Abstract
Working with 108,786 Sloan Digital Sky Survey low redshift galaxies, we have
examined the relation between galaxy mass, metallicity, radius, and star
formation rates. We subdivided the redshift range covered in our sample
0.07<z<0.3 into three narrower redshift bins, and three sets of radial size. We
describe several new relations that these subdivision reveal. We then show that
for 85-90% of the galaxies the observed gas metallicities, Zx, are consistent
with (i) a quantitative physical relation for star formation through episodic
infall of gas devoid of heavy elements; (ii) thorough mixing of infalling and
native gas before onset of star formation; (iii) a star formation rate
proportional to the 3/2 power of the infalling mass rate, M_i; and (iv)
intermittent quiescent phases devoid of star formation during which the native
gas in a galaxy exhibits a characteristic elevated gas metallicity, Zo,
dependent on galaxy mass, M*, and a characteristic ratio of stellar mass to
native mass of gas, Mg. Sets of galaxies not obeying these relations invariably
have masses M*>2.8E10 Msun and exhibit high metallicities at high star
formation rates, suggesting that for these galaxies star formation independent
of mass infall plays a significant role. A toy model for the physics of infall
accounts for the SFR Mi^(3/2) relation and permits us to estimate the mean
densities and velocities of clumps of baryonic matter traversing the dark
matter halos in which the SDSS galaxies may be embedded. The model also
reproduces the gross features of the galaxy main sequence.
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