Kennicutt (1994) proposed a simplest formula to estimate star formation rate (SFR) of the galaxies. I want to calculate SFR of some dwarf galaxies using SDSS spectroscopy and compare it with the SFR model for star forming dwarfs.
I read some papers which highlighted that when using the Hα flux for SFR estimates, we encounter the following difficulties, which may cause systematic errors: (i) contamination by [NII] emission lines, close to the Hα line, (ii) contamination by the other Hα emitters (e.g, other emission nebulae, non-thermal emitters, such as active galactic nuclei (AGN)), and (iii) internal extinction.
On the other hand, it is said that SDSS spectra data are well calibrated, so we do not need to perform any further corrections.
I am bit confused with this second statement.
Dear Sujan Prasad Gautam
Your second and third paragraphs are about different things. The third one is only about flux measurements. In principle in order to measure the flux of a line you need to worry about the proper flux calibration, response of your telescope, slit loss (galaxy larger than the slit), atmospheric attenuation, attenuation by dust in the Milky Way and so on. SDSS states that all these corrections are already done, so you can measure the flux of the line without doing anything else.
The second paragraph is about measuring SFRs from the Halpha flux. This is a more complex question, because this requires making assumptions. To the list of the issues you stated I would add
1) unknown star formation history. The calibration Halpha flux -> SFR depends on how galaxy evolved in the past. In the most simple way this can be stated that Halpha line is only sensitive to very recent star formation (last 10 Myr or so), so if you are interested in SFRs averaged over a longer period, then you can be severely off if SFR changed a lot over the timescales of 10 Myr.
2) Initial mass function (IMF). If your galaxy has a top-heavy IMF (more massive stars per unit stellar mass), then its Halpha flux will be higher for the same SFR.
These problems are nearly impossible to account for, you can only comment that they may change the SFR by a factor of a few or so.
By the way, your issue (ii) may be applicable to your dwarf galaxies in terms of Halpha emission produced in shocks, but it is unlikely that they harbour an AGN.
Hope this helps.
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