GALACTICWINDS · TESTING GALACTIC WINDS AGAINST PREDICTIONS

TESTING GALACTIC WINDS AGAINST PREDICTIONS

Project objectives: Today, astronomers have a good understanding of the formation of the large scale distribution of galaxies. But, a major discrepancy remains in the number counts of galaxies, namely the predicted number density of galaxies is too high compared to the observed number density of galaxies, and this problem is a lot worse in low-mass galaxies. In other words, it appears that the galaxy formation efficiency is somehow suppressed in this mass regime and the main goal of this project is to understand whether galactic winds can be the culprit. Indeed, galactic winds is a phenomenon often seen in star-forming galaxies where the wind is the results of the collective effect of supernovae. Galactic winds have been postulated to resolve the number count discrepancy, and are though to eject 90% to 99% of the fuel for star-formation back into the intergalactic medium. But no one has been able to measure the mass ejection rate, and thus no one has been able to confirm or refute this hypothesis. This study aimed at studying the physical properties of galactic winds (and how they might depend on the galaxy properties) in order to confirm or refute the prevailing hypothesis. In order to achieve this objectif, we have built a sample of star-forming galaxies "caught in the act" of producing super-winds where the wind signature is seen in absorption against background quasar spectra. But galaxies with a background quasar within its vicinity (100 kpc) is a rare phenomenon. So we designed several surveys to build large statistical samples of galaxies with background quasars, often referred to as galaxy-quasar pairs. First, we used the a sample of very strong absorbers and observed them with the SINFONI instrument. This SINFONI MgII Program for Line Emitters led to 14 galaxy-quasar pairs. Second, we acquired about 2 dozens galaxy-quasar pairs in the local universe at z=0.2 selected in the imaging data base from Sloan Digital Sky Survey. Last but not least, we designed a survey with the new instrument Multi Unit Spectroscopic Explorer (MUSE) for the European Very Large Telescope (VLT) in Chile, the MusE GAs Flow and Wind (MEGAFLOW), that will increase the sample size available by a factor of 10x thanks to this strategic design: we selected quasar sight-lines with multiple (N=3, 4 or 5) MgII absorbers instead of just one. The survey comprises of 22 quasar fields, of which 15 have both MUSE and UVES observations completed, yielding a total 59 galaxy-quasar pairs with a success rate of 85% in detecting the galaxy associated with the MgII absorbers. In total, we expect 85+ pairs in the whole survey. This sample will allow us to study winds in great details and allow us to determine how the wind properties might depend on the galaxy properties. Results: For these three surveys, the SINFONI survey, the SDSS survey and the MUSE surveys, we have obtained over 100 quasar-galaxy pairs. Only a fraction of these are favourable to study galactic winds. Indeed, only when the background quasar's apparent location is roughly perpendicular to the galaxy can we guarantee that the quasar line-of-sight is probing the wind material given that wind flows away from the galaxy towards the least dense regions. In order to determine the relative orientation of the galaxies with respect to the quasar line-of-sight from ground-based observations, we needed a tool to extract the galaxy's inclination and orientation from the 3D data (SINFONI & MUSE). For this purpose, we designed a software to measure the inclination of galaxies from such ground-based data. The algorithm directly compares data-cubes with a disk parametric model and uses a Markov Chain Monte Carlo (MCMC) approach with non-traditional sampling laws in order to efficiently probe the parameter space. The algorithm is available to the community at the following web site: http://galpak.irap.omp.eu. So far, we have been able to extract the wind properties on about 15 galaxies. These results appear in 3 publications (Kacprzak et al. 2014, Schroetter et al. 2015, 2016). We find that outflow rates are comparable to the star-formation rate of galaxies, and more surprisingly, that in most cases the wind material is not able to escape from the gravitational potential well of galaxies.

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