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The Lambda-Cold Dark Matter (ΛCDM) model has been extremely successful in reproducing observations and scaling relations of the large scale structure in the Universe. However, one major failure of the model is that it overpredicts the number of low mass and high mass galaxies: gas in galaxy disks cools too soon and forms too many stars. But star formation is, by and large, an inefficient process, with only ~1% of gas being converted into stars (the Kennicutt-Schmidt relation). So why does this happen? Furthermore, observations of quasar sightlines through the CGM of local galaxies reveal large reservoirs of metal-enriched gas. But metals are produced from stars within the galaxy disk. So where did these metals come from?

Galactic-scale outflows go a long way toward reconciling these questions: pressurised momentum and energy from massive stars - in the form of stellar winds, supernovae, and high energy photons or cosmic rays, collectively known as "feedback" - can sometimes collimate into a biconical outflow. In many cases, the outflows are able to eject gas from the disk into the surrounding CGM and the turbulence from them can prevent star formation in the disk and cause heating of the surround CGM gas.

Invoking these in simulations aids to reconcile the differences with observations: ejecting the cold gas and preventing accretion of new gas necessary for star formation ensures feedback from supernovae at early times prevents the build up of low mass galaxies, whilst feedback from AGN at later times prevents too many massive galaxies. Furthermore, the ejected gas is typically metal-rich, meaning large quantites of metals are dumped into the CGM, which aids to explain the observations.


However, despite the success of outflows in reconciling the above differences, much about them remains unknown. For instance, since they are generally a consequence of star formation and AGN, they vary greatly in strength and have mostly been constrained in extreme objects, rather than the general galaxy population. This has implications about the quantities of gas that galaxies are able to eject and whether the outflow quenches the galaxy. Does the amount of ejected gas depend on outflow velocity, levels of star formation, AGN activity, stellar mass, etc? Complicating this picture further, outflows don't only occur in one gas phase: when calculating the amount of ejected material, one has to account for gas in the ionised, neutral, and molecular phases. Only a few cases of very energetic galaxies have enough quality data to make such complete measurements, but these are not always representative of the general galaxy population. These measurements have to be linked to the galaxy host and some key questions are:

  • is star formation or AGN activity the primary driver of outflows?

  • do outflows quench the galaxy's star formation?

  • which gas phase dominates the outflow and what happens to the cold gas?

  • how do the outflows link with the CGM and interplay with inflows?

  • what is the structure of outflows


These are some of the major unanswered questions that astronomers are investigating, and with exciting facilities such as ALMA, Keck, and the VLT, as well as new large surveys (single spectra and integral field) such as MaNGA and SAMI, new and exciting windows onto outflows and their impoact on galaxy evolution are being opened.

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