Outflows from Star Formation May Drive Calcium into the Halos of Galaxies

Calcium (Ca) came from stars. Along with all other elements other than hydrogen, helium, and a bit of lithium, calcium was made in the fusion furnaces that power the immense amounts of energy given off by stars. To make calcium requires a star at least five times the mass of our Sun and is most efficiently produced by even more massive stars that explode as supernovae and distribute their elements to their surroundings[1]. What happens to this material and how it gets distributed around a galaxies is a key current question in the formation of galaxies.

SDSS-III members Guangtun Zhu and Brice Ménard (Johns Hopkins University) have just completed a study searching for Ca II around galaxies in SDSS Data Release 7. This paper has been submitted to the Astrophysical Journal and appears on today’s arXiv.org:

“Calcium H&K Induced by Galaxy Halos”
Guangtun Zhu, Brice Ménard

Using signatures imprinted in the spectra of distant quasars, they find that Ca II, calcium missing one of the two electrons in its outermost subshell, exists in the outskirts of galaxies, and there is a lot of it.

The search for Ca II used the Fraunhofer H&K absorption lines. These absorption lines were first discovered in the solar spectrum about 200 years ago. The wavelengths of these lines are defined by the amount of energy difference between the outermost electron of singly-ionized calcium to be in the 4s (l=0) instead of 4p state (l=1). The difference in energy between H and K is the difference between the spin of the electron (s=1/2) being in the same direction (more energy, labmda=396.8 nm, J=|l+s|=3/2) or in the opposite direction (less energy, lambda = 393.4 nm, J=|l-s|=1/2) as its orbital momentum around the nucleus[2].

When light passes through a cloud of Ca II, photons with wavelengths at 383.4 nm and 396.8 nm can be absorbed, leaving a signature in the light that passes through the cloud.

Examples of background quasars whose light passes near galaxies. Images and quasar spectra were both taken with the SDSS.

Thus the researchers set out to search for signs of a small amount of missing light at these wavelengths. However, the signature of Ca II absorption in an individual quasar spectrum is almost impossible to see. This study was thus only possible by combining the vast quantity of spectra made available by SDSS. By using the light from 100,000 background quasars that passes near galaxies in SDSS DR7, Zhu and Ménard found that significant amounts of Ca II were present up to 700,000 light years (200 kpc) away from the galaxies.

The image above shows the density of Ca II (right axis) as a function of the distance (rp) from the center of the galaxy. Significant amounts of Ca II are found all the way out to 200 kpc (700,000 light years).

The Ca II is more concentrated around star-forming galaxies than passive galaxies and is preferentially found above the plane of disk galaxies. These results are consistent with the Ca II coming from bipolar outflows from the galaxies that are driven by star formation. These outflows carry along many of the elements formed in the stars and released by supernovae.

This study is only sensitive to Ca in this ionization state where it has lost one electron. The distribution of neutral Ca or other ionization states remains unknown.

[1] http://en.wikipedia.org/wiki/Supernova_nucleosynthesis
[2] http://physics.nist.gov/PhysRefData/Handbook/Tables/calciumtable4.htm

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