Give Peas a Chance
Hi Pea Hunters,
Kevin and I wanted to give you an update with where we are in our Peas investigation. We haven’t answered all of our questions, but from detailed inspections of their spectra, it appears that the Peas are a mixed bag. A large portion of them appears to be powered by star formation, and perhaps an equal number show evidence of an active central black hole. The details of what I’ve done so far are below. Feel free to chime in with any questions or suggestions you all might have! The first thing I did was look at all of the peas that were highly rated as Green in our Pea Picker hunt. When I plotted them against a sample of randomly selected galaxies of at similar redshifts, they do stand out. This first plot is a color-magnitude diagram. In my opinion its one of the most widely used of all plots astronomers can make. Practically, this is probably because only 2 images are necessary to make this plot, but also historically because we’ve found out plots like these can tell us so much (eg. the HR diagram).
In the Pea color-magnitude diagram, you can see that the sample of typical galaxies lie at a U-R color near 3, while all of the peas identified here on this thread are much bluer. I know it might be confusing because we selected the peas to appear ‘Green’ in the SDSS images, but all that Green means there is that they are very bright in the ‘R’ band. The next step up in complexity from a color magnitude diagram is a color – color plot. I made several of these for the Peas, but if you look at the one I’ve posted here, you can see that the Peas jump out as very distinct from the galaxy sample. If we restrict our query to compact objects at the pea’s redshifts, ask for an [OIII] line to be detected in the Spectrum, and throw out luminous Quasars (QSOs) we find a sample of 439 Peas. I’ve plotted this sample of pea’s on both the color-magnitude diagram and the color-color diagram in Purple. (This Pea-search also returns all of Peas found by eye as well).
Now came the exciting part, figuring out what powers the peas. We downloaded all of their spectra from Sloan and used Gandalf to analyze their Spectral lines. Unfortunately, quite a few of our spectra had bright spectral lines we could see by eye, but did not have a high enough Signal-to-Noise ratio to make a precise measurement of the line flux. I also closely examined the fit of each spectrum and noticed that Gandalf had trouble with many of the Active Galaxies (AGN) in our sample. We’ve contacted the author of Gandalf and he is going to help us go back and fit these galaxies better so we can add them back in our sample of Peas. For now we’ve accurate spectral fits for 36 of the Peas. I’ve plotted them on a typical BPT diagram below. The BPT diagram was introduced by Baldwin, Philllips and Terlevich in 1981 and is a diagnostic, which can indicate if the gas in a galaxy is being heated by star formation (as in a starburst galaxy) or by very hot gas near a central black hole (as in an Active Galactic Nucleus). You can see most of them end up on the Starburst portion of the plot. However, this is simply because most of the fits to the AGN spectra were poor and so they were thrown out of our sample. Looking at the spectra by eye, it looks as though they are fairly evenly split between AGN and Starburst, but we’ll get a better handle on this when we can accurately fit the AGN spectra.
For the Peas powered by star formation, we can look at how many stars are forming each year. For comparison, Milky way’s star formation rate is around 1-2 solar masses per year. The typical Pea from our sample seems to have a star formation rate nearly 5 times higher than this, and for many Peas the rate at which they form stars is up to 40 times larger. Because the Peas are so much smaller then the Milky Way, these are incredibly large star formation rates.
Right now we’re working on improving our spectral analysis and searching the data archives for evidence of X-ray emission from the Peas. With X-ray emission we will be able to measure the AGN activity and where Star Formation dominates, we can get a second estimate of the star formation rate in the Peas. We are also looking for infrared emission from the peas. This will be an additional measure of their star formation rates & their stellar masses. Finally, we plan to apply for time on a larger telescope to get better spectra. These higher Signal-to-Noise spectra will allow us to measure the BPT line diagnostics better for a larger number of our Peas, and most importantly it will let us look at other spectral features that can inform us on the Peas stellar masses.
Carie & Kevin