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The "Astronomy Picture of the Day" today sent me down a rabbit-hole. It had a picture of Leo I, with a note that it may have a supermassive black hole at the center, so I went to Wikipedia for a bit more info, and I came across this note:

Typical to a dwarf galaxy, the metallicity of Leo I is very low, only one percent that of the Sun.

It's a claim made without a source, and I can't find any additional info elsewhere, so what's different about dwarf galaxies? I would think that stellar evolution would have proceeded as it does in larger galaxies. Do more of the heavy elements expelled during nova events leave the galaxy?

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    $\begingroup$ Because the rate of star formation in dwarf galaxies is so low, there simply hasn't been time for many second- and third-generation stars, which have higher metallicity, to form. Almost all the stars were formed from the galaxies' initial complement of unadulterated primordial hydrogen. $\endgroup$
    – antlersoft
    Commented Apr 27 at 18:46
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    $\begingroup$ ... and primordial helium. $\endgroup$
    – PM 2Ring
    Commented Apr 27 at 19:17
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    $\begingroup$ The "Astronomy Picture of the Day" today sent me down a rabbit-hole. Occasionally, they intentionally send you down a black hole: apod.nasa.gov/apod/image/1410/bhlens_riazuelo_2560.jpg $\endgroup$
    – Amazon Dies In Darkness
    Commented Apr 28 at 11:39

2 Answers 2

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The basic reason is that the winds and supernovae, due to high-mass star formation, drive out the enriched interstellar gas more easily from the shallow gravitational potential wells of dwarf galaxies, preventing the formation of more metal-rich stellar populations.

There is a well-known relationship between the mass of a galaxy and both the metallicities of its stellar populations and its interstellar gas (e.g., Tremonti et al. 2004). The slope of this relationship is in the sense that massive galaxies have the highest metallicity and low-mass dwarf galaxies have the lowest metallicities. The intrinsic scatter in the relationship is only about 25% in the gas metallicity over many orders of magnitude in mass.

The metallicity of the stars is related to the time-average of the metallicity in the gas - because the stars were born at a range of times in the past. The metallicity in the gas reflects the current level of enrichment, which is driven by the rate at which stars are dying but is also heavily influenced by both the outflows of enriched gas from the galaxy and the diluting inflows of pristine primordial gas.

The basic reason that dwarf galaxies end up being metal poor, both in their stellar populations and in their gas is that they cannot hang onto the interstellar gas that they enrich. That is, the outflows, driven by the winds and supernovae of massive stars and by active galactic nuclei, are more easily able to drive out enriched interstellar gas from the shallower gravitational potential wells of dwarf galaxies before it can be turned into more metal-rich stars. See Romano et al. 2023 for direct evidence of these outflows in dwarf galaxies and see Ma et al. (2015) and van Loon et al. (2021) for examples of galactic chemical evolution models featuring hydrodynamic simulations of outflow/inflow that reproduce the mass-metallicity relationship.

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Stars are the main progenitor of heavy elements - the interstellar medium is populated with such materials after a star's death.

Dwarf galaxies, with their low mass and size, have less available star-forming matter within the interstellar medium, resulting in a significantly lower stellar population than major galaxies like the Milky Way - which has nearly half a trillion stars in comparison to a couple billion.

Thus, dwarf galaxies generally have a low metallicity due to their low stellar populations.

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    $\begingroup$ "However, when stars are formed in small galaxies, they are generally of lesser mass" -- No, there is no evidence for a difference in the Initial Mass Function between galaxies of different masses (except possibly in the centers of giant ellipticals). So the fraction of stars which are low mass is the same when stars are formed in low-mass and high-mass galaxies. $\endgroup$
    – Peter Erwin
    Commented Apr 28 at 8:18
  • $\begingroup$ The fact that low-mass stars have long lifetimes is irrelevant. Most of the enrichment comes from two sources: high-mass stars (mass $> 8 M_{\odot}$) which explode as core-collapse supernovae, and intermediate-mass stars which produce metal-enhanced stellar winds and end their lives as white dwarfs -- some of which explode as Type Ia supernovae. The timescales for these have nothing to do with a galaxy's stellar mass. $\endgroup$
    – Peter Erwin
    Commented Apr 28 at 8:23
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    $\begingroup$ This sounds plausible, but surely what matters is the fraction of the material that is formed into stars. If there is less material it requires fewer stars to enrich it. You need to provide citations to work that suggests dwarf galaxies form fewer high mass stars as a proportion. $\endgroup$
    – ProfRob
    Commented Apr 28 at 10:10
  • $\begingroup$ @PeterErwin, thanks for the note, I've updated the post - I misunderstood. $\endgroup$
    – 4NT4R3S
    Commented Apr 29 at 0:51

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