Spectroscopy and star formation

Sometimes the reason is obvious and sometimes it is completely coincidental, but I am always interested by days that astro-ph seems to have themes.   Today, it was a little bit of both as many groups were releasing their white papers for the Kepler mission in 2-wheel mode, but there was also 3-4 papers on star formation in star forming galaxies.   These papers particularly caught my eye as I have been meaning to get back to my work on star forming galaxies in clusters, and in particularly, measure the star formation and metallicity from the Keck spectroscopy.

The first paper is by Stott et al.  looking at the fundamental metallicity relationship for galaxies at z=0.84 and z=1.47 selected based on narrow-band Hα imaging.  One impressive aspect is that they used FMOS on Suburu to perform near infrared spectroscopy on 400 galaxies at a time!     In the end, they have a sample of 103 galaxies for which they can estimate the mass, star formation rate, and metallicity.   They find the same amount of metals in their sample as in low redshift star forming galaxies.   Comparing their results to Mannucci et al. (2010), they find that their sample does lie on a Fundamental Metallicity Relationship between mass, metallicity, and star formation, although it is flatter than the low redshift one.  In the FMR, objects with higher stellar mass and higher star formation rates also have higher metallicities, and this relates to the inflow and outflow of material in galaxies.

The second paper is by Levesque and Richardson and looks at the ratio of [NeIII]λ3869/[OII]λ3727 for use as an diagnostic for the ionization parameter in star forming galaxies.    The ionization parameter is basically related to how much ionizing flux there is and how much material there is to be ionized.  Usually this is measured with the [OIII] λ5007/[OII]λ3727 line ratio, but this can be hard to observe at high redshift and affected by extinction.    Several interesting things arise from comparing real observations between the two line parameters as well as a large library of star forming emission line spectra:  1) There is a tight relationship in the observed galaxies as well as what is predicted from the models between these two parameters (Good it can be used to estimate the ionization parameter), 2) The predicted and observed relationship are offset from each other  (may be due to the initial ionization spectrum in the models), and 3) both diagnostics perform equally well (or poorly) at high q and both are similarly dependent on knowing the metallically as well.  Also a fun but unrelated fact, the ionization threshold for [OII] is 13.62 eV which is almost exactly the same as [HI].

The third paper is by Hong et al.  about shock-ionized gas in nearby starburst galaxies.  This is a very large paper and I have to admit to not giving it a thorough read through, but it also does have some beautiful pictures and some very interesting results.  They measured resolved line ratios from narrow-band HST imaging of nearby star-bursting galaxies.  They could then determine how much luminosity in Hα was coming from shocked gas vs. photo-ionized gas.   I have to admit that I was pretty shocked (pun intended and too easily to avoid)  to find that up to 30% of the total Hα luminosity in some of the galaxies was coming from the shocked gas.   I had no idea that much of the Hα luminosity could be coming from shocked gas in a non-AGN starbursting galaxy and I am now wondering how that affects some of our measurements of the star formation and metallicity in galaxies.   Looking a little further, it looks like the ratio of shocked to total increases for smaller mass and/or higher star formation, although the small sample size makes definitive conclusions difficult.  As I said, I have not done this paper justice, but it is definitely something worth revisiting.