Galaxy Zoo Starburst Talk

Star formation Rates

  • trouille by trouille scientist, moderator, admin

    In the latest update to Quench Tools, there are a few new columns of data that have been added for both the post-quench sample and the control sample.

    Among these new columns is the star formation rate (SFR), taken from http://www.mpa-garching.mpg.de/SDSS/DR7/sfrs.html. We found our sources in the file on that page called 'gal_totsfr_dr7_v5_2.fits.gz', which gives the total star formation rates for each of the almost 1 million galaxies in the SDSS Data Release 7. We used the median SFR value. We also gave placeholder values of '-99' (or maybe we used '-1'?) for any sources that were flagged as unreliable.

    It would be quite useful to check if those SFR appear correct in the context of the rest of the data we have on hand. For example, is there a clean-ish relationship between Halpha fluxes (or OII fluxes) and SFR in our samples of galaxies?

    I started the process in this dashboard (making sure to first remove AGN from the sample before looking at the Halpha versus SFR). I did this by filtering out sources with log(N2Ha) > -0.3. I'd prefer not to use such a blunt cut (and instead use the actual Kauffmann03 distinguishing curves between AGN and star forming galaxies), but that's unfortunately not currently possible in Tools. Another Tools update request that I've added to the list.

    You'll see in the scatter plot-11 in that dashboard that there doesn't appear to be a clear trend for our star forming galaxies between Halpha and SFR. At first, this seems troubling to me.

    Then I remembered -- the emission line fluxes aren't extinction corrected. We can trust the results of the BPT diagram because we divide one flux by another (thus canceling out the effect of extinction). But we can't trust the emission line fluxes on their own.

    So, that brings me to a request. I know a few of you have derived extinction corrected fluxes. See http://quenchtalk.galaxyzoo.org/#/boards/BGS0000001/discussions/DGS00001zv. Could you make those values available through dropbox or Google spreadsheets?

    And, could you take a moment and see if there's a clean-ish relationship between the extinction-corrected Halpha fluxes and SFR for the galaxies that lie in the BPT-star-formation-dominated regime?

    I hope there is. If there isn't, then we should take a careful look at those SFR values and decide if we're going to use them or not in our study.

    Posted

  • mlpeck by mlpeck

    I checked and got these results:

    enter image description here

    This plots the log of the star formation rate as given in the data tables against extinction corrected Hα luminosity, where the extinction correction was done using the observed Balmer decrement as outlined in Moustakas et al. (2006), except that I used a little bit different coefficient in the relationship between E(B-V) and E(Hβ - Hα) because I was using a different reddening law.

    These show the combined quench and control samples for starforming galaxies only per the BPT diagnostic. Kennicutt's calibration of the star-formation rate Hα luminosity relation is the black line. The green line is an ordinary least squares fit, which isn't really the right thing to use here. The residual standard error around the least squares line is ~ 0.25 dex.

    Obviously there's a systematic offset from the Kennicutt relation, but I'm probably using a different extinction "law", at least slightly different cosmological parameters, and Kennicutt's calibration was based on a Salpeter IMF which is likely different from what the MPA team used.

    I'd be happy to share the data, but we should probably try to be on the same page regarding cosmological parameters and extinction. If you just want a sanity check this graph shows a fairly tight relationship over 4 orders of magnitude in luminosity and sfr.

    Posted

  • mlpeck by mlpeck

    To briefly follow up on the previous post, the Hα flux and therefore luminosity was for the fiber only, while SFR was extrapolated to the entire notional galaxy. I have fiber magnitudes in my personal database: adding 0.4*(fiberMag_r-modelMag_r) to the log luminosities eliminates the offset in the above graph. The scatter remains around 0.25 dex.

    According to the DR9 schema browser physical properties of galaxies in the MPA pipeline were derived following the procedures described in Brinchmann et al. (2004). They describe a much more elaborate procedure for estimating star formation rates from emission line strengths than simply scaling them.

    Posted