Scaling Relations between warm galactic outflows and their host galaxies
Theory predicts that completely removing gas from galaxies regulates the baryon to non-baryon fraction, the star formation rate, and the mass-metallicity relation. Crucially, massive stars inject energy and momentum into the gas within galaxies with an efficiency that depends upon the stellar mass of the galaxy. Simply, low-mass galaxies lose more gas through galactic outflows because their smaller gravitational potentials necessitate less energy and momentum to completely remove gas from the gravitational potential. The energetics of the outflows can be observed, and the comparison to theory provides an important check for how galaxies form and evolve.
Here I form a sample of 48 nearby (z < 0.27) star forming galaxies observed in the ultraviolet (UV) with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). From these spectra I measure the outflow velocities from Si II absorption lines, and produce "scaling relations" between the outflow velocity and stellar mass or star formation rate (SFR). Below I give brief over view of the data, how we measure the stellar mass and SFR, and two interesting relations. For more details see the paper.
Here I form a sample of 48 nearby (z < 0.27) star forming galaxies observed in the ultraviolet (UV) with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). From these spectra I measure the outflow velocities from Si II absorption lines, and produce "scaling relations" between the outflow velocity and stellar mass or star formation rate (SFR). Below I give brief over view of the data, how we measure the stellar mass and SFR, and two interesting relations. For more details see the paper.
Sample Selection and data reductionI formed a sample of 48 star forming galaxies by searching the HST abstract archive for COS proposals observed with the G130M and G160M grating. I then remove the stellar continuum using a sophisticated multiple stellar population synthesis of Starburst99 models. The Milky Way absorption lines, and geocoronal lines are carefully accounted for. The spectra are then ready for profiles to be fit with up to ten Voigt profiles (see the red line to the left). I then measure the velocity at 90% of the continuum, and the central velocity of the fits because nearby lines contribute to the profiles.
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Host properties
I measure the stellar mass (M*) and star formation rate (SFR) of the galaxies using a consistent ancillary data set. The SFRs use a combination of UV luminosities from GALEX and infrared luminosities from WISE, while the M* use WISE luminosities. The sample is shown on the right, and is compared to the entire Sloan Digital Sky Survey (SDSS) sample in contours. The sample covers nearly four decades of M* and SFR, probing dwarf star forming galaxies and high-mass mergers. SDSS and HST images are used to classify the morphologies of the galaxies as: irregulars, spirals, compacts, or mergers.
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The outflow velocity only changes by 75% over 4 decades of SFRI then compare the "scaling relations" between maximum outflow velocity and SFR, as shown on the left (see the paper for the central velocity). This scaling relation is extremely shallow (the velocity scales as SFR^0.08). This shallow relation is crucially shows that the outflow velocity only changes by a factor of 1.75 over four decades of SFR. Therefore, massive galaxies drive outflows at very similar velocities as low-mass galaxies. Also note: mergers drive faster outflows than non-interacting galaxies. Even after accounting for the larger SFR of mergers, mergers drive 32% faster outflows than non-interacting galaxies.
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Only Low-mass galaxies lose low-ionization gas, unless high-mass galaxies undergo a mergerThe plot on the right shows how the maximum outflow velocity scales with the circular velocity of the galaxy. The escape velocity is included as a dashed line, and intersects the locus of points near a circular velocity of 200 km/s. This means that outflows from low-mass galaxies are capable of escaping the galactic potential. However, galaxies with circular velocity greater than 200 km/s tend to drive outflows slower than the escape velocity, and the galaxy retains all of the Si II gas. There is an interesting exception to this rule: there are five mergers (blue squares) and two compact galaxies that are high-mass, but drive an outflow fast enough to escape the potential.
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