The amount of line emission from a galaxy is thus a rough proxy for the rate of star formation – the greater the rate of star formation, the larger the number of large stars exciting interstellar gas into emission nebulae… Indeed, their preferred model to which they fit the trend converges towards a finite quantity of stars formed as you integrate total star formation into the future to infinity, with the total number of stars that will ever be born only being 5% larger than the number of stars that have been born at this time.
Is this a good proxy for total star formation, or only large star formation? Is it plausible that while no/few large stars are forming, many dwarfs are?
That depends on something called the “initial mass function” for a star forming region—the frequency distribution of masses produced. See http://model.galev.org/help/help_imfs.png for two estimated mass functions for our galaxy.
Until recently the consensus was that since the initial mass function was pretty similar throughout our own galaxy under very different environments, it should be similar in other places too. More recently there’s been some controversial claims that ‘Early type’ (elliptical) galaxies may have a systematically different mass functionn than spirals that also varies by galaxy mass, see http://astrobites.org/2012/02/16/the-imf-is-not-universal/ . Other research seems to contradict this, see http://astrobites.org/2014/12/08/counting-stellar-corpses-rethinking-the-variable-initial-mass-function/ . These papers become technical to a point at which I get lost pretty easily in reading them. If I am reading the paper referred to in the first link correctly though, their findings if true are consistent with one of two scenarios: either the mass functionin massive elliptical galaxies is biased towards the formation of large amounts of small stars, or it is biased towards the production of large amounts of large stars which are now dead and contributing excess compact mass in the form of dead star remnants. Both would be consistent with the data (which comes in the form of ratios of luminosity to galactic mass) but a mass function like that of most spirals would not be.
If the mass function stuff turns out true, I’m pretty sure it would distort the shape of the curve of star formation referenced in this post one way or another, but not change its ultimate form.
Is this a good proxy for total star formation, or only large star formation? Is it plausible that while no/few large stars are forming, many dwarfs are?
That depends on something called the “initial mass function” for a star forming region—the frequency distribution of masses produced. See http://model.galev.org/help/help_imfs.png for two estimated mass functions for our galaxy.
Until recently the consensus was that since the initial mass function was pretty similar throughout our own galaxy under very different environments, it should be similar in other places too. More recently there’s been some controversial claims that ‘Early type’ (elliptical) galaxies may have a systematically different mass functionn than spirals that also varies by galaxy mass, see http://astrobites.org/2012/02/16/the-imf-is-not-universal/ . Other research seems to contradict this, see http://astrobites.org/2014/12/08/counting-stellar-corpses-rethinking-the-variable-initial-mass-function/ . These papers become technical to a point at which I get lost pretty easily in reading them. If I am reading the paper referred to in the first link correctly though, their findings if true are consistent with one of two scenarios: either the mass functionin massive elliptical galaxies is biased towards the formation of large amounts of small stars, or it is biased towards the production of large amounts of large stars which are now dead and contributing excess compact mass in the form of dead star remnants. Both would be consistent with the data (which comes in the form of ratios of luminosity to galactic mass) but a mass function like that of most spirals would not be.
If the mass function stuff turns out true, I’m pretty sure it would distort the shape of the curve of star formation referenced in this post one way or another, but not change its ultimate form.