs#SDSS|SDSS]]) have found many more extremely faint dwarf galaxies (raising up the observations above). [[Instruments#LSST|LSST]] is forecasted to improve this further - Simulations like FIRE have demonstrated that careful modelling of baryonic physics in simulations actually resolves this tension - Remaining Questions: - *What is suppressing star/galaxy formation in small halos? Is it feedback?* - *What mass is associated with this change in efficiency?* > [!note] > Nora Shipp (here at MKI) has demonstrated that the tension may be resolved by carefully accounting for detectability limits of satellite dwarfs. In fact, within error, one may have "too many" satellites of a MW-like galaxy in FIRE when accounting for our lack of understanding of disruption rates in FIRE versus observations. > [!space] Too-Big-to-Fail Problem > > This is related to the missing satellites problem, where the number of predicted large halos doesn’t match the number of large galaxies observed (but the total number of satellite halos is consistent). The gravitational potential of these galaxies, however, is large enough that they should have collected enough gas and stars to form galaxies and maintain their evolution (e.g. not lose the stars through stripping). > [!space] Diversity of Shapes Problem > > Observationally, halos display diversity in the shapes of their profiles with some cuspier and some more cored profiles whereas, in simulations, halos are universally described by the NFW profile and self-similar across mass ranges (the profiles look the same when scaled). ##### (ii) Core-Cusp Problem [[Dark Matter#CDM|CDM]] simulations (like [[Galaxy Profiles#Navarro-Frenk-White Profile|NFW]]) predict a sharply rising density profile near the center of halos ("cusp"), but observations show a flatter ("cored") density profile near the centers of halos. ![[core_cusp.png|align:center]] **Resolution?** - Feedback could help remove matter from the core - [[Dark Matter#WDM|WDM (warm dark matter)]] or [[Dark Matter#SIDM|SIDM (self-interacting dark matter)]] - [[Dark Matter#MOND|MOND (Modified Newtonian Dynamics)]] ##### (iii) Cooling Flow Problem Mass cooling rates from [[Electromagnetic Spectrum|x-ray]] measurements of the [[Intracuster Medium|ICM]] suggest that gas should be cooling and forming stars at rates of $\sim 100-1000 \; {\rm M_{\odot}/yr}$ ; however, we observe star formation rates on the order of $\sim 1 \%$ of these rates in [[Galaxy Cluster|cluster]] cores (and there is not enough cool gas to account for the difference because "it happened in the past"). **Resolution?** - [[Active Galactic Nuclei|AGN]] feedback does mechanical work on the gas in the [[Intracuster Medium|ICM]] through relativistic jets originating from close to the [[Black Hole#Supermassive Black Hole|SMBH]] - This heats up the gas and prevents star formation - The details of how the jets deposit energy into the cluster is very unclear.